Chemical Coordination and Integration
The neural and endocrine systems jointly coordinate and regulate physiological functions. Neural coordination is fast but short-lived; chemical (hormonal) coordination is slower but longer-lasting.
Hormones and endocrine glands
- Endocrine glands are ductless; their secretions, the hormones, are released directly into the blood.
- Hormones are non-nutrient chemicals that act as intercellular messengers and are produced in trace amounts.
- Organised endocrine glands: hypothalamus (regulator), pituitary, pineal, thyroid, parathyroid, thymus, adrenal, pancreas, and gonads (testis, ovary). Additionally the GI tract, liver, kidney and heart produce hormones.
Hypothalamus and pituitary
- Hypothalamus (basal diencephalon) secretes releasing and inhibiting hormones (e.g., GnRH stimulates, somatostatin inhibits GH).
- Anterior pituitary (pars distalis): GH, PRL, TSH, ACTH, LH, FSH. Pars intermedia: MSH. Posterior pituitary (pars nervosa): stores and releases oxytocin and vasopressin (ADH), made by the hypothalamus.
- Disorders: gigantism (excess GH, young), pituitary dwarfism (low GH), acromegaly (excess GH, adult), diabetes insipidus (ADH impairment).
Pineal, thyroid, parathyroid, thymus
- Pineal: melatonin -> diurnal (24-hour) rhythm, sleep-wake cycle.
- Thyroid: T3, T4 (need iodine) -> BMR, RBC formation, metabolism; thyrocalcitonin (TCT) lowers blood Ca2+. Goitre/cretinism (hypo), Graves' disease/exophthalmic goitre (hyper).
- Parathyroid: PTH raises blood Ca2+ (hypercalcemic).
- Thymus: thymosins -> T-lymphocyte differentiation (cell-mediated immunity); degenerates with age.
Adrenal and pancreas
- Adrenal medulla: adrenaline and noradrenaline (catecholamines) = emergency Fight or Flight hormones.
- Adrenal cortex: glucocorticoids (cortisol; gluconeogenesis, anti-inflammatory) and mineralocorticoids (aldosterone; Na+/water reabsorption). Underproduction -> Addison's disease.
- Pancreas (Islets of Langerhans): alpha-cells -> glucagon (hyperglycemic), beta-cells -> insulin (hypoglycemic). Insulin deficiency/resistance -> diabetes mellitus.
Gonads and other sources
- Testis (Leydig cells): androgens (testosterone) -> male sex organs, spermatogenesis.
- Ovary: estrogen (secondary sex characters) and progesterone (pregnancy).
- Heart: atrial natriuretic factor (ANF) lowers BP. Kidney: erythropoietin -> RBC formation. GI tract: gastrin, secretin, CCK, GIP.
Mechanism of hormone action
- Hormones bind specific receptors. Membrane-bound receptors (peptide hormones) -> second messengers (cyclic AMP, IP3, Ca++). Intracellular receptors (steroids, iodothyronines) -> regulate gene expression.