Endocrine System 31 05 2011 14 56 00 Endocrine System Overview Endocrine system a control system which influences metabolic activities of cells by means of hormones Endocrine glands pituitary thyroid parathyroid adrenal pineal The pancreas and gonads produce both hormones and exocrine The hypothalamus has both neural functions and releases hormones Other tissues and organs that produce hormones adipose cells pockets of cells in the walls of small intestine stomach kidneys and thymus products and heart Autocrines and Paracrines Autocrines chemicals that exert effects on the same cells that Paracrines locally acting chemicals that affect cells other than secrete them those that secrete them distance chemical signals These are not considered hormones since hormones are long Hormones Hormones chemical substances secreted by cells into the extracellular fluids o Regulate the metabolic function of other cells o Have lag times ranging from seconds to hours o Tens to have prolonged effects o Are classified as amino acid based hormones or steroids Eicosanoids biologically active lipids with local hormone like activity Types of hormones Amino acid based o Amines thyroxin peptide and protein hormones Steroids gonadal and adrenocortical hormones Eicosanoids leukotrines and prostaglandins Hormone Action Hormones alter target cell activity by one of two mechanisms 1 Second messengers Regulatory G proteins Amino acid based hormones 2 Direct Gene Activation Steroid hormones The precise response depends on the type of the target cell Mechanism of Hormone Action Hormones produce one or more of the following cellular changes in target cells o Alter plasma membrane permeability o Stimulate protein synthesis o activate or deactivate enzyme systems o Induce secretory activity o Stimulate mitosis Amino Acid Based Hormone Action Second Messenger cAMP Hormone first messenger binds to its receptor which then binds to a G protein The G protein is then activated as it binds GTP displacing GDP Activated G protein activates the effector enzyme adenylate cyclase Adenylate cyclase generates camp second messenger from ATP cAMP activates protein kinases which then cause cellular effects Amino Acid Based Hormone Action PIP Calcium Hormone binds to the receptor and activates G protein G protein binds and activates phospholipase Phospholipase splits the phospholipid PIP2 into discylglycerol DAG and IP3 both act as second messengers DAG activates protein kinases IP3 triggers release of Ca2 stores Ca2 third messenger alters cellular responses Steroid Hormones This interaction prompts DNA transcription to produce mRNA The mRNA of translated into proteins which bring about a cellular effect Target cell Specificity Hormones circulate to all tissues but only activate cells referred to as target cells Target cells must have specific receptors to which hormones bind These receptors may be intracellular or located on the plasma membrane Examples of hormone activity o ACTH receptors are only found on certain cell of the adrenal cortex o Thyroxin receptors are found on nearly all cells of the body Target Cell Activation Target cell activation depends on three factors o Blood levels of the hormone o Relative number of receptors on the target cell o The affinity of those receptors for the hormone Up regulation target cells form more receptors in response to the Down regulation target cells lose receptors in response to the hormone hormone Hormone concentrations in the blood Hormones circulate in the blood in two forms free or bound o Steroids and thyroid hormone are attached to plasma proteins o All others are unencumbered o Concentrations of circulating hormone reflect Rate of release Speed of inactivation and removal from the body o Hormones are removed from the blood by Degrading enzymes The kidneys Liver enzyme systems Interaction of Hormones at Target Cells Three types of hormone interaction o Permissiveness one hormone cannot exert its effects without another hormone being present o Synergism more than one hormone produces the same o Antagonism one or more hormones opposes the action of effects on a target cell another hormone Control of hormone release Blood levels of hormones o Are controlled by negative feedback systems o Vary only within a narrow desirable range Hormones are synthesized and released in response to o Humoral stimuli o Neural stimuli o Hormonal stimuli Humoral stimuli Humoral stimuli secretion of hormones in direct response to changing blood levels of ions and nutrients Example concentration of calcium ions in the blood o Declining blood Ca2 concentration stimulates the parathyroid glands to secrete PTH parathyroid hormone o PTH causes Ca2 concentrations to raise and the stimulus is removed Neural stimuli Neural stimuli nerve fibers stimulate hormone release o Preganglionic sympathetic nervous system SNS fibers stimulate the adrenal medulla to secrete catecholamine s Hormonal Stimuli Hormonal stimuli release of hormones in response to hormones produced by other endocrine organs o The hypothalamic hormones stimulate the anterior pituitary o In turn pituitary hormones stimulate targets to secrete till more hormones Nervous System Modulation The nervous system modifies the stimulation of endocrine glands and their negative feedback mechanisms The nervous system can override normal endocrine controls o For Example control of blood glucose levels Normally the endocrine system maintains blood glucose Under stress the body needs more glucose The hypothalamus and the sympathetic nervous system are activated to supply ample glucose Major Endocrine Organs pituitary hypophysis Pituitary gland two lobed organ that secretes nine major hormones o Neurohypophysis posterior lobe neural tissue and the infundibulum hypothalamus Receives stores and releases hormones from the o Adenohypophysis anterior lobe made up of glandular tissue Synthesizes and secretes a number of hormones Pituitary Hypothalamic Relationships Posterior Lobe The posterior lobe is a down growth of hypothalamic neural tissue Has a neural connection with the hypothalamus hypothalamic Nuclei of the hypothalamus synthesize oxytoxin and antidiuretic hypophyseal tract hormone ADH These hormones are transported to the posterior pituitary Pituitary Hypothalamic Relationships Anterior Lobe The anterior lobe of the pituitary is an outpocketing of the oral mucosa There is no direct neural contact with the hypothalamus Adenophypophyseal Hormones The six hormones
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