What triggers an action potential?

As a neuroscientist with a focus on cellular and molecular neuroscience, I can explain the process that triggers an action potential. An action potential is a rapid, temporary change in the electrical potential across the membrane of a neuron, which is crucial for the transmission of signals within the nervous system. The process begins with the neuron at its resting potential, which is typically around -70 millivolts (mV). This potential is maintained by the sodium-potassium pump, which actively transports sodium ions (Na+) out of the cell and potassium ions (K+) into the cell. The neuron's membrane is also permeable to potassium ions through leak channels, allowing K+ to flow out of the cell, further contributing to the negative charge inside the cell. An action potential is triggered when the neuron receives a stimulus that causes the membrane potential to become less negative, a process known as depolarization. If the depolarization is strong enough to reach the threshold potential (usually around -55 mV), an action potential is generated. Here's what happens next: 1. **Activation of voltage-gated sodium channels**: When the threshold potential is reached, voltage-gated sodium channels in the neuron's membrane open, allowing Na+ to flow into the cell. This influx of positive charge further depolarizes the membrane, causing the rising phase of the action potential. 2. **Inactivation of sodium channels and activation of potassium channels**: As the membrane potential becomes more positive, the sodium channels inactivate, preventing further Na+ influx. Simultaneously, voltage-gated potassium channels open, allowing K+ to flow out of the cell. 3. Repolarization: The outflow of K+ helps to repolarize the membrane, restoring it to its resting potential. 4. **Hyperpolarization and return to resting potential**: For a brief period, the membrane potential may become slightly more negative than the resting potential (hyperpolarization) due to the continued efflux of K+. The sodium-potassium pump then restores the resting potential by moving Na+ out and K+ back in. 5. Refractory period: There is a brief period after an action potential during which the neuron cannot generate another action potential. This is known as the refractory period and it ensures that the action potential travels in one direction along the neuron. The entire process is all-or-nothing, meaning that once the threshold is reached, the size of the action potential does not vary with the strength of the stimulus.

Action potentials are caused when different ions cross the neuron membrane. A stimulus first causes sodium channels to open. Because there are many more sodium ions on the outside, and the inside of the neuron is negative relative to the outside, sodium ions rush into the neuron.