Hypothalamus and Pituitary Gland
ஹைப்போதலாமஸும் பிட்யூட்டரி சுரப்பியும்
The posterior pituitary makes nothing. It only stores and releases oxytocin and vasopressin that the hypothalamus makes — that is why it is not a 'true' endocrine gland.
The idea
The hypothalamus is the basal part of the diencephalon (forebrain) and regulates a wide spectrum of body functions. It contains several groups of neurosecretory cells called nuclei which produce hormones that regulate the synthesis and secretion of pituitary hormones. These hormones are of two types — releasing hormones, which stimulate secretion of pituitary hormones (e.g. gonadotrophin releasing hormone, GnRH, stimulates synthesis and release of gonadotrophins), and inhibiting hormones, which inhibit secretions (e.g. somatostatin inhibits release of growth hormone). These hormones reach the pituitary gland through a portal circulatory system and regulate the functions of the anterior pituitary; the posterior pituitary is under the direct neural regulation of the hypothalamus. The pituitary gland is located in a bony cavity called the sella turcica and is attached to the hypothalamus by a stalk. It is divided anatomically into an adenohypophysis and a neurohypophysis. The adenohypophysis consists of the pars distalis (commonly called the anterior pituitary) and the pars intermedia. The pars distalis produces growth hormone (GH), prolactin (PRL), thyroid stimulating hormone (TSH), adrenocorticotrophic hormone (ACTH), luteinizing hormone (LH) and follicle stimulating hormone (FSH). The pars intermedia secretes only one hormone, melanocyte stimulating hormone (MSH); in humans the pars intermedia is almost merged with the pars distalis. The neurohypophysis (pars nervosa, the posterior pituitary) stores and releases two hormones, oxytocin and vasopressin, which are actually synthesised by the hypothalamus and transported axonally to the neurohypophysis. Over-secretion of GH stimulates abnormal growth of the body leading to gigantism, and low secretion of GH results in stunted growth resulting in pituitary dwarfism; excess secretion of GH in adults, especially in middle age, can result in severe disfigurement, especially of the face, called acromegaly. Prolactin regulates the growth of the mammary glands and formation of milk; TSH stimulates the thyroid; ACTH stimulates the adrenal cortex to make glucocorticoids; LH and FSH stimulate gonadal activity. Oxytocin acts on smooth muscles and stimulates their contraction — in females it stimulates vigorous contraction of the uterus at childbirth and milk ejection from the mammary gland. Vasopressin acts mainly at the kidney and stimulates resorption of water and electrolytes by the distal tubules, thereby reducing water loss through urine (diuresis); hence it is also called antidiuretic hormone (ADH), and an impairment affecting its synthesis or release results in diabetes insipidus.
Why it matters
The hypothalamus-pituitary axis lets the brain control almost every other endocrine gland, so hormone levels can respond to stress, growth stage and the environment.
NEET lens
Key: hypothalamic releasing (GnRH) and inhibiting (somatostatin) hormones reach the anterior pituitary via portal circulation; posterior pituitary is under direct neural control. Pars distalis: GH, PRL, TSH, ACTH, LH, FSH; pars intermedia: MSH; neurohypophysis stores/releases oxytocin and vasopressin made by the hypothalamus (so it is not a true endocrine gland). GH: gigantism, dwarfism, acromegaly; after epiphyseal plates close in adults, excess GH cannot increase height.
Clinical significance
Synthetic oxytocin is used to induce labour and to control bleeding after delivery. Growth hormone is given to children with GH deficiency, and acromegaly is often caused by a pituitary tumour.
Introduction
The hypothalamus controls the pituitary, and the pituitary — the so-called master gland — controls many other endocrine glands.
Core concept
Hypothalamus: basal part of diencephalon; neurosecretory nuclei → releasing hormones (e.g. GnRH → gonadotrophins) and inhibiting hormones (e.g. somatostatin → inhibits GH); reach anterior pituitary through a portal circulatory system; posterior pituitary under direct neural regulation. Pituitary: in sella turcica; attached by a stalk. Adenohypophysis = pars distalis (anterior pituitary: GH, PRL, TSH, ACTH, LH, FSH) + pars intermedia (MSH; merged with pars distalis in humans). Neurohypophysis (pars nervosa, posterior pituitary): stores and releases oxytocin and vasopressin, synthesised by the hypothalamus and transported axonally. GH: over-secretion → gigantism; low → pituitary dwarfism; excess in middle-aged adults → acromegaly. PRL: mammary growth, milk formation. TSH → thyroid; ACTH → adrenal cortex (glucocorticoids). LH/FSH → gonads (LH: androgens from testis, ovulation; FSH: with androgens regulates spermatogenesis, growth of ovarian follicles). Oxytocin: uterine contraction at childbirth, milk ejection. Vasopressin/ADH: water and electrolyte reabsorption in distal tubules; deficiency → diabetes insipidus.
Go deeper
Hypersecretion of GH in adults does not increase height because the epiphyseal plates of long bones have closed after puberty; instead bones of the face, hands and feet thicken (acromegaly).
From Class 10
Class 10 called the pituitary the master gland and linked GH with dwarfism and gigantism. NEET adds the hypothalamic control and the two lobes.