NAS2 Cell Membrane Potential

What two major abilities should you gain from Section 12.4?

Describe the membrane components that establish resting membrane potential and the membrane changes that produce an action potential.

What is an excitable membrane?

A cell membrane capable of changing its electrical potential to generate signals.

Why are excitable membranes essential to nervous-system function?

They allow neurons to generate electrical signals used for sensation, integration, and response.

How does an action potential differ from the graded potentials introduced in the next section?

An action potential is an all-or-none propagated signal; graded potentials vary in size and are treated separately in the next section.

What is a transmembrane potential?

A difference in electrical charge across the cell membrane.

Why can ions not cross the phospholipid bilayer unaided?

Ions are charged and hydrophilic, so they cannot pass through the membrane’s hydrophobic core.

What type of membrane protein commonly allows ions to cross the membrane?

A transmembrane channel protein, or ion channel.

How do passive ion channels differ from active transport pumps?

Channels let ions move down electrochemical gradients without direct energy input; pumps use energy to move ions against gradients.

Why do neurons need both passive channels and active pumps?

Channels permit electrical signaling, while pumps maintain the ion gradients that make signaling possible.

What is the sodium/potassium pump?

An ATP-powered carrier protein that moves Na+ out of the cell and K+ into the cell.

Why is the sodium/potassium pump called an ATPase?

It hydrolyzes ATP to power ion transport.

Where are Na+ and K+ most concentrated at rest?

Na+ is more concentrated outside the cell; K+ is more concentrated inside.

Why does the sodium/potassium pump require energy?

It moves Na+ and K+ against their concentration gradients.

True or false: The sodium/potassium pump creates action potentials by opening a pore for ions.

False. It actively maintains Na+ and K+ concentration gradients; voltage changes mainly occur through ion channels.

What is an ion channel?

A membrane pore that allows selected charged particles to cross down an existing electrochemical gradient.

What is electrochemical exclusion?

Charge-based selectivity in which charged amino-acid side chains in a channel pore favor ions of the opposite charge.

What pore charge would favor passage of cations?

Negatively charged side chains in the pore.

What pore charge would favor passage of anions?

Positively charged side chains in the pore.

What is size exclusion in an ion channel?

Selectivity based on pore diameter and how an ion interacts with surrounding water and amino-acid side chains.

Why is a larger pore not automatically better for a smaller ion?

The smaller ion’s hydration shell may interact more strongly with water than with pore side chains, reducing selective passage.

What is a nonspecific ion channel?

A channel selective mainly by charge rather than exact ion size.

Which ions commonly pass through the nonspecific cation channels described in this section?

Na+, K+, and Ca2+, while anions are excluded.

What does it mean for an ion channel to be gated?

It opens or closes in response to a particular condition or event.

What does Figure 12.17 emphasize about cell membranes?

The phospholipid bilayer contains transmembrane proteins, including different ion-channel types.

How can you distinguish a transmembrane protein from a peripheral protein in Figure 12.17?

A transmembrane protein spans the bilayer; a peripheral protein is attached to a membrane surface or another protein.

What is the pore of a channel protein?

The internal passage through which selected ions cross the membrane.

What opens a ligand-gated channel?

Binding of a signaling molecule, or ligand, to the extracellular region of the channel.

Why is a ligand-gated channel also called an ionotropic receptor?

Ligand binding directly opens an ion pore and changes membrane charge.

What is the ligand commonly called in the nervous system?

A neurotransmitter.

What ligand opens the channel illustrated in Figure 12.18?

Acetylcholine (ACh).

Which cations are shown moving through the ligand-gated channel in Figure 12.18?

Na+, Ca2+, and K+.

Why can different cations move in opposite directions through the same open channel?

Each ion moves down its own electrochemical gradient.

What opens a mechanically gated channel?

Physical distortion of the membrane or surrounding tissue.

Give a sensory example of mechanically gated channels.

Pressure on the skin opens channels in touch receptors.

How are thermoreceptors similar to mechanically gated channels?

A temperature-induced protein change physically opens the channel.

What event opens the channel in Figure 12.19?

A mechanical change such as pressure or touch.

What opens a voltage-gated channel?

A change in the membrane’s electrical potential.

What typically happens to a voltage-gated channel as the inside becomes less negative?

Charge-sensitive parts of the protein change shape and open the pore.

What membrane-voltage change is illustrated as opening the voltage-gated channel in Figure 12.20?

A change from about -70 mV to -50 mV.

What is a leakage channel?

A channel that randomly switches between open and closed states.

Does an external stimulus directly open a leakage channel?

No. Leakage channels open and close randomly at an intrinsic rate.

What does Figure 12.21 show about leakage channels?

They can randomly open and allow ions to move down their gradients.

Why are leakage channels important even though they open randomly?

Their continuous background ion movement contributes to resting membrane potential.

What is membrane potential?

The distribution of electrical charge across a cell membrane, expressed as the voltage inside relative to outside.

In what units is membrane potential usually measured?

Millivolts (mV).

By convention, which side of the membrane is treated as the reference value of zero?

The extracellular side; the intracellular voltage is reported relative to it.

How is transmembrane voltage measured in Figure 12.22?

A recording electrode is placed inside the cell and a reference electrode outside.

What does a voltmeter reading of -70 mV mean?

The cytosol is 70 mV more negative than the extracellular fluid.

True or false: The entire cytosol carries a large net negative charge at rest.

False. Bulk intracellular and extracellular fluids are nearly neutral; charge separation is concentrated near the membrane surfaces.

