Hypothalamic Pitutary Axis & growth Hormone Flashcards
What is the HPA axis ?
The hypothalamus and pituitary gland form a complex functional unit that serves as a the major link between the endocrine and nervous system.
Where is the location of the Pituitary gland ?
Sits beneath the hypothalamus in a socket of bone called the SELLA TURCICA
What processes does the hypothalamus and P gland modulate ?
Body growth
Milk secretion
Lactation
Reproduction
Adrenal gland function
Water homeostasis
Thyroid gland function
Puberty
What are the two components of the pituitary gland ?
- Anterior Pituitary gland ( adenohypophysis)
2. Posterior pituitary gland ( neurohypophysis )
Is the posterior pituitary section a gland or not ?
No , because it doesn’t actually synthesis the hormones it secretes. It secretes hormones that are synthesised by the hypothalamus.
What is the embryological origin of the anterior pituitary gland ?
Arises from Evaginnation of oral ectoderm ( primitive gut tissue )
What is the embryological origin of posterior pituitary gland ?
Originated from neuroecroderm ( primitive brain tissue )
What structure connects the posterior Pituitary to the hypothalamus ?
Infundibulum
What is the neurocrine function of the posterior pituitary gland ?
- Oxytocin and ADH produced by neurosecretory cells in the supraoptic and paraventricular nuclei of the hypothalamus. More oxytocin is produced by the supraoptic nuclei than the paraventricular nuclei.
- These two hormones are transported down the nerve cell axons to the posterior pituitary.
- These hormones are then stored and released from posterior pituitary into general circulation to act on distant targets.
THIS IS AN EXAMPLE OF NEUROCRINE SIGNALLING
Outline the process by which hormones are stimulated and secreted by the anterior pituitary gland ?
- Hormones synthesised in the hypothalamus are transported down axons and stored in median eminence before released into HYPOPHYSEAL PORTSL SYSTEM ( connects one capillary bed to another )
- These hormones then stimulate or inhibit target endocrine cells in the ANterior pituitary gland. ( neurocrine function )
- Endocrine cells of anterior pituitary secrete variety of hormones into bloodstream to act on distant target cells ( endocrine function )
What is the function of oxytocin ( OT ) ?
Regulates the positive feedback loop during childbirth - causes uterus contractions during childbirth. And involved in the milk let down reflex when the baby begins to suckle at the breast.
What is the function of ADH ( vasopressin ) ?
Regulation of body water volume - promotes reabsorption of water through deposition of aqua porin channels in the collecting ducts of the kidney.
What are tropic hormones ?
Hormones that are released by the hypothalamus and affect the synthesis and release of hormones from thr anterior pituitary gland.
Name the 6 tropic hormones that are synthesised and secreted from the hypothalamus .
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 ( somatostatin )
What are the 6 hormones produced by the anterior pituitary gland ?
- TSH : thyroid stimulating hormone which is involved in the secretion of thyroid from thyroid gland.
ACTH : Adrenocorticotropic hormone : secretion of hormones from adrenal cortex
LH : Luteinising hormone : controls 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 and stimulates IGFS.
Outline the mechanism of HPA when the hypothalamus detects stress such as pain , fever , hypoglycaemia or low blood pressure ?
- Hypothalamus detects these signals
- Hypothalamus responds by secreting Corticotropin releasing hormone ( CRH )
- This acts on endocrine cells in the Anterior pituitary gland which results in the release of Adrenocorticotropic hormone ( ACTH) which acts on the adrenal cortex.
- Adrenal cortex responds by releasing cortisol.
- This has an effect on many target tissues.
What are the 4 main factors that influence growth ?
- Growth hormones
- Environment
- Nutrition
- Genetics
What are the two mechanisms by which cells can grow ?
Hypertrophy ( cell growth )
Hyperplasia ( increasing cell number )
Where is growth hormone produced and what two hormones regulate its secretion ?
Released from the anterior pituitary gland
It is stimulated by hypothalamic GHRH
It is inhibited by hypothalamic somatostatin
What type of hormone is growth hormone ?
Peptide hormone (191 aa) . It has a signal peptide that must be cleaved before folding.
What does GH stimulate ?
Stimulates the production of insulin-like growth factor from the liver ( indirect effects )
Or it could have direct effects - where it acts directly on growth hormone receptors
What is the function of growth hormone in childhood / teenage / adults ?
Growth hormone stimulates long bone growth - it lengthens and widens the bone BEFORE epiphyseal closure. Once those epiphyseal plates have been ossified , it can no longer grow in length , only by width.
In adults , GH and IGFS help maintain muscle and bone mass. And promotes tissue repair and healing.
Apart from GHRH and Somatostatin, what other factors promote GH secretion / inhibition ?
After onset of deep sleep , there is a surge in GH secretion. That is why it is important for babies to have uninterrupted sleep because it allows them to grow.
- Stress increases GH secretion.
- Exercise Increases GH secretion
- A decrease in glucose or fatty acids leads to an increase in GH secretion
- Fasting leads to an increase in GH secretion
- Obesity decreases GH secretion
What is the long loop negative feedback mediated by ?
Mediated by IGFS , which act on the hypothalamus and inhibit release of GHRH. But stimulate release of somatostatin from hypothalamus. They also act on the anterior pituitary gland which inhibits the release of GH.
What is the short loop mediated by ?
Mediated by GH itself by acting on the hypothalamus to stimulate somatostatin release.
Outline the mechanism of action of growth hormone on its receptor ?
- GH binds to a tyrosine kinase receptor .
- Binding of GH to its receptor results in dimerisation of the GHR . This leads to a conformational change of the receptor
- This leads to autophosphorylation of the Janus Kinases on the dimers.
- This activates signalling pathways.
- These signalling pathways leads to transcription factor activation and IGF production.
What are the two types of IGFS ?
IGF1 ( major growth factor in adults ) produced by the liver
IGF2 ( mainly involved in fetal growth )
They act on IGF receptors to module cell growth , cell number increase , increase in rate of protein synthesis and increase rate of lipolysis in adipose tissue.
Give an example of. Another hormone which influences growth
Androgens
These accelerate pubertal growth spurt. Increase muscle mass. Promote closure of epiphyseal growth plates.
