BIOL 319 Lecture 13 Review Study Notes

This set covers all materials discussed during lecture 13 for Dr. Cohn's final exam.

Do all cells have a resting membrane potential or only excitable cells?

All cells have a resting membrane potential, but excitable cells use it to generate signals.

What are excitable cells?

Cells that can respond to stimuli by generating and propagating action potentials (neurons and muscle cells).

What is the essential function of the resting membrane potential in excitable cells?

It provides the electrical gradient needed to generate action potentials.

Why is resting membrane potential important?

It allows rapid signaling and initiation of muscle contraction or nerve impulses.

How is resting membrane potential measured?

Using a voltmeter with microelectrodes placed inside and outside the cell.

Where is the reference electrode placed?

Outside the cell (extracellular fluid).

Where is the measurement electrode placed?

Inside the cell (intracellular fluid).

What does the voltmeter measure?

The voltage difference across the cell membrane.

What is the resting voltage across a cell membrane called?

Resting membrane potential (RMP).

What is the typical resting membrane potential of a skeletal muscle fiber?

Approximately -85 mV.

What is the typical resting membrane potential of a neuron?

Approximately -70 mV.

Are skeletal muscle and neuron RMPs identical?

No, but they are relatively close.

Why is the resting membrane potential negative?

Because there is a higher concentration of positive ions outside the cell than inside.

What are the main contributors to the negative resting membrane potential?

• K⁺ leaking out of the cell

• Na⁺/K⁺ pump (3 Na⁺ out, 2 K⁺ in)

• Unequal ion distribution

Compare resting membrane potentials of skeletal muscle cells vs neurons.

Skeletal muscle: ~ -85 mV
Neurons: ~ -70 mV
→ Both are negative and similar, but muscle is slightly more negative.

Why is the inside of a resting cell negatively charged relative to the outside?

Due to K⁺ leak out of the cell and the Na⁺/K⁺ pump maintaining ion gradients.

What must happen to the resting membrane potential for an action potential to occur?

It must depolarize (become less negative).

What is another name for the sodium/potassium pump?

Na⁺/K⁺ ATPase.

What is the main function of the Na⁺/K⁺ pump?

Maintains unequal Na⁺ and K⁺ concentrations across the membrane.

What is the ion ratio per pump cycle?

3 Na⁺ out, 2 K⁺ in.

In which direction are Na⁺ and K⁺ moved?

Na⁺ → out of cell
K⁺ → into cell

How much ATP is used per pump cycle?

1 ATP.

Is the Na⁺/K⁺ pump electrogenic?

Yes.

Why is the Na⁺/K⁺ pump electrogenic?

It moves more positive charges out (3 Na⁺) than in (2 K⁺), creating a net negative inside.

Does the Na⁺/K⁺ pump contribute a lot or a little directly to RMP?

A little (minor direct contribution).

Are Na⁺ and K⁺ gradients substantial?

Yes, very steep concentration gradients.

Compare Na⁺ and K⁺ gradients.

Na⁺ → high outside, low inside
K⁺ → high inside, low outside

What are leak channels?

Always-open ion channels that allow passive ion movement.

Are ions moved only by concentration gradients?

No → movement depends on both concentration AND electrical gradients.

What do potassium leak channels do?

Allow K⁺ to diffuse out of the cell.

What drives K⁺ out of the cell?

Its concentration gradient (high inside → low outside).

What does Pr⁻ stand for?

Negatively charged intracellular proteins.

What happens to Pr⁻ as K⁺ leaves the cell?

Pr⁻ stays inside, making the inside more negative.

How does Pr⁻ contribute to resting membrane potential?

It creates a negative electrical gradient inside the cell.

What happens as K⁺ leaves the cell?

Inside becomes more negative → creates an electrical gradient pulling K⁺ back in.

What happens when electrical and concentration gradients balance?

Net K⁺ movement stops (equilibrium).

What is the approximate voltage where this balance occurs?

About -90 mV.

Why does K⁺ have the greatest effect on resting membrane potential?

Because there are many more K⁺ leak channels than Na⁺ leak channels.

Does Na⁺ leak significantly affect resting membrane potential?

No → very minimal effect.

Why does Na⁺ have little effect on RMP?

Few Na⁺ leak channels + limited Na⁺ permeability.

What primarily determines resting membrane potential?

K⁺ movement through leak channels.

Explain how resting membrane potential is established.

