Hormones and Endocrine Signaling

nervous signaling characteristic

fast and typically “point-to-point”

only some cells are targeted

endocrine signaling characteristics

more diffuse

many cells have access to + encounter the secreted molecules (ex: hormones)

endocrine vs exocrine glands

exocrine glands: secrete substances through a duct onto internal or external surfaces. do not secrete hormones. ex: glands that secrete digestive enzymes, sweat glands

endocrine glands: secrete into blood

endocrine system

is a collection of endocrine cells and glands that secrete hormones into the body’s blood

endocrine cells: cells that secrete hormones

endocrine glands: specialized tissues or organs that contain several endocrine cells

hormones

chemical substances secreted into the blood by endocrine cells. they ind to receptors (typically protein) on or inside target cells to cause a response

typically function at low concentrations

how do cells respond to hormones?

any cell that has receptors for a given hormone of interest can respond to that hormone

the same hormone can bind to structurally similar receptors and trigger different responses

no receptor=no responses

pituitary gland

attached to the hypothalamus

the connection between the nervous system and the endocrine system

it has 2 lobes: anterior and posterior

posterior pituitary

receives hormones from neurosecretory cells who cell bodies are in the hypothalamus

after, hormones enter the general blood circulation

what are the 2 main hormones that are released into the posterior pituitary?

ADH (vasopressin):increases water conservation by kidneys, constricts blood vessels

oxytocin: stimulates breast milk flowa after pregnancy, stimulates uterine contractions during birth

anterior pituitary

contains non-neural endocrine cells that produce and secrete their own hormones

these cells respond to releasing hormones that are secreted by the hypothalamic neurons

tropic hormones

in response to releasing hormones the anterior pituitary cells release their own tropic hormones

main tropic hormones

Follicle stimulating hormone: growth of follicles in females, sperm production in males

Lutenizing hormone: causes ovulation

Adrenocorticotropic hormone: stimulates release of adrenaline from adrenal gland

Thyroid stimulating hormone: stimulates release of thyroid hormone from thyroid gland

how do hormones control feedback loops?

they can exert feedback inhibition on gland cells which inhibits hormone secretion

hormones secreted by peripheral glands may exert negative feedback on hypothalamic neurosecretory cells or anterior pituitary cells

pituitary tropic hormones may exert negative feedback on hypothalamic neurosecretory cells

thyroid gland

located near the trachea, is an example of a peripheral endocrine gland that is regulated by the hypothalamus and anterior pituitary

produces thyroid hormone which stimulates cell metabolism

increased amounts of thyroid hormone leads to increased metabolism

the thyroid gland produces thyroid hormones from what precursor protein?

thyroglobulin

what are called thyroid hormones?

T3 and T4

most thyroid hormone is made as T4 (also called thyroxine) but can be converted to a more active form called T3

if there is not enough iodine, what form will the thyroid hormone produce?

it will not be active (T3 or T4), instead it will be in the T0, T1, or T2 form (not active)

lots of thyroglobulin is still made but much of it remains non-iodinated, resulting in less production of T4 and T3

diagram for thyroid hormone stimulation

hypothalamus→TRH→anterior pituitary→TSH→Thyroid gland→Thyroid hormone (T4 and T3)

there is a negative feedback loop from the thyroid hormone and the hypothalamus/anterior pituitary: “long loop” negative feedback

short loop negative feedback: TSH to hypothalamus

goiter

an enlarged thyroid gland often associated with defects in thyroid stimulating hormone homeostasis

the gland enlarges from excessive stimulation of TSH receptors