Plate Forty Seven

Surface anatomy of ankle and foot.

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nervous m I

Basic Structure and Function understanding ^Vo rds astr-, starlike: astrocyte—star-

shaped neuroglial cell. ax-, axle: axon:—cylindrical nerve fiber that carries impulses away from a neuron cell body. dendr-, tree: dendrite—branched nerve fiber that serves as the receptor surface of a neuron. ependym-, tunic: ependyma— neuroglial cells that line spaces within the brain and spinal cord. -lemm, rind or peel:

neurilemma—sheath that surrounds the myelin of a nerve fiber. moto-, moving: motor neuron— neuron that stimulates a muscle to contract or a gland to release a secretion. multi-, many: multipolar neuron—neuron with many fibers extending from the cell body. oligo-, few: oligodendrocyte— small neuroglial cell with few cellular processes. peri-, all around: peripheral nervous system—portion of the nervous system that consists of the nerves branching from the brain and spinal cord. saltator-, a dancer: saltatory conduction—nerve impulse conduction in which the impulse seems to jump from node to node along the nerve fiber. sens-, feeling: sensory neuron— neuron that can be stimulated by a sensory receptor and conducts impulses into the brain or spinal cord. syn-, together: synapse—junction between two neurons. uni-, one: unipolar—neuron with only one fiber extending from the cell body.

chapter objectivi

After you have studied this chapter, you should be able to

Explain the general functions of the nervous system. Describe the general structure of a neuron. Explain how neurons are classified.

Name four types of neuroglial cells and describe the functions of each.

Explain how an injured nerve fiber may regenerate.

Explain how a membrane becomes polarized.

Describe the events that lead to the conduction of a nerve impulse.

8. Explain how a nerve impulse is transmitted from one neuron to another.

9. Distinguish between excitatory and inhibitory postsynaptic potentials.

10. Explain two ways impulses are processed in neuronal pools.

hen the five living U.S. presidents gathered at the funeral

W of former President Richard M. Nixon in April 1994, Gerald Ford, Jimmy Carter, George Bush, and Bill Clinton knew that all was not right with their compatriot Ronald Reagan. The former president was forgetful, responded inappropriately to questions, and, in the words of Gerald Ford, seemed "hollowed out." Reagan's memory continued to fade in and out, and six months later, he penned a moving letter to the public confirming that he had Alzheimer disease. The spells of forgetfulness and cloudy reasoning would come and go over the next several years, eventually increasing in frequency and duration, each day becoming like every other, for Reagan could no longer recognize events or people including his wife Nancy. His later years have been secret.

Because Alzheimer disease affects millions of families, finding a treatment is an important health care objective. Doing so requires pinpointing just what goes awry in the nervous system to trigger the cascade of destruction that ultimately strangles the brain in protein plaques and tangles. Research is focusing on replenishing the neurotransmitter that is deficient in the brain of a person with Alzheimer disease, supplying other biochemicals that stimulate nervous tissue to grow, and re placing affected cells. Data from the human genome project is helping to identify subtypes of the condition.

The ability to diagnose Alzheimer disease, even before symptoms begin, is possible for relatives of the 5% to 10% of affected individuals who have inherited the disorder. Inherited Alzheimer disease tends to have an early onset, in one's forties or fifties. At least four genes can each directly cause the disease. One gene encodes amyloid, the protein that misfolds to form intractable plaques in the brain, and the others control amyloid production. Another gene, which encodes apolipoprotein E, when abnormal increases the risk of developing Alzheimer disease. Understanding how these genes malfunction to cause Alzheimer disease will yield clues that can be used to help people with the noninherited form of the illness as well.

Genetic analysis enables a physician to predict inherited forms of Alzheimer disease before symptoms become noticeable. Such knowledge may enable families to plan better for caring for ill relatives. It may also aid physicians in distinguishing Alzheimer disease from other disorders. Finally, early diagnosis will enable researchers to test new treatments as the disease starts, when the ability to slow or prevent symptoms is more likely.

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