Myelin is a lipid-rich substance that surrounds nerve cell axons to insulate them and increase the rate at which electrical impulses are passed along the axon. Each axon comprises multiple long myelinated sections separated from each other by short gaps called “nodes of Ranvier”.
The main purpose of myelin is to increase the speed at which electrical impulses propagate along the myelinated fiber, propagating by salutatory conduction, myelin permits larger body size by maintaining agile communication between distant body parts.
That conduction speed in the nervous system has adaptative value, and the faster the better, provided the selective force driving the evolution of higher speeds:
– Reaction to predatory attack: more likely it will be able to take effective evasive measures
– More rapid conduction decreases reaction times in launching predatory strikes
– Better synchronize contraction of a distributed muscle sheet
– Evolution of larger body sizes with better communication between distant parts.
The nervous system is unique to animals. They have evolved from primitive neuro-effector cells, most of the neurons in invertebrates are unmyelinated. In mammals, the cells that form myelin are oligodendogliocytes.
It receives information received by the receivers, then elaborates and transmits a series of responses to adapt the individual to the internal and external environment. Specialized cells are the «neurons» that act quickly and effectively.
In neurons there are three parts: dendrites, axon and cell body
The axon has a myelin sheath (lipoprotein), made up of many layers of the Schwann cell membrane, there are periodic narrowings called «Ranvier nodes» every mm. The axon transmits the information by electrical impulses from one neuron to another, through changes in the membrane at 100 m / s.
The «nerve impulse» or «myelinated conduction» follows a pattern similar to a circular current flow, the depolarization jumps from one Ranvier node to another: «saltatory conduction» (conducts 50 times faster than the non-myelinated fibers). The jump of the nervous current is made in the synapse (neurite-dendrite) with «the intersynaptic space» (200 angstroms), through the «law of all or nothing«, the nervous current can thus pass from one neuron to another by driving.
Louis-Antonie Ranvier (1835-1922) was a French physician, pathologist, anatomist and histologist, who discovered the “nodes of Ranvier”, regularly spaced discontinuities of the myelin sheath, intervals along the length of a nerve fiber.
Rudolf Ludwig Carl Virchow (1821-1902) was a German physician, anthropologist, pathologist, prehistorian, biologist, writer, editor, and politician. He was “the father of modern pathology”, the founder of social medicine. Medullary sheath acquired its name of “myelin”, named by Virchow, in 1854.
Theodor Schwann (1810-1882) was a German physician and physiologist. He was considered to be the extension of cell theory to animals. He discovered Schwann cells in the peripheral nervous system, myelin covering peripheral axons.
Santiago Ramón y Cajal (1852-1934) was a Spanish neuroscientist and pathologist, specializing in neuroanatomy: histology of the central nervous system. He received the Nobel Prize in 1906 with Camillo Golgi. Establishing the neuron, or nerve cell, as the basic unit of nervous structure, neuron´s fundamental role in nervous function and in gaining a modern understanding of the nerve impulse.
Brean, Bruce P.; “The action potential in mammalian central neurons”, Nature Reviews Neuroscience, 2007
Lyons, David; “Adaptive myelination from fish to man”, Brain Research, 2016
Ganong, W.F; “Review of Medical Physiology”, Ed. Lange; 1981
Boullerne, A. I.; “The history of myelin”, Exp. Neurol; 2016
Encyclopaedia Britannica: Rudolf Virchow
Encyclopaedia Britannica: Louis A. Ranvier
Encyclopaedia Britannica: Theodor-Schwann
Encyclopaedia Britannica: Santiago Ramón y Cajal