Feedback Mechanisms

regulation of hormones in the blood

- hormones are secreted by glands

- blood carries hormones to target cells

- target cells communicate with glands

- glands stop/start producing hormones

negative feedback

increases in hormones downstream REDUCES stimulation for release of hormones upstream in the pathway

positive feedback

increases in hormones downstream leads to INCREASED stimulation for release of upstream in the pathway

calcium and parathyroid hormone - homeostatic feedback

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- negative feedback

oxytocin and lactation/childbirth

- positive feedback

- baby suckles at breast (stimulus) -> this info gets sent to the hypothalamus

- hypothalamus signals posterior pituitary to release oxytocin

- breast is stimulated to eject breast milk

- the continued suckling by the baby creates a positive feedback - more milk

- baby stops suckling = positive feedback stops

pulsatile hormone regulation

constant level of hormone over long periods of time

ex. production of sperm/eggs

acute hormone regulation

rapid increases in hormone level in response to stimuli

cyclic hormone regulation

hormone increases and decreases in predictable pattern

pineal gland

- primary product is melatonin

- melatonin plays a role in most biological rhythms

- functions continuously throughout the day

- sunlight resets clock genes

- darkness exposes clock genes

neural regulation

neurons synapse with cells producing hormone

(i.e., norepinephrine release from the adrenal gland)

endocrine regulation

hormones bind to endocrine cells, regulating release of another hormone

(i.e., FSH stimulates estrogen release)

hormone regulation by other factors (humoral)

endocrine cells respond to levels of other factors in the circulation

(i.e., glucose causes increased insulin secretion from the pancreas)

pancreas

- consists of two major types of secretory tissue which reflect the dual function: exocrine and endocrine

exocrine function of pancreas

- majority of the pancreas

- structure has large ducts

- secretes digestive enzymes and alkaline pancreatic fluid into the GI

- localized in the acinar cells

endocrine function of pancreas

- in proximity to blood supply

- releases hormones regulating carbohydrates, lipids and protein metabolism

- localized in the islet cells (Islets of Langerhans)

Islets of Langerhans: 4 cell types

- 10% alpha cells: secrete glucagon (raises glucose)

- 70% beta cells: secrete insulin (lowers blood glucose)

- 5% delta cells: secrete somatostatin (GHIH) -> cells that are active during development and regulate metabolism

- PP cells: secrete pancreatic polypeptide (self regulation)

control of insulin by glucose

1) high blood glucose -> inhibits alpha cells in pancreas -> glucagon secretion inhibited

2) high blood glucose -> stimulates beta cells in pancreas -> secretes insulin -> stimulates glycogen formation, glucose uptake and oxidation, and synthesis of protein and fat = low blood glucose

1) low blood glucose -> stimulates alpha cells in pancreas -> glucagon secretion stimulated -> secretes glucagon -> stimulates glycogen to break down into glucose = high blood glucose

2) low blood glucose -> inhibits beta cells in pancreas -> insulin secretion inhibited

neuroendocrine interaction

- stretch receptors in the digestive tract

- afferent information (sensory) to CNS

- efferent response by CNS to pancreas

- pancreas produces insulin

- insulin travels to GI

- glucose is utilized or stored

- glucose levels in blood drop

- you get hungry and need to increase glucose... repeat

adrenal glands

- basic functions of different compartments: salt, sugar, sex, and sympathy

- located above our kidneys

- contains 2 regions: adrenal cortex (outside) and adrenal medulla (inside)

salt compartment in adrenal glands

- produced in the zona glomerulosa

- releases hormone Aldosterone

- increases Na+ reabsorption in the distal tubes of the kidneys

sugar compartment in adrenal glands

- produced in the zona fasicuculata

- releases hormone Cortisol

- long term regulation of blood glucose

sex compartment in adrenal glands

- sex steroids produced in the zona reticularis

- releases hormones Androgens and Estrogens

sympathy compartment in adrenal glands

- produced in the medulla

- releases hormones norepinephrine and epinephrine

- sympathetic nervous system neurotransmitters

adrenal gland : 2 in 1

- cortex/bark and medulla/core

- cortex: an endocrine gland

- medulla: a modified sympathetic ganglion

- activity of both medulla and cortex are stress mediated

HPA and cortisol

- cortisol is secreted in a daily rhythm; levels rise in your sleep and are used throughout the day

