16.17:
Action Potential: Phases of Stimulation
An action potential has three main phases: depolarization, repolarization, and hyperpolarization.
The depolarizing phase begins after a graded potential or a stimulus triggers the axon membrane to depolarize above a threshold.
This triggers the voltage-gated sodium channels to open rapidly. The resulting influx of sodium ions increases the membrane potential that peaks at +30mV.
Shortly after this, the sodium channels inactivate, preventing further sodium influx. The voltage-gated potassium channels are now open, causing an efflux of potassium ions.
This phase is called the repolarizing phase because the efflux of positive ions decreases the membrane potential.
Due to a slight delay in the closing of the potassium channel, the membrane potential continues past the resting membrane voltage, causing a little dip. This phase is called the hyperpolarization phase.
The action of sodium-potassium ATPase pumps then restores the resting membrane potential.
Once an action potential is generated, the axon cannot initiate another action potential for a brief time known as the refractory period. It ensures the unidirectional flow of nerve impulses and prevents neurons from firing continuously.
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.