18). AI Action Potentials & Excitability

A set of 60 practice flashcards reviewing voltage-gated channels, action potential generation and propagation, refractory periods, myelination, and local anesthetics based on multiple-choice lecture review questions.

What initiates the rapid rising phase of a typical neuronal action potential?

Opening of voltage-gated $\text{Na}^+$ channels.

In which direction does $\text{Na}^+$ generally move during the rising phase of an action potential?

Into the neuron.

Why is the opening of voltage-gated $\text{Na}^+$ channels considered regenerative?

Depolarization opens additional voltage-gated $\text{Na}^+$ channels.

What most directly contributes to repolarization of the membrane?

$\text{Na}^+$ channel inactivation and increased $\text{K}^+$ conductance.

In which direction does $\text{K}^+$ generally move during repolarization?

Out of the neuron.

Why does afterhyperpolarization often occur following an action potential?

Voltage-gated $\text{K}^+$ channels remain open briefly.

Which channel change is important for the upstroke of some cardiac or other excitable-cell action potentials?

Opening of voltage-gated $\text{Ca}^{2+}$ channels.

What does $\text{Ca}^{2+}$ entry into a presynaptic terminal commonly trigger?

Synaptic vesicle fusion and transmitter release.

At the peak of a typical axonal action potential, what events are occurring?

$\text{Na}^+$ channel inactivation and $\text{K}^+$ channel activation promote the falling phase.

What is the primary role of the $\text{Na}^+/\text{K}^+\text{-ATPase}$ during repeated firing?

Maintain $\text{Na}^+$ and $\text{K}^+$ gradients over time.

How does stimulus strength relate to the amplitude of a graded potential?

Its amplitude varies with stimulus strength.

Which statement best describes the amplitude behavior of a typical action potential?

It is all-or-none after threshold is reached.

How do local graded potentials typically spread along a membrane?

Passively and decrementally.

How does an action potential propagate along an axon?

Regenerative opening of voltage-gated channels in successive regions.

What is temporal summation?

Combining postsynaptic potentials arriving close together in time.

What is spatial summation?

Combining graded inputs arising at different locations.

Which synaptic event can generate a graded potential?

Opening of ligand-gated channels at a synapse.

What typically increases as the strength of a suprathreshold stimulus increases?

Action-potential firing frequency.

Where are graded synaptic potentials often integrated to determine firing?

Axon initial segment.

Which property helps ensure that an action potential travels long distances?

Regeneration prevents progressive loss of amplitude.

What does reaching threshold mean in an excitable neuron?

Inward regenerative current becomes sufficient to trigger a spike.

Which phase of the action potential corresponds to the steep upward voltage change?

Depolarization.

Which phase of the action potential corresponds to the voltage returning toward rest after the peak?

Repolarization.

Which phase of the action potential may temporarily bring $V_m$ below resting potential?

Afterhyperpolarization.

What is the principal basis of the absolute refractory period?

Voltage-gated $\text{Na}^+$ channels are inactivated.

During the absolute refractory period, under what conditions can a second normal $\text{Na}^+$-dependent spike be initiated?

It cannot be initiated regardless of stimulus strength.

What characterizes the stimulus requirement during the relative refractory period?

A stronger-than-usual stimulus may be needed.

Which factors help explain the occurrence of the relative refractory period?

Residual $\text{K}^+$ conductance and incomplete $\text{Na}^+$ channel recovery.

Why does a typical axonal spike propagate preferentially forward?

Membrane behind the spike is refractory.

What must happen for an inactivated voltage-gated $\text{Na}^+$ channel to recover?

Repolarization permits return to a closed, activatable state.

What is saltatory conduction?

Action potentials regenerate at nodes of Ranvier.

Where are voltage-gated $\text{Na}^+$ channels especially concentrated in many myelinated axons?

Nodes of Ranvier.

What effect does myelin have on membrane resistance across internodes?

Increases it.

What effect does myelin have on internodal membrane capacitance?

Decreases it.

Why is conduction typically faster in a myelinated axon compared to an unmyelinated axon?

Less current leaks and less membrane needs charging between nodes.

What is continuous conduction?

Sequential regeneration along unmyelinated axonal membrane.

What happens to conduction speed when axon diameter increases, all else equal?

It generally increases.

Which axon type would usually conduct fastest, all else equal?

Large-diameter myelinated axon.

What functional impairment can demyelination cause?

Slowed conduction or conduction block.

Why are nodes of Ranvier essential to saltatory conduction?

They regenerate the action potential between insulated internodes.

What is the primary target of common local anesthetics?

Voltage-gated $\text{Na}^+$ channels.

What is the immediate electrophysiologic effect of voltage-gated $\text{Na}^+$ channel blockade?

Reduced inward $\text{Na}^+$ current needed for spike initiation and propagation.

Why can local anesthetics prevent sensation from reaching the CNS?

They block action-potential conduction in sensory axons.

What does use-dependent block mean in local anesthetic action?

Block can become greater with repeated channel activation.

Which channel conformations are often preferentially bound by local anesthetics?

Open and inactivated states.

If too few voltage-gated $\text{Na}^+$ channels are available, what may occur?

Failure to reach threshold or propagate a spike.

Why can a local anesthetic affect both action potential initiation and propagation?

Both processes depend on available voltage-gated $\text{Na}^+$ channels.

Which statement distinguishes a local anesthetic from a $\text{Na}^+/\text{K}^+\text{-ATPase}$ inhibitor?

A local anesthetic primarily blocks voltage-gated $\text{Na}^+$ conductance.

Why may frequent firing increase local anesthetic block?

More channels enter states with greater drug affinity.

What happens to a propagating spike if a sufficiently long axon region has strongly blocked $\text{Na}^+$ channels?

Conduction may fail across that region.

Does an action potential's height increase continuously with stimulus strength?

No; action potentials are all-or-none once threshold is reached, and firing frequency increases instead.

How does the decremental spread of a graded potential differ from action potential spread?

Graded potentials decrease in amplitude with distance, whereas action potentials regenerate to prevent loss of amplitude.

What voltage state is required for an inactivated $\text{Na}^+$ channel to return to an activatable state?

Repolarization.

In which type of axon does continuous conduction occur?

Unmyelinated axons.

What structure covers internodes to increase membrane resistance and decrease capacitance?

The myelin sheath.

What two mechanisms simultaneously promote the falling phase of an action potential?

$\text{Na}^+$ channel inactivation and voltage-gated $\text{K}^+$ channel activation.

What type of channel is typically opened at synapses to generate graded postsynaptic potentials?

Ligand-gated channels.

Why cannot a second action potential be fired during the absolute refractory period?

Because voltage-gated $\text{Na}^+$ channels are inactivated.

What effect does increased axonal diameter have on axial resistance and conduction speed?

It lowers axial resistance, which generally increases conduction speed.

What determines whether an axon reaches threshold during depolarizing inputs?

Whether inward regenerative current becomes sufficient to trigger a spike.