2- hypothalamus + pituitary physio

where’s the pituitary gland

base of the brain, sitting in the sella turcica + connected to the overlying hypothalamus via pituitary stalk (infundibulum)

structure of pituitary gland

2 lobes:

1. anterior (adenohypophysis), bigger one

2. posterior (neurohypophysis)

what controls the secretion of hormones from pituitary gland

hypothalamus

how does the hypothalamus control the secretion of anterior pituitary hormones

neurons in hypothalamus secrete releasing/inhibitory hormones into hypothalamic capillaries that form long hypothalamic-pituitary portal veins that descend to anterior pituitary → where a 2nd capillary network combines w/ the portal veins to deliver the hormones to “troph” cells that control release of anterior pituitary hormones how

what type of hormones does the anterior pituitary secrete

6 peptide hormones:

1. FSH → reproductive system

2. LH → reproductive system

3. TSH → thyroid gland

4. ACTH → adrenal glands

5. prolactin → mammary glands

6. GH → liver, bone, muscles

F L T A P G

3 levels of the pituitary hormone pathway

1. hypothalamus

2. pituitary gland

3. target gland

each one producing hormones

how is the pituitary hormone pathway regulated

hormones released by target glands can feedback inhibit hormone release from both pituitary gland + hypothalamus

which hormones are released by the hypothalamus that stimulates the pituitary

1. CRH → ACTH

2. TRH → TSH

3. GnRH/LHRH → FSH/LH

4. GHRH/somatostatin → GH

5. PRH/dopamine → PRL

how does GH (growth hormone) elicit cellular response

binds to surface receptors on muscle, adipose tissue, liver → activates JAK 2-STAT signaling pathway

4 short term effects of GH

1. lipolysis in liver

2. gluconeogenesis in liver

3. inhibition of glucose uptake in skeletal muscle

4. maintains normal pancreatic islet function

GH’s effect on insulin

• GH deficiency → decline in insulin secretion

• GH excess → rise in insulin to compensate for reduction of glucose uptake in muscle

overall function of GH

1. raises blood levels of energy substrates, fatty acids, glucose

2. inhibiting glucose uptake by muscle → ensures that enough glucose is available for other tissues, especially the CNS

long term effects of GH

secretes IGF-1 (insulin-like growth factor) → promotes linear growth between birth + puberty by stimulating proliferation of cartilage in epiphyseal (growth) plates of long bones + by increasing organ size/function

what happens once linear growth stops after puberty

GH-IGF-1 continues to regulate body composition + promote anabolic actions in muscles

GH deficiency leads to what

1. dwarfism

2. poor jaw development

3. delayed dentition with late loss of deciduous teeth

4. missing permanent teeth

GH excess leads to what

1. prepubertal → gigantism

2. postpubertal → acromegaly (thickening of bones + soft tissues of hands, head, feet), glucose intolerance, hyperinsulinemia

PRL (prolactin) is produced by which cells in the anterior pituitary

lactotrophs

T/F: PRL (prolactin) is secreted at low levels throughout life in males + females

true

PRL (prolactin) is stimulated/inhibited by what

• stimulated: TRH

• inhibited: dopamine

main function of PRL (prolactin)

breast development + milk production

excess PRL (prolactin) secretion can lead to what

• females: infertility, dysmenorrhea, galactorrhea (inappropriate milk production)

• males: decreased testosterone/sperm production, gynecomastia

most common cause of PRL (prolactin) excess secretion

prolactinoma, usually treated w/ dopamine agonist

what 2 hormones are secreted by the posterior pituitary

2 peptide hormones:

1. oxytocin (OT)

2. arginine vasopressin (AVP/ADH)

how are OT + AVP/ADH synthesized

by neurons in the hypothalamus located in supraoptic nucleus (SON) + paraventricular nucleus (PVN)

how do OT + AVP/ADH get to the posterior pituitary

OT + AVP/ADH are packaged in vesicles → travel down axons of the hypothalamus neurons that project down the pituitary stalk + terminate in posterior pituitary

how are OT + AVP/ADH made from their preprohormone form

preprohormone forms are cleaved into their bioactive forms w/ their signal peptide + inactive neurophysin fragment

when is AVP/ADH secreted

1. plasma osmolality increases: osmoreceptor cells signal to the SON + PVN in the hypothalamus

2. plasma volume decreases: sensed by vascular baroreceptors

3 effects of AVP/ADH

1. kidney: binds to basolateral side of collecting duct cells, via cAMP dependent kinases to insert aquaporins into luminal membrane → increased water reabsorption → reduced urine

2. blood vessels: vasoconstriction via PLC pathway

3. anterior pituitary: enhances ACTH secretion

AVP/ADH deficiency leads to

diabetes insipidus

2 main actions of oxytocin

1. milk let-down in lactation

2. parturition: postive feedback cycle

definition of milk let-down in lactation

neuroendocrine reflex stimulated by nipple suckling → myoepithelial cells surrounding the lactiferous ducts + acinar cells of the breast contract + eject milk from the mammary glands

definition of parturition

positive feedback cycle:

increased stretching of cervical myometrium increases oxytocin release from posterior pituitary → stimulates myometrial contractions in labor that push fetus further into cervical, further increasing stress-induced neural signals + OT release