Action Potential
An action potential occurs when a cell's membrane potential quickly increases and then decreases. This rapid depolarization triggers neighboring areas to undergo similar changes. Action potentials are observed in various excitable cells, including neurons and muscle cells in animals and some plant cells. Additionally, specific endocrine cells, such as pancreatic beta cells and certain cells in the anterior pituitary gland, also exhibit excitability.
1.0Action Potential Definition
Action potential refers to rapid, transient changes in membrane electrical potential that propagate along a cell, usually a neuron or muscle fiber, and are the basic means of information transfer both within an organism and from one organism to another. This electrical impulse is the essential mechanism by which neurons communicate with other neurons and with other cell types, including muscle cells.
2.0Action Potential Biology
Action potential is the generic term for the basic means of communication of the nervous system in biological terms. It's the result of a neuron sending information down an axon, away from the cell body, as a response to a stimulus. An action potential is a change in membrane potential resulting from the movement of particular ions, mainly Na+ and K+, across the cell membrane.
3.0Steps of Action Potential
1. Resting Membrane Potential: The neuron is at rest; inside the cell, as compared to the outside of the cell, it is having a stable negative charge at a resting potential, usually about -70 mV. The sodium-potassium pump does this actively by pumping out 3 Na+ ions for every 2 K+ ions it pumps into a cell to maintain the potential.
2. Threshold: A stimulus triggers the neuron, and provided that the stimulus is large enough to reach the threshold potential, usually about -55 mV, an action potential is initiated.
3. Depolarization: Voltage-gated Na+ channels open, allowing a rush of Na+ ions into the cell. It means that as the positive ions rush in, this makes the membrane potential less negative, moving towards a positive value.
4. Peak of Action Potential: The membrane potential peaks to about +30 to +40 mV and briefly positive inside compared to the outside of the cell.
5. Repolarization: Sodium channels are closed, and voltage-gated potassium channels are opened. As a result, K+ ions rush out of the cell. This takes back the membrane to its negative potential.
6. Hyperpolarization: The potential briefly becomes more negative than the resting potential because K+ channels shut off slowly, allowing more K+ to leave.
7. Return to Resting Potential: The K+/Na+ pump returns the membrane voltage back to the resting state; the neuron is ready to fire another action potential if it is stimulated.
Saltatory Conduction of Nerve Impulse
- This conduction occurs in myelinated nerve fibers, where the action potential leaps from one node to the next along the myelinated axon.
- This process, known as saltatory conduction, is faster than the continuous transmission in non-myelinated axons.
- Ion leakage is limited to the nodes of Ranvier, making saltatory conduction more energy-efficient.
Table of Contents
- 1.0Action Potential Definition
- 2.0Action Potential Biology
- 3.0Steps of Action Potential
- 3.1Saltatory Conduction of Nerve Impulse
Frequently Asked Questions
An action potential is triggered when a cell's membrane potential reaches a certain threshold level. This threshold is typically reached in response to a stimulus, such as a neurotransmitter binding to a receptor on a neuron.
Action potentials propagate along neurons by sequentially opening and closing voltage-gated ion channels along the axon. This chain reaction of depolarization and repolarization moves the action potential down the length of the axon.
The myelin sheath, a fatty layer that wraps around the axon of many neurons, speeds up action potential conduction. It allows the action potential to jump from one node (Node of Ranvier) to the next, a process known as saltatory conduction, which increases the efficiency and speed of signal transmission.
Yes, action potentials can also occur in non-neuronal cells such as muscle cells and some plant cells. In these cells, action potentials are involved in muscle contraction and various physiological processes.
If the action potential does not reach the threshold, it will not be generated. This is known as a subthreshold stimulus. The cell remains in its resting potential state, and no electrical signal is transmitted.
Action potentials are all-or-nothing responses that propagate along the length of the axon without decreasing in strength. In contrast, graded potentials are changes in membrane potential that vary in magnitude and decrease with distance from the stimulus source.
During the refractory period, the neuron is temporarily unable to generate another action potential. This period is divided into the absolute refractory period, where no new action potential can be initiated, and the relative refractory period, where a stronger-than-usual stimulus is required to generate another action potential.
Join ALLEN!
(Session 2025 - 26)