Animal Physiology Exam 6

endocrine primary communication system

hormones (chemical messenger) released into the bloodstream

endocrine system speed

slower than neuronal transmission because hormones travel systemically

endocrine system regulation

works together with the nervous system to maintain homeostasis, control growth and respond to stress

endocrien target cells

distant cells via bloodstream

paracrine target cells

neighboring cells

autocrine target cells

the same cell that secreted the ligand

peptide/protein

water-soluble, amino acids and polypeptides, 300-700 Da

steroid

lipid-soluble, cholesterol, 300-400kDa

Amino-acid derived (thyroid hormones)

variable solubility, tyrosine and tryptophan, ~300 Da

Eicosanoids (prostaglandins)

lipid-soluble, arachidonic acid, ~400 Da

peptide hormones

are rapidly degraded by plasma proteases, half-life depends on enzymatic clearance

steroid hormones

bind carrier proteins in plasma, cross the cell membrane and act on intracellular receptors

peptide hormone secretion

1. Translation in rough ER→ nascent peptide

2. Glycosylation/folding in Golgi apparatus

3. Storage in secretory glands

4. release triggered by intracellular Ca²⁺ influx

steroid hormones secretion

1. cholesterol→ pregnenolone

2. conversion in smooth ER→ specific thing (ex. cortisol, estrogen)

3. transport bound to plasma proteins

4. diffusion across target cell membrane to bind intracellular receptors

peptide transport in blood & receptor location

free or bound to short-lived carriers, cell-surface (GCPR or tyrosine-kinase)

Steroid transport in blood & receptor location

bound to high-affinity carrier proteins, intracellular (cytosol or nucleus)

amino-acid derived transport in blood & receptor location

variable (often bound), can be either surface or intracellular depending on specific hormone

negative feedback

an increase in a hormones level inhibits its further secretion

Positive feedback

a hormones action stimulates its own further release

ligands

(first messengers) bind receptors, receptors can be enzyme-linked, intracellular, or ion channels

infundibulum (pituitary stack)

axonal bundle that links the hypothalamic neurons to the posterior pituitary

Hypothalamo-hypophyseal portal system

2 tiered capillary network

primary capillary plexus

in the hypothalamic median eminence receives neurohormones released from hypothalamic terminals

secondary capillary plexus

in the anterior pituitary where those neurohormones diffuse to regulate pituitary hormone secretion

the portal system provides…

a direct vascular route for the hypothalamic releasing/inhibiting hormones to reach the anterior pituitary without first entering the systemic circulation

anterior pituitary transport mechanism

blood-borne via portal veins

posterior pituitary transport mechanism

axonal transport (axon terminals release hormones)

6 hypothalamic neurohormones

GHRH, GHIH, TRH, CRH, dopamine, GnRH, peptide hormones except dopamine

GHRH (growth-hormone-releasing hormone)

stimulates GH release, somatotrophs

GHIH (somatostatin)

inhibits GH release

TRH (thyrotropin-releasing hormones)

stimulates TSH release

CRH (corticotropin-releasing hormone)

stimulates ACTH release

Dopamine

inhibits prolactin release

GnRH

stimulates FSH and LH release

a releasing hormone blank while a inhibiting hormone blank

promotes, suppresses

ADH (vasopressin) and oxytocin are synthesized in

hypothalamic magnocellular neurons

peptide chains travel down axons of the infundibulum, are stored in blank and are released directly into the blank from the blank

neurosecretory vesicles, systemic circulation, posterior pituitary

follicular cells

synthesize thyroxine (T₄) and triiodothyronine (T₃); store them in the colloid as part of the thyroglobulin complex (≈ 680 kDa)

parafollicular cells

secrete calcitonin (also called “tessitomine” in the transcript) to lower blood calcium.

elevated T₃ T₄ →

inhibit TRH (hypothalamus) and TSH (anterior pituitary).

Low T₃ T₄ →

stimulate TRH → ↑ TSH → ↑ thyroid hormone production

Parathyroid hormone target organs

bone, kidney, intestine

parathyroid hormone actions

Bone: stimulates osteoclast activity → osteolysis → release of Ca²⁺ and phosphate.

Kidney: ↑ Ca²⁺ reabsorption, ↓ phosphate reabsorption; stimulates 1α‑hydroxylase → ↑ active vitamin D synthesis.

Intestine: via vitamin D → ↑ Ca²⁺ absorption.

parathyroid hormone

principal regulator of extracellular calcium, acting through bone resorption, renal reabsorption, and intestinal absorption pathways.