Human Physiology, 14th edition (2016)

(Tina Sui) #1
Endocrine Glands 347

encapsulated by the meninges covering the brain. The pineal
gland of a child weighs about 0.2 g and is 5 to 8 mm (0.2 to 0.3 in.)
long and 9 mm wide. The gland begins to regress in size at
about age seven and in the adult appears as a thickened strand
of fibrous tissue. Although the pineal gland lacks direct nervous
connections to the rest of the brain, it is highly innervated by the
sympathetic nervous system from the superior cervical ganglion.
The pineal gland secretes the hormone melatonin ( fig. 11.32 ).
The suprachiasmatic nucleus (SCN) of the hypothalamus
(chapter 8; see fig. 8.20) regulates pineal secretion of melatonin
through hypothalamic control of the sympathetic neurons that
innervate the pineal gland ( fig. 11.33 ). The SCN is also the pri-
mary center for the regulation of the body’s circadian rhythms:
rhythms of physiological activity that follow a 24-hour pattern
(chapter 8, section 8.3).
Most of the neurons of the SCN produce action potentials
starting at dawn. Action potential frequency increases toward
the middle of the day and then decreases to become mostly
silent at night. Light acts through the retinohypothalamic tract
( fig. 11.33 ) to better entrain (synchronize) this spontaneous cir-
cadian rhythm to the light/dark cycle. Circadian neural activity
of the SCN regulates the pineal gland via sympathetic nerves
( fig.  11.33 ), inhibiting the pineal gland secretion of melato-
nin during the day. As a result, melatonin secretion begins to
increase with darkness and peaks by the middle of the night.
The regulatory effect of light on the SCN, and thus the abil-
ity of light to inhibit melatonin secretion, appear to require a
recently discovered retinal pigment (chapter 10, section 10.7).
This pigment has been named melanopsin, and is found in a
population of ganglion cells; thus, it is distinct from the visual
pigments found in rods and cones. However, activation of rho-
dopsin and photopsins (in rods and cones, respectively) may also

Figure 11.30 Insulin stimulates uptake of blood
glucose. (1) Binding of insulin to its plasma membrane
receptors causes the activation of cytoplasmic signaling
molecules, which (2) act on intracellular vesicles that contain
GLUT4 carrier proteins in the vesicle membrane. (3) This causes
the intracellular vesicles to translocate and fuse with the plasma
membrane, so that the vesicle membrane becomes part of the
plasma membrane. (4) The GLUT4 proteins permit the facilitated
diffusion of glucose from the extracellular fluid into the cell.

P

Signaling
molecules

Translocation
Vesicles

Glucose

GLUT4

Insulin
Insulin
receptor

Translocation

1

3

4

2
4

3

Figure 11.31 Glucose homeostasis is maintained by insulin and glucagon. ( a ) When the plasma glucose concentration
rises after a meal, the beta cells secrete increased amounts of insulin (and alpha cells are inhibited from secreting glucagon). Insulin
then promotes the cellular uptake of blood glucose, reducing the plasma glucose concentration so that homeostasis of blood glucose
is maintained. ( b ) When the plasma glucose concentration falls, the secretion of insulin is inhibited and the secretion of glucagon is
stimulated. Glucagon promotes glycogenolysis and gluconeogenesis, so that the liver can secrete glucose into the blood and maintain
homeostasis of the blood glucose concentration.

Glucose
in plasma

Glucose
in plasma

(a) (b)


Blood Pancreatic islets

Glucagon

Cellular uptake
and utilization
of glucose

Insulin

α cells

β cells





Glucose
in plasma

Glucose
in plasma

Blood Pancreatic islets

Glucagon

Cellular uptake
of glucose
Glycogenolysis
Gluconeogenesis

Insulin

α cells

β cells





Sensor Integrating center Effector
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