The Hypothalamic pituitary axis Flashcards
location of the hypothalamus and pituitary gland
Hypothalamus and pituitary gland form a complex functional unit that serves as the
major link between the endocrine and nervous system
what structure links the hypothalamus and pituitary gland
the infundibulum
hypothalamus and pituitary gland modulate (8)
- Body growth
- Repro
- Milk secretion
- Lactation
- Adrenal gland function
- Water homeostasis
- Thyroid gland function
- Puberty
structure of the pituitary gland
consists of two parts: anterior portion and posterior portion
anteiror pitutiary gland also known as
adenohypophysis
the a.pituitary is embryologically derived from
Rathkes pouch- primitive gut tissue
why is the posterior pituitary gland technically not a gland
since doesn’t produce the hormones that is releases- releases hormones produced in the hypothalamus
what is the p.pit also known as
neurohypophysis
the p.pit is embryologically derived from
neuroectoderm (primitive brain tissue)
p.pits anatomical position in relation to the hypothalamus
PP is physically connected to the hypothalamus, since the hypothalamus drops down through the infundibulum to form the posterior pituitary
what does the posterior pituitary release
Oxytocin and ADH produced by neurosecretory cells in supraoptic and paraventricular nuclei (collection of cell bodies with a similar function) of the hypothalamus
how are Oxytocin and ADH transported from the hypothalamus to the PP
Transported down nerve cell axons to the posterior pituitary
Prime example of neurocrine signalling (where hormone is transported down the axon of a neurone before it is released to act on a tissue)
Oxytocin and ADH are hormones because
Stored and released from posterior pituitary into general circulation to act on distant targets
oxytocin action
- Milk let down
- Uterus contraction during birth- Ferguson reflex
ADH (vasopressin) action
regultion of body water volume
the anterior pituitary endocrine function
Synthesises and releases many hormones
how is the AP controlled
by the hypothalamus
- Hormones synthesised in hypothalamus are transported down axons and stored in median eminence before release into hypophyseal portal system
- These hormones stimulate (or inhibit) target endocrine cells in the a.pituitary gland (neurocrine function)
hormones released by the AP
- TSH
- ACTH
- LH
- Prolactin
- GH
TSH
- thyroid stimulating hormone- secretion of thyroid hormone from thyroid gland
ACTH
Adrenocorticotropic hormones- secretion of hormones from adrenal cortex
LH
- Luteinising hormone- ovulation and secretion of sex hormones
PRL
- Prolactin- mammary gland development and milk secretion
GH
- Growth hormone- growth and energy metabolism- stimulates IGF
The hormones produced by nerve cells in hypothalamus act via two distance neurocrine pathways
- Direct effects on distant target tissues via oxytocin and ADH hormones from PP
- Hormones secreted exclusively onto hypophyseal portal system affect endocrine cells within the anterior pituitary
tropic
hormones that are affecting the synthesis and release of another hormone
tropic hormones of the hypothalamus
6 tropic hormones that have a direct effect on the release of anterior pitutiary hormoens
- TRH
- PRH
- PHI
- CRH
- GnRH
- GHRH
- GHIH
TRH
Thryotropin releasing hormone
- stimulates release of TSH from the AP
- stimulates the thryoid gland to release T3 and T4 to icnrease metabolic rate
PRH
prolacting releasing hormone (TRH)
- minor +ve control on prolactin release in AP
- stimulates mammary glands grow and secrete milk
PIH
prolactin inhibiting hormone
- inhibits release of prolactin from the AP
- inhibits milk secretion from mammary glands
CRH
corticotropin releasing hormone
- stimulates the AP to release Adrenocorticotropic hormones
- ACTH stimulates the adrenal glands to secrete cortisol
- stimulates stress response and metabolic actions
GnRH
gonadotropin releasing hormone
- stimulates the release of LH and FSH from the AP
- LH stimulates ovaries in females and testes in male to secrete sex hormones
- FSH stimulates ovaries in females and testes in males in gamate production
GHRH
growth hormone releasing hormone
- stimulates growth hormone to be released from the AP
- the GH stimualtes many tissues including the liver to produce IGFs to increase growth
GHIH
growth hormone-inhibitng hormone (somatostatin)
- inhibits the release of GH from the AP
- reducing growth of tissue and release of IGF from the liver
how are pathways by which hypothalamic and AP hormoens are produced regulated
most often by negative feedback
(Prolactin is under tonic inhibitory control by dopamine
Anything blocking stalk will lead to prolactin disinhibition )
feedback loop for the hypothalamic pituitary-adrenala xis
- 2 levels of feedback
- ACTH can also feedback to the hypothalamus and inhibit the release of CRH
- E.g. when cortisol levels increase, negative feedback is sent to the hypothalamus (reducing CRH release) and AP (reducing ACTH release)
name the factors which influence growth
- genetics
- nutrition
- environment
- hormones (Growth Hormoen)
hyerplasia
increase in cell number
hypertrophy
increase in cell size
atrophy
decrease in cell size and number
necrosis
cell death by damage
apoptosis
programme cell death e.g. when structures such as the hands are forming
GH structure
Protein hormone (191 aa) hass a signal peptide that must be cleaved before proper folding
what stimulates GH production in AP
GHRH (growth hormone releasing hormone)
what inhibits GH production in AP
somatostatin
growth promitng effects of GH mainly promoted
indirectly via insulin-like growth factors (somatomedins)
in response to GH the liver and s. muscle produce and secrete
IGF
function of GH in children
Essential for normal growth during childhood and teenage years
- Stimulates long bone growth
- Length and width prior to epiphyseal closure
- Width after epiphyseal closure
function of GH in adults
GH and IGF help maintain muscle and bone and promote healing, tissue repair as well as modulating metabolism and body composition
when does a surge of GH secretion occur
after onset of deep sleep
what sort of sleep decreases GH secretion
REM
what else increases GH secretion
- stress
- exercise
- less glucose or fatty acids
- fasting
what else decreases GH secretion
obesity
Feedback loop of GH
Long loop
- Mediated by IGF
- Inhibits release of GHRH from hypothalamus
- Stimulates release of somatostatin from hypothalamus
- Inhibit release of GH from anterior pituitary
Short loop
- Mediated by GH itself via stimulation of somatostain release
GH deficiency can cause
pituitary dwarfism
- Proportionate type of dwarfism
- Complete or partial deficiency
- Both types respond to GH therapy
- Height below 3rd percentile on standard growth charts
- growth rate slower than expected for age
- Delayed or no sexual development during teen years
GH excess in childhood
Gigantisms
- rare
- pituitary adenoma
GH excess in adulthood
acromegaly
- large extremities: hand, feet and lower jaw
other hormones which influcne growth
isnulin, thyroid hormones, androgens, estorgens and glucocorticoids
molecular mechanism of GH
- GH binds to janus kinase (JAK) receptors
- Activating signalling pathway which leads to the activation of transcription factors and IGF
IGF action
Act through IGF receptors (distinct from GH receptors) to modulate
- Cell growth- hypertrophy
- Cell number – hyperplasia
- Increase in rate of protein synthesis
- Increase in rate of lipolysis in adipose tissue
major IGF in adults vs for fetal growth
Adult- IGF-1
Fetal growht- IGF3
what sort of signalling does IGF do
paracrine, autocrine and endocrine
IGF has some
homology with insulin receptors
e.g. can get hybrid receptros (half insulin/ half IGF receptors)
