Hypothalamo-Neurohypophysial Axis

Question 1

what supplies the posterior pituitary gland?

Answer 1

Inferior hypohyseal artery

Question 2

which hypothalamic nuclei produce neurosecretions?

Answer 2

Paraventricular and Supra-optic nuclei

note these can also be controlled by higher regions of the brain

Question 3

what lies rostral to the hypophysis?

Answer 3

Optic chiasm

Question 4

what lies caudal to the hypophysis?

Answer 4

mammillary body

Question 5

what is the difference between magnocellular and parvocellular neurones of the supraoptic and paraventricular nuclei?

Answer 5

MAGNOCELLULAR, cell bodies originate from both nuclei and have long axonal projections terminating in the neurohypophysis. vasopressinergic and oxytocinergic.

PARVOCELLULAR, cell bodies originate from the paraventricular nuclei and their shorter axonal projections terminate at the median eminence or other parts of the brain. can be vasopressinergic as VP controls corticotrophin release

Question 6

what are herring bodies?

Answer 6

they are sites of storage of neurosecretory vesicles of oxytocin and vasopressin in paraventricular and supraoptic neurones.

Question 7

similarities and differences between Arginine Vasopressin and Oxytocin.

Answer 7

Both NONAPEPTIDES. both have hexagonal high connected by disulphide bridges with tripeptide side branch.

they differ by 2 different amino acids.

Question 8

describe vasopressin synthesis

Answer 8

Preprovasopressin contains signal peptide which traffics it to golgi. this signal peptide is then cleaved producing provasopressin. this is then broken down into VASOPRESSIN, Neurophysin which prevents early breakdown and guide vasopressin to end of neurone, and glycopeptide.

Question 9

how does oxytocin production differ from vasopressin.

Answer 9

the prohormone is broken down into oxytocin, a different neurophysin and NO GLYCOPEPTIDE is produced.

Question 10

types of vasopressin receptor

Answer 10

V1 receptor- GPCR linked to PLC —> PKC and calcium

V2 receptor- GPCR linked to adenylyl cyclase —> PKA

Question 11

actions of vasopressin

Answer 11

V1a receptors: vasoconstriction in smooth muscle, glycogenolysis in hepatocytes, behavioural effects in CNS

V1b: ACTH production on corticotrophs

V2: on renal collecting duct cells to increase water reabsorption (ANTIDIURETIC) and endothelial cells to produce vWF.

Question 12

describe how vasopressin acts on a collecting duct cell

Answer 12

activation of the V2 receptor causes production of cAMP which activates protein kinase A causing aggraphores/vesicles of Aquaporin 2 (AQP2) to migrate towards the apical membrane and insert more AQP2. this allows water to flow through the AQP2 from the lumen through the cell and out into the plasma via AQP3 and AQP4 on the basolateral side.

Question 13

main functions of oxytocin

Answer 13

causes rhythmic contraction in myometrial cells in the uterus and dilates the cervix in positive feedback to deliver baby (parturition).

contraction of myoepithelial cells in breast during lactation which causes milk ejection.

Question 14

other effects of oxytocin

Answer 14

CVS- tachycardia and vasodilation UNWANTED
Renal- antidiuretic UNWANTED
CNS- tend and befriend in mothers, stimulated release of oxytocin by oestrogen (inhibited by testosterone)

Question 15

other hormones interfere with the actions of oxytocin on the uterus, what are they?

Answer 15

Progesterone surpasses effects
oestrogen enhances effects

sensitivity to oxytocin increases as gestation progresses.

Question 16

clinical uses of oxytocin

Answer 16

• induction of labour at term (iv infusion)
• stops post partum haemorrhage (iv)
• milk let down (nasal spray)
• autism for social responsiveness

Question 17

describe the vasopressin axis

Answer 17

increased plasma osmolality causes osmoreceptors to trigger release of vasopressin hence causing reabsorption of water in the kidney.

severe decrease in arterial blood pressure causes vasopressin release due to drop in baroreceptor firing rate hence reduced inhibition of AVP release. this causes increased vasoconstriction.

NEGATIVE FEEDBACK LOOPS

higher areas of the brain (eg stress) can cause vasopressin release.

Question 18

describe the neuroendocrine reflex arc for oxytocin:

Answer 18

suckling causes afferent neural pathway to stimulate production of oxytocin, it is released by the neurohypophysis into the blood stream in an efferent endocrine pathway causing milk ejection.

Question 19

dysregulation of oxytocin or vasopressin causes:

Answer 19

lack of oxytocin can be replaced by other means hence not really a problem.

lack of ADH (VP) causes diabetes insipidus which means large volumes of dilute urine (hypo-osmolar polyuria) and polydipsia (increased thirst).

Question 20

describe two causes of diabetes insipidus:

Answer 20

Cranial/cranial: lack of circulating vasopressin

Nephrogenic: resistance to vasopressin by end organs (kidney)