Supraoptic and paraventricular magnocellular neurones
Hypothalamus · ADH Oxytocin Neurophysin
Magnocellular cell bodies in the supraoptic (mainly vasopressin) and paraventricular (mainly oxytocin) nuclei make the two posterior hormones with their neurophysins and transport them down the hypothalamo-hypophyseal tract. No portal system is involved.
Detail
- Sensor
- Osmoreceptors in the organum vasculosum of the lamina terminalis respond to a 1% change in plasma osmolality; ADH release begins near 280-285 mOsm/kg and thirst near 290-295
- Override
- Baroreceptor and volume afferents override osmotic control once volume falls by about 8-10%, which is how hypovolaemia produces hyponatraemia
- Structure
- Both are nonapeptides differing at two residues, which is why oxytocin has weak antidiuretic activity
- Store
- The posterior lobe holds enough hormone for roughly a month, so damage confined to the lobe causes only transient polyuria
When it goes wrong
Lesion of the hypothalamic nuclei or high stalk (germinoma, craniopharyngioma, Langerhans cell histiocytosis, trauma)
Permanent arginine vasopressin deficiency; a low stalk or posterior lobe lesion alone is usually transient because the cell bodies survive
Transsphenoidal pituitary surgery
Triphasic response: polyuria for days, then an antidiuretic phase as stored hormone leaks (hyponatraemia around day 7-10), then permanent deficiency if neurones are lost
Practise this structure
1 question in the bank tagged Antidiuretic Hormone.