The hypothalamo adenohypophysial axis (2) Flashcards
What is the anatomy of the pituitary gland/hypophysis
The pituitary is attached to the base of the brain . It lies within a bone lined cavity called Sella Turcica
Why is Sella Turcica important
Sella Turcica is important from a clinical point of view because a tumour will be constrained by the walls of the bony cavity. Pituitary tumours may protrude out towards the brain or it may go through the bone (if it is really malignant)
Where is the hypothalamus
it lies just above the pituitary gland
How does the pituitary gland develop
The anterior lobe(adenohypophysis) is derived from glandular tissue. Its an extension of the buccal cavity that grows upwards and attaches to the base of the brain. The posterior lobe (neurohypophysis) is derived from neural tissue. It is formed from downward movement of tissue from the developing hypothalamus. It attaches to the anterior lobe; consists mainly of nerve axons and nerve terminals. These two tissues then fuse and normally lose contact with the rest of the buccal cavity.
Features of the hypothalamus
Regulates the endocrine system. it surrounds the 3rd ventricle in the brain. Optic Chiasma lies at the front (anterior) of the hypothalamus, and has an important role in sight. Mammillary body is at the back (prosterior) of the hypothalamus is important in the development of the nervous system
What is the link between the hypothalamus and the pituitary gland
The Median Eminence
What is present in hypothalamus
Hypothalamic nuclei. These are clusters of nerve cell bodies.
What are the two types of neurones within the hypothalamic
Neurones that pass through the region of median eminence and end at the NEUROHYPOPHYSIS within the pituitary gland. Neurones that terminate at the region of median eminence
What is the adenohypophysis mainly made up of
secretory cells
What is the neurohypophysis mainly made up of
nerve axons and nerve terminals
What is the hypothalamo-hypophysial portal system
The median eminence is essentially a mass of capillaries receiving blood form 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 blood down into portal vessels which run down through the pituitary stalk to terminate within the adenohypophysis.
What happens after the primary capillary plexus feeds blood down into portal vessels?
Blood drains to a second capillary plexus in the anterior pituitary – a portal network. From the secondary capillary plexus , blood would flow out thorough jugular veins via the cavernous sinus. This system is critical for control of anterior pituitary function.
What is a portal network
A portal network is where blood from the systemic circulation drains from one capillary network to another before returning to the heart.
Where is the primary capillary plexus
The median eminence which lies outside the blood-brain barrier meaning that the capillary walls have lots of holes so they are Fenestrated (leaky) Capillaries
Where is the seconday capillary plexus
in the anterior pituitary
Why does the hypothamalus plays a major part in controlling anterior pituitary function
There is important hypothalamic control over the anterior pituitary through hormones that are released by neurones which originate in the hypothalamus. The hypothalamic neurones which terminate in the median eminence lie on the surface of that primary capillary plexus. When stimulated they release neurosecretion (hormone). The portal system carries these chemicals to their target cells in the anterior pituitary. These hypothalamic hormones will bind to their target cells and stimulate release of anterior pituitary hormones into the circulation
What kind of neurosecretions are released from the hypothalamus
hypothalmic releasing and inhibiting hormones
What are the different adenohypophysial cells
somatotrophs, lactotrophs, thyrotrophs, gonadotrophs, corticotrophs
What do somatotrophs produce
Growth Hormone (Somatotrophin)
What do lactotrophs produce
prolactin
What do corticotrophs produce
AdrenoCorticoTrophic Hormone (ACTH, Corticotrophin)
What do thyrotrophs produce
Thyroid Stimulating Hormone (TSH, Thyrotrophin)
What do gonadotrophs produce
Luteinising Hormone, Follicle Stimulating Hormone (LH and FSH)
How are adenohypophysial hormones synthesised, stored and released
Synthesised as prohormones. Enzymatic cleavage of the prohormone yields the bioactive hormone. Hormone is stored in secretory granules and released by exocytosis into the bloodstream.
