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According to what I know, the reflex arc of knee jerk reflex doesn't involve interneuron, but other reflex action (e.g. removing your hand when touching hot things) do involve interneurons. Why is that the case?
The "knee jerk" reflex is a stretch reflex, as @John's answer indicates. It is unfortunately named the "knee jerk" for the response observed in the corner case where rapid, short extension of the leg at the knee is elicited by tapping the patellar tendon with a percussion hammer during a physical exam. Despite being named for this specific phenomenon, this reflex contributes to setting resting muscle tone, controlling the response to a load, and coordinating synergist and antagonist actions. You can read about this in some detail in Kandel's Principles of Neural Science, Chapter 36.
The sensory portion of this reflex involves a proprioceptive sensory organ called a muscle spindle that detects the stretch of a skeletal muscle. All human (and most mammalian) skeletal muscles have these sensory organs, so this is not specific to the quadriceps at the knee. Several are tested in a similar way to the quadriceps as part of a neurological exam (see Brust, Practice of Neural Science, Chapter 6), but, as is the case with most reflexes, they are more complex than is taught in introductory biology. This sensory organ is continuously active, sending information to the cortex, the cerebellum, synergist and antagonist muscles, as well as back to the muscle in which the organ itself is found (again, see Kandel).
Answers about "why" a physiologic system is set up in a particular way are generally speculative, but stretch reflexes in muscle are definitely different from other reflexes. Unlike, for example, the withdrawal reflex, the sensory organ (the muscle spindle) is actually part of the effector (the muscle), and provides continuous feedback to the effector, controlling its primary function (contraction/production of tension). Additionally, the processing and sensitization/desensitization that an interneuron might provide in a different reflex arc are provided by innervation and control of the spindle itself by $gamma$-motonuerones, among others.
$gamma$-motoneurone function is essential for controlling the sensitivity of muscle spindle afferents as length detectors
There are additional reflex arcs with the same sensory part that involve processing and inhibition/excitation by interneurons. But in these cases the outflow is to a separate effector (antagonist muscles). See the figure below from Brust, Chapter 6.
I did not realize it but the knee jerk reflex is a stretch reflex and thus only involves two neurons (monosynaptic). Many many more reflexes use 3 or more neurons (polysynaptic) so include interneurons.
monosynaptic are common for muscle stretch limiting reflexes, that is why I knew them by their more common name stretch reflexes and did not make the connection. They don't involve interneurons becasue they would offer little benefit, they are self regulating most of the time and only involve a single source and a single destination and will not see much benefit from modification of the signal.
This slideshare on reflexes can provide some detailed information and a place to keep researching from.
Reflex Action in Animals (explained with diagram)
Reflex action is a rapid, automatic action carried out without the intervention of the will of the animal. It is independent of the will of the animal.
Marshall Hall first observed such action in the year 1833.
During reflex action the impulse travels through a path known as reflex arc. A simple reflex arc (monosynaptic) involves a sensory or afferent neuron, an interneuron present within the spinal cord and a motor or efferent neuron. The afferent is connected to the receptors (such as skin) and the efferent is connected to the effectors (muscles or glands).
The stimulus detected by the receptors passes into the sensory or afferent neuron. These impulses enter the spinal cord through the dorsal root and initiate impulse in interneuron or association neuron. From the spinal cord the impulse is carried through the ventral root and travel along the afferent or motor nerve fibres to reach the effect or organ.
Some Important Reflex Actions:
(i) Narrowing of the pupil of eye on seeing bright light
(ii) Withdrawal of limbs when it touches hot object
(iii) Quick closing of eye lids when a flying object suddenly approaches the eye
(iv) Coughing, sneezing, yawning
(vi) Opening of the mouth on hearing a sudden loud noise.
Monosynaptic Reflex Arc:
It is a simple reflex arc involving one sensory and one motor nerve fibre. It is generally not found in vertebrates.
Polysynaptic Reflex Arc:
Reflex arc involving more than one sensory and one motor neuron.
If the controlling nerve centre of reflex are is located in spinal cord, the action is called spinal reflex. If it is brain, the action is described as cerebral reflex. There are inborn reflexes which are due to unfamiliar or unconscious stimulus such as unknowingly touching hot object, sudden appearance of a flying object in front of the eye are called unconditioned reflexes.
There are other reflexes which are acquired by the animal by learning process or experience such as secretion of saliva on looking at delicious food, getting the smell of it. Such reflexes are called conditioned reflexes. Reflex actions have many advantages. It relieves the brain of such unnecessary works by taking care of minor daily activities of the body. They are automatic and protective in nature.
Spinal reflexes include the stretch reflex, the Golgi tendon reflex, the crossed extensor reflex, and the withdrawal reflex.
Distinguish between the types of spinal reflexes
- The stretch reflex is a monosynaptic reflex that regulates muscle length through neuronal stimulation at the muscle spindle. The alpha motor neurons resist stretching by causing contraction, and the gamma motor neurons control the sensitivity of the reflex.
- The stretch and Golgi tendon reflexes work in tandem to control muscle length and tension. Both are examples of ipsilateral reflexes, meaning the reflex occurs on the same side of the body as the stimulus.
- The crossed extensor reflex is a contralateral reflex that allows the body to compensate on one side for a stimulus on the other. For example, when one foot steps on a nail, the crossed extensor reflex shifts the body’s weight onto the other foot, protecting and withdrawing the foot on the nail.
- The withdrawal reflex and the more-specific pain withdrawal reflex involve withdrawal in response to a stimulus (or pain). When pain receptors, called nociceptors, are stimulated, reciprocal innervations stimulate the flexors to withdraw and inhibit the extensors to ensure they are unable to prevent flexion and withdrawal.
- golgi tendon reflex: A normal component of the reflex arc of the peripheral nervous system. In this reflex, a skeletal muscle contraction causes the agonist muscle to simultaneously lengthen and relax. This reflex is also called the inverse myotatic reflex because it is the inverse of the stretch reflex. Although muscle tension is increasing during the contraction, the alpha motor neurons in the spinal cord that supply the muscle are inhibited. However, antagonistic muscles are activated.
- alpha motor neuron: These are large, lower motor neurons of the brainstem and spinal cord. They innervate the extrafusal muscle fibers of skeletal muscle and are directly responsible for initiating their contraction. Alpha motor neurons are distinct from gamma motor neurons that innervate the intrafusal muscle fibers of muscle spindles.
Spinal reflexes include the stretch reflex, the Golgi tendon reflex, the crossed extensor reflex, and the withdrawal reflex.
The stretch reflex (myotatic reflex) is a muscle contraction in response to stretching within the muscle. This reflex has the shortest latency of all spinal reflexes. It is a monosynaptic reflex that provides automatic regulation of skeletal muscle length.
When a muscle lengthens, the muscle spindle is stretched and its nerve activity increases. This increases alpha motor neuron activity, causing the muscle fibers to contract and thus resist the stretching. A secondary set of neurons also causes the opposing muscle to relax. The reflex functions to maintain the muscle at a constant length.
Golgi Tendon Reflex
The Golgi tendon reflex is a normal component of the reflex arc of the peripheral nervous system. The tendon reflex operates as a feedback mechanism to control muscle tension by causing muscle relaxation before muscle force becomes so great that tendons might be torn.
Although the tendon reflex is less sensitive than the stretch reflex, it can override the stretch reflex when tension is great, making you drop a very heavy weight, for example. Like the stretch reflex, the tendon reflex is ipsilateral.
The sensory receptors for this reflex are called Golgi tendon receptors, and lie within a tendon near its junction with a muscle. In contrast to muscle spindles, which are sensitive to changes in muscle length, tendon organs detect and respond to changes in muscle tension that are caused by a passive stretch or muscular contraction.
Crossed Extensor Reflex
Jendrassik maneuver: The Jendrassik maneuver is a medical maneuver wherein the patient flexes both sets of fingers into a hook-like form and interlocks those sets of fingers together (note the hands of the patient in the chair). This maneuver is used often when testing the patellar reflex, as it forces the patient to concentrate on the interlocking of the fingers and prevents conscious inhibition or influence of the reflex.
