Neurons, Synapses, and Signaling

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Voltage-gated Na+ and K+ channels respond to a change in membrane potential.

When a stimulus depolarizes the membrane, Na+ channels open, allowing Na+ to diffuse into the cell.

The movement of Na+ into the cell increases the depolarization and causes even more Na+ channels to open.

A strong stimulus results in a massive change in membrane voltage called an action potential = signal.

Slide 24

Strong depolarizing stimulus

+50

Membrane potential (mV)

–50

Threshold

Resting

potential

–100

0

2

3

4

Time (msec)

(c) Action potential = change in membrane voltage

1

5

0

Action

potential

6

Slide 25

An action potential occurs if a stimulus causes the membrane voltage to cross a particular threshold.

An action potential is a brief all-or-none depolarization of a neuron’s plasma membrane.

Action potentials are signals that carry information along axons.

Slide 26

Generation of Action Potentials: A Closer Look

A neuron can produce hundreds of action potentials per second.

The frequency of action potentials can reflect the strength of a stimulus.

An action potential can be broken down into a series of stages.

Slide 27

The role of voltage-gated ion channels in the generation of an action potential

Key

Na+

K+

+50

Action

potential

Threshold

0

1

4

5

1

–50

Resting potential

Membrane potential

(mV)

–100

Time

Extracellular fluid

Plasma

membrane

Cytosol

Inactivation loop

Resting state

Sodium

channel

Potassium

channel

Depolarization

Rising phase of the action potential

Falling phase of the action potential

5

Undershoot

2

3

2

1

3

4

Slide 28

At resting potential

Most voltage-gated Na+ and K+ channels are closed, but some K+ channels (not voltage-gated) are open.

Slide 29

When an action potential is generated

Voltage-gated Na+ channels open first and Na+ flows into the cell.

During the rising phase, the threshold is crossed, and the membrane potential increases.

During the falling phase, voltage-gated Na+ channels become inactivated; voltage-gated K+ channels open, and K+ flows out of the cell.

Cell is now repolarized but is not normal until Na+ K+ pump restores original resting potential.

Slide 30