Human Physiology, 14th edition (2016)

348 Chapter 11

The pineal gland has been implicated in a variety of physi-

ological processes. One of the most widely studied is the role of

melatonin in helping to time the births of seasonally breeding ani-

mals. To do this, melatonin influences the pituitary-gonad axis; it

stimulates this axis in short-day breeders such as sheep, but inhib-

its the axis of long-day breeders such as voles. Although there is

evidence to support an antigonadotropic effect in humans, this

possibility has not yet been proven. For example, excessive mela-

tonin secretion in humans is associated with a delay in the onset

of puberty. Melatonin secretion is highest in children between the

ages of one and five and decreases thereafter, reaching its lowest

levels at the end of puberty when concentrations are 75% lower

than during early childhood. This suggests a role for melatonin in

the onset of human puberty. However, because of much conflict-

ing data, the importance of melatonin in human reproduction is

still highly controversial.

The pattern of melatonin secretion is altered when a person

works night shifts or flies across different time zones. There

is evidence that exogenous melatonin (taken as a pill) may be

beneficial in the treatment of jet lag, but the optimum dosage

is not currently known. Phototherapy using bright fluorescent

lamps has been used effectively in the treatment of seasonal

affective disorder (SAD), or “winter depression.” Experiments

suggest that the depressed mood of SAD involves the retinal

ganglion cells that produce the light-sensitive pigment mela-

nopsin (chapter 10, section 10.7), the same cells required for

the light-induced suppression of melatonin secretion by the

pineal gland.

Figure 11.32 A simplified biosynthetic pathway for
melatonin. Secretion of melatonin by the pineal gland follows a
circadian (daily) rhythm tied to daily and seasonal changes in light.

Tryptophan

(an amino acid)

Serotonin

(a biogenic amine)

Melatonin

(a pineal gland hormone)

H 3 CO

CH 3

N

H

N

H

N

H

C

H

CC

O

H

H

N

H H

C

H

OH

C

H

NH 2

HH

C

H

C

CO

OH

NH 2

HH

Figure 11.33 The secretion of
melatonin. The secretion of melatonin by the
pineal gland is stimulated by sympathetic axons
originating in the superior cervical ganglion.
Activity of these neurons is regulated by the cyclic
activity of the suprachiasmatic nucleus of the
hypothalamus, which sets a circadian rhythm. This
rhythm is entrained to light/dark cycles by neurons
in the retina.

Day

Night

Inhibition

Retinohypothalamic

tract

Suprachiasmatic

nucleus (the

“biological clock”)

Superior cervical

ganglion

Sympathetic

neurons

Pineal

gland

Stimulation

CH 3 O

H

C

H

H

C

H

H

N

O

C

N

H

CH 3

Melatonin

influence the ability of the retina to regulate circadian rhythms.
The role of the SCN and the endocrine system (including the
secretion of melatonin) in circadian rhythms is discussed more
fully in chapter 8, section 8.3.