Nerve fibers are structures in the nervous system that arise from the cell body of neurons as thin, elongated projections. They act as a kind of conduit by transmitting electrical impulses and enabling interconnectedness between neurons. In this way, information can be processed in the nervous system and commands can be sent to the receiving organs. Diseases of the nerves thus lead to impairments in perception, motor function, and organ functionality.
What are nerve fibers?
A nerve fiber is an elongated protrusion (axon, neurite) of a nerve cell surrounded by a sheath structure (axolemma). Through depolarization of its cell membrane, which is brought about by the upstream action hill, signals are directed away from the cell body to the synapses in the form of action potentials. It thus plays a special role in the transmission of information within the organism. Based on the type of axolem, as well as by other properties, nerve fibers can be divided into different categories. If a neurite is surrounded by a myelin sheath, it is a medullary nerve fiber. In the central nervous system this is formed by oligodendrocytes, in the peripheral nervous system by Schwann cells. Marrowless fibers are encased only by the cytoplasm of Schwann cells. The direction of excitation conduction also distinguishes nerve fibers. In terms of the nervous system, afferent axons transmit impulses from sensory organs to the central nervous system. Efferent nerve fibers conduct excitations to receivers in the periphery.
Anatomy and structure
The nerve fiber can be divided into three sections based on different functionality and anatomy of certain sections: the preaxon, the axon, and the telodendron. The preaxon is the approximately 25-micrometer-long base of an axon that directly attaches to the cell body of the neuron and connects to the action hillock. It is composed of a specialized complex of proteins and is never myelinated. In addition, the initial segment has a particularly high density of voltage-gated sodium channels. The preaxon is followed by the main course of the axon, which may be wrapped by several layers of myelin, depending on the species, locality, and function. This lipid-rich and electrically insulating biomembrane is formed by glial cells (oligodendrocytes or Schwann cells). Ranvier’s lacing rings occur in regular sections – sites where the myelin sheath is absent and form the basis for saltatory conduction of excitation. The end of the axon branches in a tree-like manner to the telodendria, which are located upstream of the synapses. In this way, a neuron can connect to several other neurons or effectors.
Function and tasks
The main task of nerve fibers is to transmit action potentials from the soma in the peripheral direction and to cause a release of chemical messengers (neurotransmitters) in the synapses. Only in this way is information transmission from cell to cell or target organ made possible. Excitation conduction begins in the action hill of the cell body, where the basis for action potentials is created. The excitation threshold in the subsequent preaxon is particularly low, so that an action potential can easily be formed here. The resulting triggered depolarization of the axon membrane opens the voltage-dependent sodium channels and a depolarization wave runs over the entire nerve fiber. For physical reasons, myelination of the axon allows particularly rapid conduction over longer sections without significant attenuation. Because of the separation of the sheath layers by Schwann cells, the action potential can jump from one gap to the next. This form of excitation conduction is much faster than continuous conduction in markless nerve fibers, requires less energy, and allows thinner axons. In addition to transmitting electrical voltages, the nerve fiber is also responsible for transporting substances. Since almost all of the synthesizing activity of a nerve cell takes place in the cell body, various substances must be brought to the axon to maintain its functions. The transport directed from the cell body to the peripheral end of the axon involves proteins, which are transported only in one direction and very slowly. The axonal transport of substances in both directions, on the other hand, takes place through vesicles along the microtubules and proceeds rapidly.
Diseases and complaints
One of the most common neurological impairments in young people is brought on by multiple sclerosis. It is a chronic inflammatory disease in which the myelin sheaths of the neurites in the central nervous system are attacked and destroyed. This has a negative effect on the conduction of excitation and results, among other things, in sensory disturbances or paralysis. Together with Baló’s disease, acute disseminated encephalomyelitis (ADEM) or neuromyelitis optica (Devic’s syndrome), as well as some other clinical pictures, multiple sclerosis belongs to the demyelinating diseases (demyelinating diseases). Complaints also occur in the case of a severing of the nerve fiber (axotomy) as a result of a traumatic incident. Since ribosomes or a rough endoplasmic reticulum are only exceptionally present in the cytoplasm of the neurite, the maintenance and function of the axon must be taken over by protein synthesis in the cell body. If the nerve fiber is separated from the soma, no supply can be provided to the neurite and it dies. In the presence of severe trauma, adjacent neurons may also degenerate. Regarding the location, of the neurons affected in the surrounding area, it is necessary to differentiate between anterograde and retrograde transneural degeneration. In addition to mechanically induced damage, neurodegenerative diseases, such as Alzheimer’s and Parkinson’s diseases, or axonal degenerative polyneuropathies are also involved in the decay of axons.
