A nerve fiber is a portion of a nerve. A nerve is composed of several nerve fiber bundles. These nerve fiber bundles contain many nerve fibers.
Each nerve fiber is surrounded by the so-called endoneurium, a kind of protective mantle around each nerve fiber. The endoneurium consists of connective tissue and elastic fibres and because the blood vessels run through it, it also has an important function in feeding the Schwann cells and thus the nerve fibres. To form a nerve fiber bundle there is the so-called perineurium.
It encloses many nerve fibers and thus holds a nerve fiber bundle together. Many nerve fiber bundles together are surrounded by the so-called epineurium and form a nerve in their entirety. In general, a distinction is made between nerve fibers containing marrow and nerve fibers without marrow.
A frequently used synonym for nerve fiber is the axon or neurite, whereby strictly speaking only the axon together with the surrounding cell membrane (the axolem) constitutes a nerve fiber. The nerve fiber is used to transmit information from the cell body (soma) to the end buttons (telodendrons), which then contact a new cell body (soma) to transmit the information. The nerve fiber begins from the so-called axon hill, which is added to the cell body of a nerve cell. From there the nerve fiber reaches up to its branching into the end buttons.
Nerve fibers containing marrow
Mark-containing (myelinated) nerve fibers are characterized by the fact that the axon is surrounded by a myelin sheath. You can think of a nerve fiber as a kind of cable and the myelin sheath is an insulating layer around the cable. Myelination is different in the central nervous system (CNS) and the peripheral nervous system (PNS).
In the CNS, the myelin sheath is formed by the so-called oligodendrocytes. In PNS, on the other hand, Schwann cells form the insulating layer. However, this myelin sheath is not continuous but has repeated short interruptions in which the nerve fiber is “naked”, whereby this interruption is called RANVIER’SCHEN lacing ring.
This serves a faster excitation transmission. One calls this fast form of the excitation transmission as saltatorische excitation line. Here the excitation “jumps” from ring to ring and does not have to excite the entire length of the nerve fiber. The action potential is then formed in each lacing ring and passed on from lacing ring to lacing ring. This is much faster than the continuous propagation of excitation, as is the case with non-markless nerve fibers.
