Hypothalamo-adenohypophysial System Flashcards
What is the bony shelf that the pituitary is found in?
Sella Turcica.
What is the clinical importance of the Sella Turcica?
- A tumour will be constrained by the walls of the bone cavity.
Pituitary tumours can: - protrude out towards the brain
or - go through the bone if really malignant
Where is the hypothalamus located?
- Just above the pituitary gland
Which ventricle of the brain does the hypothalamus surround?
3rd ventricle
What are the two areas of the brain that are useful anatomical markers for distinguishing the posterior pituitary from the anterior?
Anterior (front) - optic chiasma
Posterior (back) - mammillary body
Anterior Lobe = adenohypophysis
Posterior Lobe = neurohypophysis
Describe the embryological origin of the pituitary gland.
Originates from:
Glandular Tissue:
- Extension of buccal cavity starts growing upwards
Nervous Tissue:
- Downwards movement of tissue from developing hypothalamus
- Both tissues fuse
- Eventually, the connection to the rest of the buccal cavity is cut off.
Adenohypophysis = derived from glandular tissue Neurohypophysis = derived from neural tissue
What are hypothalamic nuclei?
They are groups of neuronal cell bodies that can be grouped functionally
Where is the median eminence?
It is located between the top of the pituitary stalk and the hypothalamus.
Where is the Primary Capillary Plexus located?
Median Eminence (a mass of capillaries which receives blood from the superior hypophysial artery)
Where is the Secondary Capillary Plexus located?
In the anterior pituitary (adenohypophysis)
What connects the primary capillary plexus to the secondary?
Connected via the hypothalamo-hypophysial portal system
What is the hypothalamo-hypophysial portal system?
- Lots of neurons coming from the hypothalamic nuclei terminate on the walls of the primary capillary plexus
- Primary capillary plexus feeds blood down into the portal vessels which run down through pituitary stalk to terminate within the adenohypophysis
What happens to the blood supply after the hypothalamo-hypophysial portal system?
- The blood from the secondary capillary plexus is gathered into the cavernous sinus and out through the jugular veins
What is a key feature of the primary and secondary capillary plexus?
The capillaries are fenestrated (there are small holes in it)
Describe how the hypothalamus exerts control over the adenohypophysis.
- Some neurons from the hypothalamus terminate on the median eminence at the primary capillary plexus
- The chemicals are released directly into the primary capillaries and so they are actually HORMONES and not neurotransmitters
- There is important hypothalamic control over the anterior pituitary through hormones released by neurons originating in the hypothalamus
- The hypothalamo-adenohypophysial portal system carries these chemical to target cells in the anterior pituitary
- These hypothalamic hormones bind to adenophysial cells and stimulate release of anterior pituitary hormones into the circulation.
What are the FIVE types of adenohypophysial cells and what hormones do they produce?
- Somatotrophs - somatotrophin (growth hormones)
- Lactotrophs - prolactin
- Gonadotrophs - LH and FSH (Luteinizing Hormone, Follicle Stimulating Hormone)
- Thyrotrophs - Thyrotrophin (Thyroid Stimulating Hormone -TSH)
- Corticotrophs - corticotrophin (ACTH)
How are adenohypophysial hormones synthesised?
- Synthesised as prohormones
- Enzymatic cleavage of prohormone = bioactive hormone
- Stored in secretory granules and released by exocytosis
What are the THREE types of adenohypophysial hormone? Give examples of each.
Protein
- somatotrophin, prolactin
Glycoprotein have an alpha chain (same in all) and a beta chain (different)
- FSH, LH, thyrotrophin
Polypeptide
- ACTH
State the EIGHT hypothalamic hormones and the adenohypophysial hormones that they control.
- Somatotrophin releasing hormone (SRH) + somatostatin (SS)
= somatotrophin - Dopamine (DA) + Thyrotrophin releasing hormone (TRH)
= prolactin - Gonadotrophin releasing hormone (GnRH) + Gonadotrophin inhibitory hormone (GnIH)
= FSH + LH - TRH
= thyrotrophin - Corticotrophin releasing hormone (CRH) + vasopressin (VP)
= corticotrophin
Which of the above hormones are inhibitory?
Dopamine and Somatostatin are both INHIBITORY
Which hypothalamic hormone is dominant over its counterpart?
- SRH - dominant over somatostatin
- Dopamine - dominant over TRH
- GnRH - dominant over GnIH
State the tissues that the FIVE adenohypophysial hormones act on.
- Somatotrophin
- general body cells
- (hepatocytes in particular) - Prolactin
- breasts in lactating women
3.Thyrotrophin - thyroid
4. Gonadotrophin
- gonads:
men = testes
women = ovaries
5.Corticotrophin - adrenal cortex
Describe the two modes of action of somatotrophin
- Direct effect:
Somatotrophin can act directly on tissues by binding to somatotrophin receptors in general cells of the body
OR
- Hepatocytes:
Can act on hepatocytes and stimulate hepatocytes to produce IGF 1, which is particularly important in growth
What are the metabolic actions of somatotrophin
- Somatotrophin stimulates:
- amino acid transport into cells
- protein synthesis, increases
- lipid metabolism leading to increased fatty acid production - Increased cartilaginous growth
- Decreased glucose utilisation resulting in increased blood glucose concentration.
What stimulates somatotrophin release?
Sleep, stress, oestrogens, exercise, fasting (hypoglycaemia), amino acids, ghrelin (from stomach)
What inhibits somatotrophin release?
What has a negative effect on somatotropin release?
Direct - IGF 1 back to the pituitary
Indirect - IGF 1 back to the hypothalamus
Somatotrophin itself and somatomedins have a negative feedback effect on somatotrophin production.
What is the main effect of prolactin?
Prolactin stimulated lactogenesis in the breast.
What are some other effects of prolactin?
- Prolactin is also a natural contraceptive as it inhibits LH release
- It also upregulates LH receptors in the gonads. It decreases sexual behaviour
- Has effects on the immune system
Describe the neuroendocrine reflex of prolactin.
- There are tactile receptors in the nipple that are stimulated by the suckling of the baby
- Has an afferent pathway to the hypothalamus and stimulates:
- Release of thyrotrophin releasing hormone
- Inhibits dopaminergic neurons.
- Leads to release of prolactin
- Prolactin starts the synthesis of milk for the next suckling period.
