Fundamentals of Anatomy and Physiology

The Endocrine System

from the anterior pituitary gland, thus inhibiting the
functions of the reproductive system. Bright light inhibits
the secretion of melatonin. Studies have indicated that
melatonin regulates circadian rhythms. In bright light, with
little melatonin, people “feel good” and their fertility
increases. High levels of melatonin produced in the dark
cause individuals to feel depressed and tired, bringing on
sleep. Melatonin affects our sleep–wake patterns and
maintains our biological cycles. Nerve

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impulses originating in the retina of the eyes send light

information to the pineal gland. In dark or dim light, nerve

impulses from the eyes decrease and melatonin secretion

increases. Melatonin also plays a role in the onset of

puberty and in the female reprodu ctive cycle.

Serotonin (sayr-oh-TOH-nin) is also secreted by the

pineal gland and acts as a neurotransmitter and

-vasoconstrictor. It stimulates smooth muscle contraction

and inhibits gastric secretions.

Hypothalamus

Optic chiasm

Pituitary gland

Figure 12- 8 The location of the pineal gland.

Corpus callosum

Thalamus

Pineal

gland

Cerebellum

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As the Body Ages
As individuals age, growth hormone decreases. This causes a decrease in bone
mass, which may lead to osteoporosis. There is also a decrease in lean muscle
mass with an accompanying increase in the deposition of adipose tissue. Regular
exercising helps limit the decrease in growth hormone.^
The production of sex hormones also declines in both men and women
during the later middle years. This results in menopause in women, which is
occasionally treated with sex hormone replacement therapy.^
Secretion of thymosin from the thymus gland decreases with age, affecting
the number of lymphocytes that can mature and provide functional immunity. This can result in
susceptibility to cancers and more frequent bacterial and viral infections. Melatonin secretion from
the pineal gland also decreases in older adults, resulting in changes in sleep patterns that cause
tiredness during the daylight hours and require short naps during the day in addition to regular
nighttime sleeping.