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Ch. 15 Coordination Part 3 What begins an Action Potential? • Variety of stimuli received by the receptor cell • Stimuli: light, pressure, temperature • Receptor Cell: • End of sensory neuron • Specialized cell in sense organ • Detect specific type of stimulus and influence electrical activity of neurons • Ex. Light receptors in eyes or chemoreceptors in taste buds How Your Taste Buds Work • Papillae small bumps that cover tongue • Each contains many taste buds • Taste bud contains 50100 receptor cells sensitive to SPECIFIC chemicals (detecting 5 different tastes) • • • • • Sweet Sour Salt Bitter Savory Salt Detection • Chemoreceptors in taste buds detect Na+ ions from salt • Ions diffuse through highly selective channel proteins in cell surface membrane of microvilli of receptor cells • Leads to DEPOLARIZATION • Receptor potential generated an increase in positive charge inside of a cell • When receptor potential is large enough, it stimulates Voltagegated CALCIUM ION channels opening • Calcium ions (Ca2++) enter cytoplasm • Lead to EXOCYTOSIS of vesicles containing neurotransmitter from basal membrane • Neurotransmitter stimulates an action potential in sensory neuron • Impulse is transmitted to taste center in cerebral cortex of brain Sweetness Detection • Contain protein receptors that stimulate a G protein • G protein activates enzyme to produce cAMP • cAMP is second messenger that activates signal cascade that leads to closure of K+ ion channels • Closing K+ channels depolarizes cell produces action potential More About Action Potential • Do NOT change in size as they travel • Do NOT change in size according to intensity of stimulus • Do NOT speed up with intensity of stimulus • Speed of action potential transmission is ALWAYS the same • Frequency of action potentials change depending on strength of stimulus • Peak of Action Potential always +30 mV • Frequencies of action potentials vary • Strong stimulus produce rapid succession of action potentials • More neurons are stimulated for that ONE stimulus • Weak stimulus fewer action potentials per second • Stimulates fewer neurons for that one stimulus Brain Interpretation of Signals • Brain interprets: • FREQUENCY of action potentials arriving along the axon of a sensory neuron • NUMBER of neurons carrying action potentials • Both of these give brain info on STRENGTH of stimulus • NATURE of stimulus determined from position of sensory neuron bringing in the info • If neuron is from the retina, brain interprets signal as light • If different stimulus stimulates receptor in retina (like pressure), brain can still interpret as light sensation Factors that Affect Speed of Conduction of AP 1. Myelin • Unmyelinated neuron slow speed of conduction (0.5 m/s) • Myelinated neuron faster conduction (100 m/s) • Myelin speeds up rate at which action potential travels by insulating axon • Na+ and K+ cannot flow through parts of the cell membrane that has myelin • Action Potentials only occur at Nodes of Ranvier • All membrane proteins are located in unmyelinated portions of cell membrane (NoR) • Action potentials “jump” past portion of axon that is myelinated (1-3mm) • Called SALTATORY CONDUCTION • Increases speed of transmission of action potential by 50 x than in an unmyelinated axon 2. Diameter of axon • Thick axons faster transmission of action potential • Thin axons slower transmission of axon potential All or Nothing Law • Neurons either transmit impulses from one end to the other or they do not send an impulse at all • Action potential initiated when threshold value is reached Maintenance of Resting Potential • Axon phospholipid bilayer is impermeable to K+/Na+ ions • Na-K pump (a globular, transmembrane protein) maintains resting membrane potential • ATP used to pump 3 Na+ ions out and 2 K+ ions in • Na-K pump has binding site for ATP • Cell membrane has more K+ ion channels open than Na+ ion channels • Membrane is more permeable to K+ ions • K+ ions diffuse OUT of cell • Inside of cell becomes more negative (less positive) than the outside of the cell (due to all those K+ ions leaving) • Membrane potential INSIDE is -65 mV less than the potential outside the cell • Leaky K+ channels play a big role in maintaining resting membrane potential • Na-K pump creates an electrochemical gradient • As long as resting potential is maintained, then all voltage-gated channels are closed How Action Potentials Are Generated: • Sensory neurons respond to stimuli • Stimulus causes Na+ channels to open in cell membrane • Na+ ions enter the sell • This causes depolarization • Receptor/generator potential is created (local circuit) • If receptor potential (local circuit) is greater than the threshold (-60mv to -50mV), then the action potential is generated • This is the “all-or-nothing response” • Increased stimulus leads to increased FREQUENCY of action potentials Transmission of Action Potentials • Action potential stimulates neighboring area of membrane • Na+ ions move sideways/attracted to areas of resting potential • This creates a local circuit • This depolarization causes Na+ ion channels to open up (more depolarization) • Transmission in one direction is due to hyperpolarization/refractory periods • Myelin sheath enables a faster transmission of action potential because the action potentials must jump from one Node of Ranvier to the next • Saltatory Conduction