Basics: neuroscience and psychophysics

8 Conduction Velocity and Myelin

Learning Objectives

Understand what myelin is

Understand how myelin increases conduction velocity

Myelin

The insulation for axons in the nervous system is provided by glial cells, oligodendrocytes in the CNS, and Schwann cells in the PNS. Whereas the manner in which either cell is associated with the axon segment(s) that it insulates is different, the means of myelinating an axon segment is mostly the same in the two situations. Myelin is a lipid-rich sheath that surrounds the axon and by doing so creates a myelin sheath that facilitates the transmission of electrical signals along the axon. The lipids are essentially the phospholipids of the glial cell membrane. Myelin, however, is more than just the membrane of the glial cell. It also includes important proteins that are integral to that membrane. Some of the proteins help to hold the layers of the glial cell membrane closely together.

Gray-scale image showing cell parts. In the middle is a squished set of tight rings, which is a cross section of the myelin sheath on an axon.
Figure 8.1. Cross-section of a myelinated axon. Myelinating glia wrap several layers of cell membrane around the cell membrane of an axon segment. A single Schwann cell insulates a segment of a peripheral nerve, whereas in the CNS, an oligodendrocyte may provide insulation for a few separate axon segments. EM × 1,460,000. Visit Webscope for a chance to zoom interactively on the image and see more detail. (Micrograph provided by the Regents of University of Michigan Medical School © 2012)

Conduction velocity

Myelination improves the conduction velocity—the speed with which action potentials travel—in axons. Axon diameter also affects conduction velocity; fatter axons carry action potentials faster. So there are two factors that determine how fast an action potential travels down an axon: diameter and myelination (Suzuki, 2010). Axon diameter and myelination are correlated: the skinniest axons (< 1 micron in diameter) are also unmyelinated (often called C-fibers; a good example is found in the neurons that carry pain and temperature information from the skin to the spinal cord). They carry action potentials the most slowly, about 1 meter/second (walking speed). Axons fatter than 1 mm in diameter are generally myelinated. The fattest myelinated axons can carry action potentials faster than 100 m/s—race car speeds! Mechanical (pressure) sensors in our skins and motor neurons have fat, myelinated axons, so they can do their jobs rapidly.

Disorders of nervous tissue

Several diseases can result from the demyelination of axons. The causes of these diseases are not the same; some have genetic causes, some are caused by pathogens, and others are the result of autoimmune disorders. Though the causes are varied, the results are largely similar. The myelin insulation of axons is compromised, making electrical signaling slower.

Multiple sclerosis (MS) is one such disease. It is an autoimmune disease. The antibodies produced by lymphocytes (a type of white blood cell) mark myelin as something that should not be in the body. This causes inflammation and the destruction of the myelin in the central nervous system. As the insulation around the axons is destroyed by the disease, scarring becomes obvious. This is where the name of the disease comes from; sclerosis means hardening of tissue, which is what a scar is. Multiple scars are found in the white matter of the brain and spinal cord. The symptoms of MS include both somatic and autonomic deficits. Control of the musculature is compromised, as is control of organs such as the bladder.

Guillain-Barré (pronounced gee-YAN bah-RAY) syndrome is a demyelinating disease of the peripheral nervous system. It is also the result of an autoimmune reaction, but the inflammation is in peripheral nerves. Sensory symptoms or motor deficits are common, and autonomic failures can lead to changes in the heart rhythm or a drop in blood pressure, especially when standing, which causes dizziness.

 

Exercises

  1. Conduction Velocity and Myelin Review Quiz
    A. What type of cell provides insulation for axons in the nervous system?
    B. Astrocytes
    C. Glial Cells
    D. Ependymal Cell
    E. Microglia
  2. Match the specific cell with the nervous system that it insulates.
    A. CNS                                 __ Schwann Cells
    B. PNS                                 __ Oligodendrocytes
  3. What facilitates the transmission of electrical signals along the axon?
    A. Myelin Sheath
    B. Axolemma
    C. Glial Cell
    D. Neurofilament
  4. What type of structure is responsible for holding the layers of the Glial Cell membrane closely together?
    A. Nucleus
    B. Proteins
    C. Ribosomes
    D. Cell Membrane
  5. As axon diameter increases, conduction velocity…
    A. Decreases
    B. Increases
    C. No Change
  6. Axon diameter and myelination have what type of relationship?
    A. Inverse
    B. Direct
  7. In people who have Multiple Sclerosis, what type of white blood cell produces antibodies that mark myelin as something bad?
    A. Monocytes
    B. Neutrophils
    C. Basophils
    D. Lymphocytes
  8. Match the disease with the nervous system that it affects
    A. Guillan-Barre Syndrome                       __PNS
    B. Multiple Sclerosis (MS)                         __CNS

 

Answer Key:
1. B
2. A is Oligodendrocytes; B is Schwann Cells
3. A
4. B
5. B
6. B
7. D
8. A is PNS; B is CNS

 

CC LICENSED CONTENT, SHARED PREVIOUSLY

OpenStax, Anatomy and Physiology Section 12.2 Nervous Tissue
Provided by: Rice University.
Access for free at https://openstax.org/books/anatomy-and-physiology/pages/1-introduction
License: CC-BY 4.0
Adapted by: Cheryl Olman

References:

Susuki, K. (2010) Myelin: A Specialized Membrane for Cell Communication. Nature Education 3(9):59.

License

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Introduction to Sensation and Perception by Students of PSY 3031 and Edited by Dr. Cheryl Olman is licensed under a Creative Commons Attribution 4.0 International License, except where otherwise noted.

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