Human Physiology, 14th edition (2016)

336 Chapter 11

Figure 11.16 The hypothalamus-pituitary-thyroid
axis (control system). The secretion of thyroxine from the
thyroid is stimulated by thyroid-stimulating hormone (TSH)
from the anterior pituitary. The secretion of TSH is stimulated
by thyrotropin-releasing hormone (TRH) secreted from the
hypothalamus. This stimulation is balanced by negative feedback
inhibition (blue arrow) from thyroxine, which decreases the
responsiveness of the anterior pituitary to stimulation by TRH.

Inhibits

secretion

of TRH

Hypothalamus

Thyrotropin-

releasing

hormone

(TRH)

Anterior

pituitary

Thyroid-

stimulating

hormone

(TSH)

Growth Thyroid Thyroxine

of thyroid

Inhibits

responsiveness

to TRH

Sensor

Integrating center

Effector

Figure 11.17 The hypothalamus-pituitary-gonad

axis (control system). The hypothalamus secretes GnRH,

which stimulates the anterior pituitary to secrete

the gonadotropins (FSH and LH). These, in turn, stimulate

the gonads to secrete the sex steroids. The secretions of the

hypothalamus and anterior pituitary are themselves regulated by

negative feedback inhibition (blue arrows) from the sex steroids.

Negative

feedback

Negative

feedback

Gonadotropin-

releasing hormone

(GnRH)

Anterior

pituitary

Gonadotropins

(FSH and LH)

Sex steroid

hormones

(estrogens and

androgens)

Inhibits

responsiveness

to

GnRH

Inhibits

secretion

of GnRH

Sensor

Integrating center

Effector

Gonads

Hypothalamus

hormones, and (2) the target gland hormones can act on the
anterior pituitary to inhibit its response to the releasing hor-
mones. Thyroxine, for example, has long been known to
inhibit the synthesis and secretion of TSH by the anterior
pituitary in response to TRH stimulation ( fig. 11.16 ). Newer
evidence indicates that thyroxine also inhibits the synthesis of
TRH in the paraventricular nucleus of the hypothalamus. Sex
steroids inhibit both the secretion of GnRH from the hypo-
thalamus and the ability of the anterior pituitary to secrete the
gonadotropins (FSH and LH) in response to GnRH stimula-
tion ( fig. 11.17 ).
Evidence suggests that there are short feedback loops in
which a particular trophic hormone inhibits the secretion of its
releasing hormone from the hypothalamus. A high secretion
of TSH, for example, may inhibit further secretion of TRH by
this means.
In addition to negative feedback control of the anterior
pituitary, there is one instance of a hormone from a target
organ that actually stimulates the secretion of an anterior pitu-
itary hormone. Toward the middle of the menstrual cycle, the
rising secretion of estradiol from the ovaries stimulates the
anterior pituitary to secrete a “surge” of LH, which results in
ovulation. This is commonly described as a positive feedback
effect, to distinguish it from the more usual negative feedback

inhibition of target gland hormones on anterior pituitary secre-

tion. Interestingly, higher levels of estradiol at a later stage

of the menstrual cycle exert the opposite effect—negative

feedback inhibition—on LH secretion. The control of gonad-

otropin secretion is discussed in more detail in chapter 20,

section 20.2.

Higher Brain Function and Pituitary Secretion

The relationship between the anterior pituitary and a particular

target gland is described as an axis; the pituitary-gonad axis,

for example, refers to the action of gonadotropic hormones on

the testes and ovaries. This axis is stimulated by GnRH from

the hypothalamus, as previously described. Because the hypo-

thalamus receives neural input from “higher brain centers,” it

is not surprising that the pituitary-gonad axis can be affected

by emotions. Indeed, the ability of intense emotions to alter the

timing of ovulation or menstruation is well known.