The crossed extensor reflex is a withdrawal reflex. The reflex occurs when the flexors in the withdrawing limb contract and the extensors relax, while in the other limb, the opposite occurs. An example of this is when a person steps on a nail, the leg that is stepping on the nail pulls away, while the other leg takes the weight of the whole body.
The crossed extensor reflex is contralateral, meaning the reflex occurs on the opposite side of the body from the stimulus. To produce this reflex, branches of the afferent nerve fibers cross from the stimulated side of the body to the contralateral side of the spinal cord. There, they synapse with interneurons, which in turn, excite or inhibit alpha motor neurons to the muscles of the contralateral limb.
The withdrawal reflex (nociceptive or flexor withdrawal reflex) is a spinal reflex intended to protect the body from damaging stimuli. It is polysynaptic, and causes the stimulation of sensory, association, and motor neurons.
When a person touches a hot object and withdraws his hand from it without thinking about it, the heat stimulates temperature and danger receptors in the skin, triggering a sensory impulse that travels to the central nervous system. The sensory neuron then synapses with interneurons that connect to motor neurons. Some of these send motor impulses to the flexors to allow withdrawal.
Some motor neurons send inhibitory impulses to the extensors so flexion is not inhibited—this is referred to as reciprocal innervation. Although this is a reflex, there are two interesting aspects to it:
- The body can be trained to override that reflex.
- An unconscious body (or even drunk or drugged bodies) will not exhibit the reflex.
Golgi tendon organ: The Golgi tendon organ, responsible for the Golgi tendon reflex, is diagrammed with its typical position in a muscle (left), neuronal connections in spinal cord (middle), and expanded schematic (right). The tendon organ is a stretch receptor that signals the amount of force on the muscle and protects the muscle from excessively heavy loads by causing the muscle to relax and drop the load.
Examples of Somatic Reflexes
One of the simplest reflexes is a stretch reflex. In this reflex, when a skeletal muscle is stretched, a muscle spindle in the belly of the muscle is activated. The axon from this receptor travels to the spinal cord where it synapses with the motor neuron controlling the muscle, stimulating it to contract. This is a rapid, monosynaptic (single synapse), ipsilateral reflex that helps to maintain the length of muscles and contributes to joint stabilization. A common example of this reflex is the knee jerk reflex that is elicited by a rubber hammer striking against the patellar tendon, such as during a physical exam. When the hammer strikes, it stretches the tendon, which pulls on the quadriceps femoris muscle. Because bones and tendons do not typically pull muscles, the muscle “thinks” it is stretching very rapidly, and the reflex acts to counteract this stretch. In doing so, the “knee jerk” occurs.
Along with the monosynaptic activation of the alpha motor neuron, this reflex also includes the activation of an interneuron that inhibits the alpha motor neuron of the antagonistic muscle. This aspect of the reflex ensures that contraction of the agonist muscle occurs unopposed.
Flexor (Withdrawal) Reflex
Recall from the beginning of this unit that when you touch a hot stove, you reflexively pull your hand away. Sensory receptors in the skin sense extreme temperature and the early signs of tissue damage. To avoid further damage, information travels along the sensory fibers from the skin and into the posterior (dorsal) horn of the spinal cord. Once in the spinal cord, the sensory fibers synapse with a variety of interneurons that mediate the responses of the reflex. These responses included a strong initial withdrawal of the flexor muscle (caused by activation of the alpha motor neurons), inhibition of the extensor muscle (mediated through inhibitory interneurons), and sustained contraction of the flexor (mediated by a spinal cord neuronal circuit). Because the integration center in this reflex arc has many synapses, it is a polynaptic reflex. And as already discussed, the sensory information will also travel to the brain to develop a conscious awareness of the situation such that conscious decision-making can take over immediately after the reflex occurs.
Imagine what would happen if, when you stepped on a sharp object, it elicited a strong withdrawal reflex of your leg. You would likely topple over. In order to prevent this from happening, as the flexor (withdrawal) reflex involving the injured leg happens, an extension reflex of the opposite (contralateral) leg occurs at the same time, creating a crossed-extensor reflex. In this case, the ipsilateral limb reacts with a withdrawal reflex (stimulating flexor muscles and inhibiting extensor muscles on same side), but the contralateral extensor muscles contract so that the person can appropriately shift balance to the opposite foot during the reflex.
Reflex Actions - Does it involve the brain? [PLEASE HELP]
Is it now more correct to say that they don't involve 'the conscious areas of the brain' since reflex actions still pass through the motor neurones in the spinal cord or the brain - 'the unconscious areas of the brain'?
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(Original post by nwmyname)
Back in GCSE days, we used to say that reflex actions 'don't involve the brain.'
Is it now more correct to say that they don't involve 'the conscious areas of the brain' since reflex actions still pass through the motor neurones in the spinal cord or the brain - 'the unconscious areas of the brain'?
The brain is not required for reflexes. That isn't to say that the reflex doesn't involve the brain, however. Suppose you touch a hot object, and you pull your hand away without even thinking about it (withdrawal reflex), you will also feel pain. Therefore, there must be some signal being sent to the brain, so although your brain was not required to pull your hand away, pain signals were still sent to your brain. Another example is the patellar (knee jerk) reflex, you do not think about moving your leg, that is involuntary, however you can still feel it when your leg kicks. Therefore some afferent signal must be being sent to the brain. With autonomic reflexes on the other hand (e.g. pupillary reflex, blood pressure regulation, heart rate), there is no afferent signal that is sent to the brain, therefore these are not consciously perceived.
Most reflexes cannot be inhibited, such as the patellar reflex or the pupillary reflex (pupil constricts when light is shone in either eye), however some reflexes can be inhibited by higher centres of the brain. Take urination for example. Babies do not have control over when they urinate, it is a reflex. However, as we get older and higher centres of our brain develop, we gain the ability to be able to inhibit certain reflexes. In the case of urination, we can inhibit the reflex, allowing us to urinate at an appropriate time.
So yes, you are right that the brain is not required to initiate a reflex, however, they allow us to consciously perceive reflexes so that we know they are happening, and in some cases, higher centres of the brain enable us to inhibit particular reflexes (such as urination or defecation).
Why do some reflex actions involve interneurons, but some don't? - Biology
The nervous system is a remarkable collection of cells that governs both involuntary and voluntary behavior, while also maintaining homeostasis. Functions of the nervous system include:
·&emspCognition (thinking) and problem-solving
·&emspExecutive function and planning
·&emspLanguage comprehension and creation
·&emspEmotion and emotional expression
·&emspRegulation of endocrine organs
·&emspRegulation of heart rate, breathing rate, vascular resistance, temperature, and exocrine glands
The human nervous system is a complex web of over 100 billion cells that communicate, coordinate, and regulate signals for the rest of the body. Mental and physical action occurs when the body can react to external stimuli using the nervous system. In this section, we will look at the nervous system and its basic organization.
Note: Much of the information contained in this section is also discussed in Chapter 1 of MCAT Behavioral Sciences Review.
CENTRAL AND PERIPHERAL NERVOUS SYSTEMS
Generally speaking, there are three kinds of nerve cells in the nervous system: sensory neurons, motor neurons, and interneurons. Sensory neurons (also known as afferent neurons) transmit sensory information from receptors to the spinal cord and brain. Motor neurons (also known asefferent neurons) transmit motor information from the brain and spinal cord to muscles and glands. Interneurons are found between other neurons and are the most numerous of the three types. Interneurons are located predominantly in the brain and spinal cord and are often linked to reflexive behavior.
Afferent neurons ascend in the spinal cord toward the brain efferent neurons exit the spinal cord on their way to the rest of the body.
Different types of information require different types of processing. Processing of stimuli and response generation may happen at the level of the spinal cord, or may require input from the brainstem or cerebral cortex. Reflexes, discussed later in this section, only require processing at the level of the spinal cord. For example, when a reflex hammer hits the patellar tendon, the sensory information goes to the spinal cord, where a motor signal is sent to the quadriceps muscle, causing the leg to jerk forward at the knee. No input from the brain is required. However, some scenarios require input from the brain or brainstem. When this happens, supraspinal circuits are used.
Let’s turn to the overall structure of the human nervous system, which is diagrammed in Figure 4.9.
