OpenStax-CNX module: m57807
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The Pineal Gland
∗
Steven Telleen
Based on The Pineal Gland† by
OpenStax College
This work is produced by OpenStax-CNX and licensed under the
Creative Commons Attribution License 4.0
‡
Abstract
By the end of this section, you will be able to:
•
•
Describe the location and structure of the pineal gland
Discuss the function of melatonin
Recall that the hypothalamus, part of the diencephalon of the brain, sits inferior and somewhat anterior
to the thalamus. Inferior but somewhat posterior to the thalamus is the pineal gland, a tiny endocrine
gland whose functions are not completely understood. The pinealocyte cells that make up the pineal gland
are known to produce and secrete the amine hormone melatonin, which is derived from serotonin.
The secretion of melatonin varies according to the level of light received from the environment. When
photons of light stimulate cells containing the photosensitive pigment melanopsin in the retina of the eyes,
a nerve impulse is sent to the suprachiasmatic nucleus (SCN) in the hypothalamus, which is important in
regulating both diurnal (daily) and seasonal biological rhythms. From the SCN, the nerve signal is carried
to the spinal cord and via sympathetic nerve bers to the pineal gland, where the production of melatonin
is inhibited. As a result, blood levels of melatonin fall, promoting wakefulness. In contrast, as light levels
declinesuch as during the evening less melanopsin is activated reducing SCN stimulation, in turn reducing
the inhibitory sympathetic stimulus of the pineal gland, resulting in increased melatonin production. This
boosts melatonin levels in the blood and causes drowsiness.
Jet lag occurs when a person travels across several time zones and feels sleepy during the day or wakeful at
night. Traveling across multiple time zones signicantly disturbs the light-dark cycle regulated by melatonin.
It can take up to several days for melatonin synthesis to adjust to the light-dark patterns in the new
environment, resulting in jet lag. Some air travelers take melatonin supplements to induce sleep.
∗ Version
1.2: Sep 11, 2015 3:48 pm -0500
† http://cnx.org/content/m46693/1.3/
‡ http://creativecommons.org/licenses/by/4.0/
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:
Visit this link1 to view an animation describing the function of the hormone melatonin. What should
you avoid doing in the middle of your sleep cycle that would lower melatonin?
The secretion of melatonin may inuence not only sleepiness and wakefulness, but also appetite and body
temperature. Interestingly, children have higher melatonin levels than adults, which may prevent the release
of gonadotropins from the anterior pituitary, thereby inhibiting the onset of puberty. However, while experiments removing the pineal gland in rodents demonstrated increased pituitary production of FSH and LH,
adding melatonin to pinealectomized individuals did not return their gonadotropin levels to normal. So, a
pineal/pituitary communication mechanism other than melatonin is likely involved.
There also is evidence that melatonin inuences the immune system. The potential connection was
rst hypothesized from epidemiological data correlating the number of multiple sclerosis are ups with the
seasons. Statistically fewer are ups occurred in fall and winter when natural melatonin levels are on the
rise than in spring and summer when levels reach their lowest. Multiple sclerosis is an autoimmune disease
where cytotoxic T-cells attack the myelin sheath of neurons, disrupting neural communication. Subsequent
1 http://openstaxcollege.org/l/melatonin
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experiments with mice (and human cells in culture) have demonstrated that increased melatonin levels
increased the number of regulatory T-cells and reduced the production of cytotoxic T-cells in the immune
system, adding support to this hypothesized connection.
1 Section Review
The pineal gland is an endocrine structure of the diencephalon of the brain, and is located inferior and
posterior to the thalamus. It is made up of pinealocytes. These cells produce and secrete the hormone
melatonin in response to low light levels. High blood levels of melatonin induce drowsiness. Jet lag, caused
by traveling across several time zones, occurs because melatonin synthesis takes several days to readjust
to the light-dark patterns in the new environment. The pineal gland also appears to have a connection to
pituitary gonadotropin production; however, some studies show the connection may not be via a pathway
other than melatonin. There is both epidemiological and experimental evidence that melatonin has an eect
on regulation of T-cell production in the immune system.
2 Interactive Link Questions
Exercise 1
(Solution on p. 5.)
Visit this link2 to view an animation describing the function of the hormone melatonin. What
should you avoid doing in the middle of your sleep cycle that would lower melatonin?
3 Review Questions
Exercise 2
What cells secrete melatonin?
a.
b.
c.
d.
melanocytes
pinealocytes
suprachiasmatic nucleus cells
retinal cells
Exercise 3
The production of melatonin is inhibited by ________.
a.
b.
c.
d.
(Solution on p. 5.)
(Solution on p. 5.)
declining levels of light
exposure to bright light
the secretion of serotonin
the activity of pinealocytes
4 Critical Thinking Questions
Exercise 4
(Solution on p. 5.)
Seasonal aective disorder (SAD) is a mood disorder characterized by, among other symptoms,
increased appetite, sluggishness, and increased sleepiness. It occurs most commonly during the
winter months, especially in regions with long winter nights. Propose a role for melatonin in SAD
and a possible non-drug therapy.
2 http://openstaxcollege.org/l/melatonin
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OpenStax-CNX module: m57807
Exercise 5
4
(Solution on p. 5.)
Retinitis pigmentosa (RP) is a disease that causes deterioration of the retinas of the eyes. Describe
the impact RP would have on melatonin levels.
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Solutions to Exercises in this Module
to Exercise (p. 3)
Turning on the lights.
to Exercise (p. 3)
B
to Exercise (p. 3)
B
to Exercise (p. 3)
SAD is thought to occur in part because low levels and duration of sunlight allow excessive and prolonged
secretion of melatonin. Light therapydaytime exposure to very bright lightingis one common therapy.
to Exercise (p. 4)
The retina is important for melatonin production because it senses light. Bright light inhibits the production
of melatonin, whereas low light levels promote the production of melatonin. Therefore, deterioration of the
retinas would most likely disturb the sleep-wake pattern because melatonin production would be elevated.
Glossary
Denition 1: melatonin
amino acidderived hormone that is secreted in response to low light and causes drowsiness
Denition 2: pineal gland
endocrine gland that secretes melatonin, which is important in regulating the sleep-wake cycle
Denition 3: pinealocyte
cell of the pineal gland that produces and secretes the hormone melatonin
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