Graded Potentials & Action Potentials

Graded potentials (GPs) are

Short-lived, localized changes in membrane potential

What happens when GPs is triggered?

Opens gated ion channels

Opening of the gated ion channels result in

depolarization or hyperpolarization

Action Potentials (APs)

brief reversal of MP with a change in voltage of ~100 mV

APs are due to

opening of specific voltage-gated channels

Voltage-gated Na+ channels have two gates

Activation gates and Inactivation gates

Activation gate is

Closed at rest but open with depolarization. It allows Na+ to enter

Inactivation gate is

Open at rest and prevents more Na+ from entering

K+ channels have

one voltage-sensitive gate

K+ channels are what at rest?

Closed at rest but slowly opens with depolarization

What are the four steps to generating AP?

Resting state, depolarization, repolarization, hyperpolarization

Step 1

Resting State

What is the state of Na+ and K+ channels during resting state?

All gated Na+ and K+ channels are closed.

What channels remain open in the resting state?

Only leakage channels for Na+ and K+ are open.

What does the resting state maintain?

The resting membrane potential.

Step 2

Depolarization

What happens when depolarization begins?

Depolarizing local currents open the activation gates of voltage-gated Na+ channels.

What happens when Na+ channels open?

Na+ rushes into the cell.

What effect does Na+ influx have?

It causes more depolarization, which opens even more Na+ channels.

How does the intracellular fluid (ICF) change during depolarization?

The ICF becomes less negative.

Step 3

Repolarization

What happens to Na+ channels during repolarization?

Na+ channel inactivation gates close.

What happens to Na+ permeability during repolarization?

Membrane permeability to Na+ declines.

What channels open during repolarization?

Slow-acting voltage-gated K+ channels open.

What happens when K+ channels open?

K+ exits the cell down its electrochemical gradient.

What is the result of K+ leaving the cell?

The membrane returns to resting membrane potential.

Step 4

Hyperpolarization

Why does hyperpolarization occur?

Some K+ channels remain open, allowing excessive K+ efflux.

What happens to the inside of the membrane during hyperpolarization?

It becomes more negative than the resting state.

What is the effect of this excessive negativity?

A slight dip below the resting voltage occurs, known as hyperpolarization.

What happens to Na+ channels during hyperpolarization?

Na+ channels begin to reset.

What does repolarization reset?

Repolarization resets electrical conditions, not ionic conditions.

How are ionic conditions restored after repolarization?

Na+/K+ pumps restore ionic conditions.

All-or-None

An AP either happens completely, or does not happen at all

Threshold Voltage

-55mV to -50mV

What happens at threshold?

Voltage-gated Na+ channels open

Positive Feedback Cycle

influx of Na+ from each channel stimulates the opening of adjacent Na+ voltage-gated channels; depolarization is self-generating

Propagation

The spread of the AP down an axon