An action potential is an electrical impulse that begins in the hillock of the axon (trigger zone), propagates away from the soma, and moves along the axon, the synaptic knob and the synapse. The action potential is triggered by a depolarization of the neuron from -70 mV to -55 mV. This depolarization occurs when the neuron receives repeated stimuli or collective stimuli (local potentials) that open sodium channels and allow Na+ to flow into the neuron until the neuron reaches a threshold of -55 mV. Say no to plagiarism. Get a tailor-made essay on "Why Violent Video Games Shouldn't Be Banned"? Get an original essayWhen sodium channels open, Na+ flows into the neuron as a result of its concentration gradient (Na+ is more concentrated in the extracellular fluid, so it flows inside the neuron) and the electrical gradient (inside the neuron it is more negative, so positive sodium ions enter). As sodium diffuses into the neuron, this triggers the potassium channels to open and allows K+ to escape (as the K+ is more concentrated inside and the inside starts to become more and more positive). When the cell reaches -50 mV, the voltage change opens voltage-gated sodium channels in the trigger zone. This causes more Na+ to flow in and further depolarizes the cell, which ends up opening nearby voltage-gated sodium channels along the axon and away from the cell body. Voltage-gated potassium channels also open during the voltage change from -70 mV to -50 mV. The K+ channels here are slower to respond, but eventually allow enough potassium to escape the cell (due to the K+ concentration gradient and electrical gradient) to repolarize the cell. When the action potential reaches 0 mV, the voltage-gated sodium channels begin to close and the potassium channels come close to being all open. This causes the voltage to return to the resting membrane value after a slight hyperpolarization (when the K+ leaving the cell makes the cell slightly more negative than the resting membrane value). Resting membrane potential is reached when the voltage is restored to -70mV due to K+ and Na+ leaving or entering the cell based on their respective electrical gradients. When the shift of K+ and Na+ ultimately results in an intracellular voltage of -70 mV, both voltage-gated K+ and Na+ channels close. Although the charge is restored to -70 mV, the ions are not restored (Na+ must be more concentrated in the extracellular fluid and K+ in the intracellular fluid). This is where the Na+/K+ pump comes in. The Na+/K+ pump essentially binds Na+ in the intracellular fluid and exchanges it with K+ from the extracellular fluid. It eventually restores the ions to their original concentrations at the resting potential. The absolute refractory period is a period in which the cell does not allow a stimulus, regardless of its strength, to trigger another action potential. This period depends on voltage-gated sodium channels and lasts from the onset of the action potential until all voltage-gated sodium channels are closed. The relative refractory period is the period in which a cell can produce another action potential if the stimulus reaches the threshold. The relative refractory period includes the hyperpolarization phase until the closure of all voltage-gated potassium channels. Because the hyperpolarized membrane is at a slightly lower voltage than the resting membrane potential, the distance from the hyperpolarized state to the -50 mV threshold is greater and, therefore, requires a stimulus greater than.