The HPA Axis Flashcards
The HPA axis is a complex ——– axis
neuroendocrine
HPA Axis:
insert diagram
HPA Axis
insert slide
Hypothalamus:
- location
- relation to ventricular
system
- relation to pituitary gland
- positioned at the base of
the forebrain = diencephalic - forms the lateral walls of
the third ventricle with the
thalamus - inferior aspect/median
eminence gives rise to the
pituitary stalk, which in turn
gives rise to the posterior
pituitary gland
Which nuclei located at the base of the hypothalamus regulate anterior pituitary gland function?
tuberal nuclei
Hypothalamus components:
insert diagram
Components of the Hypothalamus:
MASTPPS
- mamillary bodies
- autonomic centers
- supraoptic nucleus
- tuberal nuclei
- preoptic areas
- paraventricular nucleus
- suprachiasmatic nucleus
What is the master endocrine gland?
the pituitary gland
Hypothalamus Components and Function:
insert slide
Lobes of the Pituitary Gland and Embryological Origins:
- anterior pituitary = adenohypophysis ->
embryonic pharynx - posterior pituitary = neurohypophysis
What connects the posterior pituitary to the hypothalamus?
The infundibulum
Pituitary Gland
insert diagram
What hormones are secreted by the anterior lobe of the pituitary gland?
- ACTH: stimulates adrenal glands
- FSH: works with LH for ovarian/testis
function - LH: works with FH for ovarian/testis
function - Growth Hormone: essential in
development for growth and affects fat
distribution/muscle mass in adults - Prolactin: stimulates breast milk
production - TSH: stimulates thyroid gland
Hormones and Target Organs:
insert diagram
Generally, anterior lobe of the pituitary gland secretes
larger peptides
*also synthesizes
Posterior Lobe of Pituitary gland Hormones:
- ADH: increases permeability of collecting
duct to absorb H2O - Oxytocin: smooth muscle contraction,
facilitating birth/milk ejection
*both are synthasised in the hypothalamus
and stored in the median eminence before
release
Posterior Lobe Hormones:
insert diagram
Pituitary Gland: Vasculature:
- capillary beds and veins form DUCTAL
system for hormone transport - arterial supply originates from the internal
carotids - the superior hypophyseal artery supplies
the primary capillary bed in the pituitary
stalk, which drains into PORTAL VEINS - the posterior lobe receives direct supply
from the inferior hypophyseal arteries,
draining into the cavernous sinus
Vasculature of the Pituitary Gland:
insert diagram
Where are the adrenal glands located?
- superior to the kidneys
- T12
What is the adrenal gland divided into and functions?
- Medulla (inner 10%) = acute stress response
- Cortex (outer 90%) = chronic stress
response, sodium/glucose homeostasis - synthesises steroid hormones
- derived from cholesterol from the diet +
cholesterol synthesised in gland
Adrenal Glands
insert diagrams
The adrenal cortex is divided into three distinct layers and function:
- outer to inner
- zona glomerulosa -> mineralcorticoids
(aldosterone) - zona fasciculata -> glucocorticoids
(cortisol, corticosterone) - zone reticularis -> androgens
(dehydroepiandosterone)
What is synthesised in the adrenal medulla?
- catecholamines
- adrenaline, noradrenaline
Adrenal Glands:
insert diagram
Synthesis of adrenocortical steroid hormones:
insert diagram
Synthesis, secretions and actions of mineralcorticoids and glucocorticoids are controlled
independently
HPA Axis:
insert diagram
Secretion of ACTH:
- manner and timings?
- affecting factors?
- pattern related to?
- pulsatile secretion (related to pulsation)
- peak in early morning, at time of waking
- nadir (lowest) in the middle of the night
- increased secretion at times of prolonged
stress - pattern is related to sleep-wake patterns:
disrupted by shift work and long-haul travel
Cortisol Secretion:
- similar to ACTH secretion, but peak and
nadir is 2 hours later - also pulsatile
Cortisol Awakening Response:
- increase of cortisol levels within the first hour
after awakening is separate from the
increase in cortisol during the second half of
the night - whilst the purpose is unclear, it is considered
to be in response to anticipated stress and
CAR provides a measure of reactivity
capacity of the HPA Axis - CAR can impaired in chronic stress
- possible: individuals with increased
vulnerability for psychosis are more sensitive
to the effects of psychosocial stress due to
abnormalities within the HPA Axis
Transport of Glucocorticoids:
- 10% - free, active form
- 90% - bound to plasma proteins:
- 75% corticosteroid-binding globulin
(CBG = single chain glycoprotein)
- 15% albumin, most abundant circulating
protein in plasma - same proteins transport other glucocorticoids
and progesterone
Cortisol levels in pregnancy:
- increase in CBG (corticosteroid-binding
globulin) - results in a compensatory increase in
circulating plasma cortisol concentrations - amount of free cortisol remains stable
What is the most physiologically important mineralcorticoid?
Aldosterone
What is the most major factor controlling the secretion of aldosterone?
RAAS (renin-angiotensin-adlsoterone)
Apart from RAAS, what is the secretion of aldosterone directly stimulated by?
- trauma
- anxiety
- hyperkalaemia
- hyponatraemia
What is the secretion of aldosterone directly inhibited by?
- Atrial Natriuretic Peptide
- ANP
What % of aldosterone during circulation is protein bound?
50%
Effect of aldosterone on sodium and potassium ion transport?
- causes expression of ion channels
- that transport sodium and potassium across
cell membrane
RAAS:
insert diagram
What is the action of aldosterone in the kidney?
- stimulates reabsorption of Na+ in distal
tubule, lesser effects in collecting duct,
ascending loop and proximal tubule - Na+ reabsorption occurs in exchange for
either K+ or H+ - control of reabsorption of Na+ influences
plasma sodium conc, which influences H2O
reabsorption in collecting duct via an effect
on ADH secretion - interaction of RAAS and ADH therefore
controls blood volume and influences blood
pressure
RAAS:
insert diagram
- interaction of RAAS and ADH therefore
controls blood volume and influences blood
pressure
Where is the growth hormone secreted from and function?
- anterior lobe of pituitary gland
- growth and affects fat distirbution/muscle
mass in adults
What are the effects of synthesising high levels of GH in adult hood?
- acromegaly
- enlargement of hands, feet, forehead, jaw
and nose
What is the most common cause of acromegaly?
Pituitary adenoma (benign neoplasia)
treat via surgery/radiotherapy
Cushing’s Syndrome: Cause:
- occurs when an individual is exposed to
excessive levels of cortisol - Endogenous Cushing’s = due to excessive
secretion of cortisol as a result of elevated
levels of circulating ACTH - elevated secretion of ACTH is most
commonly due to a pituitary tumour
Cushing’s Syndrome: Common Features:
- weight gain and adipose deposits around
midsection, upper back and face - fragile skin that bruises easily
- slow healing of wounds
- acne
Cushing’s Syndrome: Diagnosis:
- biomarkers for excessive cortisol
- check for elevated ACTH in blood for
endogenous cause
Cushing’s Syndrome: Endogenous Cause: Treatement:
- surgery
- radiotherapy
What is prolactinoma?
Excessive secretion of prolactin due to a pituitary tumour, cause of tumour is unknown
What does prolactinoma lead to?
- changes in menstrual cycle
- mood disturbances
- galactorrhoea (production of breast milk in
those who are not pregnant or recently
delivered)
What is hypopituitarism?
Reduced secretion of one, or more of the pituitary hormones and can be causes by tumours, infections, vascular failure etc
Hypopituitarism treatment:
= reversing causal mechanism eg HRT
