Hypothalamus-Pituitary Axis Part 2

Question 1

Describe the process of formation of growth hormone

Answer 1

Small bodies neurons in the arcuate nucleus secrete GHRH which binds to GHRH receptor on the surface of somatotrophs causing them to synthesis and release growth hormone.

Question 2

Describe the inhibition of growth hormone release

Answer 2

• Somatostatin is released by cells in the periventricular region. It binds to somatostatin receptors on the surface of somatotrophs (inhibits the formation of cAMP) and therefore inhibits the release of growth hormone

Question 3

Explain the feedback inhibition of growth hormone release

Answer 3

Insulin-like growth factor 1 secretion is stimulated by growth hormone.
Acts directly to inhibit GH release by supressing somatotrophs.
Acts indirectly by supressing GHRH release from arcuate nucleus in hypothalamus

Question 4

What are the physiological actions of growth hormone?

Answer 4

• Acts on liver and other tissues to cause release of Insulin-like growth factor which causes increased cartilage formation and skeletal growth, increased protein synthesis and cell growth.
• It also has anti-insulin effects on fat and carbohydrate metabolism causing increased lipolysis and increased blood glucose.
  Consequences are increased linear growth and lean body mass (important for post-nasal development) and it maintains protein synthesis and tissue function in adults

Question 5

What occurs with growth hormone deficiency?

Answer 5

Dwarfism in children due to effects on linear bone growth and decreased availability of lipids and glucose for energy. It can be treated effectively with GH therapy

Question 6

What occurs with GH excess?

Answer 6

Acromegaly (often due to pituitary tumour)
Before puberty is can cause gigantism due to excess stimulation of epiphyseal plates.
After puberty is causes periosteal bone growth causing enlarged hand, jaw and foot size. Soft tissue growth leading too enlargement of tongue and coarsening of facial features and insulin resistance and glucose intolerance (diabetes). Can treat with long-acting somatostatins until transphenoidal surgery.

Question 7

What are the physical signs of acromegaly/gigantism?

Answer 7

• Gradual enlargement of hands and feet,
• Rapid growth,
• Swelling of soft tissue,
• Skin tags,
• Muscle weakness/fatigue,
• Skin changes (oiliness, acne),
• Hirsutism,
• Coarsening of facial features (forehead, nose, lips, tongue and jaw)

Question 8

What are the other symptoms of acromegaly/gigantism?

Answer 8

• Anthralgia,
• Amenorrhoea,
• Hyperhidrosis,
• Sleep apnoea,
• Headaches,
• Paraesthesia or carpel tunnel syndrome,
• Loss of libido or impotence,
• hypertension,
• thyroid disorders,
• visual feild defects

Question 9

What causes the release of ADH?

Answer 9

• Increase in blood osmolality,
• Decrease in blood volume,
• Renin angiotensin aldosterone system

Question 10

What are the effects of ADH?

Answer 10

Recruitment of aquaporin channels causing water retention

Question 11

What occurs with deficiency of ADH?

Answer 11

Diabetes insipidus (polyuria and polydipsia). Can be caused by tumours, trauma, disease or familial disorders

Question 12

Describe features of Oxytocin

Answer 12

• Produced by hypothalamic neurons in paraventricular and supraoptic nuclei. Bound by neurophysin I and carried in the axons to pos pit.

Question 13

Explain what occurs when the suckling reflex

Answer 13

Suckling of breast sends stimuli through spinal cord to hypothalamus. Neurons from spinal cord inhibit dopamine release which leads to prolactin release, spinal cord neurons also stimulate production and release of oxytocin.
Neurons from spinal cord inhibit neurons in arcuate nucleus and preoptic area causing fall in GNRH which inhibits ovarian cycle