PHYSIOLOGY OF THE HYPOTHALAMUS
The Hypothalamus
Hypothalamic Boundaries
• Anteriorly- lamina terminalis, with anterior commissure above and the
optic chiasm below.
• Posteriorly - interpeduncular fossa.
• Dorsally -hypothalamic sulcus, marking the junction with the
thalamus.
• Ventrally -tuber cinereum, which tapers into the infundibulum.
Zones of Hypothalamus
•Hypothalamus is usually divided into 3 zones:
•Lateral Zone
•Medial Zone
•Periventricular Zone
Hypothalamic Nuclei
Hypothalamic Nuclei
• Preoptic nuclei regulate:
– Temperature, heart rate, blood pressure and bladder control
• supraoptic nucleus release antidiuretic hormone (vasopressin)
• paraventricular nucleus - antidiuretic hormone, oxytocin, water
conservation
• ventromedial nucleus controls satiety
• lateral hypothalamic nucleus /area - hunger, thirst, blood
pressure, heart rate
• suprachiasmatic nucleus - circadian rhythms
Functions of Hypothalamus
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Autonomic Nervous System Regulation
Hormone Production
Endocrine Regulation
Control of Circadian Rhythm
Interaction with Limbic System
Temperature Regulation and Feeding
Control of Autonomic Functions
ANS regulation by hypothalamus: the hypothalamus makes up 1% of brain
volume but controls temperature regulation, heart rate, blood pressure, blood
osmolarity, food and water intake, emotion and sex drives.
Hypophyseal Regulation
• Adenohypophysis (anterior pituitary) – hormone releasing factors,
inhibiting factors
• Neurohypophysis (posterior pituitary) – oxytocin, ADH/vasopressin
Hypothalamic Hypophyseal System
Cardiovascular Regulation
• Lateral hypothalamic nucleus – excitatory cardiovascular center–
increases arterial pressure, heart rate
• Posterior hypothalamic nucleus and preoptic nucleus – inhibitory
cardiovascular center – decreases arterial pressure, heart rate
• Both effects mediated through cardiovascular control centers in
reticular regions of medulla and pons
Respiratory Regulation
• Respiration – Medullary centers (CO2, O2, H+ receptors in
hypothalamus)
• Urination and defecation – spinal reflexes modulated through higher
centers, especially the cortex
Appetite
• Hunger
– Ventromedial hypothalamic nucleus – “satiety” center
– Lateral hypothalamic area – hunger center
– Glucose receptors
• Thirst
– Stimuli: cell dehydration, body fluid volume changes
Hypothalamic Control of Food Intake
Hypothalamic Control of Food Intake
• Lesions in ventromedial hypothalamic nuclei produce voracious
appetite (and rage) and obesity.
• The ventromedial nucleus is believed to be the satiety center.
• Lesions to the lateral hypothalamic area abolishes the urge to eat (loss
of appetite) resulting in anorexia and emaciation.
• This area is thought to be the hunger center
Thirst
• Receptors: Osmoreceptors, stretch receptors
– Roles of ADH, renin/angiotensin
Water Balance and Drinking
• Water deprivation
– Cell dehydration (intracellular volume decrease, e.g., fluid
deprivation or by hypertonic saline solutions)
– Increase in hypothalamic cell dehydration receptor activity
(osmoreceptors)
– Increase in supraoptic nucleus activity
– Increase in ADH release from posterior pituitary
– Water reabsorption by kidney
– Or increase in thirst center activity
– Increase in water intake
• Body fluids (extracellular volume) decrease, e.g., decrease in
volume
– Cardiopulmonary stretch receptor activity (low pressure side,
e.g., atria, great veins, pulmonary vessels)
– Increase in sympathetic activity (vasoconstriction)
– Inc in renin- angiotensin II
– Vasoconstriction and decreased excretion of salt and water by
kidney
– Angiotensin II also acts on supraoptic nucleus of hypothalamus
(ADH) and thirst center
– Result – increased arterial pressure and fluid volume
Water Balance and Drinking
Temperature
• Anterior hypothalamus – prevents temperature rise (panting, sweating,
vasodilation) – site of vascular temperature receptors
• Posterior hypothalamus prevents temperature loss - glucose oxidation,
vasoconstriction, piloerection, shivering
• Hypothalamic thermostat – affected by aspirin, alcohol, interleukins
Temperature Regulation
Sleep, Waking, Circadian Rhythm
• The suprachiasmatic nucleus is thought to be the primary “biological
clock” in mammals.
• It works in conjunction with the reticular activating system, and other
brain stem nuclei
Emotional Behavior
• Lesions in the ventromedial hypothalamic nuclei produce savage and
vicious behavior, indicative of extreme rage.
• Stimulation of the dorsomedial nuclei also produces this reaction
Nervous System Effects on Immune Function
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Immune system suppression by stress
Via hypothalamo-pituitary-adrenal axis – CRF release
Rats can be conditioned to suppress immune responses
ANS/SNS innervates immune tissues: spleen, lymph nodes, intestinal
Peyer’s patches, bone marrow
• Immune cells have receptors for neurotransmitters
• Different CNS lesions can decrease or increase immune functions.