Definition

The influx and expulsion of ions within the cell cause an action potential. The action potential, also referred to as a nerve impulse, is the electrical potential difference across the plasma membrane. Specifically, potassium and sodium ions are involved. The sodium-potassium pump and channels transport ions in and out of the cell.

The reaction of a neuron or muscle cell might vary depending on how often and for how long an action potential is produced. Each action potential (impulse) results from a rapid increase and decrease in voltage across the cellular membrane. A specific voltage change caused by an action potential requires an influx of positive ions (threshold value). It happens following a particular amount of positive electrical charge increase and internal cell membrane depolarisation.

Overview

In this context, “potential” refers to an electric potential rather than the likelihood of achieving anything. In biology, the potential is present at the inner and outer margins of cell membranes. Potential energy is constant stored energy. A ball possesses potential energy while it is motionless. A neuron has potential energy while it is not firing.

Electrical Cell Membranes

Electrical currents flow across cell membranes. They produce a charge that extends the entire length of the cell membrane by using ions from both the extracellular and intracellular sides of the cell. It is said that the cell membrane has a resting potential when not much is happening.

Ion Channels

Since ions are hydrophilic, they cannot pass through a membrane’s lipids. Specially structured proteins that form channels or tunnels are necessary to move in and out of the membrane. These channels transporting ions are called ion channels.

Specific plasma membrane-encased voltage-gated ion channels produce action potentials. When the membrane potential is close to the resting (negative) potential of the cell, these channels are closed. However, if the membrane potential rises to a specified threshold voltage, depolarizing the transmembrane potential, the channels rapidly begin to open.

Chemical Synapses

Small chemicals called neurotransmitters have the potential to activate ion channels in postsynaptic cells. Neurotransmitters are released into the synaptic cleft in response to action potentials reaching the synaptic knobs. The initiation of action potential causes the presynaptic membrane to open voltage-sensitive calcium channels. Neurotransmitter-filled vesicles move to the cell’s surface due to the calcium influx, where they discharge their components into the synaptic cleft.

Electrical Synapses

Because neurotransmitters do not need to diffuse slowly across the synaptic cleft, electrical synapses enable faster communication. Electrical synapses are therefore used whenever quick reaction and timing synchronisation are essential, such as in the heart, vertebrate retina, and the escape reflexes. By eliminating the neurotransmitter’s “middleman,” electrical synapses connect the postsynaptic and presynaptic cells.

Phases or Steps

Depolarisation, overshoot or peak phase, repolarisation, and refractory period are the phases of an action potential. The membrane potential has two more phases connected to the action potential. First, there is hypopolarisation, which comes before depolarisation, and then there is hyperpolarisation, which comes after repolarisation.

Hypopolarisation: The initial rise in membrane potential to the threshold potential is known as hypopolarisation.
Depolarisation: A significant influx of sodium ions is produced when the threshold potential activates voltage-gated sodium channels. This period is known as depolarisation.
Overshoot or Peak Phase: As the cell depolarizes, the inside of the cell becomes increasingly electropositive, approaching the electrochemical equilibrium potential of sodium, which is +61 mV. This phase of intense electropositivity is the peak phase or overshoot phase.
Repolarisation: Following the action potential firing, the potassium ion channels that remove this cation from the cell open, and the sodium ion channels close. The voltage-gated potassium channels are opened when the cell potential overshoots, which results in a significant potassium outflow and a reduction in the electropositivity of cells. This is the repolarisation phase, and its main objective is the restoration of the resting potential of the membrane.
Hyperpolarisation: This phase possesses more electronegativity in the membrane potential than in the usual resting membrane potential.
Refractory Period: The duration after a nerve impulse that a neuron must travel before it can fire again is referred to as the refractory period. Every action potential is preceded by a refractory period, further subdivided into an absolute and a relative refractory period.

During the absolute refractory period, another action potential cannot be evoked, and the relative refractory period requires a stimulus stronger than usual. The two refractory phases are driven by modifications in the states of potassium and sodium channel molecules.

Graded Potential vs Action Potential

The two forms of potential variations produced during depolarization are graded and action potentials. Graded and action potentials differ primarily because graded potentials are variable-strength signals that can travel shorter distances, and action potentials are massive depolarizations that travel longer distances.

While action potentials do not lose strength throughout transmission across the neuron, graded potentials might lose their strength as they pass through the neuron.

The action potential is produced by voltage-gated ion channels, while ligand-gated ion channels produce the graded potential.

The characteristics of each membrane potential comprise the primary distinction between graded and action potentials. The amplitude of a graded potential is lower than that of an action potential. As a result, it degrades during transmission. Action potentials do not, however, deteriorate throughout transmission.

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Frequently Asked Questions – FAQs

What is the action potential in the nervous system?

Action potentials, which are electrical impulses that go throughout the body, are nothing more than a transient shift in the membrane potential of a neuron from negative to positive caused by ions rapidly entering and exiting the cell.

Why are action potentials important?

Action potentials are essential for the proper functioning of the brain because they transmit information from the peripheral nervous system to the CNS (central nervous system) and direct instructions originating in the CNS to the periphery. Therefore, it is essential to understand their characteristics fully.

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