Figure 4.9. Major Divisions of the Nervous System
The nervous system can be broadly divided into two primary components: the central and peripheral nervous systems. The central nervous system (CNS) is composed of the brain and spinal cord. The brain consists of white matter and grey matter. The white matter consists of axons encased in myelin sheaths. The grey matter consists of unmyelinated cell bodies and dendrites. In the brain, the white matter lies deeper than the grey matter. At the base of the brain is the brainstem, which is largely responsible for basic life functions such as breathing. Note that the lobes of the brain and major brain structures are discussed in Chapter 1 of MCAT Behavioral Sciences Review.
The spinal cord extends downward from the brainstem and can be divided into four divisions: cervical, thoracic, lumbar, and sacral. Almost all of the structures below the neck receive sensory and motor innervation from the spinal cord. The spinal cord is protected by the vertebral column, which transmits nerves at the space between adjacent vertebrae. Like the brain, the spinal cord also consists of white and grey matter. The white matter lies on the outside of the cord, and the grey matter is deep within it. The axons of motor and sensory neurons are in the spinal cord. The sensory neurons bring information in from the periphery and enter on the dorsal (back) side of the spinal cord. The cell bodies of these sensory neurons are found in the dorsal root ganglia. Motor neurons exit the spinal cord ventrally, or on the side closest to the front of the body. The structure of the spinal cord can be seen in Figure 4.10.
Figure 4.10. The Spinal Cord Sensory neurons transmit information about pain, temperature, and vibration up to the brain and have cell bodies in the dorsal root ganglia toward the back of the spinal cord the motor neurons run from the brain along the opposite side of the spinal cord in the ventral root and control movements of skeletal muscle and glandular secretions.
The peripheral nervous system (PNS), in contrast, is made up of nerve tissue and fibers outside the brain and spinal cord, such as the 12 pairs of cranial and 31 pairs of spinal nerves. The PNS thus connects the CNS to the rest of the body and can itself be subdivided into the somatic and autonomic nervous systems.
The somatic nervous system consists of sensory and motor neurons distributed throughout the skin, joints, and muscles. Sensory neurons transmit information through afferent fibers. Motor impulses, in contrast, travel along efferent fibers.
The autonomic nervous system (ANS) generally regulates heartbeat, respiration, digestion, and glandular secretions. In other words, the ANS manages the involuntary muscles associated with many internal organs and glands. The ANS also helps regulate body temperature by activating sweating or piloerection, depending on whether we are too hot or too cold. The main thing to understand about these functions is that they are automatic, or independent of conscious control. Note the similarity between the words autonomic and automatic. This association makes it easy to remember that the autonomic nervous system manages automatic functions such as heartbeat, respiration, digestion, and temperature control.
One primary difference between the somatic and autonomic nervous systems is that the peripheral component of the autonomic nervous system contains two neurons. A motor neuron in the somatic nervous system goes directly from the spinal cord to the muscle without synapsing. In the autonomic nervous system, two neurons work in series to transmit messages from the spinal cord. The first neuron is known as the preganglionic neuron, whereas the second is the postganglionic neuron. The soma of the preganglionic neuron is in the CNS, and its axon travels to a ganglion in the PNS. Here it synapses on the cell body of the postganglionic neuron, which then affects the target tissue.
The first neuron in the autonomic nervous system is called the preganglionic neuron. The second neuron is the postganglionic neuron.
THE AUTONOMIC NERVOUS SYSTEM
The ANS has two subdivisions: the sympathetic nervous system and the parasympathetic nervous system. These two branches often act in opposition to one another, meaning that they are antagonistic. For example, the sympathetic nervous system acts to accelerate heart rate and inhibit digestion, while the parasympathetic nervous system, in contrast, decelerates heart rate and increases digestion.
The main role of the parasympathetic nervous system is to conserve energy. It is associated with resting and sleeping states and acts to reduce heart rate and constrict the bronchi. The parasympathetic nervous system is also responsible for managing digestion by increasing peristalsis and exocrine secretions. Acetylcholine is the neurotransmitter responsible for parasympathetic responses in the body and is released by both preganglionic and postganglionic neurons. The vagus nerve (cranial nerve X), is responsible for much of the parasympathetic innervation of the thoracic and abdominal cavity. The functions of the parasympathetic nervous system are summarized in Figure 4.11.
Figure 4.11. Functions of the Parasympathetic Nervous System
In contrast, the sympathetic nervous system is activated by stress. This can include everything from a mild stressor, such as keeping up with schoolwork, to emergencies that mean the difference between life and death. The sympathetic nervous system is closely associated with rage and fear reactions, also known as “fight-or-flight” reactions. When activated, the sympathetic nervous system:
·&emspRedistributes blood to muscles of locomotion
·&emspIncreases blood glucose concentration
·&emspDecreases digestion and peristalsis
·&emspDilates the eyes to maximize light intake
·&emspReleases epinephrine into the bloodstream
Sympathetic and parasympathetic nervous systems:
The functions of the sympathetic nervous system are summarized in Figure 4.12. In the sympathetic nervous system, preganglionic neurons release acetylcholine, while most postganglionic neurons release norepinephrine.
Figure 4.12. Functions of the Sympathetic Nervous System
Neural circuits called reflex arcs control reflexive behavior. For example, consider what occurs when someone steps on a nail. Receptors in the foot detect pain, and the pain signal is transmitted by sensory neurons up to the spinal cord. At that point, the sensory neurons connect with interneurons, which can then relay pain impulses up to the brain. Rather than wait for the brain to send out a signal, interneurons in the spinal cord can also send signals to the muscles of both legs directly, causing the individual to withdraw the foot with pain while supporting with the other foot. The original sensory information still makes its way up to the brain however, by the time it arrives there, the muscles have already responded to the pain, thanks to the reflex arc. There are two types of reflex arcs: monosynaptic and polysynaptic.
Consider the purpose of reflexes. Although it may be amusing to make your friends’ legs jump when you tap them, there is a more functional reason why this response occurs. The stretch on the patellar tendon makes the body think that the muscle may be getting overstretched. In response, the muscle contracts in order to prevent injury.
In a monosynaptic reflex arc, there is a single synapse between the sensory neuron that receives the stimulus and the motor neuron that responds to it. A classic example is the knee-jerk reflex, shown in Figure 4.13. When the patellar tendon is stretched, information travels up the sensory (afferent, presynaptic) neuron to the spinal cord, where it interfaces with the motor (efferent, postsynaptic) neuron that contracts the quadriceps muscle. The net result is extension of the leg, which lessens the tension on the patellar tendon. Note that the reflex is simply a feedback loop and a response to potential injury. If the patellar tendon or quadriceps muscles are stretched too far, they may tear, damaging the knee joint. Thus, the reflex serves to protect the muscle.
Figure 4.13. The Knee-Jerk Reflex The knee-jerk or knee extension reflex may be elicited by swiftly stretching the patellar tendon with a reflex hammer.
In a polysynaptic reflex arc, there is at least one interneuron between the sensory and motor neurons. A real-life example is the reaction to stepping on a nail described earlier, which involves the withdrawal reflex. The leg with which one steps on the nail will be stimulated to flex, using the hip muscles and hamstring muscles, pulling the foot away from the nail. This is a monosynaptic reflex, similar to the knee-jerk reflex described previously. However, if the person is to maintain balance, the other foot must be planted firmly on the ground. For this to occur, the motor neuron that controls the quadriceps muscles in the opposite leg must be stimulated, extending that leg. Interneurons in the spinal cord provide the connections from the incoming sensory information to the motor neurons in the supporting leg.
MCAT Concept Check 4.3:
Before you move on, assess your understanding of the material with these questions.
