Lesson 4: Metabolic Rate, Appetite, and Jet Lag Flashcards
Thyroxin and Metabolic Rate
Thyroxin is the hormone that controls the body’s basal metabolic rate. (the
metabolism at rest)
Thyroxin is secreted by the thyroid found in the neck.
Thyroxin also controls growth.
If insufficient thyroxin is present during childhood, physical and mental
development is affected. A condition known as cretinism may result.
A person with this condition will be short and have severe mental delays.
In adults, a lack of thyroxin results in a condition known as hypothyroidism.
Hypothyroidism results in a lowering of metabolic rate which leads to an
increase in fat accumulation and a slowing of physical and mental activity.
In adults, too much thyroxin results in a condition known as hyperthyroidism.
Hyperthyroidism results in an increase in metabolic rate which causes a
loss of body mass, increased heart rate, and an increase in physical and
mental activity.
Goiter
Both conditions may lead to goiter
A lack of iodine in the diet used to be main cause of thyroxin deficiency leading to goiter.
There has been a drop in the incidence of goitre worldwide due to a UNICEF campaign to use iodized salt.
Thyroxin and Negative Feedback
Normal levels of thyroxin are maintained through a negative feedback loop.
Thyroxin secretion is triggered by the release of a stimulating hormone from the pituitary.
When too much thyroxin is present, it inhibits the pituitary gland and production is decreased.
The pituitary is influenced by the hypothalamus
The hypothalamus responds to environmental influences such as temperature which allows metabolism to be aligned with the seasons in some animals
Thyroxin produced as a result of stimulation by the hypothalamus can speed up metabolism and increase body temperature by generating metabolic heat.
Leptin and Appetite
Leptin is a protein hormone produced
mainly by white adipose tissue.
The level of circulating leptin is a
directly proportional to the total
amount of fat in the body.
Leptin can also be produced by brown
adipose tissue, cells in the placenta,
ovaries, skeletal muscles and gastric
cells.
Leptin was discovered in 1994.
Jeffery M. Friedman studied
mutant obese mice
Leptin acts on receptor cells in the hypothalamus and inhibits appetite.
Counteracting the effects of ghrelin (a hormone from the gut) and other
hormones from the hypothalamus that stimulate hunger.
Stimulates the synthesis of α – MSH (α-Melanocyte Stimulating Hormone),
a long term appetite suppressant.
The absence of leptin or receptors for it, leads to uncontrolled food intake.
Clinical Use of Leptin
A small group humans have a homozygous mutation for the leptin gene which leads to a constant desire for food, resulting in severe obesity.
Efforts have been made to treat these patient with injections of human leptin
which led to inconclusive results.
It was found that large and frequent doses were required for small
improvements due to leptin being quickly broken down in the bloodstream
and has low potency and poor solubility
Melatonin and Circadian Rhythms
Melatonin is a hormone secreted
by the pineal gland.
The main role of melatonin is to
control the body’s circadian
rhythms (a cycle of activity that
is repeated every 24 hours)
The sleep – wake cycle is the
primary circadian rhythm that
melatonin acts on.
Control of Human Sleep
Human sleep regulation is controlled by light exposure.
Exposure to light stimulates a nerve pathway from the retina to the
hypothalamus
Cells in the supra – chiasmatic nucleus (SCN) send signals to other parts
of the brain that have a role in our feelings of sleep or wakefulness.
This signal can keep the body on an approximately 24 hour cycle of
activity.
Since this internal clock is not set to exactly 24 hours, external cues such
as light are needed to reset the clock.
During the day the pineal gland is inactive.
During the hours of darkness it is turned on by the SCN and begins to produce
melatonin, which is released into the blood.
Rising levels of melatonin cause feelings of sleepiness.
The levels of melatonin are high for about 12 hours until the following morning when light causes it to fall.
Even if the pineal gland is stimulated it will not produce melatonin unless the person is in a dimly lit environment.
Artificial indoor light can be bright enough to prevent the release of melatonin.
Melatonin and Jet Lag
Jet lag is caused by the disruption of the body’s day-night cycle caused by long
distance travel and arrival in a new time zone.
The day-night patterns no longer correspond to the new environment
Jet lag upsets the body clock because the expected patterns of light and
darkness are out of alignment.
Jet lag can be controlled by avoiding bright light so that the body clock resets.
Melatonin tablets are sometimes used to adjust the body clock but the
proper dose and time to take it are difficult to determine.
