The HPA Axis (2-3) Flashcards
What is the HPA axis?
Hypothalamus, pituitary gland, adrenal gland
→ core of the neuroendocrine system
→ regulates many body processes in response to stress
→ controls interplay of the body’s hormones and neurotransmitters
→ maintains homeostasis: osmolarity, glucose, other fuels, body temp
→ makes large and rapid changes to internal physiology: threat, mood, emotion
What are the inputs into the hypothalamus?
Complex
→ some have a positive drive
→ there’s sites of corticosteroid -ve feedback
→ there are neuronal populations known to rapidly respond to corticosteroids
Are circadian rhythms and corticosteroid secretion connected?
Corticosteroids are secreted rhythmically displaying ultradian and circadian patterns
→ lowest corticosteroid at night
Where is the pituitary gland situated?
Situated in bony base of the skull, connected to the brain via infundibulum (the pituitary stalk)
→ close spacial proximity to the hypothalamus
How is the anterior pituitary structured?
Pars tuberalis
Pars intermedia
Pars distils
How is the posterior pituitary structured?
Infundibular stalk → hypothalamic neurosecretory cells originate in the hypothalamus an extend their axons down through the pituitary stalk terminating in the posterior pituitary
→ the axons bundled together forming the hypothalamo-hypophyseal tact
Pars nervosa → receives direct input from neurones
Pituicytes → supportive glial cells, provide structural support
How does the histology of the pituitary differ?
Anterior → clusters of cells - secretory
Posterior → small blood vessels, close to herring bodies (store oxytocin and vasopressin)
Pars intermedia → basophilic cells
What is the function of oxytocin?
Peptide hormone produced by the hypothalamus
Released from the posterior pituitary - pars nervosa
→ from the neurosecretory cells in the supraoptic nucleus
Controls uterus contractions, milk ejection, emotional component, bonding, neurotransmitter, stress reduction - broad functions
What is the function vasopressin?
aka antidiuretic hormone (ADH)
Produced by the hypothalamus
Released from the posterior pituitary - pars nervosa
→ from the neurosecretory cells in the paraventricular nucleus
Controls water reabsorption → acts on kidneys to increase water reabsorption makes collecting ducts more permeable, reducing the amount of water lost in urine
Vasoconstriction → causing them to narrow and increase in peripheral vascular resistance - blood pressure regulation, increasing blood flow to vital organs
How does the gene structure and processing differ between oxytocin and vasopressin?
Very similar structure - differ in 2 amino acids
Closely related neuropeptides that are synthesised in the hypothalamus (by magnocellular neurons) and released by the posterior pituitary
OXT gene located on chromosome 20 → cysteine-tyrosine-isoleucine-glutamine-asparagine-cysteine-proline-leucine-glycine
AVP on chromosome 12 → cysteine-tyrosine-phenylalanine-glutamine-asparagine-cysteine-proline-arginine-glycine
Oxytocin → neurophysin I
Vasopressin → neurophysin II
→ carrier protein associated with their transport, packaged together into secretory vesicles, stabilises and protects from degradation
Why is neurophysin II synthesised with vasopressin?
Neurophysin II closely associated with expression of vasopressin (VP) during processing of precursor into mature hormone
→ carrier protein - keeps VP stable, plays roles in release
What is the function of vasopressin (anti-diuretic hormone)?
Primary effect → increase water retention/decrease urine output
AVP V2 receptor → G-protein coupled receptor in the kidneys
High conc. cause arteriole restriction and increased pressure
Linear relationship between plasma [ADH] and plasma osmolarity → at a certain level feeling of thirst
What are some things controlling vasopressin?
→ fall in blood volume (e.g. haemorrhage)
→ arterial pO2 and pCO2
→ other hormones: angiotensin II, epinephrine, cortisol, oestrogen
→ CNS control: pain, trauma, nausea
→ increased ambient temperature
→ alcohol absorption
What is diabetes insipidus?
a disease in which the secretion of or response to the pituitary hormone vasopressin is impaired, resulting in the production of very large quantities of dilute urine, often with dehydration and insatiable thirst.
What is the anterior pituitary pars distalis?
Part of the anterior pituitary (composed of 3 main parts - pars distalis, pars tuberalis and pars intermedia)
→ consists of clusters of hormone-secreting cells called glandular epithelial cells which secrete the majority of the hormones released by the anterior pituitary
→ including: growth hormone, thyroid-stimulating hormone, FSH, LH, prolactin
→ acidophillic cells produce GH and PRL
→ basophilic cells produce TSH, FSH, LH…
