16.17: Action Potential: Phases of Stimulation
The action potential is a complex electrical event that occurs in excitable cells, such as neurons and muscle cells. It consists of several distinct phases, each with specific characteristics.
Resting Phase:
In this phase, the cell's membrane is at its resting potential, typically around -70 millivolts (mV) for neurons. Inside the cell, there is a higher concentration of potassium ions (K+) and a lower concentration of sodium ions (Na+). Voltage-gated sodium channels are closed, and voltage-gated potassium channels are closed but capable of opening.
Depolarization Phase:
A graded potential, often an excitatory postsynaptic potential (EPSP), reaches the threshold level (typically around -55 mV). This triggers the voltage-gated sodium channels to open rapidly, allowing an influx of sodium ions into the cell. This rapid sodium influx causes a sharp increase in membrane potential, turning it more positive. The influx of sodium ions further depolarizes the membrane, leading to a positive feedback loop that triggers more sodium channels to open.
Peak of the Action Potential:
At the peak of the action potential, the sodium channels begin to inactivate or close, reducing sodium influx. Voltage-gated potassium channels start to open slowly in response to the increasing membrane potential.
Repolarization Phase:
As voltage-gated potassium channels open fully, potassium ions exit the cell. This movement of positively charged ions out of the cell helps to restore the negative membrane potential. The membrane potential gradually returns to the resting potential of around -70 mV.
Hyperpolarization Phase (Undershoot):
The movement of potassium ions continues for a brief period, causing the membrane potential to dip below the resting potential, typically around -80 mV. The delayed closure of some potassium channels contributes to this temporary hyperpolarization.
Refractory Period:
During and immediately after an action potential, it is impossible to trigger another one. This prevents the action potential from moving backward. This is called the absolute refractory period.
Following the absolute refractory period, it is possible to initiate another action potential, but it requires a stronger stimulus than usual. This is known as the relative refractory period.
The phases of an action potential are essential for transmitting electrical signals in neurons. This rapid and coordinated sequence of events allows for the unidirectional propagation of signals along the length of the neuron, enabling communication within the nervous system and with other cells.
