L2: the hypothalamo-pituitary axis

Question 1

What is the hypothalamus and what does it control?

Answer 1

• hypothalamus is a collection of brain ‘nuclei’ or centres
• controls autonomic function via brainstem autonomic centres
• controls endocrine function via the pituitary gland

Question 2

Location of hypothalamus and pituitary gland

Answer 2

1:50

Question 3

How can endocrine disorders affect vision?

Answer 3

Anatomy - pituitary gland is located close to the optic chiasma so growths or tumours on the pituitary can affect vision.

Question 4

How is the hypothalamus connected to the pituitary gland?

Answer 4

Min 3
Pituitary gland sits within the sella turcica
Anterior pituitary gland connected via the superior hypophyseal artery to hypothalamus
Neuronal link from the paraventricular nucleus (PVN) in the hypothalamus and the supraoptic nucleus will project nuclei down into the posterior pituitary.

Question 5

Anatomy of the pituitary gland

Answer 5

Split into the anterior lobe and posterior lobe, different embryonic origins.
Structurally continuous with with the hypothalamus, to which is remains attached by the hypophyseal stalk

Question 6

Which hormones are released from the anterior pituitary gland?

Answer 6

Adrenocorticotropic hormone ACTH - acts on adrenal cortex
Growth hormone GH - acts on bone and muscle
Melanocyte-stimulating hormone MSH - acts on the skin
Thyroid stimulating hormone TSH - acts on the thyroid gland
Gonadotropins (FSH, LH) - act on the testis and ovaries
Prolactin - acts on the breast

Question 7

Which hormones are released from the posterior pituitary gland?

Answer 7

Oxytocin - acts on the breast

ADH - acts on the kidney

Question 8

The hypothalamo-pituitary axis

Answer 8

13:25
Hypothalamus interacts with the anterior pituitary through a blood system - the hypothalamo-hypophyseal portal system. Hormones (peptide) from hypothalamus will go into the portal system and travel to the anterior pituitary, be released to the cells there and have their effect. These hormones are not seen in the general circulation. Vascular connection.

The hypothalamo-hypophyseal tract is a collection of axons that extend from the hypothalamus to the posterior pituitary. From the paraventricular nucleus and supra-optic nucleus, hormones are formed and travel down through the neuronal link, and are secreted directly into the blood stream and go systemically across the body. SO nucleus produces ADH as a larger preprohormone. From PVN oxytocin is produced.

Question 9

What is a portal system?

Answer 9

An enclosed system consisting of a system of capillaries.

Question 10

Actions of ADH

Answer 10

Increases water reabsorption by increasing the water permeability of collecting duct tubules
At high concentrations in the blood it will cause generalised vasoconstriction and thus increase BP

Question 11

What regulates ADH secretion

Answer 11

• ADH secretion is increased by an increase in extracellular osmolality
• ADH released in response to decreased ECV or low BP
• angiotensin II reinforces the response to hypovolaemia or hypotension and increases ADH secretion

Vice versa
- alcohol decreases ADH secretion (diuretic effect of alcohol)

Question 12

What is osmolality?

Answer 12

Number of osmoles per kilogram of solvent. Close to 285 mOsm/Kg H2O
Osmoreceptors in hypothalamus detect changes in plasma osmolality and control release of ADH

Question 13

ADH receptors

Answer 13

V1 on blood vessels. Involved in the general vasoconstriction at higher concentrations of ADH to increase BP.
V2 on renal collecting ducts. To increase renal fluid reabsorption, increasing volume.

Question 14

What diseases can involve thirst and frequent urination?

Answer 14

Diabetes insipidus is when there is a lack of ADH. Diabetes mellitus is a lack of insulin. Both involve thirst and frequent urination, and urination in much greater volumes therefore extremely dilute and a very pale yellow almost clear colour.

Question 15

What does oxytocin do?

Answer 15

Stimulates contraction of smooth muscle (myoepithelial cells) of breast and uterus. Can be given to induce labour.

Question 16

What does oxytocin play a role in?

Answer 16

• milk ejection reflex
• parturition (birth)
  Both are positive feedback.
  Neuro-endocrine reflexes - neuro-endocrine cells secrete hormones from the neural axon terminals into the blood in response to some neural signal.

Question 17

Milk ejection reflex

Answer 17

1. Receptors in nipple stimulated
2. Impulses propagated to spinal cord
3. Stimulation of hypothalamic nuclei
4. Oxytocin release from posterior pituitary
5. Milk ejected

Question 18

Hypophyseotropic hormones

Answer 18

Hypothalamic hypophyseotropic hormones control the release of anterior pituitary hormones:

• thyrotropin releasing hormone TRH stimulates the anterior pituitary to release TSH
• gonadotropin releasing hormone GnRH stimulates release of LH and FSH
• corticotropin releasing hormone CRH stimulates release of ACTH
• growth hormone releasing hormone GRH/GHRH stimulates release of growth hormone
• somatostatin inhibits release of growth hormone
• dopamine inhibits prolactin release

Question 19

How are anterior pituitary hormones release?

Answer 19

Hypothalamus has its neurosecretory neurones which release ‘releasing hormones’ that go into the hypothalamic-hypophyseal portal system and travel down to the anterior pituitary and stimulates release of other hormones that will go into the blood to affect other glands.

Question 20

What are tropic hormones?

Answer 20

ACTH, TSH, LH, FSH are all tropic hormones. This means that these hormones will nourish the glands they act on. They will cause growth of the glands they act on.

Question 21

What are the 5 types of secretory cells in the anterior pituitary?

Answer 21

Thyrotropes (make TSH), gonadotropes (make FSH and LH), corticotropes (make ACTH), somatotropes (make growth hormone), lactotropes (make prolactin).

Question 22

Action of thyroid stimulating hormone TSH

Answer 22

Stimulates growth of gland and secretion of thyroid hormone

Question 23

Action of FSH follicle stimulating hormone

Answer 23

Goes to ovaries to stimulate the development of eggs and follicles. Goes to testes and stimulates production of sperm,

Question 24

Action of luteinising hormone LH

Answer 24

In females it stimulates ovulation and corpus output to secrete progesterone and oestrogen.
In males it stimulates interstitial cells of testes to secrete testosterone.

Question 25

Action of GH growth hormone

Answer 25

Main effect is to stimulate liver to produce IGF-1

Question 26

Action of prolactin

Answer 26

In females it aids milk synthesis after delivery. In males it increases LH sensitivity thus increases testosterone secretion (less clear mechanism)

Question 27

Relevance of POMC to ACTH

Answer 27

The large precursor polypeptide POMC is the source of several important biologically active substances. POMC products in the corticotroph cell are ACTH. POMC products in the brain are gamma-MSH. Can cause skin pigmentation if there is a pituitary tumour causing overproduction of POMC.

Question 28

What is a pituitary adenoma?

Answer 28

• relatively common because when a tissue is active there is a greater chance of adenomas and carcinomas forming
• can be non-functioning or hormone-secreting
• adenoma - tumour that is not cancer
• more likely to get a lactotroph or gonatroph tumour over any other one

Question 29

Why do patients with pituitary tumours have visual field defects?

Answer 29

Cannot see from the left and right sides. Tunnel vision. This is because a tumour can put pressure on the optic nerve, so causes visual field defects.