1. What parts of the nervous system are in the central nervous system (CNS)? Peripheral nervous system (PNS)?
2. What do afferent neurons do? Efferent neurons?
3. What functions are accomplished by the somatic nervous system? The autonomic nervous system?
4. What are the effects of the sympathetic nervous system? The parasympathetic nervous system?
5. What is the pathway of neural impulses in a monosynaptic reflex? In a polysynaptic reflex?
David L. Felten MD, PhD , . Mary Summo Maida PhD , in Netter's Atlas of Neuroscience (Third Edition) , 2016
14.2 Spinal Somatic Reflex Actions and Pathways
A, Presynaptic inhibition . Some interneurons synapse on the terminal arborizations of other axons, as in the case of some afferent pools associated with muscle stretch reflexes. These axoaxonic contacts permit the modulation of neurotransmitter release from the second (target) axon terminal by depolarization of the terminal membrane, altering the influx of Ca ++ . B, Muscle stretch reflex. In the muscle stretch reflex, Ia afferents excite the homonymous LMN pool directly and inhibit the antagonist LMN pool reciprocally via Ia inhibitory interneurons. C, Recurrent inhibition. Some interneurons receive recurrent collaterals from axons (e.g., LMN axons) and project back onto the dendrites or cell body of origin of that axon, usually inhibiting that neuron. This process can help to regulate the excitability and timing of excitation of the target neurons. Collaterals of LMN axons excite Renshaw cells (large interneurons), which inhibit the LMN of origin as well as LMNs projecting to synergistic muscles. Renshaw inhibition permits wiping the slate clean, after original excitation, of pools of LMNs, requiring additional incoming stimulation in order to excite these LMNs again. D, Golgi tendon organ reflex. Ib axons from Golgi tendon organs in muscle tendons terminate on pools of interneurons that inhibit LMNs to the homonymous muscle disynaptically and excite LMNs to the antagonist muscle reciprocally. The action of this reflex as a protective mechanism to prevent damage to a muscle during generation of maximal tension on the tendon is seen in attempted passive stretch of a spastic muscle the resultant inhibition of the homonymous LMN pool is called a clasp-knife reflex. E, Flexor withdrawal reflex. A flexor reflex (also called a withdrawal reflex or a nociceptive reflex) occurs when afferents derived from a noxious stimulus terminate on pools of interneurons that excite appropriate pools of LMNs (often flexor LMNs) to bring about a protective withdrawal from the source of the noxious stimulus. These interneurons also inhibit the antagonist LMNs through reciprocal inhibition. Flexor reflexes can extend throughout the spinal cord, as happens when one touches a hot stove with a finger the result is the removal of the entire arm, or even the entire body, away from the source of heat. These flexor reflexes may involve both sides of spinal cord. F, Renshaw cell bias. Some reflex responses such as Renshaw reflexes (see part C) may result in the distribution of influence (bias) in a manner that favors a particular type of action. Renshaw cells receive inputs from axon collaterals of both flexor and extensor LMNs, but their projections are directed mainly toward the inhibition of tonic extensor LMNs (and through reciprocal inhibition with the excitation of phasic flexor LMNs). Thus, the Renshaw cell response favors flexor movements and helps to inhibit extensor movements.
This system  IPSPs can be temporally summed with subthreshold or suprathreshold EPSPs to reduce the amplitude of the resultant postsynaptic potential. Equivalent EPSPs (positive) and IPSPs (negative) can cancel each other out when summed. The balance between EPSPs and IPSPs is very important in the integration of electrical information produced by inhibitory and excitatory synapses.
The size of the neuron can also affect the inhibitory postsynaptic potential. Simple temporal summation of postsynaptic potentials occurs in smaller neurons, whereas in larger neurons larger numbers of synapses and ionotropic receptors as well as a longer distance from the synapse to the soma enables the prolongation of interactions between neurons.
GABA is a very common neurotransmitter used in IPSPs in the adult mammalian brain and retina.   GABA receptors are pentamers most commonly composed of three different subunits (α, β, γ), although several other subunits (δ,ε, θ, π, ρ) and conformations exist. The open channels are selectively permeable to chloride or potassium ions (depending on the type of receptor) and allow these ions to pass through the membrane. If the electrochemical potential of the ion is more negative than that of the action potential threshold then the resultant conductance change that occurs due to the binding of GABA to its receptors keeps the postsynaptic potential more negative than the threshold and decreases the probability of the postsynaptic neuron completing an action potential. Glycine molecules and receptors work much in the same way in the spinal cord, brain, and retina.
There are two types of inhibitory receptors:
Ionotropic receptors Edit
Ionotropic receptors (also known as ligand-gated ion channels) play an important role in inhibitory postsynaptic potentials.  A neurotransmitter binds to the extracellular site and opens the ion channel that is made up of a membrane-spanning domain that allows ions to flow across the membrane inside the postsynaptic cell. This type of receptor produces very fast postsynaptic actions within a couple of milliseconds of the presynaptic terminal receiving an action potential. These channels influence the amplitude and time-course of postsynaptic potentials as a whole. Ionotropic GABA receptors are used in binding for various drugs such as barbiturates (Phenobarbital, pentobarbital), steroids, and picrotoxin. Benzodiazepines (Valium) bind to the α and γ subunits of GABA receptors to improve GABAergic signaling. Alcohol also modulates ionotropic GABA receptors.
Metabotropic receptors Edit
Metabotropic receptors, often G-protein-coupled receptors, do not use ion channels in their structure they, instead, consist of an extracellular domain that binds to a neurotransmitter and an intracellular domain that binds to G-protein.  This begins the activation of the G-protein, which then releases itself from the receptor and interacts with ion channels and other proteins to open or close ion channels through intracellular messengers. They produce slow postsynaptic responses (from milliseconds to minutes) and can be activated in conjunction with ionotropic receptors to create both fast and slow postsynaptic potentials at one particular synapse. Metabotropic GABA receptors, heterodimers of R1 and R2 subunits, use potassium channels instead of chloride. They can also block calcium ion channels to hyperpolarize postsynaptic cells.
There are many applications of inhibitory postsynaptic potentials to the real world. Drugs that affect the actions of the neurotransmitter can treat neurological and psychological disorders through different combinations of types of receptors, G-proteins, and ion channels in postsynaptic neurons.
For example, studies researching opioid receptor-mediated receptor desensitizing and trafficking in the locus cereleus of the brain are being performed. When a high concentration of agonist is applied for an extended amount of time (fifteen minutes or more), hyperpolarization peaks and then decreases. This is significant because it is a prelude to tolerance the more opioids one needs for pain the greater the tolerance of the patient. These studies are important because it helps us to learn more about how we deal with pain and our responses to various substances that help treat pain. By studying our tolerance to pain, we can develop more efficient medications for pain treatment. 
In addition, research is being performed in the field of dopamine neurons in the ventral tegmental area, which deals with reward, and the substantia nigra, which is involved with movement and motivation. Metabotropic responses occur in dopamine neurons through the regulation of the excitability of cells. Opioids inhibit GABA release this decreases the amount of inhibition and allows them to fire spontaneously. Morphine and opioids relate to inhibitory postsynaptic potentials because they induce disinhibition in dopamine neurons. 
IPSPs can also be used to study the input-output characteristics of an inhibitory forebrain synapse used to further study learned behavior—for example in a study of song learning in birds at the University of Washington.  Poisson trains of unitary IPSPs were induced at a high frequency to reproduce postsynaptic spiking in the medial portion of the dorsalateral thalamic nucleus without any extra excitatory inputs. This shows an excess of thalamic GABAergic activation. This is important because spiking timing is needed for proper sound localization in the ascending auditory pathways. Songbirds use GABAergic calyceal synaptic terminals and a calcyx-like synapse such that each cell in the dorsalateral thalamic nucleus receives at most two axon terminals from the basal ganglia to create large postsynaptic currents.
Inhibitory postsynaptic potentials are also used to study the basal ganglia of amphibians to see how motor function is modulated through its inhibitory outputs from the striatum to the tectum and tegmentum.  Visually guided behaviors may be regulated through the inhibitory striato-tegmental pathway found in amphibians in a study performed at the Baylor College of Medicine and the Chinese Academy of Sciences. The basal ganglia in amphibians is very important in receiving visual, auditory, olfactory, and mechansensory inputs the disinhibitory striato-protecto-tectal pathway is important in prey-catching behaviors of amphibians. When the ipsilateral striatum of an adult toad was electrically stimulated, inhibitory postsynaptic potentials were induced in binocular tegmental neurons, which affects the visual system of the toad.
Inhibitory postsynaptic potentials can be inhibited themselves through a signaling process called "depolarized-induced suppression of inhibition (DSI)" in CA1 pyramidal cells and cerebellar Purkinje cells.   In a laboratory setting step depolarizations the soma have been used to create DSIs, but it can also be achieved through synaptically induced depolarization of the dendrites. DSIs can be blocked by ionotropic receptor calcium ion channel antagonists on the somata and proximal apical dendrites of CA1 pyramidal cells. Dendritic inhibitory postsynaptic potentials can be severely reduced by DSIs through direct depolarization.
