myelin

Examples of myelin in the following topics:

• Grey and White Matter

  • The basic pattern of the CNS is a central cavity surrounded by gray matter made up of neuronal cell bodies external to which is the white matter which is made up of myelinated axons.
  • A second major component of the central nervous system is white matter and it is composed of bundles of myelinated axons that connect various grey matter regions of the nervous system to each other and carry nerve impulses between neurons .
  • White matter only contains the myelinated axon tracts,  and not the cell bodies.  
  • Myelin is a lipid that forms a thin layer, known as the myelin sheath, around the axons.  
  • Myelin also gives white matter its characteristic color.

• Characteristics of Nervous Tissue

  • The axon is surrounded by a whitish, fatty layer called the myelin sheath.
  • Outside the myelin sheath there is a cellular layer called the neurilemma.
  • The myelin sheath surrounds and insulates the axon.
  • Transmission electron micrograph of a myelinated axon.
  • The myelin layer (concentric) surrounds the axon of a neuron, showing Schwann cells.

• Classification of Nerves

  • Peripheral nerve fibers are grouped based on the diameter, signal conduction velocity, and myelination state of the axons.
  • Fibers of the A group have a large diameter, high conduction velocity, and are myelinated.
  • Fibers of the B group are myelinated with a small diameter and have a low conduction velocity.
  • The lack of myelination in the C group is the primary cause of their slow conduction velocity.
  • Demonstrates the faster propagation of an action potential in myelinated neurons than that of unmyelinated neurons.

• Spinal Cord White Matter

  • The white matter of the spinal cord is composed of bundles of myelinated axons.
  • White matter tissue of the freshly cut brain appears pinkish white to the naked eye because myelin is composed largely of lipid tissue veined with capillaries.
  • Myelin acts as an insulator, increasing the speed of transmission of all nerve signals.
  • The white matter is white because of the fatty substance (myelin) that surrounds the nerve fibers (axons).
  • This myelin is found in almost all long nerve fibers and acts as an electrical insulation.

• Regeneration of Nerve Fibers

  • Such mechanisms may include generation of new neurons, glia, axons, myelin, or synapses.
  • When an axon is damaged, the distal segment undergoes Wallerian degeneration, losing its myelin sheath.
  • The distal segment, however, experiences Wallerian degeneration within hours of the injury; the axons and myelin degenerate, but the endoneurium remains.
  • The environment within the CNS, especially following trauma, counteracts the repair of myelin and neurons.
  • Micrograph of a nerve with a decrease in myelinated nerve fibres (pink) and an abnormal increase in fibrous tissue (yellow), as may be seen in nerve injuries.

• White Matter of the Cerebrum

  • White matter is composed of myelinated axons and glia and connects distinct areas of the cortex.
  • It consists mostly of glial cells and myelinated axons and forms the bulk of the deep parts of the cerebrum and the superficial parts of the spinal cord.
  • In a freshly cut brain, the tissue of white matter appears pinkish white to the naked eye because myelin is composed largely of lipid tissue that contains capillaries.
  • White matter is composed largely of myelinated axons.

• Neuroglia of the Peripheral Nervous System

  • Schwann cells provide myelination to peripheral neurons.

• Capacity of Different Tissues for Repair

  • The distal segment, however, experiences Wallerian degeneration within hours of the injury; the axons and myelin degenerate, but the endoneurium remains.
  • The hostile, non-permissible growth environment is, in part, created by the migration of myelin-associated inhibitors and other cells.
  • The environment within the CNS, especially following trauma, counteracts the repair of myelin and neurons.
  • Slower degeneration of the distal segment than that which occurs in the peripheral nervous system also contributes to the inhibitory environment because inhibitory myelin and axonal debris are not cleared away as quickly.

• Types of Nervous Tissue

  • Oligodendrocytes line up along the nerves and produce an insulating cover called myelin sheath.
  • Schwann cells surround all nerve fibers in the peripheral nervous system and form myelin sheaths around the nerve fibers.

• Pain Sensation

  • The Aδ fiber axons are myelinated and can allow an action potential to travel at a rate of about 20 meters/second towards the central nervous system.
  • Aδ fibers are thinly myelinated; therefore, they conduct signals more rapidly than unmyelinated C fibers, but more slowly than other, more thickly myelinated "A" class fibers.