Membrane Potential Changes� Used as Communication Signals

• Membrane potential changes when
• Concentrations of ions across membrane change
• Membrane permeability to ions changes
• Changes produce two types signals
• Graded potentials
• Incoming signals operating over short distances
• Action potentials
• Long-distance signals of axons
• Changes in membrane potential used as signals to receive, integrate, and send information

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Changes in Membrane Potential

• Terms describing membrane potential changes relative to resting membrane potential
• Depolarization
• Decrease in membrane potential (toward zero and above)
• Inside of membrane becomes less negative than resting membrane potential
• Increases probability of producing a nerve impulse

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Figure 11.9a Depolarization and hyperpolarization of the membrane.

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Depolarizing stimulus

Inside

positive

Inside

negative

Depolarization

Resting

potential

Membrane potential (voltage, mV)

Depolarization: The membrane potential

moves toward 0 mV, the inside becoming less

negative (more positive).

Time (ms)

+50

0

–50

–70

–100

0

1

2

3

4

5

6

7

Changes in Membrane Potential

• Terms describing membrane potential changes relative to resting membrane potential
• Hyperpolarization
• An increase in membrane potential (away from zero)
• Inside of cell more negative than resting membrane potential)
• Reduces probability of producing a nerve impulse

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Figure 11.9b Depolarization and hyperpolarization of the membrane.

© 2013 Pearson Education, Inc.

Hyperpolarizing stimulus

Membrane potential (voltage, mV)

Time (ms)

+50

0

–50

–70

–100

0

1

2

3

4

5

6

7

Hyperpolarization: The membrane potential

increases, the inside becoming more negative.

Resting

potential

Hyper-

polarization

Graded Potentials

• Short-lived, localized changes in membrane potential
• Magnitude varies with stimulus strength
• Stronger stimulus 🡪 more voltage changes; farther current flows
• Either depolarization or hyperpolarization
• Triggered by stimulus that opens gated ion channels
• Current flows but dissipates quickly and decays
• Graded potentials are signals only over short distances

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Figure 11.10a The spread and decay of a graded potential.

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Stimulus

Depolarized region

Plasma

membrane

Depolarization: A small patch of the membrane (red area)

depolarizes.

Figure 11.10b The spread and decay of a graded potential.

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Depolarization spreads: Opposite charges attract each other.

This creates local currents (black arrows) that depolarize

adjacent membrane areas, spreading the wave of depolarization.

Figure 11.10c The spread and decay of a graded potential.

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Active area

(site of initial

depolarization)

Resting potential

Membrane potential (mV)

Distance (a few mm)

Membrane potential decays with distance: Because current is

lost through the “leaky” plasma membrane, the voltage declines with

distance from the stimulus (the voltage is decremental).

Consequently, graded potentials are short-distance signals.

–70

Action Potentials (AP)

• Principle way neurons send signals
• Principal means of long-distance neural communication
• Occur only in muscle cells and axons of neurons
• Brief reversal of membrane potential with a change in voltage of ~100 mV
• Do not decay over distance as graded potentials do

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Properties of Gated Channels

• Each Na⁺ channel has two voltage-sensitive gates
• Activation gates
• Closed at rest; open with depolarization allowing Na⁺ to enter cell
• Inactivation gates
• Open at rest; block channel once it is open to prevent more Na⁺ from entering cell

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Properties of Gated Channels

• Each K⁺ channel has one voltage-sensitive gate
• Closed at rest
• Opens slowly with depolarization

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Figure 11.11 The action potential (AP) is a brief change in membrane potential in a “patch” of membrane that is depolarized by local currents.

© 2013 Pearson Education, Inc.

