3. RELATION TO THE PITUITARY GLAND
โข There are neural connections between the hypothalamus and the
posterior lobe of the pituitary gland and vascular connections
between the hypothalamus and the anterior lobe
โข Embryologically, the posterior pituitary arises as an evagination of the
floor of the third ventricle.
โข The anterior and intermediate lobes of the pituitary arise in the
embryo from the Rathke pouch, an evagination from the roof of the
pharynx
โข The portal hypophysial vessels form a direct vascular link between the
hypothalamus and the anterior pituitary
5. AUTONOMIC FUNCTION
โข Sherrington called the hypothalamus โthe head ganglion of the
autonomic system.โ
โข Stimulation of various parts of the hypothalamus, especially the
lateral areas, produces diffuse sympathetic discharge
โข And increased adrenal medullary secretion
โข It has been claimed that separate hypothalamic areas control
epinephrine and norepinephrine secretion
6. CONTROL OF POSTERIOR PITUITARY
โข The hormones secreted by the posterior pituitary gland are arginine
vasopressin (AVP) and oxytocin
โข The hormones of the posterior pituitary gland are synthesized in the
cell bodies of the magnocellular neurons in the supraoptic and
paraventricular nuclei
โข And transported down the axons of these neurons to their endings in
the posterior lobe, where they are secreted in response to electrical
activity in the endings
โข Some of the neurons make oxytocin and others make vasopressin,
and oxytocin-containing and vasopressin containing cells are found in
both nuclei
7. Actions of Oxytocin
โข Oxytocin acts primarily on the breasts and uterus, though it appears to be
involved in luteolysis as well
โข A G protein-coupled oxytocin receptor has been identified in human
myometrium, and a similar or identical receptor is found in mammary
tissue and the ovary.
โข It triggers increases in intracellular Ca2+ levels
โข Oxytocin causes contraction of the myoepithelial cells that line the ducts of
the breast.
โข Oxytocin causes contraction of the smooth muscle of the uterus
โข The sensitivity of the uterine musculature to oxytocin is enhanced by
estrogen and inhibited by progesterone
Norepinephrine-secreting neurons with their cell bodies in the hindbrain end in many different parts of the hypothalamus
Paraventricular neurons that secrete oxytocin and vasopressin project in turn to the hindbrain and the spinal cord.
Neurons that secrete epinephrine have their cell bodies in the hindbrain and end in the ventral hypothalamus
An intrahypothalamic system is comprised of dopamine secreting neurons that have their cell bodies in the arcuate nucleus and end on or near the capillaries that form the portal vessels in the median eminence.
Serotonin-secreting neurons project to the hypothalamus from the raphe nuclei.
(This system begins and ends in capillaries without going through the heart and is therefore a true portal system)
It is made up in large part of the endings of axons that arise from cell bodies in the supraoptic and paraventricular nuclei and pass to the posterior pituitaryvia the hypothalamohypophysial tract
Most of the supraoptic fibers end in the posterior lobe itself, whereas some of the paraventricular fibers end in the median eminence
Drinking is regulated by plasma osmolality and extracellular fluid (ECF) volume in much the same fashion as vasopressin secretion
Water intake is increased by increased effective osmotic pressure of the plasma by decreases in ECF volume, and by psychologic and other factors.
Osmolality acts via osmoreceptors, receptors that sense the osmolality of the body fluids.
These osmoreceptors are located in the anterior hypothalamus
The hypothalamus and related parts of the brain play a key role in the regulation of food intake
Stimulation of the hypothalamus produces autonomic responses, but the hypothalamus does not seem to be concerned with the regulation of visceral function per se. Rather, the autonomic responses triggered in the hypothalamus are part of more complex phenomena such as eating, and emotions such as rage
Oxytocin and vasopressin are typical neural hormones , that is, hormones secreted into the circulation by nerve cells.
The precursor molecules are synthesized in the ribosomes of the cell bodies of the neurons.
They have their leader sequences removed in the endoplasmic reticulum, are packaged into secretory granules in the Golgi apparatus, and are transported down the axons by axoplasmic flow to the endings in the posterior pituitary.
