Myelin
Myelin (/ˈmaɪ.əlɪn/ MY-ə-lin) is a lipid-rich material that surrounds nerve cell axons (the nervous system's electrical wires) to insulate them and increase the rate at which electrical impulses (called action potentials) pass along the axon.[1][2] The myelinated axon can be likened to an electrical wire (the axon) with insulating material (myelin) around it. However, unlike the plastic covering on an electrical wire, myelin does not form a single long sheath over the entire length of the axon. Rather, myelin ensheaths the axon segmentally: in general, each axon is encased in multiple long sheaths with short gaps between, called nodes of Ranvier. At the nodes of Ranvier, which are approximately one thousandth of a mm in length, the axon's membrane is bare of myelin.
Myelin
Nervous system
Myelin's best known function is to increase the rate at which information, encoded as electrical charges, passes along the axon's length. Myelin achieves this by eliciting saltatory conduction,.[1] Saltatory conduction refers to the fact that electrical impulses 'jump' along the axon, over long myelin sheaths, from one node of Ranvier to the next. Thus, information is passed around 100 times faster along a myelinated axon than a non-myelinated one.
At the molecular level, the myelin sheath increases the distance between extracellular and intracellular ions, reducing the accumulation of electrical charges. The discontinuous structure of the myelin sheath results in the action potential "jumping" from one node of Ranvier over a long (c. 0.1 mm – >1 mm, or 100-1000 micron) myelinated stretch of the axon called the internodal segment or "internode", before "recharging" at the next node of Ranvier. This 'jumping' continues until the action potential reaches the axon terminal.[3][4][5] Once there, the electrical signal provokes the release of chemical neurotransmitters across the synapse, which bind to receptors on the post-synaptic cell (e.g. another neuron, myocyte or secretory cell).
Myelin is made by glial cells, which are non-neuronal cells that provide nutritional and homeostatic support to the axons. This is because axons, being elongated structures, are too far from the soma to be supported by the neurons themselves. In the central nervous system (brain, spinal cord and optic nerves), myelination is formed by specialized glial cells called oligodendrocytes, each of which sends out processes (limb-like extensions from the cell body) to myelinate multiple nearby axons; while in the peripheral nervous system, myelin is formed by neurolemmocytes (Schwann cells), which only myelinate a section of one axon. In the CNS, axons carry electrical signals from one nerve cell body to another.[6][7]
The "insulating" function for myelin is essential for efficient motor function (i.e. movement such as walking), sensory function (e.g. sight, hearing, smell, the feeling of touch or pain) and cognition (e.g. acquiring and recalling knowledge), as demonstrated by the consequence of disorders that affect myelination, such as the genetically determined leukodystrophies;[8] the acquired inflammatory demyelinating disorder, multiple sclerosis;[9] and the inflammatory demyelinating peripheral neuropathies.[10] Due to its high prevalence, multiple sclerosis, which specifically affects the central nervous system (brain, spinal cord and optic nerve), is the best known disorder of myelin.
Species distribution[edit]
Vertebrates[edit]
Myelin is considered a defining characteristic of the jawed vertebrates (gnathostomes), though axons are ensheathed by a type of cell, called glial cells, in invertebrates.[15][16] These glial wraps are quite different from vertebrate compact myelin, formed, as indicated above, by concentric wrapping of the myelinating cell process multiple times around the axon. Myelin was first described in 1854 by Rudolf Virchow,[17] although it was over a century later, following the development of electron microscopy, that its glial cell origin and its ultrastructure became apparent.[18]
In vertebrates, not all axons are myelinated. For example, in the PNS, a large proportion of axons are unmyelinated. Instead, they are ensheathed by non-myelinating Schwann cells known as Remak SCs and arranged in Remak bundles.[19] In the CNS, non-myelinated axons (or intermittently myelinated axons, meaning axons with long non-myelinated regions between myelinated segments) intermingle with myelinated ones and are entwined, at least partially, by the processes of another type of glial cell the astrocyte.[20]
Invertebrates[edit]
Functionally equivalent myelin-like sheaths are found in several invertebrate taxa, including oligochaete annelids, and crustacean taxa such as penaeids, palaemonids, and calanoids. These myelin-like sheaths share several structural features with the sheaths found in vertebrates including multiplicity of membranes, condensation of membrane, and nodes.[15] However, the nodes in vertebrates are annular; i.e. they encircle the axon. In contrast, nodes found in the sheaths of invertebrates are either annular or fenestrated; i.e. they are restricted to "spots". The fastest recorded conduction speed (across both vertebrates and invertebrates) is found in the ensheathed axons of the Kuruma shrimp, an invertebrate,[15] ranging between 90 and 200 m/s[16] (cf. 100–120 m/s for the fastest myelinated vertebrate axon).