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CoursesPhysiologyNervous systemAction potential and synapsesAction potentials

CoursesPhysiologyNervous systemAction potential and synapsesAction potentials

Action potentials

Learning objectives

After completing this study unit you will be able to:

Explain the role of local potentials in initiating action potentials and the ionic changes that occur during an action potential.
Interpret graphs of action potentials, relating depolarization, repolarization, and hyperpolarization to ion movements.
Describe the significance of refractory periods in regulating action potential frequency and direction.
Differentiate between continuous and saltatory conduction, and explain how axon diameter and myelination affect conduction velocity.

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Action potentials are rapid electrical impulses that neurons use to communicate information throughout the body. They are all-or-nothing events, meaning that the action potential will fire fully once the membrane potential reaches a threshold (-55 mV).

Action potentials are initiated when an excitatory stimulus causes local potentials that depolarize the neuron's membrane. This depolarization triggers the opening of voltage-gated sodium channels, allowing Na⁺ ions to flow into the cell, further depolarizing the membrane and causing a rapid spike in membrane potential. After reaching a peak of around +30 mV, sodium channels inactivate, and voltage-gated potassium channels open, allowing K⁺ ions to leave the neuron, causing repolarization. Ion channels play a crucial role in these steps by controlling the flow of specific ions across the membrane, ensuring the proper initiation and termination of the action potential.

Refractory periods, divided into absolute and relative phases, limit the frequency of action potentials and prevent them from traveling backward along the axon. During the absolute refractory period, a neuron cannot fire another action potential due to the inactivation of sodium channels. The relative refractory period occurs when potassium channels are still open, and while a new action potential can occur, it requires a stronger stimulus.

The speed of action potential conduction is influenced by axon diameter and myelination. Larger axon diameters reduce resistance to ion flow, enabling faster signal transmission. Myelination, which insulates axons, allows for saltatory conduction, where action potentials "jump" between gaps in the myelin sheath, commonly known as the nodes of Ranvier, significantly increasing conduction velocity. In contrast, nonmyelinated axons conduct impulses more slowly via continuous conduction.

Watch the following video to learn how action potentials are generated and how axon diameter and myelination affect conduction velocity.

Explore concepts

Action potential phases

An action potential is characterized by five key phases. Have a closer look at each phase of an action potential, in relation to ion movement.

Refractory periods

Refractory periods are phases during which a neuron is unable, or less likely, to fire another action potential. Learn more about the differences between the absolute and relative refractory periods.

Continuous and saltatory conduction

Depending on whether the axon is myelinated or non-myelinated, there are two types of conduction: continuous conduction and saltatory conduction.

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Summary

Key facts about action potentials
Action potential overview	Definition: Rapid changes in neuron membrane potential that transmits signals along neurons Trigger: Initiated when the membrane potential reaches the threshold (-55 mV) due to excitatory stimuli All-or-nothing: An action potential will fully fire if the threshold is reached, or not at all if it is not
Ion channels involved	Voltage-gated sodium channels: Open at threshold, allowing Na^⁺ to enter the neuron, causing depolarization Voltage-gated potassium channels: Open after depolarization, allowing K⁺ to exit the neuron, leading to repolarization Leakage channels: Maintain resting membrane potential by allowing ions to diffuse passively down their concentration gradients
Phases	Resting membrane potential: Approximately -70 mV, maintained by sodium-potassium pumps and leakage ion channels Depolarization (and overshoot): Caused by the influx of Na⁺, making the inside of the neuron more positive Repolarization: Outflow of K⁺ ions returns the membrane potential toward resting levels Hyperpolarization (and undershoot): K⁺ channels close slowly, causing a brief dip below the resting potential
Refractory periods	Absolute refractory period: A neuron cannot fire another action potential during this time, regardless of stimulus strength, as sodium channels are inactivated Relative refractory period: A stronger-than-usual stimulus can initiate another action potential, but it is harder due to the more negative membrane potential
Types of conduction	Continuous conduction: Occurs in nonmyelinated axons, where action potentials propagate in a wave-like manner along the axon Saltatory conduction: Occurs in myelinated axons, where action potentials "jump" between the myelin sheath gaps (nodes of Ranvier), allowing faster signal transmission
Factors affecting conduction velocity	Axon diameter: Larger axons conduct impulses faster due to lower resistance to ion flow Myelination: Increases conduction speed through saltatory conduction, making it more energy-efficient

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