Ure 1142

Figure

(a, b, and c) Muscarinic receptors occur in the membranes of effector cells at the axon terminals of autonomic cholinergic neurons. Nicotinic receptors are found in the membranes of postganglionic autonomic neurons and skeletal muscle fibers.

as young adults contain the same numbers of neurons as do newborns. These extra neurons produce the extra dopamine that can lead to the hallucinations that are the hallmark of this illness.

By age thirty, the die-off of neurons accelerates somewhat, although pockets of neural stem cells lining the ventricles retain the capacity to differentiate new neurons and glia. Over an average lifetime, the brain shrinks by about 10 percent, with more loss among the gray matter than the white. Neuron loss is uneven, with cell death in the temporal lobe greatest, but very little death among certain neuron clusters in the brainstem. By age 90, the frontal cortex has lost about half its neurons—but this deficit doesn't necessary translate into loss of function.

The nervous system changes over time in several ways. The number of dendritic branches in the cerebral cortex falls. Signs of slowing neurotransmission include decreasing levels of neurotransmitters, the enzymes necessary to synthesize them, and the numbers of post-synaptic receptors. The rate of action potential propagation may decrease by 5 to 10 percent. Nervous system disorders that may begin to cause symptoms in older adulthood include stroke, depression, Alzheimer disease, Parkinson disease, and multi-infarct dementia.

Noticeable signs of a normally aging nervous system include fading memory and slowed responses and reflexes. Decline in function of the sympathetic nervous system may cause transient drops in blood pressure, which in turn may cause fainting. By the seventh decade, waning ability of nerves in the ankle to respond to vibrations from walking may affect balance, raising the risk of falling. Poor eyesight, anemia, inner ear malfunction and effects of drugs also contribute to poor balance in the later years. Because of these factors, nearly a third of individuals over age 65 have at least one serious fall a year.

Changes in sleep patterns accompany aging, reflecting the functioning of the reticular activating system. Older individuals generally sleep fewer hours per night than they once did, experiencing transient difficulty in getting to sleep and staying asleep, with more frequent movements when they are sleeping. Many have bouts with insomnia, sometimes not sleeping more than an hour or two a night. Changing electroencephalogram patterns indicate that stage IV slow wave sleep as well as

R.E.M. sleep diminishes. All of these changes may result in daytime sleepiness.

99 How does aging of the nervous system begin even before birth?

^9 How does the nervous system compensate for the steady loss of neurons that accompanies normal aging?

^9 What are some diseases that affect the aging nervous system?

□ What are some of the physical and functional signs of an aging nervous system?

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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