Neuroscience Chapter 4: Action Potentials

Questions and Answers

What best describes action potentials?

They are the only signals generated by muscle cells.

They are short-distance communication signals.

They exclusively occur in connective tissue.

They are long-distance signals characterized by reversal of charge. (correct)

What initiates the generation of an action potential?

Changes in concentrations of ions across the membrane. (correct)

A decrease in temperature affecting the membrane.

Increase in extracellular calcium concentration.

Stable ion concentrations within the neuron.

Which of the following is NOT a key characteristic of action potentials?

They can be propagated over short distances. (correct)

They are also known as spikes.

They are based on frequency and pattern.

They occur in both muscle and neuronal cells.

Which statement about graded potentials is true?

They are short-range signals that operate at local points. Signup and view all the answers

How is the neural code established for action potentials?

It is determined by the frequency and pattern of action potentials. Signup and view all the answers

What happens to sodium channels during the resting state?

Only leakage channels for sodium are open. Signup and view all the answers

Which voltage-gated sodium channel gate is responsible for preventing further sodium entry after the channel is opened?

Inactivation gate Signup and view all the answers

What is the threshold voltage that activates the main sodium channels during action potentials?

-55mV Signup and view all the answers

Which of the following statements regarding the absolute refractory period is true?

Sodium channels are inactivated. Signup and view all the answers

What is a characteristic of voltage-gated sodium channels?

They cannot reopen immediately after being activated. Signup and view all the answers

What effect does tetrodotoxin (TTX) have on sodium channels?

It clogs the Na+-permeable pore, blocking sodium channels. Signup and view all the answers

What role do voltage-gated sodium channels play during action potentials?

They allow sodium to enter the cell quickly. Signup and view all the answers

Which toxin blocks the inactivation of sodium channels, preventing them from closing?

Batrachotoxin Signup and view all the answers

What is the primary effect of Na+ influx during depolarization?

It results in depolarization and further opening of Na+ channels. Signup and view all the answers

What occurs at the threshold potential of –55 to –50 mV?

Positive feedback opens all Na+ channels. Signup and view all the answers

What is the role of K+ channels during repolarization?

K+ exists the cell down its electrochemical gradient. Signup and view all the answers

What happens to Na+ channels during repolarization?

They inactivate as K+ channels open. Signup and view all the answers

What is the membrane potential when the action potential spike reaches its peak?

+30 mV Signup and view all the answers

What is the role of potassium conductance during an action potential?

It serves to rectify or reset the membrane potential. Signup and view all the answers

How do the activation gates of sodium channels behave at the resting state?

They are closed, preventing sodium ions from entering. Signup and view all the answers

What is a defining characteristic of voltage-gated potassium channels compared to voltage-gated sodium channels?

They open later than sodium channels after depolarization. Signup and view all the answers

What happens to sodium channels after they have opened during depolarization?

They block further sodium entry due to inactivation gates. Signup and view all the answers

What maintains the resting membrane potential before an action potential occurs?

The activity of leakage channels for sodium and potassium. Signup and view all the answers

What occurs during hyperpolarization of the membrane?

Inside of the membrane becomes more negative than resting state Signup and view all the answers

What is the threshold for initiating an action potential?

A membrane depolarization of 15 to 20 mV Signup and view all the answers

How does the frequency of impulses relate to stimulus strength?

Higher frequency reflects stronger stimulus Signup and view all the answers

During which refractory period can an action potential be triggered only by an exceptionally strong stimulus?

Relative refractory period Signup and view all the answers

What is the role of sodium channels during the all-or-none phenomenon of action potentials?

They become inactive temporarily after being opened Signup and view all the answers

Which statement best describes how action potentials propagate in myelinated axons?

Action potentials jump from one node of Ranvier to another Signup and view all the answers

What mechanism prevents the back propagation of an action potential?

The absolute refractory period Signup and view all the answers

Which factors influence the rate of action potential propagation?

Axon diameter and sodium channel density Signup and view all the answers

What is the consequence of depolarization not reaching the threshold voltage?

No action potential is initiated Signup and view all the answers

What is a characteristic of the absolute refractory period?

It ensures the one-way transmission of nerve impulses Signup and view all the answers

Study Notes

Key Characteristics of Action Potentials

Action potentials are crucial for long-distance cellular communication, primarily in neurons and muscle cells.
They represent a reversal of charge relative to the extracellular space.
Neural information is transmitted through the frequency and pattern of action potentials.
Alternative names include "spike," "nerve impulse," and "discharge."

Generation of Action Potentials

Changes in membrane potential occur when ion concentrations or membrane permeability alter.
Two main types of signals:
- Graded potentials (short distances).
- Action potentials (long distances).

Steps of the Action Potential

Resting State: All gated Na+ and K+ channels are closed; only leakage channels are open, maintaining the resting membrane potential.
Depolarization:
- Voltage-gated Na+ channels open at -55mV, allowing Na+ influx; local currents cause adjacent channels to open.
- Membrane depolarizes to approximately +30mV.
Repolarization:
- Na+ inactivation gates close, causing a decrease in Na+ permeability.
- Voltage-gated K+ channels open, allowing K+ to exit, leading the membrane back to resting potential.
Hyperpolarization:
- Some K+ channels remain open, causing efflux that makes the inside of the membrane more negative, dipping below the resting potential.

All-or-None Principle

Action potentials occur only if depolarization reaches a threshold of -55mV to trigger an AP.
They either fully occur or not at all; varying stimulus strengths are encoded in the frequency of impulses.

Refractory Periods

Absolute Refractory Period: No AP can be triggered; occurs from Na+ channel opening to resetting.
Relative Refractory Period: Follows the absolute period; some Na+ channels are reset, but some K+ channels remain open, requiring a stronger stimulus for an AP.

Propagation of Action Potentials

Initiated APs propagate along axons due to local currents causing adjacent areas to depolarize.
In unmyelinated axons, each segment undergoes depolarization and repolarization.
In myelinated axons, APs occur at nodes of Ranvier, allowing faster conduction.

Voltage-Gated Channel Dynamics

Voltage-gated Na+ channels have activation and inactivation gates; dysfunction leads to channelopathies.
Voltage-gated K+ channels open at +30mV to rectify membrane potential after an AP.
Various toxins can affect channel functionality, highlighting their importance in neurotransmission.

Action Potential Characteristics

All action potentials are identical in magnitude and duration, but the CNS differentiates stimulus intensity based on impulse frequency.
Conduction velocities of APs vary widely among different axons, influenced by myelination and axon diameter.

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Explore the essential characteristics and processes involved in action potentials in this quiz. Dive into the generation, conduction, and detailed steps of action potentials, as well as understand how membrane potential changes with ion concentrations. This quiz is designed to reinforce your understanding of this critical neurological concept.