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So an action potential is initiated by Na+ flowing in, why couldn't it be initiated by K+ flowing out?
Action potential
- Thread starter md2be89
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Action potentials require a threshold potential to be reached. The threshold potential is a positive potential. Positive ions (K+) leaving the cell decrease the membrane potential, not increase it.
http://hyperphysics.phy-astr.gsu.edu/hbase/biology/actpot.html
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Because the Voltage-Gated (usually Sodium) channels won't open until the membrane is depolarized at threshold potential.
Na+ flowing in depolarizes the membrane.
K+ flowing out hyperpolarizes the membrane.
An action potential is a concerted opening of a massive number of voltage-gated (usually Sodium) channels to propagate the depolarization wave (positive feedback) down the axon of a neuron. The stop to this positive feedback is inactivation of the Sodium channel.
Na+ flowing in depolarizes the membrane.
K+ flowing out hyperpolarizes the membrane.
An action potential is a concerted opening of a massive number of voltage-gated (usually Sodium) channels to propagate the depolarization wave (positive feedback) down the axon of a neuron. The stop to this positive feedback is inactivation of the Sodium channel.
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So I guess I'm confused about the general concept - why does K+ cause the inside to be negative? Because it's more negative compared to the Na+ on the outside? Or is it something else causing the negative charge?
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The inside of a cell in negatively charged because the Na+/K+ ATPase pumps 3 sodium ions out and 2 potassium ions in, resulting in a net negative charge. The negative charge is known as the resting membrane potential.
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E_ion = +62/z log ([E_out]/[E_in])
z is the valency, E is the membrane potential at which there is no net movement of ion, if there were only that ion. The more open channels a specific ion species has, the closer the membrane potential (Vm) is to the E_ion (usually in millivolts).
If there are 10x more K+ ions inside, E_K+~-62 mV. Now since the E_Na+ is usually some positive number (let's say 62mV), there is a driving force of ~124mV for Na to go into the cell... but all the Na channels are closed! Since the cell is resting at Vm~ -60-80 mV there is little incentive for K+ ions to leave: the cell is already close to E_K+. Vm is usually measured using electrodes but it can be predicted with the Goldmann eq which takes into account the relative permeability and E_ion of each ion and their channels.
K does not cause the inside to be negative. Your cells pump K in and Na out and keeps them out, generating a negative membrane potential (closer to E_K+ since there are more open K+ leak channels).
z is the valency, E is the membrane potential at which there is no net movement of ion, if there were only that ion. The more open channels a specific ion species has, the closer the membrane potential (Vm) is to the E_ion (usually in millivolts).
If there are 10x more K+ ions inside, E_K+~-62 mV. Now since the E_Na+ is usually some positive number (let's say 62mV), there is a driving force of ~124mV for Na to go into the cell... but all the Na channels are closed! Since the cell is resting at Vm~ -60-80 mV there is little incentive for K+ ions to leave: the cell is already close to E_K+. Vm is usually measured using electrodes but it can be predicted with the Goldmann eq which takes into account the relative permeability and E_ion of each ion and their channels.
K does not cause the inside to be negative. Your cells pump K in and Na out and keeps them out, generating a negative membrane potential (closer to E_K+ since there are more open K+ leak channels).
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