The hypothalamic-pituitary axis Flashcards
Anterior pituitary hormones
Prolactin LH FSH GH ACTH TSH
Posterior pituitary hormones
Oxytocin
ADH (vasopressin)
Hypothalamus to Pituitary anatomy
Hypothalamus
- Inferior to the thalamus
- Diencephalon
- Connected to the pituitary via the infundibulum
Pituitary
- Divided into anterior (adenohypophysis) and posterior (neurohypophysis)
Portal system
- Hypothalamico-hypophyseal vessels connected to the anterior pituitary
Hypothalamico-neurohypophyseal tract
- Connects the hypothalamus to the posterior pituitary
ADH
- Structure
- Synthesis
- Secretion stimulators
9 amino acids, peptide hormone
- 7 identical to oxytocin
Made in hypothalamic nuclei as inactive precursors
- Supraoptic
- Paraventricular
Secretion stimulators
- High plasma osmolarity
- Low BP
ADH action
Acts on g-protein coupled receptors
Acts on V2 receptors in the nephrons
- Causes translocation of aquaporins into the collecting duct luminal membrane = increases absorption of water
Action on V1 receptors on smooth muscle
- Stimulates vasoconstriction= increases blood pressure, reduces haemorrhage.
- Triggers angiotensin synthesis= increased thirst and aldosterone secretion
Anterior pituitary
Adenohypophysis
- Endocrine portion of the pituitary
Connected to the hypothalamus via the portal system
- Hypothalamico-adenohypophyseal vessels
- Prevents hypothalamic hormones from entering the systemic circulation.
Posterior pituitary
Nuerohypophysis
- Neural portion of the pituitary
- Down growth of the diencephalon, contains neuronal connections from the hypothalamus.
Secretes Oxytocin and ADH.
Oxytocin
- Structure
- Synthesis
- Secretion stimulators
9 amino acids
- 7 identical to ADH
- Peptide hormone
Synthesised in hypothalamic nuclei
- Supraoptic
- Paraventricular
Synthesised more in pregnancy but this is counteracted by an increase in oxytocinase.
Secretion triggered by
- Nipple stimulation
- Genital stimulation
Oxytocin action
Acts on G-protein couple receptors (Gq)
- Triggers IP3 release
IP3 causes contraction of smooth muscle
- In breast
- In genital tract
Contraction causes:
- Milk let down in breast
- Parturition (uterus contraction)
Milk let down
- Mechanism
- Stimulants
The action of releasing milk from the mammillary glands
- Contraction of smooth muscles in the breast
- Caused by release of oxytocin
Positive feedback
- Nipple stimulation= triggers oxytocin release
- Oxytocin release= SM contraction
- SM contraction= milk let down
- Milk let down= more suckling/ nipple stimulation.
Pavlovian reflex can develop
- Nipple suckling alone not required for milk let down (i.e baby crying).
Oxytocin and childbirth
Cervical stimulation caused by the baby stimulates release of oxytocin during labour.
Positive feedback
- Stimulation of cervix = release of oxytocin
- Release of oxytocin= further contraction of uterus
- Contract of uterus= more stimulation of the cervix.
Oxytocin can be used to induce labour.
Prolactin
- Structure
- Synthesis
- Secretion control
Peptide hormone- 198 amino acids
Synthesised by lactotropic cells in the anterior pituitary. Secretion stimulated by - TRH - Mild stress - Nipple stimulation
Secretion is mainly under control by inhibition of dopamine
Prolactin action
Stimulates the synthesis of milk in mammillary glands
- Lactation
Promotes breast tissue development
Inhibits fertility
- Hence when Dopamine antagonist impair fertility
Clinical investigations into the HPA
- Signs and symptoms
- Typically predicted from the actions of the hormones - Biochemical tests
- Hormones released by the hypothalamus/ pituitary
- TSH, ACTH, etc - Imaging
- Visual defect tests
- Investigates the compression of the optic tract, caused by pituitary enlargement
- Bitemporal hemianopia
Hormones measured in biochemical tests for the HPA (10)
TSH, FT4 (free T4)
Cortisol
Prolactin
FSH, LH
Testosterone
Oestrogen and progesterone
- Observed through menstruation.
