Endocrine System.docx

Endocrine System “Internal secretion” Secrete hormones directly into blood upon stimulation Contrast – exocrine systems (salivary and sweat glands, and gastrointestinal tract) which secrete into ducts Endocrine glands are vascularized Arrangements: Endocrine organ devoted to hormone synthesis e.g....

Endocrine System “Internal secretion” Secrete hormones directly into blood upon stimulation Contrast – exocrine systems (salivary and sweat glands, and gastrointestinal tract) which secrete into ducts Endocrine glands are vascularized Arrangements: Endocrine organ devoted to hormone synthesis e.g. thyroid glands Clusters of cells within an organ e.g. islets of Langerhans (pancreas) Individual cells scattered diffusely throughout an organ e.g. gastrointestinal tract, skin Hormone Production Hypothalamus – Releasing hormones and inhibiting factors e.g. corticotrophin-releasing hormone, gonadotrophin-releasing hormone, growth hormone releasing hormone, dopamine Posterior pituitary – Arginine vasopressin, anti-diuretic hormone, oxytocin Anterior pituitary – Growth hormone, prolactin, follicle stimulating hormone, luteinising hormone, thyroid stimulating hormone and adrenocorticotrophic hormone Thyroid gland – thyroxine, triiodothyronine, calcitonin Parathyroid gland – parathyroid hormone Adrenal gland – adrenaline and cortisol Pancreas- Glucagon and Insulin Gonads – Progesterone, oestrogen and testosterone Types of hormones Steroid: Progestogens, Glucocorticoids, Mineralocorticoids, Androgens, Oestrogens Non-steroid: Polypeptide/Proteins – Growth hormone, prolactin, calcitonin, parathyroid hormone, insulin, glucagon and adrenocorticotrophic hormone Glycoproteins – Follicle stimulating hormone, luteinizing hormone, thyroid stimulating hormone, human chronic gonadotropin Eicosanoids – Prostaglandins, Leukotrienes, Prostacyclin’s Peptides – Arginine vasopressin, oxytocin, melanocytes stimulate hormone, somatostatin thyrotrophin-releasing hormone, gonadotrophin-releasing hormone Amines – Adrenaline, Noradrenaline, Melatonin, Thyroxine T4, triiodothyronine T3 Peptide hormone vs Steroid hormone Peptide – Chain of amin acids, hydrophilic, synthesized in Golgi apparatus via gene transcription and post-transitional modification, stored in large amounts at secretory granules, secreted via exocytosis, transported in blood as free hormone, received at cell surface via receptor, action is modified by second messenger system to alter activity f existing proteins. Steroid – Cholesterol-derived, Lipophilic, synthesised in mitochondria (de novo biosynthesise), Not stored usually stored as the precursor (cholesterol), Secreted via simple diffusion, bound to transport proteins in blood for movement, intracellular receptors receive them, work via specific genes being activated which produce new proteins. Hormones and actions Present in blood (very low concentration) about 10-8 – 10-10M but highly potent Slower acting than neurotransmitters Transported over long distances Require specific affinity receptors Effects can be long lasting and permanent, permissive or synergistic with other hormones. Amplitude of response controlled by, concentration of hormone, number of receptors on target cell, affinity of hormone receptors, duration of response. Causes of Endocrine disorders Hormone deficiency – destruction of glands, extra-glandular disorder, defect in biosynthesis Hormone excess – tumours, ectopic hormone production, overstimulation (hyperplasia), exogenous hormone Hormone resistance – receptor defects Hypothalamus-pituitary axis “Interface between CNS and endocrine system” Neuroendocrine control and homeostatic regulation of: Stress and immune function Reproduction Growth and development Water/Electrolyte balance Energy balance/ appetite control Sleep/wokeness Thermoregulation Hypothalamus – found in basal forebrain Involved in pituitary regulation: Hypothalamic neurosecretory cells (modified neurons) release peptides from axon terminals near capillary. Peptides include ‘releasing hormone’ – which regulate anterior pituitary – and 2 posterior pituitary hormones. Posterior pituitary (neurohypophysis), under-direct control – neural tissue comprises axons and nerve terminal endings of hypothalamic neurosecretory cells. Blood supply from inferior/middle hypophysial arteries. Direct innervation (magnocellular neurons) from hypothalamus axons in pituitary stalk release neurohypophysial hormones into synthetic blood. Anterior pituitary (adenohypophysis), under-indirect control – glandular tissue comprises cells controlled by releasing hormones (or inhibiting factors) delivered via hypophysial portal system. Blood supply from pituitary portal system. No direct innervation but parvocellular neurones terminate in median eminence of hypothalamus and release hypophysiotropic hormones into portal blood supply. *Intermediate lobe is part of the fetal pituitary source of melanocyte stimulating hormone but regresses after birth. “Tripartite” endocrine system controlled by thyroid, adrenal and gonads. “Feedback control” hormone activity dependant on feedforward drive (increased hormone output) and negative hormone feedback causing the hormone levels to fluctuate and can also alter due to biorhythms. Order of activity Stimulus Hypothalamus (hormones 1 secretion) Releasing hormone (Plasma hormone 1 in hypothalamus-pituitary portal vessel) Anterior pituitary (Hormone 2 secretion) Tropic hormone (Plasma hormone 2) Has ability to short-loop feedback Third endocrine gland (hormone 3 secretion) Required hormone (Plasma Hormone 3) Has ability to long-loop feedback Target cells of hormone 3 (response to hormone 3) Biorhythms Entrained by hypothalamic suprachiasmatic nucleus (SCN), the body’s master ‘clock’ Pulsatile patterns of hormone release depend on neural input from rhythm generator in the hypothalamic suprachiasmatic nucleus (SCN) e.g. paraventricular nucleus received circadian input from SCN, regulating hypothalamus–pituitary–adrenal (HPA) axis activity and cortisol release. Light-entrainable SCM regulates daily rhythms in many physiological systems and behaviours e.g. hypothalamic circuits for appetite and feeding.