Myelin Sheath | Cannabis Wiki

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The Myelin Sheath

The speed of an electrical signal along a neurons axon is increased by the presence of a myelin sheath- a layer of fatty insulation. 

The myelin sheath is formed differently according to where it is located:

In the peripheral nervous system (those nerves lying outside the brain and spinal cord), the myelin sheath is produced by specialized Schwann cells. These wrap themselves around the axon of a nerve cell to form a sheath of concentric circles of their cell membranes.

In the central nervous system, neurons are given their myelin sheath by cells known as oligodendrocytes, which can myelinate more than one nerve axon at a time.

Appearance 

Nerve fibers with myelin sheaths tend to look whiter than unmyelinated ones, which have a great tinge. The ‘white matter’ of the brain is composed of dense collections of myelinated nerve fibers, whereas the ‘gray matter’ is made up of nerve cell bodies and unmyelinated fibers.

Function

Each Schwann cell lies adjacent to but not touching, the next. The gap between the cells, where there is no myelin, is known as the node of Ranvier. As an electrical signal passes down the nerve, it must ‘hop’ from one node to the other, which makes it travel faster overall than if no myelin sheath was present. 

Quick Notes:

The myelin sheath insulates the axon speeding up the transmission of nerve impulses; myelinated nerve fibers can transmit information at up to a hundred fifty times the speed of unmyelinated ones.

Node of Ranvier

Node of Ranvier is the gap between Schwann cells, where no myelin is found.

Cell Membrane of Schwann Cell

The cell membrane has a particularly high content of fatty substances, which are good electrical insulators. 

Axon

Axon carries nerve impulses away from the cell body to the receiving cell.

Schwann Cell

The Schwann cell is a specialized neuron support cell that wraps itself around the axon of a nerve cell to form a sheath. 


Supporting Cells of the Central Nervous System 

Neurons are surrounded by neuroglia, a collective name given to the group of small support cells which make up about half the bulk of the central nervous system. Neuroglial cells outnumber neurons by about 10 to 1 and have a variety of functions:

Astrocytes

Astrocytes are the most abundant neuroglial cells; they are star-shaped. They anchor the neurons to their blood supply and determine what substances can pass between the blood and the brain (the so-called blood-brain barrier). 

Microglia

Like similar cells in other parts of the body, these small oval cells are specialized to ingest, or phagocytose, any invading microorganisms or dead tissue.

Oligodendrocytes

These cells provide the myelin sheath for neurons of the central nervous system.

Ependymal Cells

Lining the fluid-filled ventricles of the central nervous system, these cells may be of a variety of shapes, from flat to columnar. Are they have tiny brush-like cilia on their surfaces which beat to maintain circulation of the cerebrospinal fluid.

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