The Hypothalamus and Pituitary

The pituitary (hypothesis) and hypothalamus are morphologically and functionally intimately associated. They represent the centre of highest coordination between the endocrine and nervous systems. The hypothalamus is the control centre for the autonomic nervous system whilst the pituitary controls the endocrine system. Together they form a functional unit.

Hypothalamus

The hypothalamus is part of the brain, which lies just below the thalamus, forming the floor of the third ventricle. Numerous releasing hormones or factors are produced by the hypothalamus. These factors are transported to the pituitary where they stimulate the pituitary cells to release their respective hormones into the blood stream. The hypothalamus has a two-way connection with the cerebral cortex. A close association thus exists between the nervous system (which affects rapid co-ordination) and the endocrine system, which is involved in the slower chemical co-ordination of the body.

Pituitary

The Pituitary (Hypothesis) gland This small structure lies at the base of the brain and is connected to the hypothalamus by a small pedicle. The part of the pituitary that originates from the brain is called the posterior lobe (neurohypophysis) and the part originating from the upper palate is called the anterior lobe or adenohypophysis.

Hormones of the Anterior Pituitary

Adreno-corticotropic hormone (ACTH). It is secreted by stimulus of the corticotrophin releasing factor (CTRF) from the hypothalamus. Adreno-corticotropic hormone (ACTH) stimulates the secretion of corticosteroids by the adrenal cortex. On the other hand, corticosteroids inhibit the secretion of ACTH, a process called” negative feedback”.

Growth hormone (Somatotropin). Secretion is controlled by the growth hormone releasing factor (GHRF) from the hypothalamus. The growth hormone controls the general growth of the body (especially the longitudinal growth of the long bones).

Thyrotropic hormone (TTH). Secretion is controlled by thyrotropic releasing hormone (T-Rh). TTH stimulates growth of the thyroid gland. It controls the uptake of iodine by the thyroid and thus also the synthesis and release of the thyroid hormone.

Follicle stimulating hormone (FSH). The releasing hormone from the hypothalamus again stimulates the release of FSH by the anterior pituitary. FSH stimulates the development of the Graafian follicle in the ovary of the female. Oestrogen, which is formed in the follicle, on the other hand, inhibits the secretion of FSH when a certain level is reached.

Interstitial cell stimulating hormone (ICSH). ICSH stimulates the interstitial cells in the testis of the male to secrete testosterone (the male sex hormone).

Luteinizing hormone (LH). LH-RH causes release of LH, which is responsible for ovulation in the female animal. Ovulation takes place as soon as a specific balance between oestrogen from the follicle and LH from the anterior pituitary is reached. After ovulation LH also plays a role in the development of the corpus luteum (Yellow body).

Prolactin. Prolactin is responsible for the maintenance of secretion of the corpus luteum and is also involved in lactation. In the male it stimulates the accessory sexual organs.

Hormones of the Posterior Pituitary (Neurohypophysis)

These hormones are formed in the brain and then transported to the posterior pituitary.

Antidiuretic Hormone (ADH). Antidiuretic hormone decreases the volume of urine by increasing water resorption from the filtration solution in the kidney.

Oxytocin. This hormone acts on the mammary gland. It stimulates the flow of milk through its action on the myo-epithelial cells and smooth muscle fibres in the mammary gland – the so-called “let-down reflex”. Oxytocin is released as soon as the udder of the cow is stimulated e.g. by the pre-milk washing of the udder. The effect of the hormone only lasts for a few minutes and the cow must therefore be milked as soon as possible, otherwise there is no proper “let-down” of milk. During parturition (birth) it causes contractions of the uterus muscles and in the act of mating, it aids in the transport of spermatozoa and the ovum. The reaction of oxytocin is inhibited by adrenalin (and thus fear or stress).