This squeezes the milk out of the alveoli of the lactating breast into the large ducts (sinuses) and thence out of the nipple (milk ejection)
Milk ejection is normally initiated by a neuroendocrine reflex.
The receptors involved are touch receptors, which are plentiful in the breastโespecially around the nipple. Impulses generated in these receptors are relayed from the somatic touch pathways to the supraoptic and paraventricular nuclei.
In late pregnancy, the uterus becomes very sensitive to oxytocin coincident with a marked increase in the number of oxytocin receptors and oxytocin receptor mRNA
Oxytocin secretion is then increased during labor.
After dilation of the cervix, descent of the fetus down the birth canal initiates impulses in the afferent nerves that are relayed to the supraoptic and paraventricular nuclei, causing secretion of sufficient oxytocin to enhance labor
Oxytocin may also act on the nonpregnant uterus to facilitate sperm transport.
The passage of sperm up the female genital tract to the uterine tubes, where fertilization normally takes place, depends not only on the motile powers of the sperm but also, at least in some species, on uterine contractions.
The secretion of oxytocin is also increased by stressful stimuli and, like that of vasopressin, is inhibited by alcohol.
The anterior pituitary secretes six hormones: adrenocorticotropic hormone (corticotropin, ACTH) , thyroid- stimulating hormone (thyrotropin, TSH) , growth hormone, follicle stimulating hormone (FSH) , luteinizing hormone (LH) and prolactin (PRL)
The hypothalamus plays an important stimulatory role in regulating the secretion of ACTH, ฮฒ-LPH, TSH, growth hormone, FSH, and LH. It also regulates prolactin secretion, but its effect is predominantly inhibitory rather than stimulatory
Anterior pituitary secretion is controlled by chemical agents carried in the portal hypophysial vessels from the hypothalamus to the pituitary.
These substances used to be called releasing and inhibiting factors, but now they are commonly called hypophysiotropic hormones
There are six established hypothalamic releasing and inhibiting hormones ( Figure 17โ10 ): corticotropin-releasing hormone (CRH) ; thyrotropin-releasing hormone (TRH) ; growth hormone-releasing hormone (GRH); growth hormone- inhibiting hormone (GIH, now generally called somatostatin) ; luteinizing hormone-releasing hormone (LHRH , now generally known as gonadotropin-releasing hormone (GnRH)); and prolactin-inhibiting hormone (PIH) .
In addition, hypothalamic extracts contain prolactin-releasing activity, and a prolactin-releasing hormone (PRH) has been postulated to exist.
The FSH-stimulating activity of GnRH has been mentioned previously.
TRH stimulates the secretion of prolactin as well as TSH.
Somatostatin inhibits the secretion of TSH as well as growth hormone.
It does not normally inhibit the secretion of the other anterior pituitary hormones, but it inhibits the abnormally elevated secretion of ACTH in patients with Nelsonโs syndrome. CRH stimulates the secretion of ACTH and ฮฒ-LPH.
The GnRH-secreting neurons are primarily in the medial preoptic area, the somatostatin-secreting neurons are in the periventricular nuclei, the TRH-secreting and CRH-secreting neurons are in the medial parts of the paraventricular nuclei, and the GRH-secreting (and dopaminesecreting) neurons are in the arcuate nuclei.
Toxins from bacteria, such as endotoxin, act on monocytes, macrophages, and Kupffer cells to produce cytokines that act as endogenous pyrogens (EPs) .
There is good evidence that IL-1ฮฒ, IL-6, IFN-ฮฒ, IFN-ฮณ, and TNF-ฮฑ can act independently to produce fever.
These circulating cytokines are polypeptides and it is unlikely that they penetrate the brain.
Instead, evidence suggests that they act on the OVLT, one of the circumventricular organs.
This in turn activates the preoptic area of the hypothalamus.
Cytokines are also produced by cells in the central nervous system (CNS) when these are stimulated by infection, and these may act directly on the thermoregulatory centers.
The fever produced by cytokines is probably due to local release of prostaglandins in the hypothalamus.
