M1i: The Adrenal Medulla
- The Adrenal Medulla is innervated by PREGANGLIONIC FIBRES
- PREGANGLIONIC SYMPATHETIC FIBRE passes directly from spinal cord → via Coeliac Ganglion (no synapse) to innervate the Adrenal Medulla
- Adrenal Medulla cells are derived from embryonic neural tissue
- Their cells are like ‘POST GANGLIONIC NEURONS’
- They release NA & Adrenaline, which are NTs but act as hormones
Blood Supply
- Adrenal gland has rich blood supply
- Arterioles penetrate adrenal capsule & lose their muscular wall
- Form capillary bed to supply CORTEX
- Coalesce into medullary veins at CORTICOMEDULLARY JUNCTION
- Medulla arterioles branch down to medulla without supplying cortex
- Form capillary bed in medulla
- Empty into medullary veins
- ∴CORTICO-MEDULLARY PORTAL SYSTEM ensures there is a high [ ] of glucocorticoids (Cortisol) from Zona Fasciculata in the blood supplied to the medulla
Hormone Synthesis
- Medulla cells derived from neural crest
- Secretory cells known as CHROMAFFIN CELLS
- Formation of noradrenaline from tyrosine:
- TYROSINE → DOPA (Tyrosine hydroxylase)
- DOPA → DOPAMINE (DOPA decarboxylase)
- DOPAMINE → NA (Dopamine-β-hydroxylase)
- PNMT enzyme exclusive to Adrenal Medulla converts:
NA → Adrenaline (PNMT)
- These hormones are stored in chromaffin granules → v. high concn (0.5M)
- PNMT gene expression ↑ by CORTISOL
- Chromaffin granules also contain large amounts of ATP (0.1M) to supply E for active transport
Release
- Stress/hypoglycaemia → ↑splanchnic symp. discharge
- Preganglionic fibres supplying Adr. Medulla release ACh
- Their release causes depolarisation & Ca2+ ion secretion
- Promotes exocytosis of chromaffin granules
- Enters medullary vessels → circulation
- Prolonged effect cf. NA released at n. terminal because takes longer (10 – 30 secs) for COMT to find it & metabolise it
NB: Adr. Medulla produces other peptides: met-encephalin, Subs P, Neuropeptide Y
- It is v. important that the chromaffin granules have catecholamines ready to go
- 1° mediators of “fight-or-flight”
- Cortisol production req. several mins
- Full cortisol action takes hours