SECTION 1: CONTROL

Functions of the human body are regulated by a complex network of communication between the sensory, nervous, and endocrine systems. Information is received by various sensory systems designed for reception, such as through sight, touch, or hearing. The information is transmitted as electrical signals along a pathway of intersecting nerves as part of the nervous system, the command center for all body activity. The signals become interpreted as specific information requiring a specific response. The responses can be in the form of an increased number of electrical signals sent to a hand making it move away from a hot flame. Another example of a response is in the form of signaling an organ to turn on or off the production of an essential hormone or chemical necessary for a specific biological function. All activity in the body communicates within the network. Damage from injury or disease to the network results in dysfunction.

The Brain, Spinal Cord, and Nerves

The brain and spinal cord make up the central nervous system. The brain is a mass of nerve cells called neurons. The function of the brain is very complex and not completely understood. In general, brain cells receive messages from other parts of the body, interpret the information, and then relay messages to other parts of the brain or to other parts of the body. The brain plays a major role in thought processes such as concept formation, abstract reasoning, learning and memory. The brain stem is the lower part of the brain making the connection to the spinal cord. It functions to control vital life processes such as heartbeat, respirations, temperature, blood pressure, and the release of some hormones.

The spinal cord is basically an extension of the brain. It functions as part of the network for the communication of messages sent from the brain to the rest of the body and to receive messages from the body needing to be sent to the brain.

Nerves are neurons (nerve cells) connected in long chains that have branched out from the spinal cord. They reach the most peripheral areas of the body (the peripheral nervous system) to carry sensory or motor information. The information is transmitted in the form of electrical impulses that travel along a nerve pathway jumping from nerve cell to nerve cell. Specific chemicals called neurotransmitters (such as acetylcholine, norepinephrine, dopamine, and serotonin to name a few) play an essential role in the ability and timing of an electrical impulse to jump the space (synapse) between nerve cells. Improper balance of neurochemicals affects timing, and can result in some physical or mental dysfunction. Researchers have suggested that depression may be related to improper serotonin levels and dopamine may play a role in Parkinson's Disease.

The Aging Nervous System

Brain weight and volume declines as a result of aging. Controversy remains as to the cause. Some researchers suggest there is a loss in the total number of neurons. Others attribute the change to a shrinkage in the size of existing neurons. It has also been suggested that the brain at any age has a surplus of cells beyond what is needed for normal function, therefore the loss with aging may not matter. "Although new brain neurons do not replace lost ones, remaining neurons can sprout new connections and, within limits, repair or compensate for the short or broken circuits that occur when a neighboring neuron dies, restoring the aging brain to more or less its former condition" (Hayflick, 1994, p.163). This adaptive feature is referred to as plasticity. Dysfunction results when cell loss impairs a vital function and other cells are not able to compensate.

According to Hayflick (1994), the most profound discovery this decade is that a decline in mental capacity is not an inevitable part of aging. The amount and activity of neurotransmitter chemicals declines with age, resulting in a slower transmission of signals down nerve pathways. Therefore, activities that require information processing in the brain will be slowed. The decline of short-term memory can be attributed to the slowed retrieval process. Reaction time to noise and unpredictable stimuli tends to be slower after age seventy. The ability to learn is always present barring any disease of the brain that impairs the ability to learn. The speed of learning tends to decrease after age seventy, especially with oral material as opposed to written material used for instruction. Verbal skills and vocabulary do not change because of aging. In general, there are some elders who experience no decline in performance on mental tasks.

Endocrine System

The endocrine system consists of specific organs and tissues which function to manufacture and release chemicals called hormones. These hormones are released into the bloodstream where they are transported to a target tissue to regulate some specific activity or process. Control of the production and release of hormones is activated through a negative feedback loop. For example, too much of one hormone signals another hormone to be released to stop the first one. The main functions of the endocrine system is to coordinate body activities, control growth and development, and maintain homeostasis. Overall, the endocrine system affects virtually all cells in the human body.

The endocrine organs and the brain are highly interdependent such that a change in the activity of one can cause a change in the activity of the other. One function of this interdependence is the maintenance of homeostasis from the effects to the body due to changes in the internal and external environment.

The Aging Endocrine System

General changes associated with aging related to the endocrine system include:

1. Rate of secretion of most hormones (testosterone, estrogen, insulin, androgens, aldosterone, thyroid and growth hormone) is decreased.

2. Changes in the receptor efficiency of the target tissue.

3. Sensitivity to responses in the feedback control system is decreased.

Some specific results of these changes include enlargement of the prostate, menopause, and decreased ability to respond to the stress of extremes of temperature. The important information to remember is that the exact cause of these age-related changes have not been determined yet. Even though a blood test might demonstrate lowered levels of a hormone, the cause of the symptoms may be more related to the other two endocrine changes mentioned above. For example, the ovaries in all women eventually cease their production of ovarian estrogen, but the symptoms of menopause may vary dramatically.

THINK ABOUT IT:

It has been suggested that hormone replacement therapy might be the anti-aging panacea. Research studying the effects of artificially increasing hormones in old rats showed increased immune system efficiency and an increase in size of the thyroid. Recent research where older men were administered growth hormone showed increased muscle mass, increased bone density, and decreased fat tissue. While considered a reversal of the normal aging process, the results of these studies are still too new and lack certain details that would support depending upon them for increasing longevity.

It is known that physical changes of aging are related to the endocrine system. It has not been proven whether those changes can be attributed to low levels of hormones or the decreased receptor efficiency. Both are considered normal age changes in the endocrine system. Therefore, it is conjecture to assume that the outcome of the above studies resulted because of low levels of hormones rather than possibly a decreased ability of the target cells to respond to the hormones present. This issue is important if you consider that other studies have shown that growth hormone will accelerate the growth of an existing cancer, and has been associated with lipid and pancreas problems. If the human body was able to maintain youth and vigor with increasingly lower levels of hormones by making the target tissue more responsive to the hormone levels present, then the risks of hormone replacement therapy would be avoided.

Return to Outline

SECTION 2: REPRODUCTION

This section represents a discussion of the role reproductive system has beyond the years of fertility and reproduction.

Female

Menopause is the term that refers to the period of a woman's life when the ovaries cease to produce the hormone estradiol, one form of estrogen. It is this form of estrogen that has the most direct effect on the female reproductive organs, secondary sex characteristics, and sexual arousal. Fluctuations of this hormone prior to menopause create the monthly menstrual cycle and periods of fertility.

When the ovaries cease functioning after menopause, the adrenals continue to produce another form of estrogen called estrone. Although not as potent, it has some effect on compensating for the lack of ovarian estrogen. The amount and effect, as well as the symptoms of menopause differs among women. It has been suggested that the alteration in estrogen and other related hormones upon menopause cause women to experience a multitude of symptoms. They vary from growth of some facial hair, hot flashes, night sweats, depression, memory difficulty, and decreased vaginal secretions. Over time hormones level out and most symptoms subside.

Male

The only age-related changes to the male reproductive system is a lower production of sperm and an enlargement of the prostate gland. Current research findings suggest that decreased levels of testosterone hormone in men may be related to disease rather than age. The prostate gland, involved with the production and storage of semen, is normally found wrapped around the urethra. The reason for the enlargement of the prostate as men age has yet to be determined. The enlargement places pressure on the urethra, impairing the flow of urine. When this occurs, men experience discomfort and difficulty with urination. Sexual performance is not altered because of prostate enlargement.

Return to Outline

Return to Syllabus