The extension that transmits message from the neuron

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the extension that transmits messagefrom the neuron is called an ‘axon’. Areflex action, or simply a reflex is afundamental function of the nervoussystem.8The chain of neurons that participantsin a reflex action is called a ‘reflex arc’.It consist of at least two neuron —(1)Sensory or afferent neuron whichconveys the information about thestimulus to the central nervous system.(2) Motor or afferent neuron – whichconveys the message from the centralneuron system to the organ (effectororgan) which has to respond to thestimulus.8The function between two neurons iscalled a ‘ganglion’.THE ENDOCRINE SYSTEMHormonesThe endocrine system is a collection ofglands that secrete chemical messages we callhormones. These signals are passed through theblood to arrive at a target organ, which has cellspossessing the appropriate receptor. Exocrineglands (not part of the endocrine system) secreteproducts that are passed outside the body. Sweatglands, salivary glands, and digestive glands areexamples of exocrine glands. Hormones aregrouped into three classes based on theirstructure:1.steroids2.peptides3.amines1. STEROIDSSteroids are lipids derived from cholesterol.Testosterone is the male sex hormone. Estradiol,similar in structure to testosterone, is responsiblefor many female sex characteristics. Steroidhormones are secreted by the gonads, adrenalcortex, and placenta.2. PEPTIDESAND3. AMINESPeptides are short chains of amino acids;most hormones are peptides. They are secreted bythe pituitary, parathyroid, heart, stomach, liver,and kidneys. Amines are derived from the aminoacid tyrosine and are secreted from the thyroid andthe adrenal medulla. Solubility of the varioushormone classes varies.Synthesis, Storage, and Secretion8Steroid hormones are derived fromcholesterol by a biochemical reactionseries. Defects along this series oftenlead to hormonal imbalances withserious consequences. Oncesynthesized, steroid hormones passinto the bloodstream; they are not storedby cells, and the rate of synthesiscontrols them.8Peptide hormones are synthesized asprecursor molecules and processed bythe endoplasmic reticulum and Golgiwhere they are stored in secretorygranules. When needed, the granulesare dumped into the bloodstream.Different hormones can often be madefrom the same precursor molecule bycleaving it with a different enzyme.8Amine hormones (notably epinephrine)are stored as granules in the cytoplasmuntil needed.
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66Gist of Biology from NCERT BooksEvolution of Endocrine SystemsMost animals with well-developed nervousand circulatory systems have an endocrinesystem. Most of the similarities among theendocrine systems of crustaceans, arthropods,and vertebrates are examples of convergentevolution. The vertebrate endocrine systemconsists of glands (pituitary, thyroid, adrenal),and diffuse cell groups secreted in epithelialtissues. More than fifty different hormones aresecreted. Endocrine glands arise during

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Biology Today and Tomorrow with Physiology
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