Pituitary Gland Flashcards
What is a hormone
A chemical messenger carried from the organ where they are produced to the organ they are affected
Peptide or steroid hormones
Peptide hormones
Made as prohormones which require further processing eg cleavage
Stored in vesicles and released by exocytosis
Bind receptors on cell membranes and transducer signals using 2nd messenger systems
Steroid hormones
Made in a series of reactions from cholesterol
Released immediately upon production
Bind to intracellular receptors to change gene expression
Where is the pituitary gland
Sits in sella turcica of sphenoid bone
What is the pituitary gland suspended from
Pituitary stalk (infundibulum)
Bitemporal hemianopia
Compression of the optic chasm by a pituitary tumour prevents transmission of sensory information from the visual fields to occipital lobe
What type of neurons regulate anterior pituitary function
Hypothalamic parvocellular neurones
These are short and terminate on median eminence
They release hypothalamic releasing and inhibitory factors into capillary plexus in the median eminence
The factors are carried by hypophyseal portal circulation made of leaky fenestrated blood vessels
Features of anterior pituitary
Anatomically distinct from hypothalamus
Derived from rathkes pouch
Derived from dorsal growth of buccal cavity
Made of endocrine cells
How does the hypothalamo-pituitary portal system work
Axon terminals of hypothalamic neurosecretory cells release Thyrotropin Releasing Hormone (TRH) into hypothalamo-hypophysial portal system.
TRH travels to anterior pituitary via portal system.
TRH stimulates release of TSH (thyrotropin) from anterior pituitary thryotrophs
TSH leaves gland via blood to travel to thyroid gland to stimulate thyroid hormone release (T4- thyroxine)
Endocrine cells making up anterior pituitary
Somatotrophs
Lactotrophs
Corticotrophs
Thyrotrops
Gonadotrophs
Somatotrophs
Growth hormone
Growth hormone releasing hormone is RH
Somatostatin is inhibiting
Receptors are found in general body tissues
Lactotrophs
Dopamine inhibits it
Releases prolactin
Receptors found in mammary gland
Corticotrophs
Released adrenocorticotrophic hormone
Corticotrophin releasing hormone regulates it
Receptors found in adrenal cortex
Thyrotrophs
Makes thyroid stimulating hormone
Thyrotropin releasing hormone is RH
Receptors found in thyroid
Gonadotrophs
Released LH and FSH
Gonadrotrophin releasing hormone is RH that regulates them
Receptors are found in testes and ovaries
Outline the neuroendocrine reflex arc for milk production?
Mechanical stimulation of the nipple stimulates the touch and sensory receptors, activating the afferent pathways. Action potentials traverse along the axons through the ascending sensory pathways
Afferent signals integrate in hypothalamus and inhibit dopamine release by dopaminergic parvocellular neurones
Less dopamine in the hypothalamic-pituitary portal system results in less inhibition of anterior pituitary lactotrophs
Increased plasma prolactin increases milk secretion in mammary glands
What are the mechanisms of growth hormone action?
Secretion of growth hormone by endocrine somatotrophs from anterior pituitary directly binds to complementary target within general tissue (muscle and bone)
Growth hormone can bind to growth hormone receptors of liver → release of IGF-1 and IGF-2 (Insulin-like Growth Factor - Somatomedin)
- Both produce same effects of growth and development
- IGF-1 is the main one made in adults and children and IGF-2 is main in developing foetus
Acromegaly
Excess growth hormone secreted by Somatotrophs due to pituitary adnoma
Causes - Coarsening of facial features
- Macroglossia- bigger tongue
- Prominent nose
- Obstructive sleep apnoea (soft-tissue changes surrounding upper airway leading to narrowing → Collapse during sleep → Causes disruption to sleep patterns)
- Sweatiness
- Increased hand and feet size
- Prognathism- large jaw
- Headaches
- Carpal tunnel due to growth of soft tissues around median nerve in wrist compressing it
Causes gigantism when growth plates haven’t fused
Which hormones does the posterior pituitary release
Arginine vasopressin (AVP) also known as anti diuretic hormone
Oxytocin
What type of neurones from the hypothalamus have axon fibres sending signals to the posterior pituitary gland?
Hypothalamic magnocellular neurones
These are long
Pass down from nuclei through pituitary stalk to posterior pituitary
Which 2 hypothalamic nuclei do magnocellular axons extend from?
Supraoptic (AVP) and paraventricular (oxytocin)
Features of posterior pituitary
- It is anatomically continuous with hypothalamus
- It’s made of only neuronal tissue, not endocrinal tissue like anterior pituitary
-
Stores hormones produced in the hypothalamus, does not make the hormones itself.
Formed from down growth of diencephalon
effects of release of AVP into body
AVP is anti diuretic hormone
AVP stimulates water reabsorption in renal collecting duct which concentrates urine through basolateral V2 receptor in kidney
Also vasoconstrictor via v1 receptor
Stimulates ACTH release
AVP binds to V2 receptor,cascade of events occur causing aquaporin 2 to be inserted allowing more water to be reabsorbed by aquaporin 3
How does AVP concentrate urine and cellular level
- AVP binds to V2 receptor on collecting duct cell which results in intracellular signalling cascade
- This inserts aquaporin-2 molecules (water transport molecules) into apical membrane of duct cell
- Water diffuses down its conc grad through aquaporin-2 from urine in nephron into collecting duct cell
- It then moves through aquaporin-3 into plasma
What are the 2 physiological actions that oxytocin release causes?
- Contraction of myometrial cells in uterus at parturition (in labour) which leads to delivery of baby
- Contraction of myoepithelial cells in breast during lactation which leads to milk ejection
Outline the neuroendocrine reflex arc for milk ejection?
Mechanical stimulation of nipple and surrounding area activates afferent pathways
Afferent signals integrated in hypothalamus and stimulate oxytocin-releasing neuron activity
Action potentials travel down oxytocin neurons and oxytocin is secreted into the bloodstream
Increased plasma oxytocin increases milk ejection in mammary glands
Regulation of posterior pituitary
Two sets of hypothalamic neurone rectory cells make AVP and oxytocin. This is transported to posterior pituitary
Excitation of hypothalamic magnocellular neurons stimulates release of avp or oxytocin into posterior pituitary
