Endocrine system_2023_24.pptx

Endocrine system Endocrine glands (ductless) secrete hormones into the blood Hormones are transferred with blood to all other cells of the body Response to hormones of specific cells of the body !!! Revise signal transduction cascades to get t...

Endocrine system Endocrine glands (ductless) secrete hormones into the blood Hormones are transferred with blood to all other cells of the body Response to hormones of specific cells of the body !!! Revise signal transduction cascades to get the most of this lecture!!! HORMONES Hormones travel in the blood to their distant target cells, where they regulate or direct a particular function The same chemical messengers may be either a hormone or a neurotransmitter, depending on its source and mode of delivery to target cells (e.g. norepinephrine (=noradrenaline) is released as a neurotransmitter by nerve fibres into synaptic cleft and binds to  - adrenergic receptors, or as a hormone by the adrenal medulla) Tropic and Non-Tropic Hormones A tropic hormone: a hormone that has as its primary function the regulation of hormone secretion by another endocrine gland. For example: Thyrotropin-releasing hormone (TRH) stimulates the release of TSH from the anterior pituitary. Thyroid stimulating hormone (TSH) from the anterior pituitary stimulates the thyroid hormone secretion by the thyroid gland. A non-tropic hormone primarily exerts its effects on non-endocrine target tissue. For example, thyroid hormone, which increases the rate of oxygen consumption and metabolic activity of almost all cells. T3: triiodothyronine T4: thyroxine Hormones fall into three distinct classes according to their biochemical structure: 1. Peptides (specific amino acids arranged in a chain of varying length); hydrophilic 2. Amines (derived from the amino acid, tyrosine) 3. Steroids (neutral lipids derived from cholesterol); hydrophobic All hormones are carried by the blood, but are not transported in the same way. - Hydrophilic peptide hormones are transported simply dissolved in the plasma. - Hydrophobic steroid hormones circulate in the blood to their target cells reversibly bound to plasma proteins.  Hormones adrenalin Glucocorticoids Mineralocorticoids Androgens Estrogens Growth hormone (191 amino acids) Progestogens Vitamin D  Hormones The hydrophilic hormones are not able to pass through lipid membrane barriers of their target cells. Instead they bind to specific receptors located on the outer plasma membrane surface of the target cells. The hydrophobic hormones easily pass through the surface membrane to bind with specific receptors located inside the target cell. Target cell response differ for different hormones and differ between different target cells that respond to the same hormone. Hydrophilic Hormones, such as growth hormone or TRH, bind to receptors on the cell surface Activated receptors initiate a signalling cascade leading to regulation of enzyme activity or affect ion channels. Hydrophilic Hormones For example: Epinephrine (adrenalin) binding in liver stimulates synthesis of second messenger cAMP. Cyclic AMP binds to, and thereby activates, protein kinase A (PKA). PKA phosphorylates, and thereby activates, an enzyme phosphorylase which converts glycogen into glucose-6-P. Glucose-6-P is converted to glucose and released to blood. Hydrophobic hormones such as corticosteroids, vitamin D, thyroxine function by binding and activating specific receptors inside the target cells. Activated receptors regulate gene expression which causes the formation of new intracellular proteins, which, in turn, produce the desired effect. Hormones Once in circulation, hormones affect function of the target tissue. Some hormones exert an effect on cells of the organ from which they were released (paracrine effect), some even on the same cell type (autocrine effect).  Endocrine system functions The endocrine system coordinates functioning between different organs through hormones, which are released into the bloodstream from specific types of cells within endocrine (ductless) glands. The endocrine system controls activities that require duration rather than speed, for example: - regulates metabolism and water and electrolyte balance, which is important to maintain a constant internal environment (homeostasis). - induces adaptive changes to help the body to cope with stressful situations. - promotes smooth, sequential growth and development. - regulates red blood cell production. - controls and integrates both circulation and digestion and absorption of food (along with autonomic nervous system). Major