Endo 3- The hypothalamo-neurohypophysial system Flashcards Preview

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Flashcards in Endo 3- The hypothalamo-neurohypophysial system Deck (17):
1

What are the two HYPOTHALAMIC NUCLEI are associated with the POSTERIOR PITUITARY GLAND

o PARAVENTRICULAR nucleus; axons pass through median eminence and terminate in the
NEUROHYPOPHYSIS
o SUPRAOPTIC nucleus; axons pass through the median emninence and terminate just above the
OPTIC CHIASMA in the neurohypophysis

2

What are the two types of neurones within the paraventricular nucleus

o MAGNOCELLULAR neurones are larger (and are the majority), and pass through the median
eminence and terminate on the neurohypophysis
o PARVOCELLULAR neurones are smaller, and either terminate on the PRIMARY CAPILLARY PLEXUS
at the median eminence, or in other parts of the CNS (acting as neurotransmitters)

3

What are type of nuerones are supraoptic

magnocellular

4

Describe Magnocellular Neurones (3)

o Larger than parvocellular
o Terminate in the neurohypophysis, i.e. pass through the median eminence
o Have “swellings” along their axons, known as HERRING BODIES- Herring bodies are granules that accumulate the newly synthesised hormones within the axon/dendrites, forming swellings which then release the hormones into the general circulation

5

Describe synthesis of vasopressin

 Precursor molecule (pre-prohormone) is PRE-PROVASOPRESSIN
 This is synthesised and then processed in granules (like the herring bodies) to form PRO-VASOPRESSIN
(pro-hormone)
 Pro-vasopressin is then further processed to form:
o VASOPRESSIN (AVP- arginine vasopressin)
o NEUROPHYSIN proteins (NP- role unknown)
o GLYCOPEPTIDE (GP- role is being researched currently)
 These products are formed in EQUIMOLAR amounts
 Released as NEUROSECRETIONS of hormone and neurophysin proteins

6

Describe the synthesis of oxytocin

Oxytocin synthesis has the same sequence of events as vasopressin synthesis, although the neurophysin differs slightly, and the glycopeptide is absent.

7

Describe the structure of Oxytocin/vasopressin

 Both vasopressin and oxytocin exhibit a RING structure with 6 AMINO ACIDS
 They also have a small attached CHAIN or PRE-AMINO ACIDS
 Differ by 2 amino acids
 Difference within the ring structure; Vasopressin has Phe replaced by Ile in Oxytocin
 Difference within the chain structre; Vasopressin has Arg replaced by Leu in Oxytocin

8

Actions of Vasopressin (6)

o ANTIDIURETIC effect
o Vasoconstriction -- of smooth muscle in the vascular system, particularly arterioles
o Corticotrophin (ACTH) release (together with CRH) -- by PARVOCELLULAR neurones
o CNS effects -- neurotransmitters affect behaviour- receptors in HYPOCAMPUS
o Synthesis of blood clotting factors (VIII and Von Willbrandt factor)
o Hepatic Glycogenolysis- STRESSORS increase blood glucose concentration, therefore can be said
that stressors lead to VP secretion

9

Describe the effect of vasopressin on v1 receptors

- G-protein linked receptor which activate PHOSPHOLIPASE C (enzyme)
- Phospholipase C acts on membrane phospholipids to produce INOSITOL TRIPHOSPHATE; IP3 and DIACYL
GLYCEROL; DAG
- IP3 and DAG increase free cytoplasmic Ca2+ and other intracellular mediators (PKC)  cellular respons

o V1a receptor locations:
 Vasoconstriction in Vascular smooth muscle
 Glycogenolysis Hepatocyte
 CNS parvocellular neurones behavioural effects

o V1b (Also known as V3) receptor location:
 Adenohypophysial corticotrophs leading to ACTH production

10

Describe the effect of vasopressin on v2 receptors

- G-protein linked receptor which activate ADENYL CYCLASE (enzyme)
- Adenyl cyclase catalyses the conversion of ATP  c-AMP
- C-AMP acts as a SECOND MESSENGER MOLECULE to activate PROTEIN KINASE A (PKA)
- Protein kinase A activates other intracellular mediators  cellular response
- Cellular response: synthesis of AQUAPORINS, especially AQP2.
- AQP2 is a water protein-channel which is needed for water reabsorption in the kidney collecting duct,
which is VAOPRESSIN DEPENDENT and present in the APICAL MEMBRANE of principal cells
o Receptor location:
 Collecting duct cells  water reabsorption
 Blood clotting factors (VIII and Von Willbrandt factors)

11

Describe the receptors associated with v2 receptors

 V2 receptor lies on BASOLATERAL membrane (G-protein linked receptor with adenylate cyclase)
 Osmotic gradient across cell increases from tubule lumen to the plasma
 Synthesis of APQ2  migration of AGGRAPHORES to APICAL membrane (facing lumen) and insertion of
APQ2 into membrane
 AQP2 then acts as protein channel for water absorption from the tubule lumen into the cell
 AQP3 and AQP4 lie in the BASOLATERAL MEMBRANE, and act as protein channel for water transport out of the cell

12

What are the two roles of vasopressin

1. Water reabsorption
2. Vasoconstriction

13

Describe the response to increased plasma osmolarity

- Stimulus: increased plasma osmolarity (esp. Increase in Na+ conc), therefore water leaves collecting duct
cell
- OSMORECEPTORS in the brain respond to this increase:
o Increase activation of neurones
o Increase vasopressin secretion into general circulation
o Increased water reabsorption into nephron
- Response: decreased plasma osmolarity

14

Describe the response to decreased arterial blood pressure

- Stimulus: decreased arterial blood pressure
- BARORECEPTORS in CAROTID SINUS and AORTIC ARCH decrease frequency of stimulus
o Decreased stimulus on SYMPATHETIC nervous system
o Increased vasopressin secretion
o Increased vasoconstriction
- Response: increased arterial blood pressure

15

Actions of oxytocin (2)

 UTERUS at PARTURITION causes MYOMETRIAL cells contract leads to Delivery of baby

 BREAST during LACTATION causes MYOEPITHELIAL cells contract leasds to Milk ejection

16

Describe response to suckling

 Stimulus: suckling
 Receptors: around nipple
o Activation of the NEURAL AFFERENT LIMB
o Increased oxytocin release from the neurohypophysis
o Activation of the ENDOCRINE EFFERENT LIMB
 Response: Milk ejection

17

describe Diabetes insipidus

 Lose too much water therefore excess urine
 CENTRAL diabetes insipidus is caused by NO VASOPRESSIN
 NEPHROGENIC diabetes insipidus is caused by TISSUE INSENSITIVITY
 Net result is the same: DIURESIS, POLYURIA and POLYDIPSIA
 Polyuria: excessive urine
 Polydipsia: excessive drinking