Overview
This lecture covers the endocrine system, focusing on its mechanisms, feedback regulation, hormone pathways, key glands, and clinical examples such as diabetes, thyroid disorders, and the effects of hormones like oxytocin, cortisol, and reproductive hormones.
Endocrine vs. Paracrine Signaling
- Paracrine signaling acts locally on nearby cells, e.g., tanning and inflammation.
- Endocrine signaling uses hormones that travel through the bloodstream to distant target cells.
- The key factor in response is the presence and type of hormone receptor on the target cell.
Hormone Chemistry and Signaling
- Steroid (lipid-soluble) hormones cross cell membranes and have intracellular receptors; effects last days.
- Water-soluble hormones act via cell surface receptors; effects last hours.
- Water-soluble hormones often use second messenger systems (e.g., G protein, cAMP) for signal amplification.
Hormone Regulation and Feedback
- Hormone levels depend on production, removal (liver/kidneys), receptor number, and other hormones.
- Negative feedback loops commonly regulate hormone levels (e.g., TRH/TSH for thyroid hormone).
- Receptor down-regulation can cause hormone resistance (e.g., type 2 diabetes).
Major Endocrine Glands and Hormones
- Hypothalamus: releases -RH hormones, ADH, and oxytocin; controls pituitary.
- Pituitary gland: anterior (glandular, releases TSH, ACTH, FSH, LH, GH, prolactin); posterior (neural, releases ADH and oxytocin).
- Thyroid: produces T3/T4 (regulate metabolism), calcitonin (lowers blood calcium).
- Parathyroid: produces PTH (raises blood calcium).
- Adrenal cortex: releases mineralocorticoids (aldosterone), glucocorticoids (cortisol), and androgens.
- Adrenal medulla: releases epinephrine/norepinephrine (SNS, covered elsewhere).
- Pancreas: alpha cells (glucagon increases blood glucose), beta cells (insulin lowers blood glucose).
- Pineal gland: produces melatonin (regulates sleep and circadian rhythms).
- Gonads: testes (testosterone), ovaries (estrogen, progesterone).
Key Hormone Functions & Clinical Concepts
- Oxytocin: triggers labor contractions, letdown reflex in breastfeeding, and social bonding behaviors.
- Prolactin: stimulates breast milk production; permissive for oxytocin.
- ADH (vasopressin): promotes water retention by kidneys, increases blood pressure.
- Diabetes mellitus: Type 1 (insulin deficient), Type 2 (insulin resistant due to receptor down-regulation).
- Thyroid disorders: hyperthyroidism (excess T3/T4), hypothyroidism (iodine deficiency), goiter (enlarged gland, both conditions).
- Addison's disease: adrenal insufficiency causes skin hyperpigmentation due to ACTH/MSH overlap.
- Stress: Short-term cortisol is adaptive; chronic cortisol causes health problems (e.g., diabetes, suppressed immunity).
- Birth control pills: progesterone blocks GnRH, LH, FSH, preventing ovulation.
- Leptin: produced by adipose tissue, regulates appetite and metabolism (species-specific effects).
- Eicosanoids: local inflammatory mediators, e.g., prostaglandins; target for NSAIDs.
Key Terms & Definitions
- Hormone — chemical messenger secreted into the blood to affect distant cells.
- Second Messenger — intracellular signaling molecules (e.g., cAMP) activated by hormone-receptor binding.
- Negative Feedback Loop — process where end product inhibits its own production.
- Receptor Down-Regulation — decrease in receptor number due to persistent high hormone levels.
- Permissive Effect — one hormone enables the action of another.
Action Items / Next Steps
- Review hormone pathways and negative feedback loops.
- Memorize major hormones, their sources, and functions.
- Understand mechanisms of hormone action (second messenger vs. nuclear receptor).
- Study clinical examples (diabetes, thyroid disorders, Addison’s disease) for exam.