Excitation line

In order for the information to spread along the nerve cell and be transmitted over long distances, action potentials must be generated along the nerve again and again. Two forms of excitation conduction can be distinguished: In saltatory conduction, parts of the nerve are so well isolated in regular sections that the excitation can “jump over” from one non-isolated area to the next. These completely isolated areas are called internodes.

The short non-isolated areas in between are called Ranvier-lacing rings and contain a high number of ion channels, so that a new action potential is generated here each time, which can then jump over again to the next lacing ring. Thus, much less action potentials have to be triggered than in the case of continuous excitation conduction, where potentials have to be triggered again and again along the entire nerve at closely adjacent sections. Therefore, the saltatory excitation conduction with about 100 m/s is much faster than the continuous excitation conduction with about 1 m/s.

It takes place only at isolated neurons, the isolation is ensured by myelin, which is wrapped around the nerve cell. Pathological demyelination, such as in multiple sclerosis (MS), leads to a significant slowing of nerve conduction with partial loss of nerve function. In MS, for example, these are:

Saltatoric and
Continuous excitation conduction.
Visual disturbances,
Emotional disorders and
Muscle paralysis.

So that information can be transmitted from one cell to another, so-called synapses are necessary.

They impress as a piston-shaped bulge at the nerve endings. Every nerve cell has not only one but many synapses and therefore mostly also many connections to other cells.Between the synapse of the first neuron (presynapse, pre – before) and the second neuron (post – after) lies the synaptic cleft. When the excitation, which is passed on through the generation of action potential, arrives at the presynapse, calcium ion channels are opened by the charge change at the membrane, so that positively charged calcium flows into the presynapase and the membrane potential becomes more positive.

Through complex molecular processes, the calcium inflow ensures that prefabricated vesicles from the cell interior reach the membrane, fuse with the membrane and release their contents into the synaptic cleft. These vesicles contain neurotransmitters such as acetylcholine. These reach the membrane of the post-synapses through the synaptic cleft, where they bind to receptors specific for them.

This binding can trigger various signaling pathways.

On the one hand, ion channels can be opened again, which provide for an inflow or outflow of ions. This either makes the membrane of the target cell more negatively charged (hyperpolarization) and thus less excitable, or it becomes more positively charged (depolarization) and thus more excitable, so that when a threshold value is reached, an action potential is triggered, which is then passed on again along the nerve cell.
On the other hand, information can also be transmitted without ion channels, namely in the form of small molecules that serve as messengers (second messenger).