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Flashcards in The hypothalamic-pituitary axis Deck (18)
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1

Anterior pituitary hormones

Prolactin
LH
FSH
GH
ACTH
TSH

2

Posterior pituitary hormones

Oxytocin
ADH (vasopressin)

3

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

4

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

5

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

6

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.

7

Posterior pituitary

Nuerohypophysis
- Neural portion of the pituitary
- Down growth of the diencephalon, contains neuronal connections from the hypothalamus.

Secretes Oxytocin and ADH.

8

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

9

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)

10

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).

11

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.

12

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


13

Prolactin action

Stimulates the synthesis of milk in mammillary glands
- Lactation

Promotes breast tissue development

Inhibits fertility
- Hence when Dopamine antagonist impair fertility

14

Clinical investigations into the HPA

1. Signs and symptoms
- Typically predicted from the actions of the hormones

2. Biochemical tests
- Hormones released by the hypothalamus/ pituitary
- TSH, ACTH, etc

3. Imaging

4. Visual defect tests
- Investigates the compression of the optic tract, caused by pituitary enlargement
- Bitemporal hemianopia

15

Hormones measured in biochemical tests for the HPA (10)

TSH, FT4 (free T4)

Cortisol

Prolactin

FSH, LH

Testosterone

Oestrogen and progesterone
- Observed through menstruation.

16

Growth hormone
- Structure
- Synthesis
- Control of secretion

Peptide hormone
- Synthesised by somatotrophic cells in the anterior pituitary gland.

Hormonal secretion
- Triggered by GHRH mainly
- Inhibited by Growth hormone inhibiting hormone [GHRH]/ somatostatin.

Factors that control secretion
- Amino acids increase secretion
- Release inhibited by carbs and fatty acids.

17

GH action

Linear growth in adolescents

Protein synthesis and collagen deposition---> wound healing
- Increases tissue turnover

Triggers the activation of insulin like growth factor (IGF) in the liver

Stimulates
- Lipolysis
- Increased amino acid transport into tissues
- Increased protein and glucose synthesis in the liver

18

Insulin-like GF

Secondary messenger in hepatocytes triggered by the stimulation of GH.

Disruption in IGF-1 genes causes growth retardation- acromegaly.

IGF-1= growth in fetus