myelination

Examples of myelination 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.

• Glia

  • Glia guide developing neurons to their destinations, buffer ions and chemicals that would otherwise harm neurons, and provide myelin sheaths around axons.
  • Oligodendrocytes form myelin sheaths around axons in the CNS .
  • One axon can be myelinated by several oligodendrocytes; one oligodendrocyte can provide myelin for multiple neurons.
  • This is distinctive from the PNS where a single Schwann cell provides myelin for only one axon as the entire Schwann cell surrounds the axon.
  • Oligodendrocytes form the myelin sheath around axons.

• 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.

• Introducing the Neuron

  • Some axons are covered with myelin, a fatty material that wraps around the axon to form the myelin sheath.
  • Periodic gaps in the myelin sheath are called nodes of Ranvier.
  • The myelin sheath is not actually part of the neuron.
  • Myelin is produced by glial cells (or simply glia, or "glue" in Greek), which are non-neuronal cells that provide support for the nervous system.
  • This neuron diagram also shows the oligodendrocyte, myelin sheath, and nodes of Ranvier.

• 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.

• Nerve Impulse Transmission within a Neuron: Action Potential

  • Myelin acts as an insulator that prevents current from leaving the axon, increasing the speed of action potential conduction.
  • Diseases like multiple sclerosis cause degeneration of the myelin, which slows action potential conduction because axon areas are no longer insulated so the current leaks.
  • A node of Ranvier is a natural gap in the myelin sheath along the axon .
  • Nodes of Ranvier are gaps in myelin coverage along axons.

• 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 containing capillaries.
  • White matter is composed largely of myelinated axons.

• 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.