Module 1 Lesson 6 - 5
Lesson 6 — The Nerve Impulse: Transporting the Message
Myelin and Impulse Transmission
Read page 372
How does depolarization of one section trigger depolarization on the next section of the axon? Depolarization spreads because sodium ions enter and start diffusing along the inside of the membrane. As it spreads, the previous section of the axon where
the depolarization just occurred cannot have another action potential due to the refractory period. In the next section of the axon, the sodium ions depolarize the cell membrane, making it positive. The voltage-gated sodium channels open and, as
more positive ions rush into the cell, the depolarization is spread unidirectionally farther along the axon.
Refresh your memory with the basic structure of a neuron on page 372 of the textbook. Some axons are wrapped in Schwann cells that produce a fatty material called myelin. Myelin acts as an insulator.
Myelin is not present on all nerve cells. Myelinated neurons can conduct nerve impulses over 100 metres per second whereas unmyelinated neurons are much slower, with speeds of only 0.5 metres per second. Although the axon of the myelinated neuron is not
in contact with the sodium rich extracellular fluid outside the neuron, rapid communication occurs. The nodes of Ranvier facilitate this rapid communication. These nodes lack myelin because they are between individual Schwann cells, and in these
nodes the axon is in contact with the extracellular fluid.
This means the action potentials can occur only at the nodes of Ranvier. Instead of depolarizing every surface of the cell membrane, the neural impulse can "jump" from one node to the next one all along the axon in what is called saltatory conduction.
Because action potential needs to occur only in the nodes of Ranvier, saltatory conduction permits a greatly increased speed of transmission.
The distance between two nodes is optimal to ensure that depolarization spreads quickly. If the two nodes are too far apart, the sodium ions are unable to depolarize the cell membrane to trigger the voltage-gated sodium channels. If the two nodes are
too close together, an increased number of action potentials are needed and the result is slower propagation.
Multiple Sclerosis
Watch and Listen
Watch the following segment of BiologiX 01 on multiple sclerosis.
Multiple sclerosis (MS) is a disease of the white matter tissue of the central nervous system. The white matter is made up of myelinated nerve fibres responsible for transmitting communication signals both within the CNS and between the CNS and the PNS. When the myelin sheaths of nerves of the CNS are damaged, nerve impulses are slowed significantly or even stopped.
People with MS can experience partial or complete loss of any function that is controlled by, or passes through, the brain or the spinal cord. As such, this disease results in the weakening of the skeletal muscles.
Note Figure 11.17 on page 378 of your textbook, which shows lesions in the brain where the white matter has been destroyed. All myelinated motor neurons from the CNS to the skeletal muscles of the body are affected. This results in loss of muscle coordination and function.
Did You Know?
The fast-reacting giant axons of the squid that allow it to "jet-propel" itself away from danger are several millimetres in diameter, or 100 to 1000 times the diameter of a human axon. However, the squid's giant axons conduct impulses at only about 30 metres per second, which is far below human capabilities.Self-Check
One way to model the action potential is to line up several dominoes and initiate a cascade event in which each domino knocks down in succession the next domino.
- The finger has to contact the first domino just hard enough to cause it to fall. Which response does this represent in a real neuron?
- When the dominoes start to fall, they all fall in succession. What does this action represent in the real neuron?
- The dominoes always fall in one direction. Contrast this with the direction of impulse transmission in a real neuron.
- No matter how many times the dominoes fall, they always move at the same speed and intensity. What principle does this represent in the real neuron?
Self-Check Answers
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The falling dominoes demonstrate part of the all-or-none response. The action potential is generated by the stimulus or it is not (just as the domino either falls or it does not). The stimulus must reach threshold potential or depolarization does not
occur.
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In a real neuron, this could be described as a wave of depolarization along an axon or the propagation of the action potential along the neuron. As well, the impulse travels in only one direction in a neuron.
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The wave of depolarization proceeds in one direction, similar to the dominoes falling in one direction. The previous domino pushes on the next one, causing it to fall and so on. In the neural membrane, when the sodium gates open and sodium ions rush into
the axon, they cannot diffuse out. They diffuse along the axon, and when they reach the next section, the positive charges reduce the net negative charge, thereby depolarizing the next section to threshold and stimulating more depolarization.
Because an action potential just occurred in the previous section and the net negative charge has been reduced, the impulse cannot go backward. In the domino analogy, the previous domino has fallen and it cannot fall again.
- The dominoes always fall at the same speed and intensity. This event is also explained by the all-or-none response in a neuron. An axon cannot respond with a mild or a strong response; it can only respond or not respond. A stimulus that is strong enough to reach the threshold potential in the neuron generates an action potential and a wave of depolarization is initiated. The depolarization in the next section does not occur faster or slower or more powerfully than in the last section; it just occurs.