What effects can other molecules (not hormones) produced by Adenohypophysial cells
paracrine or autocrine effects
Give an example of hormone production from precursor prohormone
Example: POMC —–> CORTICOTROPHIN + Pro-gMSH + bLPH
What 3 forms can adenohypophysial hormones be
proteins, glycoproteins and polypeptides
Example of protein adenohypophysial hormones and how many amino acids they are made up of
Somatotrophin (growth hormone) - 191aa
Prolactin - 199aa
Example of glycoprotein adenohypophysial hormones and how many amino acids they are made up of
Glycoproteins consist of alpha and beta subunits. The alpha subunit is common to all, beta subunit is different and hence determines the characteristics of the molecule
Thyrotrophin (Beta subunit - 110aa)
LH and FSH (Beta subunit - 115aa)
Example of polypeptide adenohypophysial hormones and how many amino acids they are made up of
Corticotrophin (ACTH) - 39aa
What does each adenohypophysial hormone have
two hypothalamic hormones that control them: one which stimulates release of the adenohypophysial hormone and one that inhibits it
What is somatotrophin releasing hormone (SRH) /growth hormone releasing hormone (GHRH)
a hypothalamic hormone that stimulates the release of somatotrophin. It is dominant over somatostatin
What is somatostatin (SS)
a hypothalamic hormone that inhibits the release of somatotrophin
what is dopamine
a hypothalamic hormone that inhibits the release of prolactin. it is dominant over thyrotrophin releasing hormone
what is Thyrotrophin releasing hormone (TRH)
a hypothalamic hormone that stimulates the release of prolactin and thryotrophin
What are the target cells of thyrotrophin
the thyroid
What is Gonadotrophin releasing hormone (GnRH)
a hypothalamic hormone that stimulates the release of Luteinising hormone (LH) and Follicle stimulating hormone (FSH). It is dominant over gonadotrophin inhibitory hormone
What are the target cells of gonadotrophins (LH and FSH)
testes in men and ovaries in women
What are Corticotrophin releasing hormone (CRH) and Vasopressin (VP)
hypothalamic hormones that stimulate the release of ACTH (corticotrophin)
What are the target cells of somatotrophin (growth hormone)
general body tissues, particularly in the liver (endocrine functions)
What are the target cells of prolactin
breasts - lactating women
What are the target cells of corticotrophin
adrenal cortex
What is the action of somatotrophin
It stimulates various metabolic actions in body tissues and is involved in growth. It binds to receptors on the hepatocytes (liver cells) and stimulates the production of hormone from those cells. The molecules produced Somatomedins aka Insulin-like Growth Factor (IGF) I and II
What is IGF I is particularly important for
growth and development
What two ways does somatotrophin work in
Direct Effect - binding to somatotrophin receptors in general cells of the body. Hepatocytes (indirectly) - stimulating hepatocytes to produce IGF I
What are the metabolic actions of somatotrophin/growth hormone
Stimulation of amino acid transport into cells (e.g. muscle) and protein synthesis. Increased gluconeogenesis. Stimulation of lipolysis leading to increased fatty acid production. Increased cartilaginous growth and somatic cell growth
What stimulates the release of growth hormone releasing hormone/somatotrophins from the hypothalamus
Certain glucogenic amino acids (arginine). hypoglycaemia (fasting). Exercise. Stress. Oestrogen. Sleep. Ghrelin (from stomach)
What are growth hormones released as
pulses
What is the direct negative feedback in the control of somatotrophin
somatomedians (mainly IGF 1) directly stops the release of somatotrophins from the anterior pituitary.
What is the indirect negative feedback in the control of somatotrophins
somatomedians (mainly IGF 1 ) has an indirect effect on the hypothalamus to reduce the release of GHRH. Somatotrophin also has an effect on the hypothalamus
What is the neuro endocrine reflex arc for prolactin
Suckling of the breast causes stimulation of the tactile receptors. The afferent neural pathway is stimulated to higher centres. This suppresses the hypothalamic dopaminergic neurones. There is direct stimulation of thyrotrophin releasing hormone - this leads to release of prolactin. This is the efferent endocrine pathway. Prolactin causes milk production is post partum breast
