Small gaps of exposed cellular membrane that are not covered by the myelin sheath in a myelinated axon. Nodes of Ranvier contain clusters of voltage-gated Na+ ion channels that can regenerate the action potential as it propagates along the axon. 

Nodes of Ranvier

A nerve impulse travels significantly faster along an axon wrapped in a myelin sheath compared to an unmyelinated axon of the same diameter. Which statement best analyzes the underlying mechanism for this increased speed?

Consider two neurons of equal length. Neuron X's axon is wrapped in a fatty sheath that is interrupted by small, regularly spaced gaps. Neuron Y's axon lacks this sheath entirely. If both neurons are stimulated at one end with sufficient intensity to generate an electrical signal, which statement best analyzes the resulting signal propagation?

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.

Google

Brandeis University

California Polytechnic State University, San Luis Obispo

Large shift in membrane potential that lasts a short time, which causes a neuron to fire. These large shifts in membrane potential (i.e., electrical signals) are how information is transmitted within a neuron. 

Action Potential

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

Breedlove, S. & Watson, N. (2017). Behavioral neuroscience, eighth edition. Oxford University Press.

Behavioral Neuroscience 8th Edition

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

The reduction of the electrical potential of the membrane. During depolarization, 
voltage-gated Na+ ion channels begin to open, thus letting some Na+ ions pass through the membrane. The action potential is sparked when the rushing in of the positive Na+ ions into the neuron reduces the electrical potential to around -40 mV.

Depolarization

Hyperpolarization is a phase of a neuron's action potential where the membrane potential becomes more negative than its resting state, reaching approximately -80mV. This increased negativity occurs after repolarization, as potassium (K+) ion channels remain open, allowing positively charged K+ ions to continue leaving the cell. This outflow of positive ions is driven by repulsion from the positively-charged sodium ions that had previously entered the cell.

Hyperpolarization

The all-or-none property dictates that an action potential is either triggered completely or not at all. Once the threshold of excitation is met, the action potential is generated and propagated at its full, consistent strength along the entire axon, without diminishing over distance. This principle explains why the brain perceives a signal from a distant body part, such as a toe, with the same intensity as one from a closer part, like the nose, as the signal's strength is maintained throughout its journey.

All-or-None Property

A change in the electrical potential of the membrane due to the binding of neurotransmitters in the synapses. These potentials determine whether a neuron fires or not.

Postsynaptic Potentials

Saltatory Conduction

The passing of information received from an action potential through the synapse. It occurs in the following steps:
- Exocytosis of neurotransmitter-containing vesicles occurs as the incoming action potential depolarizes the presynaptic membrane, leading to a rush of Ca 2+ entering the terminal
- As a result of exocytosis, transmitters are released into the synaptic cleft and bind with receptor molecules in the postsynaptic membrane
- These bindings cause ion channels in the postsynaptic membrane to open, creating a local EPSP or IPSP in the postsynaptic neuron
- The effects of the released transmitters are halted through either degradation or reuptake
- Some transmitters bind to autoreceptors, which further regulates the release of transmitters

Synaptic Transmission

The speed of an action potential, also known as the conduction velocity,  generally depends on two things: 1.) The diameter of the axon, and 2.) The myelination of the axon. Axons that have larger diameters conduct impulses faster because depolarization is allowed to more rapidly spread throughout the interior of the cell. Axons that are myelinated also conduct impulses faster than axons that are unmyelinated. This is because myelin resists the flow of ionic currents, and so it channels the depolarization down the axon interior until it reaches a small gap in the cellular membrane that lacks myelin, where it can be regenerated. Therefore, myelination allows action potentials to "jump" between nodes along the axon (see "Saltatory Conduction").  

Conduction Velocity

The firing of the action potential only occurs at a particular point. In sound, this means the potential only fires at a certain frequency. Having an accurate phase-locked representation generally needs averaging across several cells rather than a single cell. This helps with telling where you are in a cycle, because the neuron will only fire at a certain point in that cycle. 

phase-locked action potentials

The period of time immediately following an action potential during which a neuron's responsiveness to subsequent stimulation is either completely absent or significantly reduced.

Refractory Phase

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.

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.

Learn Before

Related

Learn After