What triggers an action potential?

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.

How do action potentials propagate along 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.

What is the role of the myelin sheath in action potential conduction?

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.

Can action potentials occur in non-neuronal cells?

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.

What happens if the action potential does not reach the threshold?

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.

How is the action potential different from graded potentials?

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.

What happens during the refractory period after an action potential?

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.

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NEET Biology

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.

Resting potential of -70mV in the steps of action 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.

Image showing the peak of action potential

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.

Image showing the Repolarization in an action 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.

The image of the graph showing the return to resting potential in an action potential

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.

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