Download hypothalamus

The nervous system
Ahmed
L5,
Dr.Abdul-Aziz
Integrative and behavioral functions
Many neuronal circuits in the brain are integrated to produce complex
phenomena such as emotions, motivated behavior, consciousness,
language, memory, and cognition. Three areas of the brain are
particularly important for the implementation of integrative functions:
1. The hypothalamus is the major controller of the endocrine and ANS. It
is a key site for the control of homeostatic functions and motivated
behaviors, including eating, circadian rhythms, and the sex drive.
2. The reticular formation consists of several well-defined nuclei that
give origin to monoaminergic neurons; the widespread connections of
these neurons form the diffuse modulatory systems of the brain. The
functional concept is one of an ascending reticular activating system for
the forebrain, which is essential for determining the level of
consciousness and general arousal.
3. The limbic system is the seat of emotions, and is formed from a series
of cortical and subcortical structures that have reciprocal connections
with the reticular formation and hypothalamus. Dysfunction of the limbic
system and diffuse modulatory systems underlie psychiatric diseases
such as major depression, bipolar disorder, and schizophrenia.
THE HYPOTHALAMUS
The hypothalamus consists of groups of nuclei that orchestrates many
homeostatic functions via the autonomic and endocrine systems. It is
also a key output pathway for the limbic system, playing a role in the
expression of emotions. The hypothalamus occupies the small area
below the thalamus and extends from the optic chiasm anteriorly to the
mamillary bodies posteriorly; the lower part of the third ventricle lies in
the center of the hypothalamus.
Afferent inputs
The hypothalamus receives many afferent inputs:
1.Collaterals from the visceral and somatic sensory pathways (e.g., via
the medial lemniscus and the reticular formation).
2.Afferent fibers from the frontal lobe and parts of the limbic system
link the hypothalamus with the higher centers for mood and emotion.
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The nervous system
Ahmed
L5,
Dr.Abdul-Aziz
The major efferent connections of the hypothalamus are:
1.Descending pathways that control the peripheral ANS. The
hypothalamus sends efferent fiber to the brainstem nuclei for
parasympathetic outflow and to the lateral horn of the spinal cord for
sympathetic outflow.
2.Output to the endocrine system is via connections with the pituitary
gland, via the hypothalamohypophyseal tract, to the posterior pituitary
and hypophyseal portal blood supply to the anterior pituitary.
3.Output is conveyed to the limbic system through several pathways
(e.g., the mammillothalamic tract).
Function of hypothalamus
REGULATION OF FOOD INTAKE
Body weight is determined by the balance between food intake and
energy expenditure. The hypothalamus is the main site where food
intake is regulated. There are two areas of the hypothalamus that are
important in the regulation of eating:
1. The ventromedial nucleus is the satiety center; damage to this area
causes unrelenting hunger. Corticotropin-releasing hormone (CRH) and
the cocaine and amphetamine-regulated transcript (CART), which are
among the neurotransmitters in the ventromedial nucleus that suppress
the desire to eat( anorexigenic factors).
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The nervous system
Ahmed
L5,
Dr.Abdul-Aziz
2. The lateral hypothalamic area is the feeding center; Neuropeptide Y
(NPY) and the orexins, which are neurotransmitters in the lateral
hypothalamic area that stimulate eating(orexigenic factors).damage to
this area causes profound loss of the desire to eat.
long-term factors related to the energy stores in adipose tissue.
Short-term and long-term factors regulation of food intake
The drive to eat is influenced by
1. short-term factors related to the daily pattern of meals, the short
term, hunger is induced by hypoglycemia and by the gastrointestinal
peptide hormone ghrelin. After a meal is consumed, the sensation of
satiety is mediated via the vagus nerve, due to distention of the stomach
and by the release of the gastrointestinal hormone cholecystokinin.
2.the major long-term regulator of eating is leptin, a polypeptide
hormone released by adipocytes, which stimulates the expression of
cocaine and amphetamine-regulated transcript (CART) and inhibits
Neuropeptide Y (NPY) thereby inhibiting eating.
Plasma leptin concentration reflects the size of the total body fat store;
there is a feedback loop in which high levels of body fat should increase
leptin levels and decrease feeding. Patients who are obese are poorly
responsive to leptin, which may contribute to the development and
maintenance of overeating.
figure: Long-term control of eating in relation to body energy stores
occurs by leptin signaling to the hypothalamus.
