3 - Hypothalamo-Neurohypophysial System Flashcards
What is a hypothalamic nucleus?
A collection of cell bodies that send their axons to a particular place
What are supraoptic neurones?
- magnocellular
- contain herring bodies
- either oxytocinergic or vasopressinergic
What are the two main hypothalamic nuclei and where are they located?
- paraventricular: next to 3rd ventricle
- supra-optic: above optic chiasm
What are the 2 types of neurones from the hypothalamic nuclei and where do they travel?
- 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
What are herring bodies?
- sites of storage in neurones
- contain hormones
- found along axons
What are paraventricular neurones?
- mainly magnocellular
- sometimes parvocellular
What is the difference in structure between arginine vasopressin and oxytocin?
- AVP has phenylalanine (Phe) and arginine (Arg)
- oxytocin has isoleucine (Ile) and leucine (Leu)
What is the synthesis process of vasopressin?
- pre-provasopressin cleaved to pro-vasopressin
- pro-vasopressin cleaved to vasopressin, neurophysin and glycopeptide in nerve axon
How does the synthesis of oxytocin differ to that of vasopressin?
- different neurophysin produced
- no glycopeptide produced
- pre-prooxytocin splits in 2
What is the purpose of neurophysin?
- protects hormone
- ensures hormone reaches end of axon
Which two enzymes are produced in the posterior pituitary and what are they?
- AVP (arginine vasopressin)
- oxytocin
- both are nonapeptides
What are the physiological actions of oxytocin?
- 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)
How do V1 vasopressin receptors work?
- 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
What are the two types of V1 vasopressin receptors, where are they found and what do they bring about?
- V1a: arterial smooth muscle (vasoconstriction), hepatocytes (glyogenolysis) and CNS neurones (behavioural)
- V1b: corticotrophs (ACTH production)
What is the mechanism of action of V2 receptors?
- 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)
What is the purpose of aquaporins (AQ2)?
- 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
What is the difference between polyuria and polydipsia?
- polyuria: frequent urination
- polydipsia: frequently drinking water
How is vasopressin used in the homeostatic control of water?
- 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
How is vasopressin used in the homeostatic control of BP?
- vasopressin causes vasoconstriction
- vasoconstriction increases BP
- increased BP increases baroreceptor firing rate
- increased firing rate inhibits vasopressin production
How is oxytocin used in the homeostatic control of lactation?
- 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
What are the features and signs of diabetes insipidus?
- cranial: absence of circulatory vasopressin
- nephrogenic: kidney resistance to ADH
- signs: polyuria, polydipsia and dilute urine
