Lecture 13 — Ion Channels & Membrane Potential

What is an ion channel?

A transmembrane aqueous pore that allows ions to cross.

Do ion channels use active transport?

No; ions diffuse through channels down electrochemical gradients.

What is an ion selectivity filter?

Narrow channel region that determines which ions can pass.

What determines ion channel selectivity?

Pore size, shape, and charge distribution.

What is a gated ion channel?

Channel that opens or closes in response to a stimulus.

What opens a voltage-gated channel?

Change in membrane potential.

What opens a ligand-gated channel?

Ligand binding.

What opens a stress-gated channel?

Mechanical force/stress.

What is membrane potential?

Difference in electrical charge across a membrane.

At resting membrane potential, which side is more negative?

Inside of the cell.

Where is Na+ concentration highest?

Outside the cell.

Where is K+ concentration highest?

Inside the cell.

What does the Na+/K+ pump do at rest?

Pumps Na+ out and K+ in.

What do K+ leak channels do?

Allow K+ to leave the cell.

How does K+ leaving affect cell charge?

Makes the inside more negative.

What is an action potential?

Traveling wave of electrical activity across a neuron membrane.

What must a stimulus reach to trigger an action potential?

Threshold.

What happens during depolarization?

Voltage-gated Na+ channels open and Na+ enters.

During depolarization, does the inside become more positive or negative?

More positive.

What happens during repolarization?

Voltage-gated K+ channels open and K+ leaves.

During repolarization, what happens to Na+ channels?

They become inactivated.

If a drug prevents repolarization, what is the likely target?

Voltage-gated K+ channels.

What is the refractory period?

Period when the membrane resists another action potential.

What happens to Na+ channels during the refractory period?

They remain temporarily inactivated.

What helps restore resting conditions?

Na+/K+ pump and K+ leak channels.

Why does an action potential normally travel forward?

Recently activated regions are refractory.

What is a neuron’s cell body?

Region containing the nucleus and major cellular machinery.

What do dendrites do?

Receive signals.

What does the axon do?

Conducts action potentials away from the cell body.

What happens when an action potential reaches the nerve terminal?

Voltage-gated Ca2+ channels open.

What happens when Ca2+ enters the nerve terminal?

It triggers neurotransmitter release.

What is a neurotransmitter?

Chemical signal released from a neuron.

What is the synaptic cleft?

Gap between a neuron and its target cell.

What do neurotransmitters bind on the target cell?

Ligand-gated ion channels.

Why is cytosolic Ca2+ normally kept low?

Ca2+ acts as an important intracellular signal.

Where is Ca2+ stored in muscle cells?

Sarcoplasmic reticulum.

What happens during muscle activation?

Cytosolic Ca2+ rises and promotes contraction.

What pumps Ca2+ back into the sarcoplasmic reticulum?

ATP-driven Ca2+ pumps/SERCA.

What is myelin?

Insulating covering around axons.

What are Nodes of Ranvier?

Gaps between myelin segments where the axon membrane is exposed.