The all-or-none phenomenon is related to an action potential in the way that once a neuron's membrane potential gets to the threshold of excitation, the neuron will either fire an action potential all the way or not at all (i.e., there can't be any "partial" firing of a neuron). 

All-or-None Phenomenon in Action Potential of Neuronal Communication

A graph of an action potential demonstrates the specific millivolt (mV) changes across a neuron's membrane during different signaling phases. The process begins at a resting potential of roughly -70mV. Upon reaching the threshold of excitation at approximately -55mV, the neuron fires, causing a spike to a peak action potential of 30mV. As the cell recovers, it undergoes repolarization (dropping to around 5mV) and then hyperpolarization, where the charge dips to roughly -80mV before eventually returning to its baseline resting state.

Action Potential Diagram

A neuron's membrane has just received a stimulus strong enough to trigger an electrical impulse. Which of the following statements best breaks down the sequence of ion movements that allows this impulse to travel along the axon?

During the action potential, the electrical charge across the membrane changes dramatically. The millivolts through the phases are approximately -70mV at resting potential, -55mV at threshold of excitation, 30mV at peak action potential, 5mV at repolarization, and -80mV at hyperpolarization.

Action Potential Voltage Phases

The action potential is an electrical signal that typically moves from the cell body down the axon to the axon terminals.

Action Potential Propagation

When the action potential arrives at the terminal button, the synaptic vesicles release their neurotransmitters into the synaptic cleft.

Neurotransmitter Release

An action potential is the electrical signal that travels down an axon when a neuron receives a sufficiently strong stimulus. This impulse operates on an all-or-none principle, meaning it either fires completely or not at all. The propagation of the signal involves the movement of sodium (Na+) and potassium (K+) ions across the neuronal membrane.

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It is important to understand how electrical signals move through a neuron and onto the next neuron in order to facilitate communication.

Neuronal Communication 

The threshold of excitation represents the critical level of membrane potential that a neuron must reach to initiate an action potential. This specific charge level (approximately -55mV) is attained when an influx of positive ions enters the cell, making the internal environment sufficiently positive. Once this exact threshold is surpassed, the neuron becomes active and inevitably triggers an action potential.

Threshold of Excitation

Spielman, R. M., Jenkins, W. J., & Lovett, M. D. (2020, April 22). Psychology (2nd ed.). OpenStax. https://openstax.org/books/psychology-2e/pages/1-introduction

OpenStax Psychology (2nd ed.) Textbook

Action Potential

Between signals, a neuron's membrane potential is maintained in a state of readiness known as the resting potential. In this condition, ions—such as sodium (Na+) and potassium (K+)—accumulate closely on either side of the cell membrane. This separation of charges acts much like a stretched rubber band storing potential energy, leaving the ions prepared to rush across the membrane as soon as the neuron activates and its gates open.

Resting Potential

The membrane potential is the difference in electrical charge between the intracellular and extracellular fluids across the neuronal membrane. This electrical difference serves as the energy source for the neural signal.

Membrane Potential

When two neurons repeatedly become stimulated, the signal transmission between them gets enhanced. This enhancement is known as long-term potentiation (LTP). 

Long-term Potentiation (LTP)

The process of neuronal communication is described as an electrochemical event because it involves two distinct stages. The electrical stage is the movement of the action potential down the axon, while the chemical stage is the transmission of the signal across the synaptic space via neurotransmitters.

Neuronal Communication as an Electrochemical Event

Based on the patient's medication described in the case study, explain how the normal process of communication between two neurons is interrupted, leading to reduced neural activity.

Pharmacological Intervention in Neural Pathways

Saltatory conduction is the process by which an action potential propagates along a myelinated axon, appearing to 'jump' in a leapfrog fashion between the Nodes of Ranvier. At each node, an influx of sodium ions generates the action potential. These ions then diffuse to the adjacent section of the axon, raising its charge past the threshold of excitation and triggering a new influx of sodium ions at the next node. This mechanism allows the electrical signal to travel rapidly down the axon.

Saltatory Conduction

Based on the provided scenario, which stimulus (or stimuli) will cause the neuron to fire an electrical impulse? Explain your reasoning, focusing on the importance of the neuron's critical firing level.

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