Neural impulses occur when a stimulus depolarizes a cell membrane, prompting an action potential which sends an “all or nothing” signal.
Transmission of a signal within a neuron (in one direction only, from dendrite to axon terminal) is carried out by the opening and closing of voltage-gated ion channels, which cause a brief reversal of the resting membrane potential to create an action potential. As action potential travels down the axon, the polarity changes across the membrane. Once the signal reaches the axon terminal, it stimulates other neurons.
Once the threshold potential is reached, the neuron completely depolarizes. Depolarization causes the opening of voltage-gated calcium channels, and the influx of calcium into the cell triggers neurotransmitter release.
As soon as depolarization is complete, the cell “resets” its membrane voltage back to the resting potential. The actions of the sodium-potassium pump help to maintain the resting potential, once it is established. Recall that sodium-potassium pumps bring two K+ ions into the cell while removing three Na+ ions per ATP consumed. At this point, the sodium channels return to their resting state, ready to open again if the membrane potential again exceeds the threshold potential.
Action potentials are considered an all-or-none event, meaning that they have the same magnitude – there is no “big” or “small” action potential. Therefore, either a full action potential is fired or the neuron does not reach the threshold potential.
The formation of an action potential consists of the following significant events:
- A stimulus from a sensory cell or another neuron causes the target cell to depolarize toward the threshold potential.
- If the threshold of excitation is reached, voltage-gated Na+ channels open, and the membrane depolarizes.
- At the peak action potential, K+ channels open and K+ begins to leave the cell. At the same time, Na+ channels close.
- The membrane becomes hyperpolarized as K+ ions continue to leave the cell. The hyperpolarized membrane is in a refractory period and cannot fire.
- The K+ channels close, and the Na+/K+ transporter restores the resting potential.
Formation of an action potential. The action potential formation is divided into five steps. (1) Resting potential. (2) The threshold of excitation/Depolarization. (3) Peak action potential/Repolarization. (4) Hyperpolarization. (5) Hyperpolarization up to re-establishment of resting state.
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Key Points
• A stimulus causing the cell membrane to depolarize past the threshold of excitation, making all sodium ion channels to open, forms the action potential.
• When the potassium ion channels are opened and sodium ion channels are closed, the cell membrane becomes hyperpolarized as potassium ions leave the cell.
• The action potential travels down the axon as the membrane of the axon depolarizes and repolarizes.
• Depolarization causes calcium influx into the cell, which triggers neurotransmitter release.
• Action potential is all-or-nothing, meaning that they have the same size-no big or small action potentials; therefore, it is either a full action potential is fired or the neuron does not reach the threshold potential.
• Sodium-potassium pumps move two potassium ions inside the cell as three sodium ions are pumped out to maintain the negatively-charged membrane inside the cell; this helps maintain the resting potential.
Key Terms
Action potential: a short term change in the electrical potential that travels along the cell
Depolarization: a decrease in the difference in voltage between the inside and outside of the neuron
Hyperpolarize: to increase the polarity of something, especially the polarity across a biological membrane
Resting potential: the nearly latent membrane potential of inactive cells
Threshold potential: the critical level to which a membrane potential must be depolarized to initiate an action potential
Resting potential: the resting membrane potential of a neuron is about -70 mV. at rest, there are relatively more sodium ions outside the neuron and more potassium ions inside that neuron
Sodium–potassium pump: found in many cell (plasma) membranes powered by ATP, the pump moves sodium and potassium ions in opposite directions
ATP: is an organic compound that provides energy to drive many processes in living cells