The hypothalamo-adenohypophysial axis Flashcards
Where does the pituitary gland sit?
Lies in a bone lined cavity - sella turcica (turkish saddle)
tumour will be constrained in the walls of the bony cavity
where is the hypothalamus with respect to the pituitary gland?
Hypothalamus lies above the pituitary gland
What are the lobes of the pituitary gland?
Anterior ( Adenohypophysis) and posterior (neurohypophysis)
Where is the posterior lobe developed from?
It is developed embryologically from neural tissue (down growth)
Where does the anterior lobe develop from?
It is not nerual tissue, it is glandular tissue- upgrowth from roof of the mouth. It has a secretory function and a bloodsupply.
Both tissues will fuse and normally lose contact with the rest of the buccal cavity
What are the hypothalamic nuclei?
In the hypothalamus, there are lots of bundles of neurones and they can be grouped functionally- hypothalamic nuclei
What is median eminence?
An area that lies between the top of the pituitary stalk and the hypothalamus
Describe this photo
The posterior lobe (neurohypophis) is made up of nerve axons.
Some of the axons comign from the hypothalamic nuclei terminate in the median eminence
Describe the blood supply in the pituitary
Median eminence is a mass of capillaries recieving blood from the superior hypophysial artery.
Lots of neurones coming from the hypothalamic nuclei terminate on the walls of the primary capillary plexus.
The primary capillary plexus feeds down portal vessels which go down the pituitary stalk to terminate within the adenohypophysis.
This is the hypothalao-hypophysial portal system.
From here the blood is gathered into the cavernous sinus and out through the jugular veins.
Primary capillary plexus- in the median eminence. It lies outside the blood-brain barrier- the capillary walls have fenestrations
Secondayr capillary plexus- in the anterior pituitary
Descritpion of the hypothalamo-hypophysial axis
- Acitvated neurones will release chemicals from their nerve endings- these should be neurotransmitters. The chemicals are released directly into the primary capillaries (hormones).
- There is important hypothalamic control over the anterior pituitary through hormones that are released by neurones which originate in the hypothalamus.
- The portal system carries the chemicals to their target cells in the anterior pituitary
- The hypothalamic hormones will bind to their target cells and stimulate the release of anterior pituitary hormones into the circulation
What does the hypothalamus do?
It is a source of release hormone and release inhibiting
What is the process for releasing adenohypophysial hormones?
What are the 5 adenohypophysial cells?
How are adenohypophysial hormones synthesised?
They are synthesised as prohormones
Enzymatic cleavage of the pro-hormone yeilds the bioactive hormone- they are stored in secretory granules and are released by exocytosis.
Adenohypophysial cells also produce other molecules which have paracrine/ autocrine effects. E.g POMC
Active hormone is corticotrophin
What are all adenohypophysial hormones?
Proteins/ glycoproteins/ polypeptides
What do the anterior pituitary hormones (adenohypophysial) need to work?
Hypothalamic hormones need to regulate them.
The blue ones stimulatory, red ones are inhibitory.
For each adenohypophysial hormone there are two hypothalamic hormones that control them: one that stimulates the release and one that inhibits it.
What are the target cells of the adenohypophysial hormones?
Describe the action of somatotrophin
It works on many tissues in the body and stimulates various metabolic actions. Liver is the main target tissue of somatotrophin.
Somatotrophin binds to receptors on the hepatocytes and stimulates the production of hormones from those cells.
The live produces somatomedins (IGF1 and IGF2- insulin growth like factor 1)
IGF is important for growth
What are the two pathways that somatotrophin takes?
- Direct effect- binding to somatotrophin receptors in general cells of the body
- hepatocytes- stimulating them to produce IGF 1
How is somatotrophin controlled?
Growth Hormone releasing hormone (GHRH) is the main one from the hypothalamus (stimulatory effect)
Somastatin (SS) has the opposite effect (inhibitory)
Certain glucogenic amino acids can stimulate growth hormone release- after a meal you have a lot of amino acids in your blood. Amino acids move into cells and increase proteinsynthesis.
Which factors stimulate GHRH release?
- Amino acids
- Hypoglycaemia (potent stimulus- the ultimate effect of growth hormone is to increase blood glucose levels)
- Exercise
- Stress
- Oestrogen
- Sleep
- Ghrelin (comes from the stomach and goes directly to the anterior pituitary gland whilst the others are stimulated from the hypothalamus)
*hypothalamic hormones are released in pulses
What are the inhibitors of somatotrophin?
You can have direct inhibition- IGF 1 back to the pituitary
Indirect inhibiton - IGF 1 goes back to the hypothalamus and inhibits production of GHRH (autonegative feedback loop)
What does somatotrophin do?
Metabolic actions (either by itself or by somatotrophin)
- Stimulation of amino acids transport into cells
- stimulation of protein synthesis
- Increased cartilaginous growth
- Stimulation of lipid metabolism leading to increased fatty acid production
- Decreased glucose utilisation resulting in increased blood glucose concentration
What is prolactin? and what are its effects?
It is a hormone that is released from the anterior pituitary gland and has many effects:
- breast lactogenesis (post-partum women)- main effect
- effects on the immune system
- increase in LH receptors
- Renal reabsorption
How does prolactin work in breast lactogenesis?
It has an effect on LH receptors on target tissues- when prolactin levels are high, it inhibits LH release.
Prolactin inhibits LH
Prolactin is released in high amounts when the baby is suckling (stimulus)- during this period, the mother would not want to have periods and become pregnant again.
How is prolactin controlled?
Baby suckles on the breast (stimulus)
There are tactil receptors on the nipple which send signals down the afferent neural pathway. This pathway ends at the hypothalamus
The two hypothalamic hormones associated with prolactin are dopamine (inhibitor) and thyrotrophin releasing hormone (stimulator) are released.
When the afferent pathway is stimulated, dopamingergic neurones are inhibited and there is direct stimulation of thyrotropin releasing hormone. This leads to release of prolactin.
Prolactin starts the synthesis of milk for the next suckling period.
Oestrogen and iodothyronines also have an effect.
What is the prolactin release an example of?
Neuroendocrine reflex arc
Suckling of breast- tactile receptors- afferent nerve pathway (neural)
higher centres- hypothalamus dopamine/trh - adenohypophysis- prolactin- milk production post partum (endocrine)
Both working together
