Action Potential / Spike Potential in an Unmyelinated Nerve Fiber

Vocabulary flashcards covering key terms and concepts related to the generation, stages, and ion channel mechanisms of nerve action potentials in unmyelinated fibers.

Action Potential

A rapid, self-propagating change in membrane potential that travels along the membrane of an excitable cell, beginning with depolarization and ending with repolarization.

Spike Potential

Another name for an action potential, emphasizing its brief, sharp voltage change.

Excitable Tissue

Tissue (e.g., nerve or muscle) capable of generating and propagating action potentials.

Resting Membrane Potential

The baseline electrical charge across the membrane (≈ –90 mV in large neurons) when the cell is not firing.

Polarized Membrane

State of the membrane during rest when the inside is negative relative to the outside.

Depolarization

Phase during which membrane potential becomes less negative and briefly positive due to Na⁺ influx.

Overshoot

Part of depolarization where the membrane potential becomes positive beyond 0 mV.

Repolarization

Return of membrane potential to its resting negative value, mainly via K⁺ efflux.

Afterpotential (Undershoot)

Transient hyperpolarization following repolarization before the membrane returns to rest.

Voltage-Gated Sodium (Na⁺) Channel

Membrane protein with activation and inactivation gates that opens rapidly on depolarization to allow Na⁺ influx.

Activation Gate (Na⁺ Channel)

Outer gate of the Na⁺ channel that opens quickly when the membrane depolarizes toward –70 to –50 mV.

Inactivation Gate (Na⁺ Channel)

Inner gate of the Na⁺ channel that closes more slowly, terminating Na⁺ entry during an action potential.

Voltage-Gated Potassium (K⁺) Channel

Channel that opens with slight delay during depolarization, allowing K⁺ efflux to speed repolarization.

Na⁺-K⁺ Pump

ATP-driven transporter that restores ion gradients by pumping 3 Na⁺ out and 2 K⁺ into the cell.

Leak Channels

Non-gated Na⁺ and K⁺ channels that contribute to the resting membrane potential.

Threshold Stimulus

The minimal depolarizing input required to trigger an action potential.

Threshold Potential

Membrane voltage (~ –65 mV in large fibers) at which Na⁺ influx exceeds K⁺ efflux, initiating the action potential.

Positive Feedback Vicious Cycle

Self-amplifying loop in which rising depolarization opens more Na⁺ channels, causing further depolarization.

Sodium Permeability

Ease with which Na⁺ crosses the membrane; increases up to 5000-fold during Na⁺ channel opening.

Potassium Permeability

Ease with which K⁺ crosses the membrane; rises during repolarization due to K⁺ channel opening.

Conductance Shift

Change from dominant Na⁺ conductance (depolarization) to dominant K⁺ conductance (repolarization).

Self-Propagation

Ability of an action potential to regenerate itself along adjacent sections of membrane without decrement.

Unmyelinated Nerve Fiber

Axon lacking myelin in which action potentials propagate continuously rather than by saltatory conduction.

Overshoot Peak

Highest positive membrane voltage (≈ +20 to +40 mV) reached during an action potential.

Resting Stage

Phase before an action potential when the membrane is polarized at –90 mV.

Initiation of Action Potential

Moment when threshold is reached, Na⁺ channels open, and the positive feedback loop starts.

Recovery Phase

Period after repolarization when ion gradients are re-established by pumps and channels.