environmental influences

low temperature axis

• low temperature detected

• thyrotropin releasing hormone (TRH) released from hypothalamus

• stimulates endocrine gland in anterior pituitary to release thyroid stimulating hormone (TSH)

• TSH will bind to thyroid gland to release thyroid hormones

  • inc metabolism and heat production in body

tropic hormone

• hormone which acts on another endocrine gland to stimulate their own hormones

is there any neuronal contact between the lobes?

no

magnocellular neurons

• found in the hypothalamus supraoptic and paraventricular nucleus

• extends to the posterior pituitary

• stores oxytocin and ADH (vasopressin) in granules for release

posterior pituitary

• mainly nervous tissue and neurons

• axon terminal of magnocellular neurons

• release of oxytocin and ADH

how does the anterior pituitary and posterior pituitary interact

• no neuronal connection

• hypothalamic portal blood system

parvocellular neurons

• really short

• found in lateral wall of 3rd ventricle

• extend blood supply to just above the anterior pituitary

median eminence

• area at the superior end where the neurons synapse with the blood vessels

how do neurohormones travel through the hypothalamic portal blood system

• neurohormones released from axon termini of hypothalamic neurons → primary plexus

• primary plexus → secondary plexus 

• secondary plexus → fenestrations in capillary walls

• react with receptors in target anterior pituitary cells 

hypothalamohypophyseal tract

• projection of the magnocellular neurons down the infundibular stalk

• connects the hypothalamus to the pituitary

hypophysiotropic region

• found in the hypothalamus

• regulates the anterior pituitary gland

• formed from several hypothalamic nuclei - secrete in rhythms controlled by CNS

half life of hypothalamic releasing hormones and conc

• 3-10 mins

• released in nanograms

half life of anterior pituitary releasing hormones and conc

• minutes to hours

• micrograms

half life and conc of peripheral pituitary hormones

• hours to days

• miligrams

pituitary actions of releasing hormones derived from the hypothalamus

Hypothalamic factor	Anterior pituitary hormone
GnRH GHRH Somatostatin TRH Dopamine CRH	Gonadotrophins (LH, FSH) Growth Hormone Growth Hormone (inhibits) TSH, Prolactin Prolactin (inhibits) Adrencorticotropic Hormone

in vitro demo of the HPA and result

• assay of ACTH from cells in test tube

• over time the cells will stop secreting

• add fresh hypothalamic tissue and stimulates secretion of hormone again

→ therefore hypothalamic cells produce and release a soluble factor that can stimulate the release of ACTH from pituitary cells

ACTH pathway

Corticotropic releasing hormone from hypothalamus

Corticotrope cells in anterior pituitary will stimulate endocrine gland adrenal cortex to release glucocorticoids

cortisol will feed back to corticotrope cells to stop ACTH being release

• also feedback to cells within adrenal cortex

in vivo demonstration of the HPA

• clamping and separation of the infundibular stalk between hypothalamus and anterior pituitary gland

• blood supply between hypothalamus to pituitary was reduced or prevented

• caused degeneration of cells → dystrophia adiposa genitalis

• pituitary is one organ

results from in vivo clamping of the stalk

• weight gain

• genital and mammory atrophy - small testes

• anterior pituitary atrophy

reasons behind results from ablation of the stalk 

• hypothalamus did not signal GnRH to the anterior pituitary

• anterior pituitary could not release FSH or LH

• FSH, LH do not reach ovary or testes so no sex hormones

• prolactin from hypothalamus still released - development of mammary gland

craniopharyngioma

• squashed the infundibular stalk → prevented hormones reaching pituitary

• obesity

• failed to enter puberty

• visual abnormalities (optic nerve nearby)

• small testes