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26 Cards in this Set
- Front
- Back
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The Endocrine System |
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Systems |
The endocrine system together with the nervous system coordinate and integrate the activity of all the cells in the human body
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System Names |
Pineal gland, Hypothalamus, Pituitary gland, Thyroid gland, Parathyroid glands (on dorsal aspect or thyroid gland), Thymus gland, Adrenal glands, Pancreas, Ovary (female), Testis (male) |
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Endocrine System (ES) |
The ES consists of widely distributed, highly vascular endocrine glands, e.g. Thyroid gland Discrete groups of endocrine cells; Gastrin producing G cells in the gastric glands of the stomach, Insulin producing beta cells of islets of Langerhans in the pancreas |
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Hormones #1 |
The ES uses hormones to co-ordinate and integrate cellular activity Hormones are chemical messengers secreted by endocrine cells into their interstitial fluid Most hormones are transported via the circulatory system to respective target cells They regulate the metabolic function of the target cell Hormones produce prolonged effects on target cells |
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Hormones #2 (Localling Acting Chemical Messengers) |
Autocrine mode of action: the secreted hormone acts on the same cell that released the hormone e.g. prostaglandins released by smooth muscle cells Paracrine mode of action: the released hormone acts on adjacent cells |
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Types of Hormones (Amino Acid Derivatives) |
Amino acid derivatives: relatively small hormone molecules built from tyrosine or tryptophan Tyrosine derivatives include thyroid hormones, dopamine, adrenaline and noradrenaline Dopamine, adrenaline and noradrenaline collectively constitute the catecholamines The main hormone produced from tryptophan is melatonin secreted by the pineal gland |
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Types of Hormones (Peptide and Protein Hormones) |
Peptide protein hormones: these are also built from amino acids, variable in size from short amino acid chains to small proteins; e.g. oxytocin (9 aa long) to parathormone (84 aa long) |
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Type of Hormones (Lipid Derivatives) |
Steroid hormones: synthesised from cholesterol, e.g. the gonadal hormones oestrogen and testosterone Eicosanoids: synthesised from fatty acid arachidonic acid, include leukotrienes released by activated white blood cells or leukocytes which control tissue response to injury, and prostaglandins which function as paracrine hormones that regulate cellular activity auch as blood clotting |
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Specificity of Hormone Action |
Hormones circulate to all tissues but generally only affect the activity of a discrete group of cells (their target cells)
Target cells express receptors that specifically recognise the hormone that the cells respond, e.g. insulin receptors, thyroxine receptors are found on nearly all cells of the body |
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Mechanisms of Action Hormones |
Hormones exert their effects indirectly through receptors Hormone receptors may be: integral membrane proteins or intracellular receptor Receptors couple hormones to intracellular molecules (2nd messengers) that mediate the cells (pre-programmed) response to the hormone |
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cAMP 2nd Messenger #1 |
Binding of hormone (1st messenger) e.g. glucagon, to its receptor causes a change in the shape of the receptor this facilitates its binding to a G protein (enzyme complex) The G protein becomes activated as it binds GTP, displacing GDP |
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cAMP 2nd Messenger #2 |
Activated G protein stimulates the effector enzyme adenylate cyclase Adenylate cyclase generates cAMP (the 2nd messenger) from ATP cAMP activates protein kinases (enzymes) by phosphorylation, which mediate cellular responses to the hormone A protein kinase is an enzyme that phosphorylates another molecule |
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PIP - Ca2+ 2nd Messenger |
Hormone binds to the receptor and activates the G protein G protein binds and activates a phos pholipase enzyme Phopholipase hydrolysis the phospholipid PIP2 into diacylglycerol (DAG) and IP3 (2nd messengers) DAG activates protein kinases: IP3 triggers release of Ca2+ stores Ca2+ alters cellular responses |
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Steroid Hormones #1 |
Lipid soluble steroid hormones ( and thyroid hormones) diffuse readily across the phospholipid bilayer of the plasma membrane of their target cells In the target cell, they bind to and activate their specific intracellular receptor |
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Steroid Hormones #2 |
The hormone-receptor complex translocate to the nucleus where it binds to a DNA-associated receptor protein This interaction triggers gene transcription to produce mRNA molecules (slow process) Each mRNA is translated into the corresponding protein, these mediate the appropriate cellular response to the hormone |
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Mechanism of Hormone Action |
Cellular responses mediated by intracellular signalling pathways in response to hormones include: alteration of plasma membrane permeability, stimulation of protein synthesis, activation or deactivation of enzyme systems (e.g. glucagon), increased glandular secretions (e.g. parietal cells), stimulation of cell proliferation (EGF), promotion of cell survival & growth (e.g. nerve growth factor) |
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Hormone Activation of Target Cells #1 |
Target cell activation depends on three factors: circulating hormone concentration, relative number of receptors expressed on target cells, receptor affinity for the hormone |
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Hormone Activation of Targte Cells #2 |
Relative number of receptors on target cells can vary: up-regulation due to low circulating hormone levels - target cells express more receptors in response to the decline in hormone level in blood - target cell becomes more sensitive to hormone, down-regulation due to prolonged exposure to high concentrations of hormone - target cells express fewer receptors in response to the elevated circulating hormone levels it prevents excessive stimulation of target cells - target cells become less sensitive to hormone |
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Circulation of Hormones in the Blood |
Hormones circulate in the blood in two forms - free or bound Steroids and thyroid hormone are attached to plasma proteins, prevents loss in kidneys & increases solubility of lipid-derived hormones in blood All others are unbound and circulate freely in blood |
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Modulation of Hormone Actions #1 |
Hormonal effects can be complex - multiple hormones may act on the same target cell at the same time Hormonal interactions may produce three types of effects in target cells |
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Permissive Effect |
Permissive effect - one hormone requires the presence of another hormone in order to produce its effects e.g. reproductive system hormones require the presence of thyroid hormones in order to regulate the development of the reproductive system |
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Modulation of Hormone Actions #2 |
Synergistic effect (potentiation) - more than one hormone produce the same effects on a target cell; combined effects may be far greater than individual effects, gastrin and histamine stimulation of gastric acid secretions from parietal cells Antagonistic effect - one or more hormones opposes the action of another hormone e.g. insulin decreases blood glucose levels while glucagon produces opposite effect |
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Regulation of Hormone Release #1 |
The concentration of hormones in the blood: are controlled by negative feedback systems, vary only within a narrow desirable range Hormones are synthesised and released in response to humoral, neural and hormonal stimuli |
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Regulation of Hormone Release #2 |
Humoral stimuli - secretion of hormones in direct response to changing blood levels of ions and nutrients e.g. PTH & concentration of calcium ions in the blood; Insulin & blood glucose Declining blood Ca2+ concentration stimulates the parathyroid glands to secrete parathyroid hormone (PTH) PTH causes Ca2+ concentrations to rise and the stimulus is removed |
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Hormonal Stimulation of Hormone Release |
Hormonal stimuli - release of hormones in response to hormones produced by other endocrine organs The hypothalamic (master endocrine organ) hormones stimulate the anterior pituitary to release a variety of hormones In turn, pituitary hormones stimulate targets to secrete other hormones |