Along these lines, inhibitory postsynaptic potentials are useful in the signaling of the olfactory bulb to the olfactory cortex.  EPSPs are amplified by persistent sodium ion conductance in external tufted cells. Low-voltage activated calcium ion conductance enhances even larger EPSPs. The hyperpolarization activated nonselective cation conductance decreases EPSP summation and duration and they also change inhibitory inputs into postsynaptic excitation. IPSPs come into the picture when the tufted cells membranes are depolarized and IPSPs then cause inhibition. At resting threshold IPSPs induce action potentials. GABA is responsible for much of the work of the IPSPs in the external tufted cells.
Another interesting study of inhibitory postsynaptic potentials looks at neuronal theta rhythm oscillations that can be used to represent electrophysiological phenomena and various behaviors.   Theta rhythms are found in the hippocampus and GABAergic synaptic inhibition helps to modulate them. They are dependent on IPSPs and started in either CA3 by muscarinic acetylcholine receptors and within C1 by the activation of group I metabotropic glutamate receptors. When interneurons are activated by metabotropic acetylcholine receptors in the CA1 region of rat hippocampal slices, a theta pattern of IPSPs in pyramidal cells occurs independent of the input. This research also studies DSIs, showing that DSIs interrupt metabotropic acetylcholine-initiated rhythm through the release of endocannabinoids. An endocannabinoid-dependent mechanism can disrupt theta IPSPs through action potentials delivered as a burst pattern or brief train. In addition, the activation of metabotropic glutamate receptors removes any theta IPSP activity through a G-protein, calcium ion–independent pathway.
Inhibitory postsynaptic potentials have also been studied in the Purkinje cell through dendritic amplification. The study focused in on the propagation of IPSPs along dendrites and its dependency of ionotropic receptors by measuring the amplitude and time-course of the inhibitory postsynaptic potential. The results showed that both compound and unitary inhibitory postsynaptic potentials are amplified by dendritic calcium ion channels. The width of a somatic IPSP is independent of the distance between the soma and the synapse whereas the rise time increases with this distance. These IPSPs also regulate theta rhythms in pyramidal cells. On the other hand, inhibitory postsynaptic potentials are depolarizing and sometimes excitatory in immature mammalian spinal neurons because of high concentrations of intracellular chloride through ionotropic GABA or glycine chloride ion channels.  These depolarizations activate voltage-dependent calcium channels. They later become hyperpolarizing as the mammal matures. To be specific, in rats, this maturation occurs during the perinatal period when brain stem projects reach the lumbar enlargement. Descending modulatory inputs are necessary for the developmental shift from depolarizing to hyperpolarizing inhibitory postsynaptic potentials. This was studied through complete spinal cord transections at birth of rats and recording IPSPs from lumbar motoneurons at the end of the first week after birth.
Glutamate, an excitatory neurotransmitter, is usually associated with excitatory postsynaptic potentials in synaptic transmission. However, a study completed at the Vollum Institute at the Oregon Health Sciences University demonstrates that glutamate can also be used to induce inhibitory postsynaptic potentials in neurons.  This study explains that metabotropic glutamate receptors feature activated G proteins in dopamine neurons that induce phosphoinositide hydrolysis. The resultant products bind to inositol triphosphate (IP3) receptors through calcium ion channels. The calcium comes from stores and activate potassium conductance, which causes a pure inhibition in the dopamine cells. The changing levels of synaptically released glutamate creates an excitation through the activation of ionotropic receptors, followed by the inhibition of metabotropic glutamate receptors.
Human Nervous System: Function and Types (with diagram)
1. Sensory input, that is, the detection of stimuli by the receptors, or sense organs (e.g., eyes, ears, skin, nose and tongue).
2. Transmission of this input by nerve impulses to the brain and spinal cord, which generate an appropriate response.
3. Motor output, that is, carrying out of the response by muscles or glands, which are called effectors.
Two types of cells constitute the nervous system— neurons and neuroglia. The neurons conduct impulses and the neuroglia support and protect the neurons. A neuron consists of a cell body called cyton, and two types of processes—dendrite and axon.
Dendrites or Dendron’s:
These are hair like processes connected to the cyton. They receive stimulus, which may be physical, chemical, mechanical or electrical, and pass it on to the cyton.
It is the cell body, with a central nucleus surrounded by cytoplasm.
From one side of the cyton arises a cylindrical process filled with cytoplasm. This process is called axon. It is the longest part of the neuron. It transmits impulse away from the cyton. Its tip has a swelling called axon bulb. Generally, a neuron has one axon.
The ending of an axon may be branched. These endings are called synaptic terminals. The gap between a synaptic terminal and the dendrite of another neuron or an effecter cell is called a synapse.
How do we feel a hot or cold object? How do we feel pain? Why do different things have different smells and tastes? There are thousands of receptor cells in our sense organs. They detect stimuli such as heat, cold, pain, smells and tastes.
There are different types of receptors such as algesireceptors (for pain), tango receptors (for touch), gustatoreceptors (for taste), olfactoreceptors (for smell), and so on. The stimulus received by a receptor is passed on in the form of electrical signals through the dendrites of a neuron to the cyton of the neuron.
The cyton transmits only strong impulses. Weak impulses are not further transmitted. An impulse passed on by the cyton travels along the axon of the neuron. When it reaches the end of the axon, it causes the axon bulb to release a chemical which diffuses across the synapse and stimulates the dendrites of the adjacent neuron.
These dendrites in turn send electrical signals to their cell body, to be carried along the axon. In this way, the sensation from the receptor is passed on to the brain or spinal cord. A signal from the brain is similarly passed on to the effector, which carries out the appropriate response.
Eat some sugar. You will find it tastes sweet. If you block your nose with your fingers there is no difference in its taste. It still tastes sweet because sugar has no smell that can also contribute to the taste.
Block your nose again while eating lunch. You will find that the blocked nose makes a difference in appreciating the taste of various food items. When an item has taste as well as smell, it needs the gustatoreceptors on the tongue as well as the olfactoreceptors in the nose to transmit its stimuli to the brain for the full appreciation of its taste.
For example, you may not be able to distinguish between mashed papaya and mashed banana with your nose blocked and eyes closed. The gustatoreceptors and olfactoreceptors together make us appreciate any food better. This is the reason why food seems tasteless when you have a cold and your nose is blocked.
In humans and vertebrates, the nervous system may be divided into the (1) central, (2) peripheral, and (3) autonomic nervous system.
Central nervous system:
The central nervous system consists of the brain and the spinal cord.
It is the most important coordinating centre in the body. It is lodged in the brain box, or cranium, which protects it. The brain is covered by membranes called meninges. Between the membranes and the brain and also inside the brain, there is a characteristic fluid, called cerebrospinal fluid. This also protects the brain.
The brain may be divided into three parts—forebrain, midbrain and hindbrain:
1. The forebrain (cerebrum) is the anterior part, consisting of two large hemispheres divided by a longitudinal fissure. The surface of the hemispheres has many folds and is called cerebral cortex. The cerebral cortex consists of numerous neurons, and the folds serve to increase the surface area so that the maximum number of neurons can be present.
The cerebral hemispheres are seats of intelligence and voluntary action. The forebrain also contains olfactory lobes, which are the centres of smell and the diencephalon, which has centres of hunger, thirst, etc. To the floor of the diencephalon is attached the pituitary gland.
2. The midbrain includes optic lobes, which are the centres of vision.
3. The hindbrain is the posterior part, located below the forebrain. It consists of the cerebellum, pons and medulla oblongata. The cerebellum is the coordination centre, and maintains the body’s posture and balance. It also controls some precise voluntary actions such as those involved in writing and speech.
The medulla oblongata in the brain stem is the centre of involuntary actions, like swallowing, coughing, sneezing, salivation, vomiting, heartbeat and breathing. The medulla oblongata is continued into the spinal cord. The pons relays information between the cerebellum and the cerebrum.
It is a long cord which arises from the medulla oblongata and rims through the vertebral column (backbone). The vertebral column protects the spinal cord. The spinal cord is also covered by meninges.
