7.1- The Hypothamic Pituitary Axis Flashcards
Where does the pituitary gland sit in relation to the hypothalamus?
Sits below the hypothalamus in a socket of bone called the sella turcica
What do the hypothalamus and pituitary form together?
A complex functional unit
What are the two components of the pituitary gland?
Anterior pituitary
posterior pituitary
Is physically connected to the hypothalamus
What is the neurocrine function of the posterior pituitary?
Oxytocin and ADH
- produced by neurosecretory cells in the supraoptic and paraventricular nuclei of the hypothalamus
- transported down nerve axons to posterior pituitary
- stored and released from posterior pituitary in general circulation to act on distant target sites
What’s the function of the anterior pituitary?
Hormones made in hypothalamus are transported down axons and stored in the median eminence before released into the hypophyseal portal system
These hormones stimulate target endocrine cells in the anterior pituitary gland (neuroendocrine function)
Endocrine cells of ant pit secrete hormones into bloodstream for endocrine function
Hormones in ant pit also have autocrine and paracrine function
What are the two distinct neurocrine pathways for hormones made by nerve cells in the hypothalamus?
Direct effects on distant target tissues with oxytocin and ADH are released from posterior pituitary
Hormones secreted into hypophyseal portal system affect endocrinecells within anterior pituitary
What’s the role of ADH and oxytocin?
Oxytocin - milk and uterus contractions during birth
ADH - regulation of body water volume
What’s a tropic hormone?
Hormone that affects the release of other hormones in the target tissue
What are the 6 tropic hormones of the hypothalamus and what do they do?
TRH - thyrotropin releasing hormone PIH - prolactin release inhibiting hormone (dopamine) CRH - corticotropin releasing hormone GnRH - gonadotropin releasing hormone GHRH - growth hormone releasing hormone GHIH - growth hormone inhibiting hormone
What do tropic hormones from the hypothalamus effect?
Have direct effects on the release of anterior pituitary hormones
What hormones are produced by the anterior pituitary?
TSH - thyroid stimulating hormone
( secrete thyroid hormone from thyroid gland)
ACTH - adrenocorticotropic hormone
( secreting of hormones from adrenal cortex)
LH - leutinising hormone
( ovulation and secretion of sex hormones)
FSH - follicle stimulating hormone
( development of eggs and sperm)
PRL - prolactin
( mammary gland development and milk secretion)
GH - growth hormone
( growth and energy metabolism)
How would the negative feedback loop of the release of corticotropin releasing hormone work?
1) stimulus e.g pain, fever, low blood pressure
2) change detected by hypothalamus
3) corticotropin releasing hormones (CRH) goes to ant pit
4) ant pit releases Adrencocorticotropic hormone
5) goes to adrenal cortex and cortisol is released
6) levels of cortisol rise = less CRH produced by hypothalamus
What is hyperplasia?
Cell growth due to increase in number
How tumours grow
What is growth hormone responsible for
- stimulates long bone growth = length and width prior to epiphyseal closure, width after epihyseal closure
- in adults GH and IGF help maintain muscle and bone mass and promote healing and tissue repair as well as modulating metabolism and body composition
What inhibits growth hormone?
Somatostatin
How do growth promoting effects exert?
Via insulin like growth factors (somatomedin)
In response to GH cells of the liver and skeletal muscles produce and secrete IGF
What movements cause levels of growth hormone to fluctuate?
Increase
- surge in GH after deep sleep
- surge if stressed
- exercise increases
- fasting: decrease in glucose or fatty acids
Decrease
- rapid eye movement sleep = decrease in GH
- increase in glucose or free fatty acids = decrease in secretion
- obesity
How is GH secretion regulated in its negative feedback loop?
Long loop
- mediated by IGF
- inhibit release of GHRH from hypothalamus
- stimulates the release of somatostatin from hypothalamus
- inhibit release of GH from anterior pituitary
Short loop
- mediated by GH itself via stimulation of somatostatin release.
What is the consequence of growth hormone deficiency?
In childhood, results in pituitary dwarfism
- proportionate type of dwarfism
- complete or partial deficiency
- both types respond to GH therapy
- delayed or no sexual development during teen years
What are the consequences of growth hormone excess?
In childhood, results in gigantism
In adulthood leads to acromegaly = large extremities
- hands
- feet
- lower jaw
How does GH exert its effects on cells?
GH receptors activate Janus kinases (JAK)
- binding causes phosphorylation of JAK and after phosphorylation of GH receptor
- activation of signalling pathways
- transcription factor activation and IGF production
What are insulin like growth factors?
Main types
- IGF1 (fetal growth) and IGF2 (adults)
- binding proteins modlate their availability
- actions can be paracrine autocrine and endocrine
- act through IGF receptors to modulate hypertrophy, hyperplasia, increase the rate of protein synthesis, increase the rate of lipolysis.
What hormones other than GH influence growth?
- insulin:interacts with IGF
- thyroid hormones: enhance GH secretion
- androgens: accelerate pubertal growth spurt, increase muscle mass
- estrogens: decrease somatic growt, promote closure of epiphyseal plates
- glucocorticoids: inhibit somatic growth