Endocrine Glands Hypothalamus? Endocrine Glands and Their Hormones Hypothalamus About the size of a pearl Directs a multitude of important functions in the body: - It is the control centre for many autonomic functions of the peripheral nervous system - As a part of the limbic system structure, the hypothalamus also influences various emotional responses - Connections with structures of the endocrine and nervous systems enable the hypothalamus to play a vital role in maintaining homeostasis The real “master gland” Endocrine Glands and Their Hormones Hypothalamus Neurosecretory neuronal cell bodies in hypothalamus produce hormones vasopressin and oxytocin. When the neurons are excited, neuronal terminals in posterior pituitary release vasopressin and oxytocin into systemic blood: vasopressin vasopressin vasopressin receptor receptor in in nephrons arterioles throughout body conservation of water vasoconstriction during urine formation Endocrine Glands and Their Hormones Hypothalamus Blood vessel connections between the hypothalamus and pituitary gland allow hypothalamic hormones to control pituitary hormone secretion: - Corticotropin-releasing hormone (CRH) - Thyrotropin-releasing hormone (TRH) - Gonadotropin-releasing hormone (GnRH) - Growth hormone-releasing hormone (GHRH) - Growth hormone-inhibiting hormone (GHIH, somatostatin) - Prolactin-inhibiting hormone (PIH, dopamine) Endocrine Glands and Their Hormones The Pituitary Gland - located at the base of the brain - secretes the largest number of different hormones and therefore has the widest range of effects on the body functions. - can be divided into 2 parts: anterior lobe and posterior lobe Endocrine Glands and Their Hormones The Pituitary Gland – initially thought to be the “master gland” =vasopressin From intermediary lobe Endocrine Glands and Their Hormones The Anterior Pituitary The anterior pituitary (used to be called the "master gland“) responds to chemical messages from the bloodstream to produce numerous hormones that trigger the action of other endocrine glands. Secretes: - growth hormone (GH, somatotropin) - thyroid stimulating hormone (TSH) - adrenocotropic hormone (ACTH) - follicle stimulating hormone (FSH) - luteinizing hormone (LH) - prolactin (PRL) Controls the activity of many other endocrine glands (thyroid, ovaries, adrenal, etc.). Endocrine Glands and Their Hormones The Posterior Pituitary The posterior pituitary is controlled by the nervous system. It secretes two hormones produced by the hypothalamus: - vasopressin, which causes blood pressure to rise and regulates the amount of water in the blood and consequently in cells and ECM. - oxytocin, which causes the uterus to contract during childbirth and lactation to begin. Released during orgasm in both sexes. Endocrine Glands and Their Hormones The Pineal Gland - a pine cone-shaped gland - produces and secretes several important hormones including melatonin Melatonin is a natural sleep-inducing agent. One of the melatonin most widely accepted roles is helping to keep the body inherent circadian rhythms in synchrony with the light-dark cycle by regulating activity levels over the course of the day. Daylight reduces melatonin production such that blood levels are usually high at night and low during the day. Exposure of the retina to light (particularly blue light) suppresses melatonin secretion by the pineal gland Melanopsin-expressing intrinsically SCN relays the message to the photosensitive retinal ganglion pineal gland → suppression of cells innervate the hypothalamic melatonin release suprachiasmatic nucleus (SCN) superior cervical ganglion Endocrine Glands and Their Hormones Thyroid and parathyroid Thyroid Gland: Located in the front of the neck. Thyroid gland secretes thyroid hormone, thyroxine which regulates the body overall metabolism. Thyroxine can reduce concentration and lead to irritability when the thyroid is overactive, and cause drowsiness and a sluggish metabolism when the thyroid is under-active. Parathyroid Glands: There are 4 parathyroid tiny pea-shaped glands located behind the thyroid. They secrete parathyroid hormone, parathormone and have absolute control over calcium and phosphate in the blood and tissue fluids. This, in turn, affects the excitability of the nervous system. Endocrine Glands and Their Hormones Adrenal Glands Small, triangular-shaped endocrine glands located on the top of each kidney. Each adrenal gland is approximately 3 inches wide, and a half inch thick Adrenal glands functions: - maintain salt levels in the blood - maintain blood pressure - help control kidney function - control overall fluid concentrations in the body Each has two parts: an outer covering, the adrenal cortex, and an inner core, the adrenal medulla. Endocrine Glands and Their Hormones Adrenal Glands The cortex and medulla of the adrenal gland secrete different hormones. The adrenal cortex is essential to life, but the medulla may be removed with no life-threatening effects. Both the cortex and medulla influence the body responses to stress. Endocrine Glands and Their Hormones Adrenal Glands: Adrenal Cortex The adrenal cortex consists of three different regions, with each region producing a different group or type of hormones. All cortical hormones are synthesized from cholesterol. Chemically, all the cortical hormones are considered steroids: - Mineralocorticoids are secreted by the outermost region of the adrenal cortex. The principal mineralocorticoid is aldosterone, which acts to conserve sodium ions and water in the body. - Glucocorticoids are secreted by the middle region of the adrenal cortex. The principal glucocorticoid is cortisol, which increases blood glucose levels. - Gonadocorticoids (sex hormones). These are secreted by the innermost region. Male hormones, androgens, and female hormones, estrogens, are secreted in minimal amounts in both sexes by the adrenal cortex, but their effect is usually masked by the hormones from the testes and ovaries. Endocrine Glands and Their Hormones Adrenal Glands: Adrenal Medulla - develops from neural tissue - synthesizes from tyrosine and secretes two hormones: epinephrine (adrenaline) and norepinephrine (noradrenaline) - these two hormones (catecholamines) are secreted in response to stimulation by sympathetic nerves, particularly during stressful situations. - receptors for catecholamines are widely distributed throughout the body - effects: increased heart rate and blood pressure, blood vessel constriction in the skin and gastrointestinal tract, bronchiole dilation, and increased metabolism, all of which are characteristic of the fight-or-flight response. - a lack of hormones from the adrenal medulla produces no life threatening effects. - hypersecretion, usually from a tumour, causes prolonged or continual sympathetic responses. Endocrine Glands and Their Hormones Neuroendocrine Glands of the Pancreas Located deep in the abdomen behind the stomach, the pancreas is primarily a digestive organ. It also contains extremely important endocrine cells which secrete: insulin, glucagon, somatostatin, and others. They control blood sugar and overall glucose metabolism, help control other endocrine cells of the digestive tract. Endocrine Glands and Their Hormones Neuroendocrine glands of the pancreas (the islets of Langerhans): Small groups of cells in the pancreas that function as an endocrine gland. The α (or A) cells secrete the hormone glucagon The β (or B) cells secrete insulin The δ (or D) cells secrete somatostatin The pancreatic polypeptide (PP) cells secrete pancreatic polypeptide SOME  SOME Heart Kidney Stomach Duodenum Skin Liver Can you access the most up-to-date books online/in the library? Regulation of hormone secretion Direct feedback: increased blood glucose + Pancreatic beta cells - decreased insulin blood glucose Increased cellular uptake of glucose  Negative feedback control Hormonal hypothalamus is a prominent feature of negative feedback - TRH hormonal control systems. negative feedback: the output of a system counteracts a change in input, thus maintaining a pituitary controlled variable within a narrow change around a set TSH level. thyroid thyroxin Positive and negative feedback: Positive and negative feedback:  The endocrine system is complex A single endocrine gland may A single hormone may have produce multiple hormones more than one type of target with different control cell and therefore can induce mechanisms and different more than one type of effect functions (e.g. the anterior (e.g. vasopressin promotes water pituitary gland releases six reabsorption by kidneys and different types of hormones). vasoconstriction of arterioles). A single hormone may be A single target cell may be secreted by different influenced by more than one hormone. Cells may contain an array endocrine glands (e.g. of different receptors for responding in somatostatin is produced by different ways to different hormones. hypothalamus and pancreas).  Neurology and endocrinology Robert Sapolsky Stanford University http://www.youtube.com/watch?v=kAfz0yAcOyQ Relevant for understanding the major principles and complexities in neuronal and hormonal signalling (including neural circuits in the retina)