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.