How do the sodium-potassium pumps help establish the resting potential?

They actively transport 3 Na⁺ out for every 2 K⁺ in, so there are fewer positive ions inside.

Why does the inside of the axon become negative at rest?

More K⁺ diffuse out than Na⁺ diffuse in, because the membrane is more permeable to K⁺.

What causes depolarisation?

Voltage-gated sodium ion channels open and Na⁺ diffuse into the axon, reversing the potential to about +40 mV.

What causes repolarisation?

Sodium ion channels close and voltage-gated potassium ion channels open, so K⁺ diffuse out.

What is the all-or-nothing principle?

If the threshold is reached, an action potential of fixed size always occurs; below it, none occurs.

How does a stronger stimulus change the response of a neurone?

It produces a higher frequency of action potentials, not larger ones.

How does an action potential pass along a non-myelinated axon?

Na⁺ diffuse sideways (local circuits) and depolarise the next region, so the wave moves along.

What is saltatory conduction?

The jumping of the action potential between nodes of Ranvier in a myelinated axon.

Why is conduction faster in myelinated axons?

Myelin insulates the axon so depolarisation occurs only at nodes, so there are fewer depolarisations along the length.

What is the refractory period?

The time after an action potential when voltage-gated sodium ion channels cannot open, so no new action potential can start.

Give two roles of the refractory period.

Makes action potentials discrete and unidirectional, and limits the frequency of impulses.

How do axon diameter and temperature affect conduction speed?

A larger diameter lowers resistance, and a higher temperature speeds diffusion of ions, so both increase speed.

Where are the nodes of Ranvier?

The gaps between Schwann cells in the myelin sheath, where the axon membrane is exposed.

Nerve impulsesAQA A-Level Biology: Flashcards

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What is the resting potential and its approximate value?

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What is the resting potential and its approximate value?: The potential difference across the membrane of an axon at rest, about −70 mV (inside negative).
How do the sodium-potassium pumps help establish the resting potential?: They actively transport 3 Na⁺ out for every 2 K⁺ in, so there are fewer positive ions inside.
Why does the inside of the axon become negative at rest?: More K⁺ diffuse out than Na⁺ diffuse in, because the membrane is more permeable to K⁺.
What causes depolarisation?: Voltage-gated sodium ion channels open and Na⁺ diffuse into the axon, reversing the potential to about +40 mV.
What causes repolarisation?: Sodium ion channels close and voltage-gated potassium ion channels open, so K⁺ diffuse out.
What is the all-or-nothing principle?: If the threshold is reached, an action potential of fixed size always occurs; below it, none occurs.
How does a stronger stimulus change the response of a neurone?: It produces a higher frequency of action potentials, not larger ones.
How does an action potential pass along a non-myelinated axon?: Na⁺ diffuse sideways (local circuits) and depolarise the next region, so the wave moves along.
What is saltatory conduction?: The jumping of the action potential between nodes of Ranvier in a myelinated axon.
Why is conduction faster in myelinated axons?: Myelin insulates the axon so depolarisation occurs only at nodes, so there are fewer depolarisations along the length.
What is the refractory period?: The time after an action potential when voltage-gated sodium ion channels cannot open, so no new action potential can start.
Give two roles of the refractory period.: Makes action potentials discrete and unidirectional, and limits the frequency of impulses.
How do axon diameter and temperature affect conduction speed?: A larger diameter lowers resistance, and a higher temperature speeds diffusion of ions, so both increase speed.
Where are the nodes of Ranvier?: The gaps between Schwann cells in the myelin sheath, where the axon membrane is exposed.

Exam questions on Nerve impulses

A neurone has a resting potential of −70 mV and a threshold potential of −55 mV. In an experiment, three separate stimuli depolarised the membrane to −62 mV, −50 mV and −30 mV. Only two of the stimuli produced action potentials.
Explain why one stimulus produced no action potential and the other two produced action potentials of the same size.2 marks
Neurologists measure the speed of conduction along motor neurones. In a healthy adult at 37 °C the speed along one neurone is 60 m s⁻¹. In a patient with multiple sclerosis, a condition that destroys patches of the myelin sheath, the speed along the same neurone is 8 m s⁻¹.
Explain why the speed of conduction is lower in the patient.2 marks
A researcher stimulates an axon repeatedly. After each action potential the axon cannot produce another for 2.0 ms, because the voltage-gated sodium ion channels are closed and the membrane potential has to return to its resting value. The researcher increases the strength of the stimulus.
Explain the importance of the refractory period for the transmission of nerve impulses.3 marks

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Written by the Exaim team, led by Shaun Daswani (Head of Upper Secondary, Improve ME Institute; MSc Financial Mathematics, Imperial College London; BSc, UCL) and Jason Daswani (operational lead, Improve ME Institute; LSE).