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The nervous system
Ahmed
L5,
Dr.Abdul-Aziz
REGULATION OF BODY TEMPERATURE
Body temperature is one of the key physiologically controlled variables
and is regulated by negative feedback via the hypothalamus. Body
temperature is stable when the rate of heat generation from cellular
metabolism equals the rate of heat loss to the environment.
Heat exchange occurs via four processes:
1. Radiation transfers heat as electromagnetic energy between objects
that are not in contact.
2. Conduction of heat occurs down a temperature gradient between
objects that are in direct contact.
3. Convection is the transfer of heat in fluids and occurs in the direction
of a temperature gradient. For example, when immersed in cold water,
the body heats the surrounding water, setting up convection currents
that carry warm water away from the body. Another example of
convection is the transfer of heat from the body core to the body
surface via circulating blood.
4. Evaporation is an example of unidirectional heat transfer in which
body heat is used to evaporate water on the skin. The rate of
evaporation depends on the gradient of water vapor pressure (i.e.,
evaporation cannot occur when the humidity of air is 100 %).
Afferent input regarding body temperature is derived from two
sources:
1. Peripheral thermoreceptors in the skin provide information about
body surface temperature that is used to anticipate threats to core body
temperature.
2. Central thermoreceptors, mostly consist of temperature sensitive
neurons in the hypothalamus that monitor the core body temperature.
Warm receptors are most abundant and are localized in the preoptic
area of the hypothalamus. Physiologic responses are dominated by
changes in core temperature rather than by changes in skin temperature.
The normal set-point value for core body temperature of about 37 C
(98.6 F) is determined by the hypothalamus; a negative feedback
response occurs if body temperature deviates from the set point.
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The nervous system
Ahmed
L5,
Dr.Abdul-Aziz
Temperature regulation:
1.Skin circulation is regulated by the adrenergic sympathetic tone;
active vasoconstriction minimizes heat loss in response to cold. A
reduction in sympathetic tone occurs when the body is hot, which
vasodilates blood vessels in the skin and results in increased heat loss.
Further vasodilation occurs as part of the sweating response.
2.Metabolic rate may vary acutely in response to body temperature
changes. A low body temperature increases cellular metabolism via
activation of the sympathetic nervous system and through increased
thyroid hormone secretion.
3.Shivering occurs when the core body temperature falls below
approximately 35 C (95 F); it is coordinated by the hypothalamus and
occurs first in the proximal muscles. Shivering generates a large amount
of heat from muscle but is a short-term response.
4. Sweating is the most important response to increased body
temperature due to the large amount of heat needed to evaporate
water. Sweat is a hypotonic saline solution secreted by the eccrine sweat
glands in the skin in response to cholinergic postganglionic sympathetic
innervation.
(Note: Sweating can be viewed as a symptom of many underlying
pathologic conditions. Because the sympathetic nervous system
innervates sweat glands (albeit through cholinergic receptors), as in
myocardial infarction, amphetamine intoxication, or hypoperfusion
states (e.g., hypovolemic shock).
5.Behavioral changes that minimize both heat and cold stress occur in
response to thermal discomfort; for example, wearing more clothing,
eating more, and increasing physical activity in cold weather, or seeking
shade, eating less, and reducing activity in hot weather.
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The nervous system
Ahmed
L5,
Dr.Abdul-Aziz
Fever is a regulated increase in body temperature in response to
infection or disease. The mechanism of fever generation involves several
phases :
 Secretion of cytokines by immune cells in response to an
infection; cytokines function as circulating pyrogens (e.g.,
interleukin 6).2
 Access of cytokines to the brain across the leaky blood-brain
barrier at the OVLT, one of the circumventricular organs.
 Generation of prostaglandin E2 (PGE2) by the capillary endothelial
cells of the OVLT stimulates the hypothalamus to raise the set
point for body temperature. This accounts for the efficacy of
cyclooxygenase inhibitors (e.g., NSAIDs) at reducing fever because
PGE2 formation is reduced.
Hypothermia is defined as core body temperature below 35 C (95 F), the
point at which compensatory heat conserving mechanisms begin to fail.
To prevent significant morbidity and mortality, patients should be
removed from the cold environment and rewarmed.
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