Chapter 16
The Endocrine System
597
16
mutually exclusive—some endocrine organs respond to multi-
ple stimuli.
Nervous System Modulation
Te nervous system can modify both “turn-on” factors (hormo-
nal, humoral, and neural stimuli) and “turn-off” factors (feed-
back inhibition and others) that affect the endocrine system.
Without this added safeguard, endocrine system activity would
be strictly mechanical, much like a household thermostat. A
thermostat can maintain the temperature at or around its set
value, but it cannot sense that your grandmother visiting from
Florida feels cold at that temperature and reset itself accordingly.
You must make that adjustment. In your body, it is the nervous
system that makes certain adjustments to maintain homeostasis
by overriding normal endocrine controls.
For example, the action of insulin and several other hor-
mones normally keeps blood glucose levels in the range of 90–
110 mg/100 ml of blood. However, when your body is under
severe stress, blood glucose levels rise because the hypothala-
mus and sympathetic nervous system centers are strongly ac-
tivated. In this way, the nervous system ensures that body cells
have sufficient fuel in case vigorous activity is required.
Neural Stimuli
In a few cases, nerve fibers stimulate hormone
release. Te classic example of neural stimuli is the response
to stress, in which the sympathetic nervous system stimulates
the adrenal medulla to release norepinephrine and epinephrine
(Figure 16.4b).
Hormonal Stimuli
Finally, many endocrine glands release
their hormones in response to hormones produced by other
endocrine organs. For example, releasing and inhibiting hor-
mones produced by the hypothalamus regulate the secretion
of most anterior pituitary hormones, and many anterior pi-
tuitary hormones in turn stimulate other endocrine organs to
release their hormones (Figure 16.4c). As blood levels of the
hormones produced by the final target glands increase, they
inhibit the release of anterior pituitary hormones and thus
their own release.
Tis hypothalamic–pituitary–target endocrine organ feed-
back loop lies at the very core of endocrinology, and it will come
up many times in this chapter. Hormonal stimuli promote
rhythmic hormone release, with hormone blood levels rising
and falling in a specific pattern.
Although these three types of stimuli are typical of most
systems that control hormone release, they are by no means
(a)
Humoral Stimulus
(b)
Neural Stimulus
(c)
Hormonal Stimulus
Capillary (low Ca
2+
in blood)
Parathyroid
glands
Thyroid gland
(posterior view)
PTH
Parathyroid
glands
CNS (spinal cord)
Medulla of
adrenal gland
Preganglionic
sympathetic
fibers
Capillary
Hypothalamus
Thyroid
gland
Adrenal
cortex
Gonad
(Testis)
Hormone release caused by altered levels
of certain critical ions or nutrients.
Stimulus:
Low concentration of Ca
2+
in
capillary blood.
Response:
Parathyroid glands secrete
parathyroid hormone (PTH), which
increases blood Ca
2+
.
Stimulus:
Action potentials in preganglionic
sympathetic fibers to adrenal medulla.
Response:
Adrenal medulla cells secrete
epinephrine and norepinephrine.
Stimulus:
Hormones from hypothalamus.
Response:
Anterior pituitary gland secretes
hormones that stimulate other endocrine glands
to secrete hormones.
Hormone release caused by neural input.
Hormone release caused by another hormone
(a tropic hormone).
Anterior
pituitary
gland
Figure 16.4
Three types of endocrine gland stimuli.
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