Module 1 The Nervous System

Lesson 1.1.7

1.1.7 page 5

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Myelin and Impulse Transmission

In lesson 4, you learned about the basic structures of the neuron. You discovered that some axons are wrapped in Schwann cells that produce a fatty material called myelin. Myelin acts as an insulator. In the following video clip, you will see the significance of insulation to communication in myelinated nerves versus unmyelinated nerves.

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Myelinated vs. Nonmyelinated Neurons

myelinated neuron: a neuron whose axon is wrapped by Schwann cells which produce a myelin sheath; these neurons make up the white matter in the brain and the spinal cord and transmit nerve impulses very quickly

unmyelinated neuron: a neuron that does not have Schwann cells and therefore lacks a myelin sheath; these neurons make up the grey matter in the brain and spinal cord and transmit nerve impulses much more slowly than myelinated neurons

saltatory conduction: rapid transmission of a nerve impulse along an axon resulting from the action potential jumping from one node of Ranvier to another, skipping the myelinated regions of the membrane

In the video, you saw that 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. Even though the axon of the myelinated neuron is not in contact with the sodium rich extracellular fluid outside the neuron, there is rapid communication. The nodes of Ranvier facilitate this rapid communication.

In Lesson 3, you learned that these nodes lack myelin because they are between individual Schwann cells. Here, the axon is in contact with the extracellular fluid. The smooth wave-like nerve impulse that you studied in such detail in the previous sections does not apply to myelinated neurons because action potentials can only occur at the nodes of Ranvier. Scientists found that the neural impulse “jumps” from one node to the next all along the axon in what is called saltatory conduction. For more detail on this process read pages 377 – 378. Figure 11.16 on page 378 provides a good illustration of this process. Add this information to your course folder in a format that you prefer.

Saltatory conduction is like a skillfully thrown rock skipping along a water surface, permitting a greatly increased speed of transmission. The speed of impulses is also increased in axons of large diameter. The fast-reacting giant axons of the squid that allow it to “jet-propel” itself away from danger are several millimeters 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 your own capabilities.

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Multiple Sclerosis

In Lesson 4, you were introduced to the disorder multiple sclerosis (MS), and some of the ways it affects communication in the nervous system.

Self-Check

MS is a disorder capable of slowing down, or even stopping impulse transmission. Prepare a paragraph, outlining the relationship between myelinated nerve function and the symptoms of MS. Indicate the specific losses of myelinated nerve function caused by MS. After autochecking your answer, put it in your Biology 30 Course Folder for easy access when studying.

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Self-Check Answers

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 significantly slowed 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.