Why can a very small separation of charge generate powerful neuronal signals?

The charge difference is concentrated across the extremely thin membrane, creating a biologically significant voltage.

What is the resting membrane potential?

The steady transmembrane voltage of an unstimulated excitable cell, commonly represented as -70 mV.

What resting membrane potential value is most commonly used for neurons in this section?

-70 mV.

Is -70 mV the exact resting membrane potential of every cell?

No. The exact value varies among cells; -70 mV is a common representative value.

How does extracellular Na+ concentration compare with intracellular Na+ concentration at rest?

It is about 10 times greater outside the cell.

Where is K+ concentration higher at rest?

Inside the cell.

What major anions contribute to the negative interior of the resting cell?

Phosphate ions, negatively charged proteins, and negatively charged membrane-associated molecules.

What is a membrane leaflet?

One side, or one layer, of the lipid bilayer.

How do Na+ and K+ leakage affect resting potential?

Na+ leakage tends to make the inside less negative; K+ leakage tends to make it more negative.

What restores Na+ and K+ that leak across the resting membrane?

The Na+/K+ pump.

True or false: Resting membrane potential is static because no ions move.

False. It is a dynamic steady state maintained by continuous leakage and active pumping.

Why is ion leakage plus pumping not merely wasted energy?

Together they maintain the electrochemical conditions needed for rapid electrical signaling.

What is an action potential?

A rapid, stereotyped change in membrane voltage that serves as an electrical signal in nervous tissue.

Put the main phases of an action potential in order.

Resting state → depolarization → repolarization → hyperpolarization → return to resting potential.

What is depolarization?

A change in membrane potential toward zero or a more positive value.

Which ion’s entry initiates the rapid depolarizing phase described in this section?

Na+ entering the cell.

Why does Na+ rush into the cell when Na+ channels open?

Both its concentration gradient and the negative interior favor inward movement.

To approximately what voltage does the membrane rise during depolarization?

+30 mV.

Why can Na+ continue entering even after membrane voltage reaches 0 mV?

Its strong concentration gradient still drives inward movement.

What is repolarization?

The return of membrane voltage toward the negative resting value after depolarization.

Which ion movement produces repolarization?

K+ leaves the cell through voltage-gated K+ channels.

What is hyperpolarization?

A temporary period when membrane potential becomes more negative than resting potential.

Why does hyperpolarization occur?

Voltage-gated K+ channels close with a delay, so K+ continues leaving after the resting voltage is passed.

How large is the voltage change from -70 mV to +30 mV?

100 mV, or 0.1 V.

Why does the text compare the membrane to a battery?

Charge is stored across the membrane and can be released under appropriate conditions.

What variables are plotted in Figure 12.23?

Membrane voltage against time.

What phase follows the +30 mV peak in Figure 12.23?

Repolarization.

What portion of the graph lies below -70 mV?

The hyperpolarization phase.

Where does the graph end after hyperpolarization?

Back at the resting membrane potential.

What kinds of channels can begin the initial depolarization toward threshold?

Ligand-gated or mechanically gated Na+ channels activated by a neurotransmitter or sensory stimulus.

What is threshold in this section?

The membrane voltage, about -55 mV, at which many voltage-gated Na+ channels open and an action potential begins.

What threshold voltage is used in this section?

-55 mV.

True or false: A depolarization to -60 mV triggers a full action potential.

False. It does not reach the -55 mV threshold.

What happens if depolarization reaches -55 mV or above?

Many voltage-gated Na+ channels open and a full action potential is initiated.

What does all-or-none mean for an action potential?

Once threshold is reached, the full stereotyped action potential occurs; below threshold, it does not occur.

Does a stronger stimulus produce a higher action-potential peak?

No. Individual action potentials peak at approximately the same voltage.

How can a stronger stimulus encode greater intensity if action potentials do not get larger?

It can increase the frequency or number of action potentials.

Compare a subthreshold and suprathreshold stimulus.

A subthreshold stimulus produces no action potential; a suprathreshold stimulus produces the same full-sized action potential.

True or false: Stronger pain occurs because each action potential is physically larger.

False. Greater stimulus intensity is represented mainly by more frequent action potentials, not larger ones.

How many gates does a voltage-gated Na+ channel have?

Two: an activation gate and an inactivation gate.

What does the activation gate of a voltage-gated Na+ channel do?

It opens when threshold is crossed, allowing Na+ to enter.

What does the inactivation gate of a voltage-gated Na+ channel do?

It closes shortly after activation, stopping further Na+ entry.

Describe the Na+ channel gates at rest, at threshold, at the peak, and during reset.

Rest: activation closed/inactivation open; threshold: activation opens; peak: inactivation closes; after repolarization: activation closes, then inactivation reopens.

Why does Na+ influx stop near the peak of depolarization?

The inactivation gates close.

At what voltage does the activation gate open?

Approximately -55 mV.

When does the activation gate close again?

As repolarization carries the membrane back below about -55 mV.

How many gates does the voltage-gated K+ channel have in this description?

One.

To what voltage is the voltage-gated K+ channel sensitive?

Approximately -50 mV.

How does K+ channel opening compare in timing with Na+ channel opening?

K+ channels open more slowly, around the time Na+ influx peaks and Na+ channels inactivate.

What closes the voltage-gated K+ channel?

Repolarization past about -50 mV, followed by a slight closing delay.

Which channel delay directly produces the hyperpolarizing overshoot?

Delayed closure of voltage-gated K+ channels.