Myelin sheath is the term used to describe the covering of the neurites of a nerve cell, which can be up to one meter long. The myelin sheath protects the nerve fiber, insulates it electrically, and allows much faster transmission speeds than nonmyelinated nerve fibers. Myelin sheaths are composed of special lipids, phospholipids, and structural proteins and are each interrupted after about one to one and a half millimeters by a so-called Ranvier’s lacing ring.
What is the myelin sheath?
A nerve cell or neuron usually consists of the cell body, short projections (dendrites) close to the cell body, and a neurite, which in humans can reach a length of more than one meter. While the dendrites are usually not sheathed, most neurites are protected by a myelin or myelin sheath and are then called axons. Typically, the myelin sheath is interrupted by a so-called Ranvier’s lacing ring after every 0.2 to 1.5 millimeters of length, so that the axon‘s appearance is somewhat reminiscent of a string of elongated pearls. The myelin sheaths electrically insulate the nerve process and not only provide protection, but also allow a much higher speed in the transmission of nerve stimuli through so-called saltatory stimulus transmission, which “jumps” from lacing ring to lacing ring. The structural substance of the myelin sheaths consists mainly of lipids such as cholesterol and phospholipids as well as special structural proteins. The structure and composition of myelin sheaths is somewhat reminiscent of the plasmalemma, the cell membrane of human and animal cells.
Anatomy and structure
The myelin sheaths of peripheral nervous system (PNS) axons are formed by Schwann cells and those of the central nervous system (CNS) by oligodendrocytes. Both cell types belong to the glial cells, which perform support functions for the neurons and, like the neurons themselves, originate from the ectoderm. Schwann cells each wrap a section of an axon spirally with a myelin layer that is exactly similar in composition to their plasmalemma, their cell membrane. Thus, axons may well be wrapped with up to 50 double layers of the cell membrane. In the CNS, projections grow out of the soma of the oligodendrocytes, making contact with the axons and enveloping them with a myelin sheath. A dendrocyte can thereby “wrap” axonal segments of several axons simultaneously. The regular interruptions of the myelin sheaths in the form of Ranvier’s lacing rings at intervals of 0.2 to 1.5 millimeters play an important role in stimulus transmission. Ranvier’s lacing rings leave very narrow interstices of about one micrometer each, where the nerve tracts are virtually bare with no electrical insulation.
Function and tasks
The myelin sheaths of axons perform several functions, all of which are individually important to the interaction of the nervous system and account for its functionality. The myelin sheath provides mechanical protection and at the same time electrical insulation to the neurite running inside, which is interrupted only at Ranvier’s lacing rings. The regular interruptions of the insulation have a crucial importance for the speed and the way of action potential transmission. In the resting state, the axon has the so-called resting potential inside, which is characterized by an excess of negatively charged proteins and positively charged potassium ions compared to an excess of negatively charged chloride and positively charged sodium ions in the extracellular space outside the plasma membrane of the axon. The slightly negative resting potential (membrane potential) is maintained by ion channels and by actively controllable sodium–potassium pumps in the membrane. If the neuron receives a specific stimulus, it is depolarized, the electrical conditions briefly reverse, and the action potential is generated via voltage-gated sodium and potassium ion channels, but this action potential only lasts for about 0.1 to 0.2 milliseconds. The action potential in the axon depolarizes the next following lacing ring and establishes an action potential. This means that the relatively slow and cumbersome stimulus conduction is bridged by continuous transmission of the action potential and is replaced by the erratic (saltatory) stimulus conduction from lacing ring to lacing ring.The “nerve speed” thus increases from about 1 to 2 m/sec in neurites without myelin sheaths to up to 120 m/sec in axons with thick myelin sheaths. Another function of the myelin sheaths is to supply the nerves.
Diseases
The most important diseases and disorders directly related to myelin sheaths are diseases that lead to the degradation, demyelination of nerves. Demyelination of the axons – as demyelination is also called – is based either on genetic defects known to trigger hereditary motor-sensitive neuropathies or, for example, the autoimmune disease multiple sclerosis (MS). Other causes such as excessive chronic alcohol consumption, diabetic neuropathy, Lyme disease, or myelin degradation as an undesirable side effect of medications are also possible culprits. Hereditary motor-sensitive neuropathies are manifested by a gradual degradation of the myelin layers or there are problems with the structure or synthesis of the myelin sheaths from the outset. The genetic disease Krabbe disease is a special situation, because there is no degradation of the myelin, but an accumulation of harmful degradation products from the myelin metabolism due to missing enzymes. Demyelination of the axons can also occur due to toxic effects or due to a deficiency of certain B vitamins such as B6 and B12, from which alcoholics often suffer. The autoimmune disease MS, whose causes are not (yet) fully understood, is relatively common in Central Europe and affects women about twice as often as men. The chronic inflammatory disease of the CNS leads to multiple or multiple (multiple) zones in the white matter that are affected by demyelination with the resulting symptomatic consequences.
