Session 8: The HPA Axis and Growth Hormone (NOT DONE) Flashcards
Briefly explain the hypothalamic pituitary axis (HPA).
It’s a major link between the nervous and endocrine systems acting together.
Where can you find the hypothalamus?
Below the thalamus
Where is the pituitary gland?
Beneath the hypothalamus in a socket of bone at the base of the skull.
The pituitary gland is embedded in a socket of bone. What is it called?
Sella turcica
The pituitary consists of two parts. Which?
The anterior lobe also called the anterior pituitary or the adenohypophysis. The posterior lobe also called the posterior pituitary or the neurohypophysis.
What’s the major differences between the anterior pituitary and the posterior pituitary? (Origin)
The AP comes from gut ectoderm from the Rathke’s pouch and grows upward from the roof of the mouth. The PP comes from neuroectoderm and grows downwards from the diencephalon of the developing brain.
How is the hypothalamus connected to the posterior pituitary?
By a structure called the infundibulum or the pituitary stalk. It is a structure with axons going down it connecting the two.
What does the posterior pituitary synthesise?
The posterior pituitary doesn’t synthesise any hormones itself, it stores and secrete oxytocin and antidiuretic hormone (vasopressin) however.
Explain the release of oxytocin and ADH.
They are synthesised by the hypothalamus and then transported to the axon terminals of the posterior pituitary by neurocrine signalling where they are stored until the hypothalamus tells the posterior pituitary to release the hormones into the blood stream. This is done by neuronal inputs into the hypothalamus.
What stimulates the secretion of oxytocin?
The stimulus of suckling in the milk let-down reflex is transmitted via neurons from the breast to the hypothalamus resulting in release of oxytocin from the posterior pituitary. Also during childbirth the stimulus of pressure on the cervix and uterine wall is again transmitted to the hypothalamus via neuronal input and release oxytocin from posterior pituitary.
Explain the mode of travel and action of oxytocin.
It is secreted into the blood stream from posterior pituitary gland and travels to mammary glands to cause milk to be released by activating oxytocin receptors on myoepithelial cells surrounding the mammary alveoli causing them to contract squeezing milk into the duct system. In the case of childbirth it goes into general circulation and imitates powerful uterine contractions by activation of oxytocin receptors on uterine smooth muscle cells.
Why would synthetic oxytocin (pitocin) be used?
Often administered to increase uterine tone and control bleeding just after birth.
Explain how ADH works.
It causes a reduction in urine production. ADH binds to receptors that are present on the distal tubular epithelium of the collecting ducts in kidneys. When they are activated by ADH there is an increase in permeability by inducing translocation of aquaporin water channels in plasma membrane allowing more reabsorption of water back into blood.
What stimulates/inhibits ADH?
ADH is stimulated by the detection of low or high blood osmolality in the blood by osmoreceptors (this also controls thirst). A high blood osmolality stimulates secretion of ADH. A low blood osmolality inhibits secretion of ADH. Also alchohol inhibits release of ADH causing polyuria and polydipsia.
ADH also has an effect on vascular system. What?
It increases peripheral vascular resistance by activating ADH receptors on the smooth muscle cells of blood vessels to cause vasoconstriction and an increase in arterial blood pressure. Particularly important in restoring blood pressure in hypovolaemic shock during haemorrhage.
Apart from synthesising Oxytocin and ADH for release from posterior pituitary, what else does the hypothalamus synthesise?
They synthesise and secrete tropic hormones to work on the anterior pituitary which is a gland!
Define tropic hormone.
A hormone which affect the release of another hormone (stimulating or inhibitory).
What tropic hormones are synthesised and released by the hypothalamus?
TRH (Thyrotropin releasing hormone) PRH (prolactin releasing hormone) PIH (prolactin inhibiting hormone = dopamine) CRH (Corticotropin releasing hormone) GnRH (Gonadotropin releasing hormone) GHRH (Growth hormone releasing hormone) GHIH (Groth hormone inhibiting hormone = somatostatin)
Explain the mode of transport of a tropic hormone.
It is synthesised and released by the hypothalamus down axons and stored in a structure caleld the median eminence which can be found just above the anterior pituitary.
They are then released from the median eminence into a local system of blood vessels called the hypophyseal portal system. The blood vessels running away from the median eminence run directly into the anterior pituitary, this means that the AP is directly exposed to the tropic hormones which will either stimulate or inhibit target endocrine cells withing the anterior pituitary gland by binding to specific GPCRs on their surface.
What are the different types of negative feedback of the hypothalamus and AP?
There are four types:
Short loop
Indirect long loop
Direct long loop
Ultra short loop
Explain the short loop.
The hormone secreted by the AP will inhibit further release of the tropic hormone of the hypothalamus.
