How does axon diameter affect speed

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A canonical statement I have frequently read is that "large diameter axons conduct action potentials at faster velocities than small diameter axons". After recently learning the effect of increased capacitance on the cell membrane (namely, that more charge is required for depolarization to occur...Are large cell bodies of neurons harder to depolarize than small cell bodies of neurons?), I find this canon to be rather confusing.

I have attached a picture to make my question clearer:

How does axon diameter affect speed

In the above picture, both neurons have the same myelin spacing. So, my question is why should the action potential in Axon B traverse the distance X1 to X2 more quickly than in Axon A. Shouldn't the increased capacitance of Axon B (due to its increased diameter) increase the time it takes for the action potential to travel between the two points?

Bryan Krause

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asked Apr 22, 2019 at 4:24

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Why should the action potential in Axon B traverse the distance X1 to X2 more quickly than in Axon A?

The larger the diameter of the axon, the less the longitudinal resistance, so the current can propagate along the axon more easily.

Shouldn't the increased capacitance of Axon B (due to its increased diameter) increase the time it takes for the action potential to travel between the two points?

Both, the capacitance and the number of ion channels increase quadratically whit the diameter. So the increase in capacitance is compensated by an increase in total ionic current.

Wiki cable therory

answered Apr 22, 2019 at 19:48

herachoheracho

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There are several factors affecting the rate and speed of an action potential:

1. Myelin sheath - This covers some of the nodes and acts as an electrical insulator where the action potential travels from one node of ranvier to the next by saltatory conduction. 

2. Diameter of the axon - the larger the diameter of an axon increases the rate and speed of conductance as there is less leakage of ions.

3. Temperature - The higher the temperature the faster the conductance. This is because enzymes work faster at a high temperature which control the sodium-potassium pump needed to create action potentials.

Basics: neuroscience and psychophysics

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.

How does axon diameter affect speed
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.

  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.

How diameter of axon affects the speed of impulse?

Larger diameter axons have a higher conduction velocity, which means they are able to send signals faster. This is because there is less resistance facing the ion flow.

Does the size of an axon affect conduction velocity?

Conduction velocity is decreased in shorter axons, and vice versa, to allow isochronic spiking at the target nuclei. The much longer, contralateral axon branch is excluded from this strategy: conduction time to the other hemisphere is more than twice as long as the time needed for a signal to reach ipsilateral targets.

How does myelin thickness and axon diameter affect speed of impulse conduction?

By acting as an electrical insulator, myelin greatly speeds up action potential conduction (Figure 3.14). For example, whereas unmyelinated axon conduction velocities range from about 0.5 to 10 m/s, myelinated axons can conduct at velocities up to 150 m/s.

Does the diameter of the axon matter?

Axon diameter directly affects nerve function. It is well known that in myelinated axons, nerve conduction velocity is directly proportional to axon diameter (1–4).