Coordination and control endocrine system AF 2024 (2).pdf

Coordination and Control: Endocrine System Dr Allison Fulford [email protected] What does the endocrine system do? Cell communication A control system, acting in partnership with the nervous system, to control other systems of the body Made up of a collection of endocrine glands & tissues, and...

Coordination and Control: Endocrine System Dr Allison Fulford [email protected] What does the endocrine system do? Cell communication A control system, acting in partnership with the nervous system, to control other systems of the body Made up of a collection of endocrine glands & tissues, and their hormone products Controls all major body processes Exocrine vs endocrine glands ‘Endo-crine’: internal secretion Endocrine glands secrete hormones directly into the bloodstream upon stimulation Contrast with ‘exocrine’ glands (eg. salivary glands, sweat glands & glands of the gastrointestinal tract) which secrete into ducts Endocrine glands are therefore highly vascularised Endocrine tissues Three arrangements 1. Endocrine organ devoted to hormone synthesis eg thyroid gland 2. Distinct clusters of cells within an organ eg. Islets of Langerhans (pancreas) islet 3. Individual cells scattered diffusely throughout an organ, eg. gastrointestinal tract, skin MAJOR ENDOCRINE GLANDS A F E B G C D Endocrine gland Major hormone product/s Hypothalamus Releasing hormones & inhibiting factors (eg. corticotrophin-releasing hormone, CRF; gonadotrophin-releasing hormone, GnRH; Growth hormone releasing hormone, GHRH; Thyrotrophin-releasing hormone, TRH; dopamine) Posterior pituitary Arginine vasopressin (AVP)/Anti-diuretic hormone (ADH); Oxytocin Anterior pituitary Adrenocorticotrophic hormone (ACTH) Growth hormone Prolactin Follicle stimulating hormone (FSH) Luteinising hormone (LH) Thyroid stimulating hormone (TSH) Thyroid gland Thyroxine (T4); Tri-iodothyronine (T3); Calcitonin Parathyroid gland Parathyroid hormone (PTH) Adrenal gland Adrenaline; Cortisol Pancreas Glucagon; Insulin Gonads Progesterone Oestrogen Testosterone Modes of hormone action: A? B? Local diffusion C? Circulation D? Circulation Chemical nature of hormones is related to…. How they are synthesised & secreted Nature of hormone receptor they bind to Ability to bind to other proteins (eg. transport proteins) How they partition in tissues How they are degraded Hormones Steroid Progestagens Glucocorticoids Mineralocorticoids Androgens Oestrogens Chemical classes of hormone Non-steroid Polypeptide/Proteins Growth hormone, Prolactin, Calcitonin, Parathyroid hormone, Adrenocorticotrophic hormone (ACTH) Insulin, Glucagon Eicosanoids Prostaglandins Leukotrienes Prostacyclins Glycoproteins FSH, LH, TSH, Human Chorionic Gonadotropin (hCG) Peptides AVP, Oxytocin, Melanocyte stimulating hormone, Somatostatin Thyrotrophin-releasing hormone, Gonadotrophin-releasing hormone Amines Adrenaline Thyroxine T4 Noradrenaline Tri-iodothyronine T3 Melatonin Major differences between peptide and steroid hormones Hormone Property Peptide/protein (eg. oxytocin, vasopressin) Steroid (eg. glucocorticoid, cortisol) Structure Chains of amino acids Cholesterol-derived Solubility Hydrophilic Lipophilic Synthesis Gene transcription & post-translational modification in Golgi De novo biosynthesis (stepwise modification of cholesterol in mitochondria & SER) Storage Large amounts in secretory granules Not stored (nb. cholesterol precursor stored as esters) Secretion Exocytosis Simple diffusion Transport in blood As free hormone Bound to transport proteins Receptor Cell surface receptor Intracellular receptor Mechanism of action Activation of second messenger system to alter activity of existing proteins Activation of specific genes to produce new proteins Hormones & actions ▪ Present in blood in v. low concs (10-8 10-12M)- highly potent ▪ Diffuse/widespread ▪ Slower acting than neurotransmitters ▪ Can be transported over long distances ▪ Require specific, high affinity receptors ▪ Effects can be long-lasting & determinative (permanent), permissive or synergistic with other hormones Amplitude of response is determined by: ▪ Concentration of hormone ▪ Number of receptors on target cells ▪ Affinity of hormone receptors ▪ Duration of exposure Hormones can alter many different cell functions Causes of endocrine disorders Hormone deficiency Destruction of gland Extraglandular disorder Defect in biosynthesis Hormone excess Tumours of endocrine gland Ectopic hormone production Overstimulation (hyperplasia) Exogenous hormone Hormone resistance Receptor defects Organisation of hypothalamus & pituitary (neuroendocrine control) Hypothalamus-pituitary