A cross sections of the spinal cord shows the central canal, which is filled with cerebrospinal fluid. Around the canal are clusters of cytons, which form the grey matter.
The peripheral part has mainly axons and is called white matter. From each side of the spinal cord two roots, the dorsal and the ventral root, arise.
The dorsal root is joined by a nerve called sensory nerve, which picks up sensations from the sense organs (receptors). From the ventral root arises the motor nerve, which takes messages from the spinal cord to the muscles or glands (effectors).
What happens when you touch something hot or your finger is pricked by a needle? You immediately pull your hand away, without even thinking why you are doing so. Such sudden involuntary responses to stimuli are examples of reflex action. The response may be different when your conscious thought process is involved. For example, when a doctor pricks you with an injection needle to inject a medicine into your arm, you do not withdraw your arm immediately.
Your conscious thinking tells you that the medicine is being administered to cure your disease. In this case, a message from the spinal cord goes to the cerebrum, the thinking part of your brain, and your thinking brain directs your arm to bear the pain and not pull away.
The spinal cord is the centre of reflex action. Reflex actions are produced by reflex arcs, which may be formed anywhere along the spinal cord, nearest to the receptor and effecter. A reflex arc is formed by a sensory nerve and a motor nerve joined by a connecting nerve present in the spinal cord.
As the impulses do not have to travel all the way to the brain and back, the detection of stimuli and the completion of responses are faster.
Reflex action is an extremely quick action, which does not involve any thinking by the brain. If someone hits your leg with a hammer the leg is immediately withdrawn. In this type of reflex action the impact of the hammer (stimulus) received by the receptor is sent to the spinal cord through the sensory nerve. The message is received by the connecting nerve in the spinal cord.
The connecting nerve then sends a response through the motor nerve to the muscles (effectors) to pull the leg away. Thus, reflex action is a sudden, involuntary motor response to a stimulus. The flow of food in the alimentary canal, blinking in strong light or in response to a sudden movement in front of the eye, sneezing, coughing, yawning, hiccupping, shivering, etc., are also reflex actions.
Peripheral nervous system:
The peripheral nervous system includes 12 pairs of cranial nerves arising from the brain and 31 pairs of spinal nerves arising from the spinal cord. The nerves from the brain and the spinal cord connect the skeletal muscles and control their activity according to the directions and demands of the body. These nerves are, therefore, related to voluntary acts, i.e., they act according to our will.
Autonomic nervous system:
The autonomic nervous system controls and integrates the functions of internal organs like the heart, blood vessels, glands, etc., which are not under the control of our will.
The autonomic nervous system has two subdivisions: sympathetic and parasympathetic. The organs receive both sympathetic and parasympathetic nerves. The two types of nerves have opposite effects on the organs, i.e., if one is stimulatory, the other is inhibitory.
How does the nervous tissue cause the muscles to act?
When an electrical signal from a nerve cell reaches a synapse it causes the axon bulb to release a chemical. This chemical, which is discharged at the junction between the nerve cell and the muscle cell, causes the cell membrane of the muscle cell to move some ions in the muscle cell. This triggers a series of changes, ultimately causing the muscle to contract or relax.
All movements, e.g. touching your nose, require motor neurons to fire action potentials that results in contraction of muscles. In humans,
150,000 motor neurons control the contraction of
600 muscles. To produce movements, a subset of 600 muscles must contract in a temporally precise pattern to produce the right force at the right time. 
Motor units and force production Edit
A single motor neuron and the muscle fibers it innervates are called a motor unit. For example, the rectus femoris contains approximately 1 million muscle fibers, which are controlled by around 1000 motor neurons. Activity in the motor neuron causes contraction in all of the innervated muscle fibers so that they function as a unit. Increases in action potential frequency (spike rate) in the motor neuron cause increases in muscle fiber contraction, up to the maximal force.   The maximal force depends on the contractile properties of the muscle fibers. Within a motor unit, all the muscle fibers are of the same type (e.g. type I (slow twitch) or Type II fibers (fast twitch)), and motor units of multiple types make up a given muscle. Motor units of a given muscle are collectively referred to as a motor pool.
The force produced in a given muscle thus depends on: 1) How many motor neurons are active, and their spike rates 2) the contractile properties and number of muscle fibers innervated by the active neurons. To generate more force, increase the spike rates of active motor neurons and/or recruiting more and stronger motor units.
Recruitment order Edit
Motor units within a motor pool are recruited in a stereotypical order, from motor units that produce small amounts of force per spike, to those producing the largest force per spike. The gradient of motor unit force is correlated with a gradient in motor neuron soma size and motor neuron electrical excitability. This relationship was described by Elwood Henneman and is known as Henneman's size principle, a fundamental discovery of neuroscience and an organizing principle of motor control. 
For tasks requiring small forces, such as continual adjustment of posture, motor units with fewer muscle fibers that are slowly-contracting but less fatigueable are used. As more force is required, motor units with fast twitch, fast-fatigeable muscle fibers are recruited.
The nervous system produces movement by selecting which motor neurons are activated, and when. The finding that a recruitment order exists within a motor pool is thought to reflect a simplification of the problem: if a particular muscle should produce a particular force, then activate the motor pool along its recruitment hierarchy until that force is produced.
But then how to choose what force to produce in each muscle? The nervous system faces the following issues in solving this problem. 
- Redundancy. Infinite trajectories of movements can accomplish a goal (e.g. touch my nose). How is a trajectory chosen? Which trajectory is best?
- Noise. Noise is defined as small fluctuations that are unrelated to a signal, which can occur in neurons and synaptic connections at any point from sensation to muscle contraction.
- Delays. Motor neuron activity precedes muscle contraction, which precedes the movement. Sensory signals also reflect events that have already occurred. Such delays affect the choice of motor program.
- Uncertainty. Uncertainty arises because of neural noise, but also because inferences about the state of the world may not be correct (e.g. speed of on coming ball).
- Nonstationarity. Even as a movement is being executed, the state of the world changes, even through such simple effects as reactive forces on the rest of the body, causing translation of a joint while it is actuated.
- Nonlinearity. The effects of neural activity and muscle contraction are highly non-linear, which the nervous system must account for when predicting the consequences of a pattern of motor neuron activity.
Much ongoing research is dedicated to investigating how the nervous system deals with these issues, both at the behavioral level, as well as how neural circuits in the brain and spinal cord represent and deal with these factors to produce the fluid movements we witness in animals.
"Optimal feedback control" is an influential theoretical framing of these computation issues. 
Response to stimuli Edit
The process of becoming aware of a sensory stimulus and using that information to influence an action occurs in stages. Reaction time of simple tasks can be used to reveal information about these stages. Reaction time refers to the period of time between when the stimulus is presented, and the end of the response. Movement time is the time it takes to complete the movement. Some of the first reaction time experiments were carried out by Franciscus Donders, who used the difference in response times to a choice task to determine the length of time needed to process the stimuli and choose the correct response.  While this approach is ultimately flawed, it gave rise to the idea that reaction time was made up of a stimulus identification, followed by a response selection, and ultimately culminates in carrying out the correct movement. Further research has provided evidence that these stages do exist, but that the response selection period of any reaction time increases as the number of available choices grows, a relationship known as Hick's law. 
Closed loop control Edit
The classical definition of a closed loop system for human movement comes from Jack A. Adams (1971).  A reference of the desired output is compared to the actual output via error detection mechanisms, using feedback, the error is corrected for. Most movements that are carried out during day-to-day activity are formed using a continual process of accessing sensory information and using it to more accurately continue the motion. This type of motor control is called feedback control, as it relies on sensory feedback to control movements. Feedback control is a situated form of motor control, relying on sensory information about performance and specific sensory input from the environment in which the movement is carried out. This sensory input, while processed, does not necessarily cause conscious awareness of the action. Closed loop control  is a feedback based mechanism of motor control, where any act on the environment creates some sort of change that affects future performance through feedback. Closed loop motor control is best suited to continuously controlled actions, but does not work quickly enough for ballistic actions. Ballistic actions are actions that continue to the end without thinking about it, even when they no longer are appropriate. [ citation needed ] Because feedback control relies on sensory information, it is as slow as sensory processing. These movements are subject to a speed/accuracy trade-off, because sensory processing is being used to control the movement, the faster the movement is carried out, the less accurate it becomes.
