MEH 15 - The HPA Axis + Growth Hormone Flashcards
What is the hypothalamic pituitary (HPA) axis?
- A complex functional unit of the hypothalamus and pituitary gland that serves as the major link between nervous and endocrine systems.
- It modulates a wide variety of processes
Describe the anatomy of the HPA axis
- The hypothalamus is connected to the posterior pituitary gland via the infundibulum (pituitary stalk).
- The pituitary gland is split into the anterior and posterior pituitary (adeno + neurohypophysis).
- Anterior is derived from primitive gut tissue, posterior from the neuroectoderm.
What is the neurocrine function of the posterior pituitary?
- Oxytocin + ADH synthesised by neruosecretory cells in supraoptic + paraventricular nucleus in hypothalamus, transported down nerve cell axons in posterior pituitary
- They’re stored + released from posterior pituitary, NOT synthesised here.
What is the function of the anterior pituitary gland?
- Hormones (releasing) synthesised in the hypothalamus stored in the median eminence before being released into the hypophyseal portal system.
- These releasing hormones stimulate (or inhibit) endocrine cells in the anterior pituitary
- Endocrine cells of anterior pituitary release a variety (6) hormones into the blood stream, but also have autocrine + paracrine effects.
NB: Oxytocin + ADH to posterior via axons, the rest to anterior via hypophyseal portal system.
What is the role of oxytocin + ADH?
Oxytocin = Positive feedback loop for uterus contractions during childbirth + milk let down reflex (milk let down into duct when child suckles)
ADH = Causes expression of AQP’s in CD cells of kidney to regulate body water volume
What are the 6 tropic (releasing) hormones released by the hypothalamus that have effects on the release of hormones from the anterior pituitary?
1) Thyrotropin releasing hormones (TRH) - stimulates TSH
2) Prolactin release-inhibiting hormone (PIH) - dopamine
3) Corticotropin releasing hormone (CRH) - stimulates ACTH
4) Gonadotropin releasing hormone (GnRH) - stimulates LH + FSH
5) Growth hormone release hormone (GHrH) - stimulates GH
6) Growth hormone inhibiting hormone (GHIH) - AKA: somatostatin
What are the 6 hormones released by the anterior pituitary + their general effects?
1) TSH - stimulate secretion of T3/T4 from thyroid
2) ACTH - stimulate cortisol release from adrenal cortex
3) LH - ovulation and secretion of sex hormones
4) FSH - development of sperm and eggs
5) PRL - mammary gland development + milk secretion
6) GH - Growth + energy metabolism, stimulates IGF’s
How are hypothalamic and anterior pituitary hormone pathways regulated?
- Negative feedback, which can be long loop (from the hormone released from the endocrine gland) or short loop (from the anterior pituitary).
- E.g.: Cortisol inhibits at hypothalamus + anterior pituitary (long loop).
What factors are involved the control of growth?
- Multiple - genetics, nutrition, environment + hormones
- Multiple hormones, much most important regulator is growth hormone (GH)
What stimulates/inhibits GH?
What is the role of GH?
- Peptide (191 AA) hormone produced in anterior pituitary, stimulated by GHRH and inhibited by somatostatin.
- Growth-promoting effects exerted indirectly via insulin-like growth factors (IGF’s) secreted from liver and skeletal muscle (in response to GH)
What are the effects of GH in childhood and during adult years?
- GH stimulates long bone growth in children, both length + width before epiphyseal closure + width after closure. IGF also stimulates bone + cartilage growth.
- In adults, GH + IGF’s help maintain muscle and bone mass and promote healing/tissue repair as well as modulating metabolism.
What factors act as inputs to the hypothalamus affecting GHRH + somatostatin levels (and therefore GH)?
- Deep sleep = surge in GH
- REM sleep = decrease in GH
- Stress = increase in GH
- Exercise = increase in GH
- decrease in glucose/fatty acids = increase in GH (vice versa for increase)
- Fasting = increase GH secretion
- Obesity = decrease GH secretion
What are the long loop + short loop negative feedback mechanism controlling GH secretion?
Long loop = mediated by IGF’s, inhibit GHRH from hypothalamus, stimulates somatostatin. Inhibits GH from anterior pituitary.
Short loop = Mediated by GH to stimulate somatostatin from hypothalamus.
What occurs as a result of GH deficiency in childhood?
- Pituitary dwarfism - proportional dwarfism, delayed or no sexual development during teen years + very slow growth rate.
What occurs as a result of GH excess in childhood + adulthood?
Childhood = Gigantism, caused by pituitary adenoma Adulthood = Acromegaly - large hands, feet, lower Jaw
