Anatomy & Physiology: The Hypothalamus and Pituitary Glands

The hypothalamus makes up the lower region of the diencephalon and lies just above the brain stem. The pituitary gland (hypophysis) is attached to the bottom of the hypothalamus by a slender stalk called the infundibulum. The pituitary gland consists of two major regions: the anterior pituitary gland (anterior lobe or adenohypophysis) and the posterior pituitary gland (posterior lobe or neurohypophysis).

The hypothalamus oversees many internal body conditions. It receives nervous stimuli from receptors throughout the body and monitors chemical and physical characteristics of the blood, including temperature; blood pressure; and nutrient, hormone, and water content. When deviations from homeostasis occur or when certain developmental changes are required, the hypothalamus stimulates cellular activity in various parts of the body by directing the release of hormones from the anterior and posterior pituitary glands. The hypothalamus communicates directives with these glands by one of the following two pathways:

Communication between the hypothalamus and the anterior pituitary occurs through chemicals (releasing hormones and inhibiting hormones) that are produced by the hypothalamus and delivered to the anterior pituitary through blood vessels in the infundibulum. The releasing and inhibiting hormones are produced by specialized neurons of the hypothalamus, called neurosecretory cells. The hormones are released into a capillary network (primary plexus) and transported through veins (hypophyseal portal veins) to a second capillary network (secondary plexus) that supplies the anterior pituitary. The primary plexus and the hypophyseal portal veins are in the infundibulum and the secondary plexus is in the anterior pituitary. The hormones then diffuse from the secondary plexus into the cells of the anterior pituitary, where they initiate the production of specific hormones by the anterior pituitary. The releasing and inhibiting hormones secreted by the hypothalamus and the hormones produced in response by the anterior pituitary are listed in Table 1. Many of the hormones produced by the anterior pituitary are tropic hormones (tropins), hormones that stimulate other endocrine glands to secrete their hormones.
Communication between the hypothalamus and the posterior pituitary occurs through neurosecretory cells that span the short distance between the hypothalamus and the posterior pituitary (through the infundibulum). Hormones produced by the cell bodies of the neurosecretory cells are packaged in vesicles and transported through the axon, and stored in the axon terminals that lie in the posterior pituitary. When the neurosecretory cells are stimulated, the action potential generated triggers the release of the stored hormones from the axon terminals to a capillary network within the posterior pituitary. Two hormones, oxytocin and antidiuretic hormone (ADH), are produced and released in this way. Their functions are summarized in Table 1.

