3 - Hypothalamo-Neurohypophysial System

Question 1

What is a hypothalamic nucleus?

Answer 1

A collection of cell bodies that send their axons to a particular place

Question 2

What are supraoptic neurones?

Answer 2

• magnocellular
• contain herring bodies
• either oxytocinergic or vasopressinergic

Question 3

What are the two main hypothalamic nuclei and where are they located?

Answer 3

• paraventricular: next to 3rd ventricle

- supra-optic: above optic chiasm

Question 4

What are the 2 types of neurones from the hypothalamic nuclei and where do they travel?

Answer 4

• magnocellular: extend into posterior pituitary from hypothalamus (pass through median eminence and terminate in neurohypophysis
• parvocellular: pass to other parts of brain or terminate in median eminence

Question 5

What are herring bodies?

Answer 5

• sites of storage in neurones
• contain hormones
• found along axons

Question 6

What are paraventricular neurones?

Answer 6

• mainly magnocellular

- sometimes parvocellular

Question 7

What is the difference in structure between arginine vasopressin and oxytocin?

Answer 7

• AVP has phenylalanine (Phe) and arginine (Arg)

- oxytocin has isoleucine (Ile) and leucine (Leu)

Question 8

What is the synthesis process of vasopressin?

Answer 8

• pre-provasopressin cleaved to pro-vasopressin

- pro-vasopressin cleaved to vasopressin, neurophysin and glycopeptide in nerve axon

Question 9

How does the synthesis of oxytocin differ to that of vasopressin?

Answer 9

• different neurophysin produced
• no glycopeptide produced
• pre-prooxytocin splits in 2

Question 10

What is the purpose of neurophysin?

Answer 10

• protects hormone

- ensures hormone reaches end of axon

Question 11

Which two enzymes are produced in the posterior pituitary and what are they?

Answer 11

• AVP (arginine vasopressin)
• oxytocin
• both are nonapeptides

Question 12

What are the physiological actions of oxytocin?

Answer 12

• contraction of myometrial cells for delivery of baby (fundus to cervix)
• dilates cervix (suppressed by progesterone and enhanced by oestrogen)
• contraction of myoepithelial cells for milk ejection during lactation
• constricts umbilical arteries and veins (cardiovascular)
• anti-diuretic (vasopressin like effects in renal)

Question 13

How do V1 vasopressin receptors work?

Answer 13

• linked via G proteins to phospholipase C (acts on membrane phospholipids to produce IP3 and DAG)
• IP3 and DAG increase cytoplasmic Ca2+ to produce a cellular response

Question 14

What are the two types of V1 vasopressin receptors, where are they found and what do they bring about?

Answer 14

• V1a: arterial smooth muscle (vasoconstriction), hepatocytes (glyogenolysis) and CNS neurones (behavioural)
• V1b: corticotrophs (ACTH production)

Question 15

What is the mechanism of action of V2 receptors?

Answer 15

• linked via G proteins in adenyl cyclase (acts on ATP to form cAMP)
• cAMP activates PKA, activates intracellular mediators to produce a cellular response (e.g. aquaporins - AQP2)

Question 16

What is the purpose of aquaporins (AQ2)?

Answer 16

• found in vesicles (aggraphores)
• migrate and insert into apical membrane
• tubule lumen side (apical membrane) of collecting duct cell to allow water molecules in
• found in collecting duct cells and endothelial cells

Question 17

What is the difference between polyuria and polydipsia?

Answer 17

• polyuria: frequent urination

- polydipsia: frequently drinking water

Question 18

How is vasopressin used in the homeostatic control of water?

Answer 18

• osmoreceptors in hypothalamus detect increased plasma osmolality (cause secretion of vasopressin)
• vasopressin increases CD cell membrane permeability to water (increases water reabsorption)
• increased water reabsorption decreases plasma osmolality and vasopressin release

Question 19

How is vasopressin used in the homeostatic control of BP?

Answer 19

• vasopressin causes vasoconstriction
• vasoconstriction increases BP
• increased BP increases baroreceptor firing rate
• increased firing rate inhibits vasopressin production

Question 20

How is oxytocin used in the homeostatic control of lactation?

Answer 20

• receptors around nipple detect sucking
• response sent via neural afferent limb of neuroendocrine reflex arc
• more oxytocin released as response
• oxytocin travels down endocrine efferent limb and causes more milk ejection

Question 21

What are the features and signs of diabetes insipidus?

Answer 21

• cranial: absence of circulatory vasopressin
• nephrogenic: kidney resistance to ADH
• signs: polyuria, polydipsia and dilute urine