- major adrenal cortical activator is stress

- deficient cortisol leads to poor processing of stress

major effects of too much cortisol

- inhibition of protein synthesis increases lipolysis

- increases blood glucose levels

- suppresses immune function

- facilitates effects of catecholamines

stimulation of ACTH secretion increases by

- decreased cortisol

- sleep-wake transitions

- stress

- psychological disorders (anxiety, depression)

- serotonin

- acetylcholine

- CRH and ADH (Arginine Vasopressin) synthesized by hypothalamus

inhibition of ACTH secretion produced by

- increased cortisol

- Enkephalins

- opiods

- ACTH

- somatostatin

- GABA

hormone production in the ovary

theca cells -> Androgen

corpus luteum -> Progesterone

granulosa cells -> Estrogen/Inhibin and Activin

hormone production in the testis

leydig cells -> Testosterone

sertoli cells -> Sperm development

gonadotrophs

- cells in anterior pituitary that produce LH and FSH

- synthesis and secretion stimulated by GnRH

- FSH secretion controlled by inhibin

pulsatile secretion of GnRH and inhibin cause distinct patterns of LH and FSH secretion

Luteinizing hormone (LH) and Follicle stimulating hormone (FSH)

- pulsatile patter of secretion

- LH pulses are biphasic (every 1 min, then large pulse at 1 hour) -> up until puberty

- FH pulses and uniphasic

- Diurnal (during puberty)

- LH/FSH more pronounced during puberty

- cyclic in females - ovarian cycle with LH surge at time of ovulation

- not cyclic in males, but constant pulses of LH causes pulses of testosterone to be produced

HPG Axis Female

- feedback in the female varies with stage of ovarian cycle, both negative and positive feedback are responsible

- follicular phase

- preovulatory surge

- luteal phase

follicular phase

maturing follicle secretes E2, P is low

preovulatory surge

follicle makes enough E2 to reach threshold for switch to PFB; initiates LH/FSH surge, ovulation, formation of corpus luteum

luteal phase

corpus luteum secretes P for NFB on HPG axis, low [LH] allows corpus luteum demise, decrease in P, and menstration

regulation of LH and FSH - positive feedback

estradiol at high plasma concentrations in late follicular phase of ovarian cycle stimulates GnRH and LH surge which stimulates ovulation

- controls the female reproductive cycle

regulation of LH and FSH - negative feedback

- inhibin produced by testes and ovaries - decreases

- testosterone from leydig cells - synthesis stimulated by LH. feeds back to inhibit GnRH production from hypothalamus and down-regulates GnRH

- progesterone - suppresses ovulation, basis for contraceptives. Works at both the level of pituitary and hypothalamus

prolactin in regulation of LH and FSH

prolactin inhibits GnRH release,

prolactin inhibition of GnRH can produce post-partum contraceptive effect

LH pulses in males

- LH increases production of testosterone from the testis

- testosterone feeds back upon the pituitary to inhibit LH release

HPG Axis in Males

- negative feedback overall

- without feedback, GnRH pulses are stimulated by adrenergic neurons & inhibited by opioid neurons of the hypothalamus

- Gonadal T is converted to E2 in brain -> inhibits adrenergic cells, normally they increase GnRH pulse amplitude

- T converted to DHT in blood -> stimulates endorphin cells, normally they decrease GnRH pulse frequency

negative feedback in HPG axix for Males

inhibiting adrenergic cells decreases GnRH pulse amplitude; stimulating endorphin cells decreases GnRH pulse frequency