Nerve Impulse

Why are neurons described as “excitable” cells?

They can dramatically change their internal electrical charge in response to stimuli, allowing them to communicate with each other.

What is membrane potential?

The difference in electrical charge between the inside and outside of a cell.

What is the resting membrane potential (RMP) of a neuron?

About –70 mV when the neuron is at rest or inactive.

Why is the inside of a resting neuron negatively charged?

Negatively charged proteins and ions inside the neuron cannot move across the membrane, creating an overall negative charge inside.

Where is the concentration of K⁺ higher?

Inside the neuron.

Where is the concentration of Na⁺ higher?

Outside the neuron.

What is a stimulus in terms of a neuron?

Anything that can change the resting membrane potential of –70 mV in either direction.

What happens during depolarization?

Na⁺ channels open, Na⁺ enters the neuron, and the membrane potential becomes less negative and moves toward zero.

Which ion is primarily responsible for depolarization?

Na⁺, because Na⁺ enters the neuron through open channels.

What is the threshold?

The level of stimulation that must be reached to trigger an action potential; in neurons it is about –55 mV.

What happens when a neuron reaches threshold?

It generates an action potential and begins sending an electrical signal along its axon.

What is an action potential?

A change in membrane potential caused by a threshold stimulus that produces an electrical signal, or impulse.

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

Once the neuron reaches threshold, the action potential occurs; there is no turning back.

What happens to Na⁺ channels after the action potential reaches its peak?

Na⁺ channels close and K⁺ channels open.

What happens during repolarization?

K⁺ moves out of the neuron, causing the membrane potential to return toward its normal negative state.

Which ion is primarily responsible for repolarization?

K⁺, because K⁺ moves out of the neuron.

What causes hyperpolarization?

Too much K⁺ leaves the neuron, causing the membrane to become temporarily more negative than its normal resting potential.

What is the purpose of the Na⁺/K⁺ pump?

It maintains the resting membrane potential by pumping 3 Na⁺ out of the neuron for every 2 K⁺ pumped in.

What does the Na⁺/K⁺ pump require to function?

ATP.

How does an action potential move along an axon?

An action potential in one region stimulates the adjacent region, causing a series of action potentials to travel down the axon.

Why does an action potential normally travel in only one direction?

The refractory period prevents another action potential from being generated immediately behind the one that just occurred.

What is the refractory period?

The time after an action potential when another action potential cannot be generated.

What is continuous conduction?

Action potential conduction along an unmyelinated axon; it flows continuously and is relatively slow.

What is saltatory conduction?

Action potential conduction along a myelinated axon in which the impulse jumps between the Nodes of Ranvier.

Why is saltatory conduction faster than continuous conduction?

The action potential jumps between the Nodes of Ranvier along a myelinated axon.

How does myelination affect impulse conduction?

Myelination allows saltatory conduction, which is much faster than conduction along an unmyelinated axon.

How does axon diameter affect impulse speed?

A larger axon diameter results in a faster impulse.

What is the sequence from resting potential to an action potential?

RMP → threshold stimulus → Na⁺ channels open → Na⁺ enters → depolarization → K⁺ channels open → K⁺ exits → repolarization → adjacent axon region is stimulated → action potentials continue down the axon.

What happens to the membrane potential during depolarization compared with the RMP?

It becomes less negative and moves toward zero.

What happens to the membrane potential during repolarization?

It becomes more negative and returns toward the normal resting membrane potential.

A neuron is at –70 mV and receives a stimulus that brings it to –55 mV. What has happened?

The neuron has reached threshold and will generate an action potential.

A neuron has reached threshold. Can the action potential be stopped halfway through?

No. It is an all-or-none response once threshold is reached.

If Na⁺ is rushing into a neuron, which phase is occurring?

Depolarization.

If K⁺ is rushing out of a neuron, which phase is occurring?

Repolarization.

If too much K⁺ leaves and the neuron becomes more negative than its RMP, what is occurring?

Hyperpolarization.

Which type of conduction occurs in a myelinated axon?

Saltatory conduction.

Which type of conduction occurs in an unmyelinated axon?

Continuous conduction.

What are the four key numbers to remember for nerve impulses?

RMP = –70 mV; threshold = –55 mV; Na⁺/K⁺ pump = 3 Na⁺ out and 2 K⁺ in; the pump uses ATP.