Human Physiology, 14th edition (2016)

The Central Nervous System 227

and ADH stimulates the kidneys to reabsorb water and thus to

excrete a smaller volume of urine. Neurons in the hypothala-

mus also produce hormones known as releasing hormones

and inhibiting hormones that are transported by the blood to

the adenohypophysis (anterior pituitary). These hypothalamic

releasing and inhibiting hormones regulate the secretions of

the anterior pituitary and, by this means, regulate the secre-

tions of other endocrine glands (chapter 11, section 11.3).

Regulation of Circadian Rhythms

Within the anterior hypothalamus ( fig. 8.20 ) are bilaterally located

suprachiasmatic nuclei (SCN). These nuclei contain about 20,000

neurons that function as “clock cells,” with electrical activity that

oscillates automatically in a pattern that repeats about every twenty-

four hours. The SCN functions as the master regulator of the body’s

circadian rhythms (from the Latin circa 5  about;  diem 5  day).

These are the physiological processes—including metabolism,

sleep, body temperature, blood pressure, hormone secretion, and

many others—that repeat at approximately 24-hour intervals. For

these to function properly, the neuron clocks of the SCN must be

entrained (synchronized) to the day/night cycles.

Nonmammalian vertebrates—fish, amphibians, reptiles,

and birds—have photosensitive cells in their brains that can

Experimental heating of this hypothalamic area results in
hyperventilation (stimulated by somatic motor nerves), vaso-
dilation, salivation, and sweat-gland secretion (regulated by
sympathetic nerves). These responses serve to correct the tem-
perature deviations in a negative feedback fashion.
The coordination of sympathetic and parasympathetic
reflexes is thus integrated with the control of somatic and
endocrine responses by the hypothalamus. The activities of the
hypothalamus are in turn influenced by higher brain centers.

Regulation of the Pituitary Gland

The pituitary gland is located immediately inferior to the
hypothalamus. Indeed, the posterior pituitary derives embryon-
ically from a downgrowth of the diencephalon, and the entire
pituitary remains connected to the diencephalon by means of a
stalk (chapter 11, section 11.3). Neurons within the supraoptic
and paraventricular nuclei of the hypothalamus ( fig. 8.20 ) pro-
duce two hormones— antidiuretic hormone (ADH), which is
also known as vasopressin, and oxytocin. These two hormones
are transported in axons of the hypothalamo-hypophyseal tract
to the neurohypophysis (posterior pituitary), where they are
stored and released in response to hypothalamic stimulation.
Oxytocin stimulates contractions of the uterus during labor,

Anterior pituitary

(adenohypophysis)

Posterior pituitary

(neurohypophysis)

Pituitary gland

Optic chiasma

Suprachiasmatic nucleus

Preoptic area

Anterior nucleus

Paraventricular nucleus

Median eminence

Mammillary body

Ventromedial nucleus

Posterior nucleus

Dorsomedial nucleus

Supraoptic nucleus

Figure 8.20 A diagram of some of the nuclei within the hypothalamus. The hypothalamic nuclei, composed of neuron
cell bodies, have different functions.