442
UNIT 3
Regulation and Integration of the Body
12
body homeostasis. Few tissues in the body escape its influence.
Its chief homeostatic roles are to:
Control the autonomic nervous system.
Recall that the au-
tonomic nervous system (ANS) is a system of peripheral
nerves that regulates cardiac and smooth muscle and secre-
tion by the glands. Te hypothalamus regulates ANS activity
by controlling the activity of centers in the brain stem and
spinal cord. In this role, the hypothalamus influences blood
pressure, rate and force of heartbeat, digestive tract motility,
eye pupil size, and many other visceral activities.
Initiate physical responses to emotions.
Te hypothalamus
lies at the “heart” of the limbic system (the emotional part of
the brain). It contains nuclei involved in perceiving pleasure,
fear, and rage, as well as those involved in biological rhythms
and drives (such as the sex drive).
Te hypothalamus acts through ANS pathways to initiate
most physical expressions of emotion. For example, a fear-
ful person has a pounding heart, high blood pressure, pallor,
sweating, and a dry mouth.
Regulate body temperature.
Te body’s thermostat is in the
hypothalamus. Hypothalamic neurons monitor blood tem-
perature and receive input from other thermoreceptors in
the brain and body periphery. Accordingly, the hypothala-
mus initiates cooling (sweating) or heat-generating actions
(shivering) as needed to maintain a relatively constant body
temperature (see Figure 24.26, p. 946).
Ventral nuclei
Medial
dorsal
nucleus
Anterior
nuclei
Reticular
nucleus
Ventral
anterior
Ventral
lateral
Ventral
postero-
lateral
Lateral
geniculate
body
Medial
geniculate
body
Pulvinar
Lateral
dorsal
nucleus
Lateral
posterior
nucleus
Preoptic
nucleus
Supraoptic
nucleus
Suprachiasmatic
nucleus
Anterior
hypothalamic
nucleus
Dorsomedial
nucleus
Paraventricular
nucleus
Fornix
Anterior
commissure
Posterior
hypothalamic
nucleus
Lateral
hypothalamic
area
Ventromedial
nucleus
Optic
chiasma
Infundibulum
(stalk of the
pituitary gland)
Pituitary
gland
Arcuate
nucleus
Mammillary
body
(a)
The main thalamic nuclei.
(The reticular nuclei that “cap” the
thalamus laterally are depicted as curving translucent structures.)
(b) The main hypothalamic nuclei.
Figure 12.11
Selected structures of the diencephalon.
Regulate food intake.
In response to changing blood levels
of certain nutrients (glucose and amino acids) or hormones
(cholecystokinin, ghrelin, and others), the hypothalamus
regulates feelings of hunger and satiety.
Regulate water balance and thirst.
When body fluids become
too concentrated, hypothalamic neurons called
osmoreceptors
are activated. Osmoreceptors excite hypothalamic nuclei that
trigger the release of antidiuretic hormone (ADH) from the
posterior pituitary. ADH causes the kidneys to retain water.
Te same conditions also stimulate hypothalamic neurons in
the
thirst center
, causing us to feel thirsty and drink more fluids.
Regulate sleep-wake cycles.
Acting with other brain regions, the
hypothalamus helps regulate sleep. Its
suprachiasmatic nucleus
(our biological clock) sets the timing of the sleep cycle in response
to daylight-darkness cues received from the visual pathways.
Control endocrine system function.
Te hypothalamus acts
as the helmsman of the endocrine system in two important
ways. First, its
releasing
and
inhibiting hormones
control the
secretion of hormones by the anterior pituitary gland. Sec-
ond, its
supraoptic
and
paraventricular nuclei
produce the
hormones ADH and oxytocin.
Homeostatic Imbalance
12.4
Hypothalamic disturbances cause a number of disorders includ-
ing severe body wasting, obesity, sleep disturbances, dehydra-
tion, and emotional imbalances. For example, the hypothalamus
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