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.

Action Potential

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.

Predicting Neuronal Firing

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.

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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 

If the membrane potential becomes "less negative", it is said to be depolarized.  This can occur by positive ions moving into the cell or negative ions moving out of the cell.  

Depolarization

When a neuron receives signals at its dendrites, typically from neurotransmitters released by an adjacent neuron binding to its receptors, small pores or gates open on the neuronal membrane. This event allows sodium (Na+) ions, propelled by both charge and concentration differences, to flow into the cell. With this influx of positive ions, the cell's internal charge becomes more positive.

Sodium Influx During Signal Reception

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

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

Threshold of Excitation

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

Action potentials are changes in membrane potential that cause cells to become depolarized.  A single action potential only involves a small portion of the total cell membrane.  However, this signal can be propagated through the entire membrane as they do not diminish in strength over distance.  This makes action potentials reliable for long-distance signals.  

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