Nerves are parallel nerve fibers that are surrounded by connective tissue and can conduct electrochemical impulses to peripheral organs, muscles and the central nervous system. A nerve cell basically consists of the soma (cell body, cell nucleus with dendrites), the axon and the synapses (axon terminals). A single nerve fiber is surrounded by the "endoneurium". Several nerve fibres are surrounded by the "perineurium" and together form a nerve fascicle. Nerve fascicles are in turn grouped by the epineurium to form the anatomical structure of a peripheral nerve.
In order to pass information from one cell to the next, the so-called "synaptic cleft" must be bridged. This represents a 20 to 30 nm wide gap between two cells. The synaptic cleft forms the synapse together with the axon end button of the transmitter cell and the dendrite of the receiver cell.
The axon end button contains small vesicles, which in the case of chemical synapses contain messenger substances such as the neurotransmitters adrenaline and dopamine. When an electrical impulse reaches the end button, the vesicles fuse with the presynaptic cell membrane, i.e. the one in front of the synaptic cleft, and the messenger substances are released into the synaptic cleft. The electrical impulse is converted into a chemical signal.
At the cell membrane of the recipient cell, i.e. the postsynaptic membrane, there are docking sites (receptor molecules) for the messenger substances. When a transmitter binds to a receptor molecule, an electrical signal is again triggered in the recipient cell, which can propagate along the cell. In this way, nerve impulses are transmitted from cell to cell. Exchange rates of 1 to 100 m/s are achieved. The speed of nerve conduction depends, among other things, on whether the nerve is surrounded by a fat-rich biomembrane (myelin sheath).
We distinguish between multipolar (for sensory and motor tasks), unipolar (only sensory perception), bipolar (reception and transmission of signals from rods and cones in the cornea of the eye) and pseudounipolar (with only one extension from the soma) nerve cells. Furthermore, afferent (conducting signals from sensory cells to the central nervous system) and efferent (conducting signals from the central nervous system to muscles and glands) neurons are differentiated. Furthermore, central (CNS) and peripheral parts (PNS) of the nervous system can be distinguished. The cranial nervous system is a special form of PNS. The PNS can be divided into somatic (arbitrary) and autonomous (vegetative) parts.