action potential

Definition

Target type: biologicalprocess

A process in which membrane potential cycles through a depolarizing spike, triggered in response to depolarization above some threshold, followed by repolarization. This cycle is driven by the flow of ions through various voltage gated channels with different thresholds and ion specificities. [GOC:dph, GOC:go_curators, GOC:tb, ISBN:978-0-07-139011-8]

An action potential is a rapid, transient change in the electrical potential across the plasma membrane of a neuron or muscle cell, which transmits signals along the cell. It is a fundamental process in the nervous system, enabling communication between neurons and the initiation of muscle contractions.

The action potential arises from the coordinated movement of ions across the cell membrane, primarily sodium (Na+) and potassium (K+). These ions flow through voltage-gated channels, specialized proteins embedded in the membrane that open or close in response to changes in membrane potential.

**Steps in Action Potential Generation:**

1. **Resting potential:** In the resting state, the neuron maintains a negative membrane potential, typically around -70 mV. This is due to a higher concentration of K+ inside the cell and Na+ outside the cell, maintained by the sodium-potassium pump.

2. **Depolarization:** When a stimulus, such as a neurotransmitter, triggers the opening of Na+ channels, Na+ ions rush into the cell, causing the membrane potential to become less negative. This is known as depolarization. If the depolarization reaches a threshold level, typically around -55 mV, an action potential is triggered.

3. **Rising phase:** Once the threshold is reached, the influx of Na+ ions becomes self-reinforcing, leading to a rapid and dramatic rise in membrane potential. This phase is known as the rising phase or upstroke of the action potential. The membrane potential reaches a peak value of around +30 mV.

4. **Repolarization:** As the membrane potential reaches its peak, the Na+ channels begin to inactivate, while K+ channels open. This allows K+ ions to flow out of the cell, bringing the membrane potential back towards its resting value. This phase is called repolarization.

5. **Hyperpolarization:** In some cases, the membrane potential may briefly become more negative than the resting potential, a phenomenon called hyperpolarization. This is due to the continued outflow of K+ ions.

6. **Refractory period:** After an action potential, there is a brief period during which the neuron is less excitable. This is known as the refractory period, and it prevents the neuron from generating another action potential immediately.

**Propagation:** Action potentials travel along the axon of a neuron by a process called saltatory conduction. The myelin sheath, a fatty insulating layer that surrounds the axon, speeds up the conduction by limiting the leakage of ions across the membrane. The action potential jumps from one node of Ranvier, a gap in the myelin sheath, to the next.

**Importance of Action Potentials:**

Action potentials play a crucial role in various physiological functions, including:

- **Neural communication:** Transmission of signals between neurons at synapses.
- **Muscle contraction:** Activation of muscle fibers to generate force.
- **Sensory perception:** Transduction of sensory stimuli into electrical signals.
- **Hormone release:** Regulation of hormone secretion by endocrine cells.

**Factors Affecting Action Potential:**

Several factors can influence the characteristics of action potentials, including:

- **Temperature:** Higher temperatures tend to increase the speed of conduction.
- **Ion concentration:** Changes in the concentration of ions like Na+ and K+ can alter the resting potential and threshold.
- **Myelination:** Myelination increases the speed of conduction.
- **Presence of drugs or toxins:** Some substances can block ion channels or alter their function.

Action potentials are a fundamental process in the nervous system, ensuring rapid and efficient communication between neurons and the generation of physiological responses.'
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Proteins (34)

Compounds (154)