Hypothalamus And Pituitary (Physiology)

Question 1

Describe the vascular and neuronal connections between the hypothalamus and the pituitary

Answer 1

Vascular connections

• The anterior pituitary (adenohypophysis) is linked to the hypothalamus by a complex of blood vessels called the hypophyseal portal system.
• This system consists of a network of primary capillaries in the hypothalamus, a group of portal venules that travel down the infundibulum, and a complex of secondary capillaries in the adenohypophysis.
• The hypothalamus controls the anterior pituitary by secreting hormones that travel down the hypophyseal portal system and diffues into the pituitary tissue.

Neuronal connections

• Hormones of the posterior pituitary (neurohypophysis) are made by certain neuroendocrine cells in the hypothalamus.
• The axons of these cells pass down the infundibulum as a bundle called the hypothalamo-hypophyseal tract and end in the neurohypophysis.
• Hormones are made in the neurosomas and move down the nerve fibres by axoplasmic flow to the neurohypophysis. They are stored in the nerve endings until a nerve signal coming down the same axons triggers their release.

Question 2

List the main pituitary hormones and discuss their actions

Answer 2

Anterior pituitary hormones
- Follicle-stimulating hormone (FSH) - Secreted by pituitary cells called gonadotropes. Stimulates the secretion of ovarian sex hormones and the development of the follicles that contain the eggs. Stimulates sperm production in the testes.

• Luteinising hormone (LH) - Secreted by gonadotropes. Stimulates ovulation. Stimulates the corpus luteum to secrete progesterone, a hormone important in pregnancy. Stimulates the testes to secrete testosterone.
• Thyroid-stimulating hormone (TSH) - Secreted by thyrotropes. Stimulates growth of the thyroid gland and the secretion of thyroid hormone which effects metabolic rate and body temperature.
• Adrenocorticotrophic hormone (ACTH) - Secreted by corticotropes. Stimulates the adrenal gland’s cortex to secrete hormones called glucocorticoids (especially cortisol), which regulates glucose, protein, and fat metabolism, and are important in the body’s response to stress.
• Prolactin (PRL) - Secreted by lactotropes. During pregnancy lactotropes increase in size and number, and prolactin secretion increases proportionately, but it has no effect on the mammary glands until after the woman has given birth. Then it stimulates them to synthesise milk.
• Growth hormone (GH) - Secreted by somatotropes. Stimulates mitosis and cellular differentiation and thus stimulates tissue growth throughout the body.

Posterior pituitary hormones
- Antidiuretic hormone (ADH) - Increases water retention by the kidneys, reduces urine volume, and helps prevent dehydration. Also functions as a brain neurotransmitter and is usually called arginine vasopressin which refers to its ability to cause vasoconstriction.

• Oxytocin (OT) - It surges in both sexes during sexual arousal and orgasm. Possibly aids in the propulsion of semen through the male reproductive tract and in stimulating uterine contractions that help transport sperm up the female tract. Functions in feelings of sexual satisfaction and emotional bonding between partners. In childbirth it stimulates labour contractions, and in lactating mothers it stimulates the flow of milk from deep in the mammary gland to the nipple.

Question 3

Describe how secretion of pituitary hormones is regulated

Answer 3

• Hypothalamic control enables the brain to monitor conditions inside and outside the body and to stimulate or inhibit the release of anterior lobe hormones in response.
• For example, in times of stress, the hypothalamus triggers adrenocorticotrophic hormone secretion which leads to cortisol secretion and mobilisation of materials needed for tissue repair.
• The posterior pituitary is controlled by neuroendocrine reflexes.
• For example, the suckling of an infant triggers a neuroendocrine reflex mediated by oxytocin. Stimulation of the nipple sends nerve signals up the spinal cord and brainstem to the hypothalamus and from there to the posterior pituitary. This causes the release of oxytocin which stimulates the release of milk.

Question 4

Explain how the levels of trophic hormones are modulated by feedback from their target hormones

Answer 4

• Target organs regulate the pituitary and hypothalamus through various feedback loops.
• Most often this is through negative feedback inhibition. The pituitary stimulates another endocrine gland to secrete its hormone, and that hormone feeds back to the pituitary or hypothalamus and inhibits further secretion of the pituitary hormone.
• Take the hypothalamo-pituitary-thyroid axis as an example:

The hypothalamus secretes thyrotropin-releasing hormone (TRH). TRH stimulates the anterior pituitary to secrete thyroid-stimulating hormone (TSH). TSH stimulates the thyroid gland to secrete thyroid hormone (TH). TH stimulates the metabolism of most cells throughout the body. TH also inhibits the release of TSH by the pituitary. If thyroid hormone secretion drops, TSH secretion rises and stimulates the thyroid to secret more hormone. This feedback keep thyroid levels oscillating around a set point.

Question 5

Briefly outline the effects of pituitary hormone hyposecretion and hypersecretion and explain these effects in terms of normal pituitary function

Answer 5

Primary hyposecretion

• Too little hormone is secreted due to abnormality within gland.
• Can be caused by lack of iodine in diet, autoimmune diseases, cancer.
• A congenital absence or underdevelopment of the pituitary gland causes panhypopituitarism, wide-spectrum hyposecretion of multiple hormones affected by the pituitary, requiring lifelong multi-hormone replacement therapy.

Secondary hyposecretion
- Gland is normal but too little hormone is secreted due to deficiency of its tropic hormone.

Hypersecretion

• Caused by tumours that result in the overgrowth of function endocrine tissue leading to continuous excess hormone secretion.
• Primary hypersecretion is caused by an abnormality within the gland and secondary is caused by excessive stimulation from outside the gland.
• Excess ACTH causes corticosteroid excess (Cushing’s disease). Excess prolactin causes impaired reproductive function. Excess ADH causes fluid retention and low plasma osmolality. Excess GH causes gigantism in children and acromegaly in adults.

Question 6

What are the benefits of the hypothalamic-hypophyseal portal system

Answer 6

• Minimises dilution of hormones.

- Allows rapid response.