11.1 - Hypothalmic Pituitary Axis + Growth Hormone Flashcards
(30 cards)
What is the hypothalamic pituitary axis and what do they modulate
- The hypothalamus and the pituitary gland
- Together form a complex functional unit that serves as the major link between the endocrine and nervous system
modulate
- Body growth
- Reproduction
- Adrenal gland function
- Water homeostatis
- Puberty
- Thyroid gland function
- Lactation
- Milk secretion
The two parts of the pituitary gland
anterior PG
- Aka adenohypophysis
- Related to glands
- Mainly glandular epithelial tissue
posterior pituitary
- Aka neurohypophysis
- Consists of mainly neural secretory tissue
- Physically connected to hypothalamus (hypothalamus drops down through infundibulum to form posterior pituitary)
What protects the pituitary gland
A socket of bone called the sella turcica
Neurocrine function of the posterior pituitary
- Posterior pituitary is not a gland, only a site of release
- Oxytocin and ADH produced by neurosecretory cells in supraoptic and paraventricular nuclei of the hypothalamus
- Transported down nerve cell axons to the posterior pituitary
- stored and released from posterior pituitary into general circulation to act on distant targets
- Note: posterior pituitary doesn’t synthesise these hormones, just releases them
- neurocrine signalling = where hormones travel down axon before being released
Where do the two parts of the pituitary gland originate from embryologically
anterior pituitary arises from evagination of oral ectoderm (rathke’s pouch) – primitive gut tissue
posterior pituitary originates from neuroectoderm (primitive brain tissue)
Anterior pituitary function
- Gland as it produces the hormones it releases
- Hormones are synthesised in the hypothalamus and transported down axons
- The hormones are stored in median eminence before release into the hypophyseal portal system
- These hormones stimulate (or inhibit) target endocrine cells in the anterior pituitary gland (neurocrine function)
- Endocrine cells of anterior pituitary secrete a variety of hormones into bloodstream to act on distant target cells (endocrine function)
- Anterior pituitary hormones also effect neighbouring cells (autocrine and paracrine function)
Why is there a portal system in the anterior pituitary gland
Portal system allows for concentration of hormones without being diluted by passing through the rest of the circulatory system
The hormones produced by nerve cells in the hypothalamus act via two distinct neurocrine pathways…
- Direct effects on distant target tissues (via oxytocin and ADH) from posterior pituitary – neurocrine = passed down neurone axon before being released
- Hormones secreted exclusively into hypophyseal portal system affect endocrine cells within the anterior pituitary
Two hormones produced in the hypothalamus released from the posterior pituitary
oxytocin (OT) - milk let down and uterus contractions during birth
antidiuretic hormone (ADH) aka vasopressin - regulation of body water volume
Tropic hormones of the hypothalamus (6)
these all have direct effects on the release of anterior pituitary hormones
* TRH thyrotropin releasing hormone RELEASING
* PIH prolactin release-inhibiting hormone (aka dopamine) which has a negative effect on prolactin production INHIBITORY
* CRH corticotropin releasing hormone RELEASING
* GnRH gonadotropin releasing hormone (regulates FSH + LH) RELEASING
* GHRH growth hormone releasing hormone RELEASING
* GHIH growth hormone-inhibitory hormone (somastostatin) INHIBITORY
Tropic vs trophic hormones
trophic - affects growth
tropic - affects production + release of another hormone
Some hormones can be both eg growth hormone
Hormones produced by the anterior pituitary
the release of these are regulated by the tropic hormones of the hypothalamus (a couple cards back)
- TSH = thyroid stimulating hormone (secretion of thyroid hormone from thyroid gland)
- ACTH = adrenocorticotropic hormone (secretion of hormones from adrenal cortex)
- LH = luteinising 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 – stimulates IGFs)
Negative feedback
the pathways by which hypothalamic and anterior pituitary hormones are produced are often regulated by negative feedback
Eg hypothalamus produces H1, which stimulates anterior pituitary to secrete H2, which has a positive effect on target endocrine gland, stimulation the secretion of H3, which has a positive effect on the target tissue.
Negative feedback means that hormones H2/H3 etc could have an inhibitory effect on the hypothalamus.
Negative feedback example - the hypothalamic-pituitar-adrenal axis
Eg stimulus (such as stress, pain, fever etc) stimulates hypothalamus to produce CRH, which travels by the portal system to the anterior pituitary, stimulating the release of ACTH, which is released into the bloodstream and stimulates adrenal cortex to release cortisol, which has a positive effect on target tissues.
cortisol has a negative effect on anterior pituitary and hypothalamus – dampening CRH production at hypothalamus and ACTH release from anterior pituitary.
Endocrine control of growth
Growth is multifactorial and is influenced by many factors such as:
- Genetics
- Environment
- Nutrition
- Hormones (eg growth hormone is the most important endocrine regulator of postnatal growth)
Define necrosis and apoptosis
☞ necrosis = cell death by damage
☞ apoptosis = programmed cell death (physiological mechanism to control cell death)
Define atrophy
This term covers both a decrease in cell size and/or a decrease in cell number
Define hyperplasia and hypertrophy
hyperplasia is increase in cell number – eg in cancer
hypertrophy is increase in cell size – eg in muscle growth
Growth hormone – features
- Produced in the anterior pituitary
- Stimulated by hypothalamic GHRH
- Inhibited by hypothalamic somastostatin
- Protein hormone that has a signal peptide that must be cleaved off before proper folding
- Growth-promoting effects mainly exerted indirectly via insulin-like growth factors, aka IGFs, (somatomedins)
- In a response to GH, cells of the liver and skeletal muscle produce and secrete IGFs
What is growth hormone stimulated and inhibited by
- Stimulated by hypothalamic GHRH
- Inhibited by hypothalamic somastostatin
Growth hormone - importance for growth
essential for normal growth during childhood and teenage years
GH stimulates long bone growth
☞ before epiphyseal plate closure, bone can grow length and widthways
☞ after, can only grow wider
☞ epiphyseal plate closes during puberty
IGFs stimulate bone and cartilage growth
in adults
- GH and IGFs help to maintain muscle and bone mass
- GH and IGFs help to promote healing and tissue repair
- Also modulates metabolism and body composition
Control of GH secretion
- growth hormone releasing hormone (GHRH) stimulates GH secretion
-
somatostatin aka growth hormone inhibiting hormone decreases GH secretion
The CNS regulates GH secretion by inputs into hypothalamus, effecting GHRH and somatostatin levels
What factors stimulate and reduce GH secretion
stimulates GH secretion
- After onset of deep sleep
- Stress eg trauma, surgery or fever
- Exercise
- A decrease in glucose or fatty acids
- Fasting
decreases GH secretion
- REM (rapid eye movement sleep)
- Increase in glucose or free fatty acid
- Obesity
☞ GH secretion is regulated by long and short loop feedback (negative feedback)
Next slide
GH secretion is regulated by long loop and short loop feedback
long loop negative feedback
- Mediated by IGFs
- Inhibits release of GHRH from hypothalamus
- Stimulates release of somatostatin from hypothalamus
- Inhibits release of GH from anterior pituitary
short loop negative feedback
- Mediated by GH itself
- Via stimulation of somatostatin release
