VPB 121: Veterinary Physiology - II (2+1)

VEGETATIVE FUNCTIONS OF HYPOTHALAMUS

• Cardio vascular regulation
  • Stimulation of the posterior and lateral hypothalamus increases the arterial pressure and the heart rate, but the stimulation of the preoptic‚ area depresses the blood pressure and heart rate.
• Regulation of body temperature
  • The preoptic and anterior hypo-thalamus is concerned with regulation of body temperature, panting and sweating. Temperature is monitored peripherally and centrally by thermosensitive neurons . Hypothalamic receptors sense both cold and warm and monitor core body temperature. These receptors are present in the preoptic area of the anterior hypothalamus . Posterior hypothalamus integrates peripheral and central thermal sense and modify the heat loss or  heat generating mechanisms. Hypothalamus is more sensitive to central thermoceptors.

• Regulation of body water
  • Hyperosmolality of the body fluid stimulates the thirst centre in the lateral hypothalamus. The hyperosmolality (hemoconcentration) in turn stimulates the ADH secretion from the supraoptic nuclei‚ that aids in water reabsorption from the collecting tubules and ducts of the kidney.
• Regulation of thirst and water balance
  • Hypothalamus senses the external condition and control thirst. This region of the brain detects circulating hormones due to hyperosmolarity posed by Na+ as they lie in the zone of incomplete blood brain barrier system. Subfornical organ (SFO) , Organum vasculosum of lamina terminalis (OVLT) in the median preoptic nucleus are collectively known as circumventricular organs that detect osmotic conditions. Detection is mediated by both osmotic and hormone receptors. Osmotic receptors monitor the osmolarity of CSF that cushions the hypothalamus. Hormone receptor to bind Angiotensin II in this area exerts renal water sparing action. Perception of osmolarity is mediated by above said mechanism. They send signals to the thirst center of dorsomedial hypothalamic nuclei and drives thirst behaviour.
• Regulation of uterine contraction and milk ejection
  • Neuroendocrine reflex stimulation of paraventricular nuclei‚ (entry of fetus into the birth canal stimulates sensory nerves, which passes via the spinal cord to hypothalamus and releases oxytocin) causes increased uterine contraction. Tactile stimulation of udder sends sensory impulses to hypothalamus causing release of oxytocin, which aids milk ejection by stimulating the contraction of the myoepithelial cells surrounding the alveoli of the mammary gland.
• Regulation of feed intake
  • The ventromedial region of the hypothalamus‚ is referred as feeding centre. Stimulation of this centre causes hyperphagia, whereas the lateral region of hypothalamus known as satiety centre, on stimulation reduces feeding behaviour. Stimulation of optic chiasma area of hypothalamus causes increased gastric secretions.
• Sleep and wakefulness
  • Stimulation of the anterior hypothalamus induces sleep, while the stimulation of mammillary body (caudal hypothalamus) activates the ascending reticular activating system (RAS) to produce wakefulness.
• Reproductive functions
  • Median eminence of the hypothalamus releases GnRH in a pulsatile manner, which regulates the FSH and LH release from the pituitary gland. Estrogen and progesterone acts on the hypothalamus and causes negative feed back effect on GnRH release; the estrogen also produces a positive feed back effect on GnRH release. Thus the functions of ovary and testes are regulated by the hypothalamus.In seasonal breeders and birds hypothalamus plays a key role to effect photoperiodism and thereby the breeding performance is regulated.
  • Melatonin released from the pineal gland regulates the duration of light, and activates/inhibits GnRH release from hypothalamus in short and long-day breeders. Until puberty, hypothalamic GnRH release is highly sensitive to  oestrogen and is kept in check by the negative feed back effect of oestrogen. At the time of puberty, the sensitivity of hypothalamus to oestrogen is reduced resulting in pulsatile GnRH release and puberty occurs.
• Regulation of anterior pituitary functions
  • Hypothalamus  controls the anterior  pituitary  gland secretions through neuro-secretory peptides and amines known as the releasing and inhibitory hormones. E.g. TRH, GHRH, GHIH, PIH, CRH, GnRH, PRH. The TRH and CRH are released from paraventricular nucleus, GnRH from preoptic area, GHIH from anterior hypothalamic area, GHRH and PIH from arcuate nucleus. These regulatory hormones pass from the hypothalamus to adenohypophysis through the hypothalamic-hypophyseal portal system.
• Regulation of emotional status
  • Hypothalamus controls both fear and aggressiveness. The posterior hypothalamus regulates sympathetic response of the emotional status via catecholamines.