Action Potential: Definition, Steps, Facts, Phases, FAQs

Definition & Basic Concepts

The action potential is the brief fluctuation in the electrical potential that occurs in the membrane of a neuron or muscle cell during the transmission of a nerve impulse or the contraction of a muscle fibre.

Action potentials are the main way the cells of the nervous system communicate over long distances and underlie nearly all higher brain functions like perception, action, and thought.

Phases of Action Potential

Action potential occurs in several phases:

Threshold Potential

Lowest membrane potential that can be used to initiate an action potential.

Depolarisation

Open sodium channels, allow sodium ions to enter, the membrane is thus depolarised.

Repolarisation

Open potassium channels, allowing potassium ions to exit, hence repolarising the membrane.

Hyperpolarisation / Afterhyperpolarisation

At this point, the membrane potential becomes slightly more negative than at rest.

Mechanism & Ion Channels

The mechanism and ion channels are discussed below:

Role of Na⁺ Channels in Rising Phase

Voltage-gated Na⁺ channels open rapidly during depolarization, allowing sodium ions to rush into the neuron and generate the rising phase of the action potential.

Role of K⁺ Channels in Falling Phase

Voltage-gated K⁺ channels open as Na⁺ channels close, letting potassium ions exit the neuron and producing repolarization.

Voltage-Gated Ion Channels

These specialized membrane proteins open and close in response to changes in membrane potential, enabling the rapid ionic shifts required for nerve impulse conduction.

Propagation Of Action Potential

The propagation of action potential:

Saltatory Vs. Continuous Conduction

• Saltatory conduction occurs in myelinated axons.

• The action potentials jump between the nodes of Ranvier.

Myelin Sheath & Nodes Of Ranvier

• The myelin sheath insulates and speeds up the conduction by reducing ion leakage across the membrane.

• They are small gaps in the myelin sheath, where action potentials are regenerated

Factors Affecting Action Potential

There are several factors determining action potential kinetics.

Temperature

Higher temperatures increase the conduction velocity.

Axon Diameter

The greater the diameter the less resistance, so conduction takes more rapidly.

Myelinisation

Myelinated axons conduct impulses faster than unmyelinated axons because of the saltatory conduction in myelinated axons.

Action Potential NEET MCQs (With Answers & Explanations)

Important questions asked in NEET from this topic are:

• Phases of action potential

• Factors affecting action potential

Practice Questions for NEET

Q1. Choose the incorrect statement

1. All the nerve fibres carry information in the form of the nerve impulse

2. The interior of the axon is filled with extracellular fluid

3. Generally, the solutes in ECF and axoplasm are in ionic form

4. The electrical events in the nerve fibres are governed by differential permeability of the axolemma to sodium and potassium ions.

Correct answer: 2) The interior of the axon is filled with extracellular fluid

Explanation:

All the nerve fibres carry information in the form of the nerve impulse. The nerve impulse is the sum total of physical and chemical disturbances created by a stimulus (electrical, chemical or mechanical) in a neuron or nerve fibre which results in the movement of a wave along the nerve fibre. The nerve fibre or axon is like a cylinder. The interior of the axon is filled with axoplasm (i.e., the cytoplasm of the nerve cell). The exterior of the axon is covered with a thin membrane, the axon membrane or axolemma. Axolemma is selectively permeable for eimple organic and inorganic molecules. The axon is immersed in the extracellular fluid (ECF). Through axolemma movement of solute takes place between the axoplasm and ECF. Generally, the solutes in ECF and axoplasm are in ionic form.

Hence, the correct answer is option 2) The interior of the axon is filled with extracellular fluid.

Q2. What is the function of the axon in a neuron?

1. To receive signals from other neurons

2. To support and protect the cell body

3. To transmit signals to other neurons

4. To synthesize neurotransmitters

Correct answer: 3) To transmit signals to other neurons

Explanation:

The function of the axon in a neuron is to transmit signals to other neurons or other cells such as muscles or glands. The axon is a long, slender projection of the neuron that carries electrical signals away from the cell body and toward the axon terminals, where neurotransmitters are released to communicate with other neurons or cells. The axon is covered by a myelin sheath, which helps to insulate and speed up the transmission of electrical signals.

Hence, the correct answer is option 3) To transmit signals to other neurons.

Q3. Resting membrane potential is maintained by

1. Hormones

2. Neurotransmitters

3. Ion pumps

4. None of the above

Correct answer: 3) Ion Pumps

Explanation:

The sodium-potassium pump and leak channels are examples of ion pumps and channels that preserve a cell's resting membrane potential:
For every ATP molecule, this pump transports two potassium ions into the cell and three sodium ions out of it. The potassium ion gradient produced by this action adds to the resting membrane potential. The electrical potential across a cell's plasma membrane when it is not activated is known as the resting membrane potential. Because more cations are released than are absorbed, the inside of the cell stays negatively charged in relation to the extracellular fluid.

Hence, the correct answer is option 3) Ion pumps.

Also Read:

• MCQs on Introduction to Nerve Impulse

• Lobes of Cerebrum

• Cerebrospinal Fluid

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