Equilibrium Potentials and Neuronal Action Potentials

Flashcards covering equilibrium potentials, ion gradients, neuronal thresholds, action potentials, and basic neuron anatomy.

What is an equilibrium potential for an ion?

The membrane voltage at which the electrical gradient exactly opposes the chemical gradient so the net movement of that ion stops.

What is the typical resting membrane potential of most excitable cells?

Approximately –70 mV to –90 mV, negative inside relative to outside.

Where is sodium concentration higher, inside or outside the cell?

Outside the cell.

Where is potassium concentration higher, inside or outside the cell?

Inside the cell.

Why does Na⁺ naturally move into a resting cell?

It follows both a strong chemical gradient (higher outside) and an electrical attraction to the negative interior.

Why does K⁺ naturally move out of a resting cell?

Its large chemical gradient overrides the electrical attraction pulling it inward.

What is the numerical value of Na⁺’s equilibrium potential (Eₙₐ)?

+60 mV.

What happens to Na⁺ influx at +60 mV?

Net Na⁺ influx stops because the interior is now too positive to attract additional Na⁺.

What is the numerical value of K⁺’s equilibrium potential (Eₖ)?

–90 mV.

Why does K⁺ efflux stop near –90 mV even if channels remain open?

The inside becomes negative enough to attract K⁺, balancing its chemical drive to leave.

Define depolarization in membrane physiology.

A shift toward a more positive interior membrane voltage.

Why is the term “depolarization” called a misnomer by some?

Because the polarity actually reverses (inside becomes positive, outside becomes negative) rather than merely losing polarity.

What is a graded potential?

A local, variable-size depolarization (or hyperpolarization) that spreads across the dendrites and cell body.

What is an action potential?

A rapid, all-or-none depolarization–repolarization wave that propagates along an axon.

At what structure must threshold be reached to trigger an axonal action potential?

The axon hillock.

What is the typical neuronal threshold voltage?

–55 mV.

Primary function of dendrites on a neuron?

Increase surface area for synaptic input and receptor/channel placement.

Primary function of axon terminals?

Release neurotransmitters onto downstream cells.

What do synaptic vesicles store?

Neurotransmitters.

Which ion’s influx triggers exocytosis of synaptic vesicles?

Ca²⁺ (calcium).

Give three example neurotransmitters mentioned in the lecture.

Acetylcholine, dopamine, serotonin (others mentioned: norepinephrine).

When do voltage-gated Na⁺ and K⁺ channels begin to open?

Both start opening at threshold (≈ –55 mV).

Why does the action potential peak at about +30 mV instead of +60 mV?

Voltage-gated Na⁺ channels inactivate quickly, stopping Na⁺ entry before Eₙₐ is reached.

What causes the repolarization and hyperpolarization phases?

K⁺ efflux through slowly closing voltage-gated K⁺ channels.

How does the membrane return from –90 mV hyperpolarization to –70 mV resting?

Continuous Na⁺ influx through always-open sodium leak channels.

What is the charge effect of the Na⁺/K⁺ ATPase pump?

It exports 3 Na⁺ and imports 2 K⁺, making the outside slightly more positive than the inside.

Distinguish chemical gradient from electrical gradient.

Chemical gradient is due to concentration differences; electrical gradient is due to charge differences across the membrane.

Why is only a small amount of Na⁺ needed to depolarize a cell?

Even tiny charge changes significantly shift the thin membrane’s voltage.

In what units is membrane potential measured?

Millivolts (mV).

How do neurotransmitters propagate a signal to the next neuron or cell?

They diffuse a few nanometers across the synaptic cleft and bind receptors that open ion channels or activate signaling pathways.

Effect of acetylcholine on the heart versus skeletal muscle?

It slows heart rate (myocardial cells) but causes skeletal muscle fibers to depolarize and contract.