Excitation lines

Different forms of excitation lines

When an excitement has been taken up by the dendrites, transmitted via the soma of the nerve cell to the axon hillock, it is decided whether an action potential is triggered. Stimulus transmission through electrical impulses is a commonality shared by all living beings. Nevertheless, there are e.g. in the excitation transmission differences:

The continuous excitation line

In a continuous excitation conduction, the excitation by the axon is relayed by means of a progressive formation of the action potential. Consequently, depolarization must take place at each site of the axon.
Continuous excitation is especially in invertebrates such as squid or earthworms the form of excitation transmission. Cuttlefish have up to 1mm thick axons (giant axon), due to evolutionary reasons: For the speed of the excitation line can be increased in the continuous formation of an action potential only by increasing the diameter. As a result, the internal resistance of the axon decreases and the action potential can be formed faster.

The saltatory excitation line

In contrast to the aforementioned cuttlefish, vertebrates have an outward isolation of the axon. High-fat lipids form the so-called myelin sheaths and continuously surround the axon. They are separated only by the Ranvierschen Schnürrringe. In this way, an action potential can only be formed on the uninsulated Ranvier's Schnürring. Compared to the continuous excitation line, the saltatory runs much faster. The excitement 'jumps' within the axon from Ranvier's Schnürring to Ranvier's Schnürring and bridges the outwardly insulating myelin sheaths. A depolarization must and can be done only on the un-insulated Schnürrringen. In continuous conduction, myelin sheaths do not exist and these areas need to be depolarized continuously, which takes more time.

Comparison of continuous and saltatory excitation

Continuous excitation lineSaltatoric excitation line
To be found at:invertebratesvertebrates
speedMax. 30 m / sMax. 100 m / s
Size of the axonsup to 1 mm thick (giant axon in squid)from a diameter considerably thinner
Isolation of the axonOnly the natural isolation of the axon (not very effective)lipid-rich myelin sheaths isolate the axon
depolarizationcontinuously on the entire axononly on the Ranvier Schnürring