Two types of ion channels leakage nongated channels

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Two Types of Ion Channels Leakage (nongated) channels randomly alternate between open and closed positions nerve cells have more K+ than Na+ leakage channels as a result, membrane permeability to K+ is higher explains resting membrane potential of -70mV in nerve tissue Gated channels open and close in response to a stimulus results in neuron excitability voltage-gated open in response to change in voltage ligand-gated open & close in response to particular chemical stimuli (hormone, neurotransmitter, ion) mechanically-gated open with mechanical stimulation See pg. 428Tortora 12e or pg. 460 in 13e for a more complete explanation.
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Resting Membrane Potential
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Resting Membrane Potential Negative ions along inside of cell membrane & positive ions along outside potential energy difference at rest is -70 mV cell is “polarized” Resting potential exists because concentration of ions different inside & outside extracellular fluid rich in Na+ and Cl- cytosol full of K+, organic phosphate & amino acids membrane permeability differs for Na+ and K+ 50 – 100x greater permeability for K+ inward flow of Na+ can’t keep up with outward flow of K+ Na+/K+ pump removes Na+ as fast as it leaks in
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Graded Potentials Small deviations from resting potential of -70mV hyperpolarization = membrane has become more negative depolarization = membrane has become more positive
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Action Potential Series of rapidly occurring events that change and then restore the membrane potential of a cell to its resting state Ion channels open, Na+ rushes in (depolarization), K+ rushes out (repolarization) All-or-none principal = with stimulation, either happens one specific way or not at all (lasts 1/1000 of a second) Travels (spreads) over surface of cell without dying out
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Depolarizing Phase of Action Potential Chemical or mechanical stimulus caused a graded potential to reach at least (-55mV or threshold) Voltage-gated Na+ channels open & Na+ rushes into cell in resting membrane, inactivation gate of sodium channel is open & activation gate is closed (Na+ can not get in) when threshold (-55mV) is reached, both open & Na+ enters inactivation gate closes again in few ten-thousandths of second only a total of 20,000 Na+ ions actually enter the cell, but they change the membrane potential considerably(up to +30mV) Positive feedback process
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Repolarizing Phase of Action Potential When threshold potential of -55mV is reached, voltage-gated K+ channels open K+ channel opening is much slower than Na+ channel opening which caused depolarization When K+ channels finally do open, the Na+ channels have already closed (Na+ inflow stops) K+ outflow returns membrane potential to -70mV If enough K+ leaves the cell, it will reach a -90mV membrane potential and enter the after-hyperpolarizing phase K+ channels close and the membrane potential returns to the resting potential of -70mV
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Refractory Period of Action Potential Period of time during which
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