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The hypothalamus is the part of the brain where activity of the autonomic
nervous system and endocrine glands, which directly control various
systems of the body, is integrated with input from other centers
that give rise to emotions and behavior. The hypothalamus thus serves
to ensure that (1) the organism responds appropriately to deviations
from various internal set points (including those for temperature,
volume, osmolality, satiety, and body fat content), (2) the responses
to such deviations from a set point include coordinated activity of
the nervous and endocrine systems, and (3) the emotions and behavior
being manifested are appropriate for reflex responses being triggered
to correct the deviations from internal set points. The following
description outlines the integrative function of the hypothalamus
in regard to the coordination of endocrine and CNS responses.
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Intravascular volume loss from any cause activates autonomic
neural responses, mainly via the sympathetic nervous system to retain
fluid and electrolytes, maintain blood pressure through vascular
smooth muscle contraction, and maintain cardiac output by increasing
heart rate. The effect of these immediate neural responses is reinforced
by activation of several hormonal systems. In response to a decrease
in intravascular volume, the renin-angiotensin-aldosteronesystem
(RAAS) is activated and sodium is retained. Additionally increasing
osmolarity triggers thirst and leads to release of vasopressin (antidiuretic
hormone [ADH]) from hypothalamic neurons that
end in the posterior pituitary, resulting in free water absorption
in the kidney. In short, the body maintains intravascular volume
by regulating sodium reabsorption through aldosterone, while it
regulates osmolarity by increasing fluid intake (thirst) and free
water retention by vasopressin.
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Emotions interplay with these systems to coordinate appropriate
behavioral and hormonal responses. Fear and pain activate limbic,
hypothalamic and other centers to coordinate respective defensive
(fight or flight) and recuperative stereotypic behaviors. These
emotional responses to various stressors (eg, perceived threat to
body; fear) also activate the sympathetic nervous system and the
hypothalamic-pituitary-adrenal (HPA) axis, which coordinate the
mammalian stress response through preparing the body for fight and
flight and through mobilization of energy stores. Any kind of stress
(eg, physical, mental, metabolic stress) leads to the release of
corticotropin-releasing hormone (CRH) from the hypothalamus and
consequent adrenocorticotropin (ACTH; pituitary) and cortisol (adrenal
cortex) secretion. For example, starvation leads to the activation
of the HPA axis and ultimately cortisol-mediated increased gluconeogenesis
to maintain basic physiologic functions.
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The pituitary gland is the partner of the hypothalamus on the
body side of the mind-body interface. Once viewed as the “master
gland” in regulation of neuroendocrine systems, the pituitary
is now known to be a “middle manager” responding
to input from both the brain (via the hypothalamus) and the body
(via the various peripheral endocrine glands).
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The basic framework for hypothalamic-pituitary function is the neuroendocrine
axis, a cascade of interacting hormonal products from various
regions of the CNS to the hypothalamus, anterior pituitary gland,
peripheral endocrine end organs, and peripheral target tissues.
Some neuroendocrine axes involve ...