Electrical Signals in Neurons and Action Potential Conduction

Vocabulary flashcards covering graded potentials, action potential dynamics, channel gating, refractory periods, and conduction velocity.

Graded potentials

Variable strength electrical signals that travel over short distances, lose strength as they spread through the cell, and can be either depolarizing or hyperpolarizing.

Action potentials

Very brief, large depolarizations of uniform strength (all-or-none) that travel long distances through a neuron without losing strength.

Excitatory Postsynaptic Potential (EPSP)

A depolarizing graded potential that brings the membrane potential closer to threshold, increasing the likelihood of firing an action potential.

Inhibitory Postsynaptic Potential (IPSP)

A hyperpolarizing graded potential that moves the membrane potential further from threshold, decreasing the likelihood of firing an action potential.

Trigger zone

The region comprising the axon hillock and axon initial segment (AIS), containing a high concentration of voltage-gated $Na^+$ channels where an action potential is initiated if membrane potential reaches approximately $-55\,mV$.

Rising phase

The depolarization phase of an action potential where depolarizing stimuli open voltage-gated $Na^+$ channels at $-55\,mV$, allowing rapid $Na^+$ entry that depolarizes the cell to approximately $+30\,mV$.

Falling phase

The repolarization phase of an action potential where voltage-gated $Na^+$ channels close/inactivate and slower voltage-gated $K^+$ channels open, allowing $K^+$ efflux from the cell to the extracellular fluid.

After-hyperpolarization phase

The undershoot phase of an action potential where voltage-gated $K^+$ channels remain open beyond $-70\,mV$, causing the membrane potential to temporarily dip below the resting membrane potential.

Absolute refractory period

A period lasting $1\text{--}2\,msec$ after an action potential initiation during which a second action potential cannot be triggered regardless of stimulus strength because voltage-gated $Na^+$ channels are inactivated.

Relative refractory period

A period lasting $2\text{--}5\,msec$ following the absolute refractory period during which a second action potential can be initiated only if a larger-than-normal depolarizing stimulus is applied.

Conduction

The movement or propagation of an action potential along an axon, in which a new action potential is sequentially generated in each adjacent section of the membrane.

Current leak

The loss of ions through open membrane channels as a graded potential spreads, contributing to its decrease in strength over distance.

Squid giant axon

An axon from a squid measuring $0.8\,mm$ in diameter, which offers low internal resistance to current flow and serves as a model for studying action potential conduction velocity.