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.