Myelin

Myelin

Myelin is a lipid based or a fatty substance that covers the axons of neuronsThe functional cells of the nervous system that transmit signals.. You can think of myelin like the rubber coating that comes on a lot of electrical cords. That rubber coating is there to prevent the electricity from running through the wires and arcing away. It keeps the electricity from fizzling out into space. The insulation on an electric cord makes sure the electricity from the wall in your house is transmitted safely. It then powers whatever electrical device you want.

The myelin covering on axons ensures the action potentialA rapid, temporary electrical charge that travels along neurons, allowing signal transmission. can travel long distances of axons. The cell membrane of an axon of any neuron can be given the more specific name of axolemmaThe cell membrane of an axon.. Like any part of the cell membrane, there are proteinsLarge molecules made of amino acids with various functions in the body. embedded in the axolemma. These proteins include voltage gated channelsProtein passages in the cell membrane that allow specific molecules to pass through., leak channels, and occasionally, a sodium(Na⁺): Major ECF cation; important for fluid balance, nerve function. and potassium(K⁺): Major ICF cation; essential for muscle and nerve function. pump. What is not in the axolemma of an axon are ligand gated channels.  Why would we need channels that connection for neurotransmittersChemicals released by neurons to transmit signals across a synapse. here?  An axon is the site of propagation, not depolarizationThe loss of electrical charge across a membrane, triggering an action potential. from neurotransmitters. 

Oligodendrocytes of the CNS

In the central nervous systemComposed of the brain and spinal cord; integrates and processes information. , the brain and the spinal cordThe central nervous system structure that relays signals between the brain and body., the neuroglial cellsThe basic structural and functional units of life. that make myelin are called oligodendrocytesGlial cells in the CNS that form the myelin sheath around axons.. When a myelinated neuron in the central nervous systemThe organ system that controls body functions using electrical and chemical signals. is injured, its myelin covering can be damaged on the axon. Microglia come in and clean up the debris. As they do this, they secrete certain factors or proteins.  These factors cause the maturation of pre oligodendrocytes to mature oligodendrocytes. The mature oligodendrocyte then attaches to the axon of a neuron or axons of many neurons.  Each of their many processes make individual myelin sheaths. 

Schwann Cells of the PNS

In the peripheral nervous systemPNS All nervous tissue outside the CNS; includes nerves and ganglia., Schwann cellsPNS glial cells that form the myelin sheath around peripheral nerve fibers. are the myelin-making cells.  These cells wrap around the axon differently than the oligodendrocytes of the central nervous system.  Whereas one oligodendrocytes could form many myelin sheaths around numerous axons, each Schwann cell creates one of those myelin sheaths.  When an axon of a neuron in a nerve such as the ulnar or the sciatic nerveThe largest nerve in the body, arising from the sacral plexus. becomes damaged, the first step is still the same. The initial action is to clean up the debris.  In the peripheral nervous system, macrophages do this clean up while the Schwann cells detach from the injury site.  These “repair” Schwann cells form a regeneration tubeA guide formed by Schwann cells for axonal regrowth after injury in the PNS. through which the regenerating axon can grow.  Once the axon has regenerated the injury site, the repair Schwann cells start to regenerate the individual sheaths of myelin.