The big picture

Resting state

1

2

Depolarization

Membrane potential (mV)

+30

0

–55

–70

Action

potential

2

3

4

1

1

0 1 2 3 4

Threshold

Time (ms)

Repolarization

Hyperpolarization

3

4

The AP is caused by permeability changes in the

plasma membrane:

Membrane potential (mV)

–70

–55

+30

0

Time (ms)

Action

potential

Na⁺

permeability

K⁺ permeability

Relative membrane

permeability

0 1 2 3 4

4

1

1

2

3

Outside

cell

Inside

cell

Activation

gate

Inactivation

gate

Closed

Opened

Inactivated

The events

The key players

Voltage-gated Na⁺ channels

Closed

Opened

Outside

cell

Inside

cell

Voltage-gated K⁺ channels

Sodium

channel

Potassium

channel

Activation

gates

Inactivation

gate

Resting state

Depolarization

Repolarization

Hyperpolarization

1

4

3

2

Generation of an Action Potential:�Resting State

• All gated Na⁺ and K⁺ channels are closed
• Only leakage channels for Na⁺ and K⁺ are open
• This maintains the resting membrane potential

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Generation of an Action Potential: Depolarizing Phase

• Depolarizing local currents open voltage-gated Na⁺ channels
• Na⁺ rushes into cell
• Na⁺ influx causes more depolarization which opens more Na⁺ channels 🡪 ICF less negative
• At threshold (–55 to –50 mV) positive feedback causes opening of all Na⁺ channels 🡪 a reversal of membrane polarity to +30mV
• Spike of action potential

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Generation of an Action Potential:�Repolarizing Phase

• Repolarizing phase
• Na⁺ channel slow inactivation gates close
• Membrane permeability to Na⁺ declines to resting state
• AP spike stops rising
• Slow voltage-gated K⁺ channels open
• K⁺ exits the cell and internal negativity is restored

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Generation of an Action Potential:�Hyperpolarization

• Some K⁺ channels remain open, allowing excessive K⁺ efflux
• Inside of membrane more negative than resting state
• This causes hyperpolarization of the membrane (slight dip below resting voltage)
• Na⁺ channels begin to reset

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Figure 11.11 The action potential (AP) is a brief change in membrane potential in a “patch” of�membrane that is depolarized by local currents. (1 of 3)

© 2013 Pearson Education, Inc.

Action

potential

Threshold

Time (ms)

Membrane potential (mV)

+30

0

–70

0

1

2

3

4

–55

1

1

Resting state. No

ions move through

voltage-gated

channels.

1

Figure 11.11 The action potential (AP) is a brief change in membrane potential in a “patch” of�membrane that is depolarized by local currents. (1 of 3)

© 2013 Pearson Education, Inc.

Depolarization

is caused by Na⁺

flowing into the cell.

Action

potential

Threshold

Time (ms)

+30

0

–70

0

1

2

3

4

–55

2

1

2

1

Resting state. No

ions move through

voltage-gated

channels.

1

Membrane potential (mV)

Figure 11.11 The action potential (AP) is a brief change in membrane potential in a “patch” of�membrane that is depolarized by local currents. (1 of 3)

© 2013 Pearson Education, Inc.

Depolarization

is caused by Na⁺

flowing into the cell.

Repolarization is

caused by K⁺ flowing

out of the cell.

Action

potential

Threshold

Time (ms)

+30

0

–70

0

1

2

3

4

–55

2

1

2

3

3

1

Resting state. No

ions move through

voltage-gated

channels.

1

Membrane potential (mV)

Figure 11.11 The action potential (AP) is a brief change in membrane potential in a “patch” of�membrane that is depolarized by local currents. (1 of 3)

© 2013 Pearson Education, Inc.

Resting state. No

ions move through

voltage-gated

channels.

Depolarization

is caused by Na⁺

flowing into the cell.

Repolarization is

caused by K⁺ flowing

out of the cell.

Hyperpolarization is

caused by K⁺ continuing to

leave the cell.

Action

potential

Threshold

Time (ms)

Membrane potential (mV)

+30

0

–70

0

1

2

3

4

–55

1

2

1

2

3

4

3

4

1