Na⁺/K⁺ pump creates ion gradients → K⁺ diffuses out through leak channels → Pr⁻ remains inside → inside becomes negative → electrical gradient balances concentration gradient → stable negative RMP forms.

Which contributes MOST to resting membrane potential?

K⁺ leak channels.

Which contributes LEAST directly to resting membrane potential?

Na⁺/K⁺ pump (directly).

Pump vs Leak Channels (main difference)?

Pump → creates gradients (active)
Leak channels → create RMP (passive)

What is an action potential?

A rapid, temporary change in membrane potential used for signaling in neurons and muscle cells.

Which voltage-gated channel opens first during an action potential?

Voltage-gated Na⁺ channels.

What is the threshold potential?

The membrane voltage required to trigger an action potential.

What is the approximate threshold potential?

About -55 mV.

What happens when threshold potential is reached?

Voltage-gated Na⁺ channels open rapidly → depolarization begins.

What ion enters the cell during depolarization?

Na⁺ (sodium).

What effect does Na⁺ influx have on membrane potential?

It makes the membrane rapidly more positive.

How long do voltage-gated Na⁺ channels stay open?

~0.5 milliseconds.

What happens to membrane potential during this time?

It rises quickly toward positive values.

Which channel opens second during an action potential?

Voltage-gated K⁺ channels.

When do K⁺ channels open relative to Na⁺ channels?

They open as Na⁺ channels begin to inactivate.

What ion moves during repolarization?

K⁺ leaves the cell.

What effect does K⁺ efflux have on membrane potential?

It makes the membrane more negative (repolarization).

How long does repolarization take?

About ~0.5 milliseconds.

Are Na⁺ and K⁺ channel activities staggered?

Yes.

Why is staggered channel activity important?

It ensures a clear depolarization phase followed by repolarization.

What would happen if Na⁺ and K⁺ channels overlapped significantly?

The action potential would be weakened or fail (signals would cancel out).

What is after-hyperpolarization?

A phase where membrane potential becomes more negative than resting.

Why does after-hyperpolarization occur?

K⁺ channels stay open longer than needed → excess K⁺ leaves the cell.

What restores ion gradients after an action potential?

Na⁺/K⁺ pump.

What values are restored after the action potential?

-85 mV (muscle) or -70 mV (neurons).

Which channel is responsible for depolarization?

Voltage-gated Na⁺ channels.

Which channel is responsible for repolarization?

Voltage-gated K⁺ channels.

What is the correct order of events in an action potential?

Threshold reached → Na⁺ channels open → depolarization → Na⁺ channels close → K⁺ channels open → repolarization → hyperpolarization → return to rest.

Which opens first: Na⁺ or K⁺ channels?

Na⁺ channels.

Which ion causes the rapid rise of the action potential?

Na⁺.

Which ion causes the fall of the action potential?

K⁺.

Why is precise timing of Na⁺ and K⁺ channels critical?

It allows a rapid, directional, and non-overlapping electrical signal.

How does Na⁺ entry at one location influence Na⁺ entry at the next location?

Na⁺ influx causes local depolarization, which brings adjacent membrane to threshold, opening more Na⁺ channels.

Why is Na⁺ movement considered a positive feedback mechanism?

Because Na⁺ entry causes more depolarization, which opens more Na⁺ channels, leading to even more Na⁺ entry.

Describe the sequence of Na⁺ movement during propagation.

Na⁺ enters at position 1 → depolarizes position 2 → triggers Na⁺ entry at position 2 → repeats forward.

What is meant by a self-propagating chain reaction in action potentials?

Each depolarized segment triggers the next segment, allowing the signal to move along the membrane.

How does positive feedback drive action potential propagation?

Depolarization at one segment brings the next segment to threshold, continuously regenerating the action potential.

Why does the action potential keep “reinventing itself”?

Because each new section of membrane reaches threshold and generates a new depolarization.

Why does an action potential not travel backward?

The previous segment is in a refractory period and cannot be reactivated.

What property prevents backward propagation?

Refractory period (Na⁺ channels are inactivated).

What allows an action potential to travel long distances without losing strength?

Continuous regeneration via positive feedback at each segment.

What type of feedback drives action potential propagation?

Positive feedback.

What stops the signal from reversing direction?

Refractory period behind the action potential.

Summarize action potential propagation in one sentence.

Na⁺-driven positive feedback causes a self-propagating wave that moves forward only due to refractory periods behind it.