The Endocrine System
327
Thus far, our discussion of neural control of hormone
release has been limited to the role of the autonomic ner-
vous system (see Figure 11–12). However, one large group
of hormones—those secreted by the hypothalamus and its
extension, the posterior pituitary—are under the direct con-
trol not of autonomic neurons, but of neurons in the brain
itself (see Figure 11–12). This category will be described in
detail in Section B.
Control by Other Hormones
In many cases, the secretion of a particular hormone is directly
controlled by the blood concentration of another hormone.
There are often sequences in which the only function of the
fi rst hormones in the sequence is to stimulate the secretion of
the next. A hormone that stimulates the secretion of another
hormone is often referred to as a
tropic hormone.
The tropic
hormones usually stimulate not only secretion but also the
growth of the stimulated gland. When referring to growth-
promoting actions, the term
trophic
is often used. These types
of hormonal sequences are covered in detail in Section B.
In addition to stimulatory actions, some hormones such
as those in a multihormone sequence inhibit secretion of other
hormones.
Types of Endocrine Disorders
Because there is such a wide variety of hormones and endo-
crine glands, disorders within the endocrine system may vary
considerably in terms of symptoms. For example, endocrine
disease may manifest as an imbalance in metabolism, leading to
weight gain or loss; as a failure to grow or develop normally in
early life; as an abnormally high or low blood pressure; as a loss
of reproductive fertility; or as mental and emotional changes,
to name a few. Despite these varied symptoms, which depend
upon the particular hormone affected, essentially all endocrine
diseases can be categorized in one of four ways. These include
(1) too little hormone (
hyposecretion
); (2) too much hormone
(
hypersecretion
); (3) reduced responsiveness of the target cells
(
hyporesponsiveness
) to hormone; and (4) increased respon-
siveness of the target cells (
hyperresponsiveness
).
Hyposecretion
An endocrine gland may be secreting too little hormone
because the gland cannot function normally, a condition
termed
primary hyposecretion
.
Examples of primary hypo-
secretion include (1) partial destruction of a gland such as
the adrenal cortex, leading to decreased cortisol secretion,
(2) an enzyme defi ciency resulting in decreased synthesis of
the hormone, and (3) dietary defi ciency of iodine, leading
to decreased secretion of thyroid hormones. There are many
other causes such as infections and exposure to toxic chemi-
cals, all having the common denominator of damaging the
endocrine gland.
The other major cause of hyposecretion is
secondary
hyposecretion
.
In this case, the endocrine gland is not dam-
aged initially, but is receiving too little of its tropic hormones.
One way to differentiate the two major causes of hypo-
secretion is to administer the tropic hormone. If the target
Autonomic
ganglion
Endocrine
gland cell
Hormone
Hormone
(epinephrine)
Hormones
Adrenal
medulla
Posterior
pituitary
Anterior
pituitary
Hormones
Hormones
Central nervous system
Hypothalamus
Autonomic nervous system
+
+
or
or
+
Figure 11–12
Pathways by which the nervous system infl uences hormone secretion. The autonomic nervous system controls hormone secretion by the adrenal
medulla and many other endocrine glands. Certain neurons in the hypothalamus, some of which terminate in the posterior pituitary, secrete
hormones. The secretion of hypothalamic hormones from the posterior pituitary and the effects of other hypothalamic hormones on the
anterior pituitary are described later in this chapter. The
B
and
E
symbols indicate stimulatory and inhibitory actions, respectively.
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