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
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