axis Interface between central nervous system and endocrine system Neuroendocrine control & homeostatic regulation of: Stress & immune function Reproduction Growth and development Water/electrolyte balance Energy balance/appetite control Thermoregulation Sleep/wakefulness Hypothalamus * Found in basal forebrain close to ‘optic chiasm’ ‘Ancient’ part of brain. Small region with many diverse neurones Involved in pituitary regulation: Hypothalamic neurosecretory cells (modified neurones) release peptides from axon terminals near to capillaries Peptides include ‘releasing hormones’ –which regulate anterior pituitaryand two posterior pituitary hormones Pituitary (hypophysis): Two glands in one Posterior Lobe (neurohypophysis) Anterior Lobe (adenohypophysis) (under DIRECT control) (under INDIRECT control) Pituitary stalk Neural tissue comprises axons & nerve terminal endings of hypothalamic neurosecretory cells Glandular tissue comprises cells controlled by releasing hormones (or inhibitory factors) delivered via hypophysial portal system Summary: pituitary blood supply & regulation Posterior pituitary (neurohypophysis) Blood supply from inferior/middle hypophysial arteries Direct innervation (magnocellular neurones) from hypothalamus- axons form pituitary stalk, release neurohypophysial hormones into systemic blood Anterior pituitary (adenohypophysis) Blood supply from pituitary portal system No direct innervation but parvocellular neurones terminate in median eminence of hypothalamus and release hypophysiotrophic hormones into portal blood supply *Nb the intermediate lobe is part of the fetal pituitary, a source of melanocyte stimulating hormone, but this regresses in human after birth Pituitary (‘master gland’) secretes 8 hormones Anterior lobe Posterior lobe Posterior pituitary hormones: Anterior pituitary hormones: (i) Hormones with tropic action: Follicle stimulating hormone (FSH) Luteinising hormone (LH) Adrenocorticotrophic hormone (ACTH) Thyroid stimulating hormone (TSH) Oxytocin Arginine-vasopressin (ADH) (ii) Other hormones Growth hormone Prolactin “Tripartite” neuroendocrine system is prominent in the control of major glands, namely Thyroid gland Adrenal gland Gonads (ovary/testis) Releasing hormone Tropic hormone Hormone Feedback control HP axis activity depends on a fine balance between: Feedforward drive (increased hormone output) AND hormone negative feedback* Appreciate that hormone levels fluctuate! & may be influenced by biorhythms Biorhythms are entrained by the hypothalamic suprachiasmatic nucleus (SCN), the body’s ‘master clock’ Circadian (daily) rhythm in plasma corticosterone secretion (24h, rat, S. Lightman) SCN Anterior pituitary Pulsatile patterns of hormone release depend, in part, on neural input from rhythm generators in the hypothalamic suprachiasmatic nucleus (SCN). eg. paraventricular nucleus receives circadian input from the SCN, regulating hypothalamo-pituitary-adrenal (HPA) axis activity and cortisol release Nb. Light-entrainable SCN regulates daily rhythms in many physiological systems and behaviours, eg. hypothalamic circuits for appetite and feeding Overview of endocrine system- key points Hormones are produced in: Endocrine glands and tissues Hypothalamic neurosecretory cells Hormones are classified by chemical structure or mode of action They are transported in blood, body fluids or diffuse locally in extracellular fluid and act via specific receptors in target tissues Hormone levels are subject to feedback control to achieve homeostasis Neuroendocrine control involves tight interaction between hypothalamus and pituitary, controlling activity of major glands including thyroid, adrenal and gonads Pituitary is of critical importance & has a unique blood supply. It releases 8 major hormones (2 posterior pituitary hormones & 6 anterior pituitary hormones) into the blood However two major glands are not under pituitary control (pancreas and parathyroid glands). Their activity is controlled by counter-regulatory hormones (eg. pancreas and plasma glucose homeostasis/ parathyroids and blood calcium levels) Brain inputs to hypothalamus allows factors such as biorhythms and stress to influence endocrine systems Key take-home messages Hormones provide exquisite control over all body functions They are potent and their levels fluctuate throughout the day in response to rhythms and physiological stimuli Feedback controls levels within tight limits to achieve homeostasis Endocrine problems are common (eg. diabetes type 1, obesity, thyroid disorders) and are most often caused by too little or too much hormone being released, or a problem with its hormone receptor