Action Potential

Action Potential

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  Action Potential Through Neuron PREPARED BY RASHIDULHASAN ROBEL B. PHARM (RU), M. PHARM (RU) REG. NO. A4968 PGD-HRM (BIM) EMBA (ULAB)
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  Action Potential An action potentialis a rapid rise and subsequent fall in voltage or membrane potential across a cellular membrane with a characteristic pattern. It occurs in different cells, called excitable cells, which include neurons, muscle cells, endocrine cells etc. 5 Steps of Action Potential 1. Resting Membrane potential 2. Threshold (early depolarization) 3. Depolarization 4. Repolarization 5. Hyperpolarization (Undershoot) & Refractory period (Na⁺/K⁺ Pumps)
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  Membrane Potential Positive or negativelycharged electrolyte can cause a difference in electrical charge between intra-cellular & extra-cellular portion of a neuron. This is called membrane potential. Nerve impulse can influence the movement of electrically charged ions across the cell membrane.
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  Step-1 Resting Membrane Potential During resting stateof neuron, K⁺ concentration is higher inside the cell membrane with anionic proteins & Na⁺ & Clˉ concentration is higher outside the membrane. The resting membrane potential of a neuron is about -70 mV (mV means millivolt). It means that the inside of the neuron is 70 mV less than the outside.
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  Step-2 Threshold Early depolarization iscalled threshold. The action potential starts by a stimulus causing the resting potential moving from -70mV to 0mV. When the depolarization reaches about -55 mV, a neuron will fire an action potential. This is the threshold. If the neuron does not reach this critical threshold level, then no action potential will start. In this step, some Na-channel open which cause Na⁺ to move through the channel into the cell due to stimulus. Remember, sodium has a positive charge, so the inside of neuron becomes more positive and becomes depolarized. Outside of neuron becomes negative.
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  Step-3 Depolarization After reaching thresholdlevel, More Na-channel open which cause huge Na⁺ to move through the channel into the cell. So the inside of neuron becomes more positive and becomes depolarized. And the membrane potential reaches from -55mV to +40mV.
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  Step-4 Repolarization After depolarization (hugeNa⁺ inside the neuron & membrane potential reaches +40mV), Na-channel inactivated & K-channel open which cause K⁺ to move through the channel outside the cell. So the outside of neuron becomes more positive and becomes repolarized. Clˉ moves from outside to inside that turns into negative. And the membrane potential falls from +40mV to -70mV.
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  Step-5 Hyper polarization During hyperpolarization, The voltage-gatedK-channels stay open a little longer than needed to bring the membrane back to its resting potential. But Na-channels become closed .This results in a phenomenon called undershoot, in which the membrane potential more lower (more negative -75mV) than its resting potential -70mV. After that, K-channels are closed. No more K⁺ moves from inside to outside of membrane.
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  Refractory Period The slow closureof the voltage-gated potassium channels, which results in undershoot, also contributes to the refractory period by making it harder to depolarize the membrane (even once the voltage-gated sodium channels have returned to their active state). During refractory period, both Na & K-channels remain closed. The refractory period ensures that an action potential will only travel forward down the axon, not backwards.
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  Refractory Period During refractory period, InterestinglyNa⁺/K⁺ ATPase pumps are activated by an ATP molecule. Then the pump allows 2K⁺ move inside the cell in exchange of 3Na⁺ outside that results into reaching resting membrane potential.
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  New Action Potential When the actionpotential reaches the axon terminal (nerve ending), it causes neurotransmitter-containing vesicles to fuse with the membrane, releasing neurotransmitter molecules into the synaptic cleft (space between neurons). When the neurotransmitters bind to ligand-gated ion channels on the receiving cell, it may cause depolarization of that cell, causing it to undergo its own action potential.
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  +40 mV -75 mV ❶ ❷ ❸❹ ❺ Schematic Diagram of Action Potential ❶Resting Membrane potential ❷Threshold (early depolarization) ❸Depolarization ❹Repolarization ❺Hyperpolarization (Undershoot) (Refractory period) -55 mV ❶
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  Figure of Action Potential
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  Movement of electro-chemical charge throughaxon
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