Table 1. Hormone Functions
Abbrevi-ation	Name	Classification	Target	Action
Key to the classification abbreviations used in Table 1 H = hormone RH = releasing hormone IH = inhibiting hormone PP = polypeptide hormone GP = glycoprotein hormone P = protein AA = amino acid derivative hormone S = steroid hormone E = eicosanoid
From the Hypothalamus
GHRH	Growth hormone RH	PP	Anterior pituitary	Stimulates release of GH
GHIH	Growth hormone IH (somatostatin)	PP	Anterior pituitary	Inhibits release of GH
TRH	Thyrotropin RH	PP	Anterior pituitary	Stimulates release of TSH and GH
GnRH	Gonadotropin RH	PP	Anterior pituitary	Stimulates release of LH and FSH
PRH	Prolactin RH	PP	Anterior pituitary	Stimulates release of PRL
PIH	Prolactin IH (dopamine)	PP	Anterior pituitary	Inhibits release of PRL
CRH	Corticotropin RH	PP	Anterior pituitary	Stimulates release of ACTH
From the Anterior Pituitary (Tropic Hormones)
TSH	Thyroid stimulating hormone (thyrotropin)	GP	Thyroid gland	Stimulates secretion of T₃ and T₄
ACTH	Adrenocorticotropic hormone	PP	Adrenal cortex	Stimulates secretion of glucocorticoids
FSH	Follicle stimulating hormone	GP	Ovaries and testes	Regulates oogenesis and spermatogenesis
LH	Luteinizing hormone	GP	Ovaries and testes	Causes ovulation and release of testosterone
From the Anterior Pituitary (Not Tropic Hormones)
PRL	Prolactin	P	Mammary glands	Stimulates production of milk
hGH	human Growth Hormone (somatotropin)	P	Bones, muscles, and cells in general	Stimulates growth
From the Posterior Pituitary
OT	Oxytocin	PP	Uterus and mammary glands	Produces uterine contractions and release of milk
ADH	Antidiuretic hormone (vasopressin)	PP	Kidneys and sweat glands	Prevents dehydration
From the Thyroid Gland
T₄	Thyroxine	AA	Bone and general cells	Increases metabolism
T₃	Triiodothyronine	AA	Bone and general cells	Increases metabolism
CT	Calcitonin	PP	Bone and general cells	Decreases blood Ca²⁺
From the Parathyroid Gland
PTH	Parathyroid hormone	PP	Bone, kidneys, and small intestine	Increases blood Ca⁺²
From the Adrenal Medulla
EPI	Epinephrine (adrenaline)	AA	Blood vessels, liver, and heart	Increases blood glucose levels
NE	Norepinephrine (noradrenaline)	AA	Blood vessels, liver, and heart	Increases blood glucose levels
From the Adrenal Cortex
—	Mineralocorticoids (aldosterone)	S	Kidneys	Allows retention of Na⁺ and release of K⁺; increases BP
—	Glucocorticoids (such as cortisol)	S	General body cells	Increases blood glucose levels
—	Androgens	S	General body cells	Stimulates onset of puberty
From the Pancreas
—	Glucagon (from alpha cells)	PP	Liver	Increases blood glucose levels
—	Insulin (from beta cells)	PP	Liver, muscle, and general cells of the body.	Decreases blood glucose levels
—	Somatostatin (from delta cells)	PP	Alpha and beta cells, and adenohypophysis	Inhibits glucagon and insulin release and inhibits release of GH
—	Pancreatic polypeptide (from F cells)	PP	Delta cells	Inhibits somatostatin and pancreatic enzyme release
From the Ovaries
—	Estrogen	S	General cells of the body	Determines secondary sex characteristics
—	Progesterone	S	Uterus	Prepares the body for pregnancy; involved in the menstrual cycle
—	Relaxin	PP	Pelvis and cervix	Prepares the birth canal and cervix dilation
—	Inhibin	P	Anterior pituitary	Inhibits release of FSH
From the Testes
—	Testosterone	S	General body cells	Determines secondary sex characteristics and plays minor role in spermatogenesis
—	Inhibin	P	Anterior pituitary	Inhibits release of FSH
From the Pineal Gland
—	Melatonin	AA	General body cells	Regulates the biological clock and circadian rhythms
From the Kidneys
EPO	Erythropoietin	GP	Bone marrow	Stimulates RBC formation
—	Calcitriol(vitamin D)	S	Small intestines	Increases Ca⁺² absorption
From the Placenta
—	Estrogen	S	Uterus	Maintains pregnancy
—	Progesterone	S	Endometrium of the uterus	Maintains pregnancy
hCG	human Chorionic Gonadotropin	GP	Corpus luteum of the ovaries	Stimulates release of estrogen and progesterone
hPL	human Placental Lactogen	P	Mammary glands	Prepares the mammary glands for lactation
From the Gastrointestinal Tract
—	Gastrin	PP	Chief (C) and Parietal (P) cells of the stomach	Stimulates release of HCl and pepsinogen, respectively
GIP	Glucose-dependent insulinotropic peptide.	PP	Beta cells of the pancreas	Stimulates the release of insulin
—	Secretin	PP	Pancreas and liver	Stimulates production of buffers from the pancreas and bile from the liver
CCK	Cholecystokinin	P	Pancreas and gallbladder	Stimulates release of enzymes from the pancreas and bile from the gallbladder
—	Serotonin	AA	Stomach	Stimulates stomach muscle contraction
From the Heart
ANP	Atrial natriuretic peptide	P	Kidneys	Causes loss of Na⁺ and water; decreases BP
From Adipocytes
—	Adipokines	E	General cells of the body	Is involved in anti-inflammatory responses and insulin sensitizing for increased glucose intake
From General Cells
PG	Prostaglandins	E	General cells of the body	Is involved in numerous tasks, including the inflammatory response
LT	Leukotrienes	E	General cells of the body	Is involved in numerous tasks, including the inflammatory response

CliffsNotes Poll

The Hypothalamus and Pituitary Glands

Cite this article

Ask Cliff

Related Topics:

CliffsNotes Poll

The Hypothalamus and Pituitary Glands

Cite this article

Ask Cliff