Info

- Neuron nucleus

- Neuron cell body

- Unmyelinated region of axon

Myelinated

\ region of axon

Axon

Groove Ranvier

Node of Ranvier

Schwann cell nucleus-Myelin -

Axon

Node of Ranvier

Schwann cell nucleus-Myelin -

Node of Ranvier

Groove Ranvier

Myelin

Axon

Myelin

Axon

Myelinated

\ region of axon

Enveloping Schwann cell

Schwann cell nucleus

Unmyelinated axon

Longitudinal groove

Enveloping Schwann cell

Schwann cell nucleus

Longitudinal groove

Unmyelinated axon

Figure

(a) The portion of a Schwann cell that winds tightly around an axon forms the myelin sheath. The cytoplasm and nucleus of the Schwann cell, remaining on the outside, form the neurilemmal sheath. (b) Light micrograph of a myelinated axon (longitudinal section) (300x micrograph enlarged to 650x). (c) An axon lying in a longitudinal groove of a Schwann cell lacks a myelin sheath.

axon or a group of axons may lie partially or completely in a longitudinal groove of Schwann cells.

Axons that have myelin sheaths are called myelinated (medullated) axons, and those that lack these sheaths are unmyelinated axons (fig. 10.5). Groups of myelinated axons appear white. Masses of such axons impart color to the white matter in the brain and spinal cord, but here another kind of neuroglial cell called an oligodendrocyte produces myelin. In the brain and spinal cord, myelinated axons lack neurilemmal sheaths.

Unmyelinated nerve tissue appears gray. Thus, the gray matter within the brain and spinal cord contains many unmyelinated axons and neuron cell bodies. Clinical Application 10.2 discusses multiple sclerosis, in which neurons in the brain and spinal cord lose their myelin.

H List the general functions of the nervous system. Describe a neuron.

Explain how an axon in the peripheral nervous system becomes myelinated.

Clinical Application

Multiple Sclerosis

In 1964, at age 20, skier Jimmie Huega won the Olympic bronze medal in the slalom event. In 1967, his vision blurred, and then his legs became slightly numb. Jimmie ignored these intermittent symptoms, and after a while they disappeared.

Three years later, the symptoms returned, and this time Jimmie sought medical help. On the basis of his symptoms, which affected more than one body part and occurred sporadically, physicians diagnosed multiple sclerosis (MS). Today, diagnosis also includes a magnetic resonance imaging scan (MRI), which can detect brain and spinal cord lesions.

Jimmie is still active. For many of the 300,000 people in the United States with MS, the progressive deterioration causes permanent paralysis. For some patients, taking a drug based on the immune system bio chemical beta interferon can prevent flare-ups and mitigate symptoms.

In MS, the myelin coating in various sites through the brain and spinal cord forms hard scars, called scleroses, that block the underlying neurons from transmitting messages. Muscles that no longer receive input from motor neurons stop contracting, and eventually they atrophy. Symptoms reflect the specific neurons affected. Shortcircuit-ing in one part of the brain may affect fine coordination in one hand; if another brain part is affected, vision may be altered.

What might destroy myelin in MS? A virus may cause the body's immune system to attack the cells producing myelin. This would happen if viruses lay latent in nerve cells, then emerged years later bearing proteins also found on nerve cells. The immune system, interpreting the proteins as foreign, would attack the viruses as well as the neurons (an autoimmune response).

A virus is suspected for a few reasons: viral infections can strip neurons of their myelin sheaths; viral infections can cause repeated bouts of symptoms; and most compelling, MS is much more common in some geographical regions (the temperate zones of Europe, South America, and North America) than others, suggesting a pattern of infection. ■

Myelinated And Unmyelinated Neurons

Figure

A transmission electron micrograph of myelinated and unmyelinated axons in cross section (30,000x).

Essentials of Human Physiology

Essentials of Human Physiology

This ebook provides an introductory explanation of the workings of the human body, with an effort to draw connections between the body systems and explain their interdependencies. A framework for the book is homeostasis and how the body maintains balance within each system. This is intended as a first introduction to physiology for a college-level course.

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