Open loop control Edit
The classical definition from Jack A. Adams is:  “An open loop system has no feedback or mechanisms for error regulation. The input events for a system exert their influence, the system effects its transformation on the input and the system has an output. A traffic light with fixed timing snarls traffic when the load is heavy and impedes the flow when the traffic is light. The system has no compensatory capability.”
Some movements, however, occur too quickly to integrate sensory information, and instead must rely on feed forward control. Open loop control is a feed forward form of motor control, and is used to control rapid, ballistic movements that end before any sensory information can be processed. To best study this type of control, most research focuses on deafferentation studies, often involving cats or monkeys whose sensory nerves have been disconnected from their spinal cords. Monkeys who lost all sensory information from their arms resumed normal behavior after recovering from the deafferentation procedure. Most skills were relearned, but fine motor control became very difficult.  It has been shown that the open loop control can be adapted to different disease conditions and can therefore be used to extract signatures of different motor disorders by varying the cost functional governing the system. 
A core motor control issue is coordinating the various components of the motor system to act in unison to produce movement. The motor system is highly complex, composed of many interacting parts at many different organizational levels
Peripheral neurons receive input from the central nervous system and innervate the muscles. In turn, muscles generate forces which actuate joints. Getting the pieces to work together is a challenging problem for the motor system and how this problem is resolved is an active area of study in motor control research.
In some cases the coordination of motor components is hard-wired, consisting of fixed neuromuscular pathways that are called reflexes. Reflexes are typically characterized as automatic and fixed motor responses, and they occur on a much faster time scale than what is possible for reactions that depend on perceptual processing.  Reflexes play a fundamental role in stabilizing the motor system, providing almost immediate compensation for small perturbations and maintaining fixed execution patterns. Some reflex loops are routed solely through the spinal cord without receiving input from the brain, and thus do not require attention or conscious control. Others involve lower brain areas and can be influenced by prior instructions or intentions, but they remain independent of perceptual processing and online control.
The simplest reflex is the monosynaptic reflex or short-loop reflex, such as the monosynaptic stretch response. In this example, Ia afferent neurons are activated by muscle spindles when they deform due to the stretching of the muscle. In the spinal cord, these afferent neurons synapse directly onto alpha motor neurons that regulate the contraction of the same muscle.  Thus, any stretching of a muscle automatically signals a reflexive contraction of that muscle, without any central control. As the name and the description implies, monosynaptic reflexes depend on a single synaptic connection between an afferent sensory neuron and efferent motor neuron. In general the actions of monosynaptic reflexes are fixed and cannot be controlled or influenced by intention or instruction. However, there is some evidence to suggest that the gain or magnitude of these reflexes can be adjusted by context and experience. 
Polysynaptic reflexes or long-loop reflexes are reflex arcs which involve more than a single synaptic connection in the spinal cord. These loops may include cortical regions of the brain as well, and are thus slower than their monosynaptic counterparts due to the greater travel time. However, actions controlled by polysynaptic reflex loops are still faster than actions which require perceptual processing.  While the actions of short-loop reflexes are fixed, polysynaptic reflexes can often be regulated by instruction or prior experience.  A common example of a long loop reflex is the asymmetrical tonic neck reflex observed in infants.
A motor synergy is a neural organization of a multi-element system that (1) organizes sharing of a task among a set of elemental variables and (2) ensures co-variation among elemental variables with the purpose to stabilize performance variables.   The components of a synergy need not be physically connected, but instead are connected by their response to perceptual information about the particular motor task being executed. Synergies are learned, rather than being hardwired like reflexes, and are organized in a task-dependent manner a synergy is structured for a particular action and not determined generally for the components themselves. Nikolai Bernstein famously demonstrated synergies at work in the hammering actions of professional blacksmiths. The muscles of the arm controlling the movement of the hammer are informationally linked in such a way that errors and variability in one muscle are automatically compensated for by the actions of the other muscles. These compensatory actions are reflex-like in that they occur faster than perceptual processing would seem to allow, yet they are only present in expert performance, not in novices. In the case of blacksmiths, the synergy in question is organized specifically for hammering actions and is not a general purpose organization of the muscles of the arm. Synergies have two defining characteristics in addition to being task dependent sharing and flexibility/stability. 
"Sharing" requires that the execution of a particular motor task depends on the combined actions of all the components that make up the synergy. Often, there are more components involved than are strictly needed for the particular task (see "Redundancy" below), but the control of that motor task is distributed across all components nonetheless. A simple demonstration comes from a two-finger force production task, where participants are required to generate a fixed amount of force by pushing down on two force plates with two different fingers.  In this task, participants generated a particular force output by combining the contributions of independent fingers. While the force produced by any single finger can vary, this variation is constrained by the action of the other such that the desired force is always generated.
Co-variation also provides "flexibility and stability" to motor tasks. Considering again the force production task, if one finger did not produce enough force, it could be compensated for by the other.  The components of a motor synergy are expected to change their action to compensate for the errors and variability in other components that could affect the outcome of the motor task. This provides flexibility because it allows for multiple motor solutions to particular tasks, and it provides motor stability by preventing errors in individual motor components from affecting the task itself.
Synergies simplify the computational difficulty of motor control. Coordinating the numerous degrees of freedom in the body is a challenging problem, both because of the tremendous complexity of the motor system, as well as the different levels at which this organization can occur (neural, muscular, kinematic, spatial, etc.). Because the components of a synergy are functionally coupled for a specific task, execution of motor tasks can be accomplished by activating the relevant synergy with a single neural signal.  The need to control all of the relevant components independently is removed because organization emerges automatically as a consequence of the systematic covariation of components. Similar to how reflexes are physically connected and thus do not require control of individual components by the central nervous system, actions can be executed through synergies with minimal executive control because they are functionally connected. Beside motor synergies, the term of sensory synergies has recently been introduced.  Sensory synergy are believed to play an important role in integrating the mixture of environmental inputs to provide low-dimensional information to the CNS thus guiding the recruitment of motor synergies.
Synergies are fundamental for controlling complex movements, such as the ones of the hand during grasping. Their importance has been demonstrated for both muscle control and in the kinematic domain in several studies, lately on studies including large cohorts of subjects.    The relevance of synergies for hand grasps is also enforced by studies on hand grasp taxonomies, showing muscular and kinematic similarities among specific groups of grasps, leading to specific clusters of movements. 
Motor Programs Edit
While synergies represent coordination derived from peripheral interactions of motor components, motor programs are specific, pre-structured motor activation patterns that are generated and executed by a central controller (in the case of a biological organism, the brain).  They represent at top-down approach to motor coordination, rather than the bottom-up approach offered by synergies. Motor programs are executed in an open-loop manner, although sensory information is most likely used to sense the current state of the organism and determine the appropriate goals. However, once the program has been executed, it cannot be altered online by additional sensory information.
Evidence for the existence of motor programs comes from studies of rapid movement execution and the difficulty associated with changing those movements once they have been initiated. For example, people who are asked to make fast arm swings have extreme difficulty in halting that movement when provided with a "STOP" signal after the movement has been initiated.  This reversal difficulty persists even if the stop signal is presented after the initial "GO" signal but before the movement actually begins. This research suggests that once selection and execution of a motor program begins, it must run to completion before another action can be taken. This effect has been found even when the movement that is being executed by a particular motor program is prevented from occurring at all. People who attempt to execute particular movements (such as pushing with the arm), but unknowingly have the action of their body arrested before any movement can actually take place, show the same muscle activation patterns (including stabilizing and support activation that does not actually generate the movement) as when they are allowed to complete their intended action. 
Although the evidence for motor programs seems persuasive, there have been several important criticisms of the theory. The first is the problem of storage. If each movement an organism could generate requires its own motor program, it would seem necessary for that organism to possess an unlimited repository of such programs and where these would be kept is not clear. Aside from the enormous memory requirements such a facility would take, no motor program storage area in the brain has yet been identified. The second problem is concerned with novelty in movement. If a specific motor program is required for any particular movement, it is not clear how one would ever produce a novel movement. At best, an individual would have to practice any new movement before executing it with any success, and at worst, would be incapable of new movements because no motor program would exist for new movements. These difficulties have led to a more nuanced notion of motor programs known as generalized motor programs.  A generalized motor program is a program for a particular class of action, rather than a specific movement. This program is parameterized by the context of the environment and the current state of the organism.
