Physiology of the Hypothalamus

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

•  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

•  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.