An important issue for coordinating the motor system is the problem of the redundancy of motor degrees of freedom. As detailed in the "Synergies" section, many actions and movements can be executed in multiple ways because functional synergies controlling those actions are able to co-vary without changing the outcome of the action. This is possible because there are more motor components involved in the production of actions than are generally required by the physical constraints on that action. For example, the human arm has seven joints which determine the position of the hand in the world. However, only three spatial dimensions are needed to specify any location the hand could be placed in. This excess of kinematic degrees of freedom means that there are multiple arm configurations that correspond to any particular location of the hand.
Some of the earliest and most influential work on the study of motor redundancy came from the Russian physiologist Nikolai Bernstein. Bernstein's research was primarily concerned with understanding how coordination was developed for skilled actions. He observed that the redundancy of the motor system made it possible to execute actions and movements in a multitude of different ways while achieving equivalent outcomes.  This equivalency in motor action means that there is no one-to-one correspondence between the desired movements and the coordination of the motor system needed to execute those movements. Any desired movement or action does not have a particular coordination of neurons, muscles, and kinematics that make it possible. This motor equivalency problem became known as the degrees of freedom problem because it is a product of having redundant degrees of freedom available in the motor system.
Related, yet distinct from the issue of how the processing of sensory information affects the control of movements and actions is the question of how the perception of the world structures action. Perception is extremely important in motor control because it carries the relevant information about objects, environments and bodies which is used in organizing and executing actions and movements. What is perceived and how the subsequent information is used to organize the motor system is a current and ongoing area of research.
Model based control strategies Edit
Most model based strategies of motor control rely on perceptual information, but assume that this information is not always useful, veridical or constant. Optical information is interrupted by eye blinks, motion is obstructed by objects in the environment, distortions can change the appearance of object shape. Model based and representational control strategies are those that rely on accurate internal models of the environment, constructed from a combination of perceptual information and prior knowledge, as the primary source information for planning and executing actions, even in the absence of perceptual information. 
Inference and indirect perception Edit
Many models of the perceptual system assume indirect perception, or the notion that the world that gets perceived is not identical to the actual environment. Environmental information must go through several stages before being perceived, and the transitions between these stages introduce ambiguity. What actually gets perceived is the mind's best guess about what is occurring in the environment based on previous experience. Support for this idea comes from the Ames room illusion, where a distorted room causes the viewer to see objects known to be a constant size as growing or shrinking as they move around the room. The room itself is seen as being square, or at least consisting of right angles, as all previous rooms the perceiver has encountered have had those properties. Another example of this ambiguity comes from the doctrine of specific nerve energies. The doctrine presents the finding that there are distinct nerve types for different types of sensory input, and these nerves respond in a characteristic way regardless of the method of stimulation. That is to say, the color red causes optical nerves to fire in a specific pattern that is processed by the brain as experiencing the color red. However, if that same nerve is electrically stimulated in an identical pattern, the brain could perceive the color red when no corresponding stimuli is present.
Forward models Edit
Forward models are a predictive internal model of motor control that takes the available perceptual information, combined with a particular motor program, and tries to predict the outcome of the planned motor movement. Forward models structure action by determining how the forces, velocities, and positions of motor components affect changes in the environment and in the individual. It is proposed that forward models help with the Neural control of limb stiffness when individuals interact with their environment. Forward models are thought to use motor programs as input to predict the outcome of an action. An error signal is generated when the predictions made by a forward model do not match the actual outcome of the movement, prompting an update of an existing model and providing a mechanism for learning. These models explain why it is impossible to tickle yourself. A sensation is experienced as ticklish when it is unpredictable. However, forward models predict the outcome of your motor movements, meaning the motion is predictable, and therefore not ticklish. 
Evidence for forward models comes from studies of motor adaptation. When a person's goal-directed reaching movements are perturbed by a force field, they gradually, but steadily, adapt the movement of their arm to allow them to again reach their goal. However, they do so in such a way that preserves some high level movement characteristics bell-shaped velocity profiles, straight line translation of the hand, and smooth, continuous movements.  These movement features are recovered, despite the fact that they require startlingly different arm dynamics (i.e. torques and forces). This recovery provides evidence that what is motivating movement is a particular motor plan, and the individual is using a forward model to predict how arm dynamics change the movement of the arm to achieve particular task level characteristics. Differences between the expected arm movement and the observed arm movement produces an error signal which is used as the basis for learning. Additional evidence for forward models comes from experiments which require subjects to determine the location of an effector following an unvisualized movement 
Inverse models Edit
Inverse models predict the necessary movements of motor components to achieve a desired perceptual outcome. They can also take the outcome of a motion and attempt to determine the sequence of motor commands that resulted in that state. These types of models are particularly useful for open loop control, and allow for specific types of movements, such as fixating on a stationary object while the head is moving. Complementary to forward models, inverse models attempt to estimate how to achieve a particular perceptual outcome in order to generate the appropriate motor plan. Because inverse models and forward model are so closely associated, studies of internal models are often used as evidence for the roles of both model types in action.
Motor adaptation studies, therefore, also make a case for inverse models. Motor movements seem to follow predefined "plans" that preserve certain invariant features of the movement. In the reaching task mentioned above, the persistence of bell-shaped velocity profiles and smooth, straight hand trajectories provides evidence for the existence of such plans.  Movements that achieve these desired task-level outcomes are estimated by an inverse model. Adaptation therefore proceeds as a process of estimating the necessary movements with an inverse model, simulating with a forward model the outcome of those movement plans, observing the difference between the desired outcome and the actual outcome, and updating the models for a future attempt.
Information based control Edit
An alternative to model based control is information based control. Informational control strategies organize movements and actions based on perceptual information about the environment, rather than on cognitive models or representations of the world. The actions of the motor system are organized by information about the environment and information about the current state of the agent.  Information based control strategies often treat the environment and the organism as a single system, with action proceeding as a natural consequence of the interactions of this system. A core assumption of information based control strategies is that perceptions of the environment are rich in information and veridical for the purposes of producing actions. This runs counter to the assumptions of indirect perception made by model based control strategies.
Direct perception Edit
Direct perception in the cognitive sense is related to the philosophical notion of naïve or direct realism in that it is predicated on the assumption that what we perceive is what is actually in the world. James J. Gibson is credited with recasting direct perception as ecological perception.  While the problem of indirect perception proposes that physical information about object in our environment is not available due to the ambiguity of sensory information, proponents of direct perception (like Gibson) suggest that the relevant information encoded in sensory signals is not the physical properties of objects, but rather the action opportunities the environment affords. These affordances are directly perceivable without ambiguity, and thus preclude the need for internal models or representations of the world. Affordances exist only as a byproduct of the interactions between an agent and its environment, and thus perception is an "ecological" endeavor, depending on the whole agent/environment system rather than on the agent in isolation.
Because affordances are action possibilities, perception is directly connected to the production of actions and movements. The role of perception is to provide information that specifies how actions should be organized and controlled,  and the motor system is "tuned" to respond to specific type of information in particular ways. Through this relationship, control of the motor system and the execution of actions is dictated by the information of the environment. As an example, a doorway "affords" passing through, but a wall does not. How one might pass through a doorway is specified by the visual information received from the environment, as well as the information perceived about one's own body. Together, this information determines the pass-ability of a doorway, but not a wall. In addition, the act of moving towards and passing through the doorway generates more information and this in turn specifies further action. The conclusion of direct perception is that actions and perceptions are critically linked and one cannot be fully understood without the other.
Behavioral dynamics Edit
Building on the assumptions of direct perception behavioral dynamics is a behavioral control theory that treats perceptual organisms as dynamic systems that respond to informational variables with actions, in a functional manner.  Under this understanding of behavior, actions unfold as the natural consequence of the interaction between the organisms and the available information about the environment, which specified in body-relevant variables. Much of the research in behavioral dynamics has focused on locomotion, where visually specified information (such as optic flow, time-to-contact, optical expansion, etc.) is used to determine how to navigate the environment   Interaction forces between the human and the environment also affect behavioral dynamics as seen in by the Neural control of limb stiffness.
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