Describe the structure of a sensory neurone.

A long dendron carries impulses to a cell body just outside the CNS, then a short axon into the CNS.

What is the role of a relay neurone?

Connects sensory and motor neurones within the CNS.

What is the function of Schwann cells?

They wrap around axons to form the insulating myelin sheath.

What are nodes of Ranvier?

Gaps between Schwann cells where the axon membrane is exposed and ions can cross.

What is the resting potential and how is it maintained?

About -70 mV; the sodium-potassium pump moves 3 Na⁺ out and 2 K⁺ in, and the membrane is more permeable to K⁺.

What happens to the membrane during depolarisation?

Voltage-gated Na⁺ channels open and Na⁺ diffuses in, so the potential rises towards +40 mV.

What is the threshold potential?

About -55 mV, the potential at which enough Na⁺ channels open to trigger an action potential.

What causes repolarisation?

Na⁺ channels close and voltage-gated K⁺ channels open, so K⁺ diffuses out.

What does all-or-nothing mean?

A stimulus above threshold gives a full-size action potential; one below gives none.

Why is the refractory period important?

Na⁺ channels cannot open, so impulses are discrete and travel in one direction only.

What is saltatory conduction?

The impulse jumps from node of Ranvier to node, since depolarisation occurs only at the nodes.

Why is conduction faster in myelinated axons?

Insulation means depolarisation only at nodes, so local circuits bridge long distances and fewer regions depolarise.

Neurones and nerve impulsesEdexcel A-Level Biology A: Flashcards

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Describe the structure of a motor neurone.

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Describe the structure of a motor neurone.: Cell body in the CNS with many dendrites and a long axon carrying impulses to an effector.
Describe the structure of a sensory neurone.: A long dendron carries impulses to a cell body just outside the CNS, then a short axon into the CNS.
What is the role of a relay neurone?: Connects sensory and motor neurones within the CNS.
What is the function of Schwann cells?: They wrap around axons to form the insulating myelin sheath.
What are nodes of Ranvier?: Gaps between Schwann cells where the axon membrane is exposed and ions can cross.
What is the resting potential and how is it maintained?: About -70 mV; the sodium-potassium pump moves 3 Na⁺ out and 2 K⁺ in, and the membrane is more permeable to K⁺.
What happens to the membrane during depolarisation?: Voltage-gated Na⁺ channels open and Na⁺ diffuses in, so the potential rises towards +40 mV.
What is the threshold potential?: About -55 mV, the potential at which enough Na⁺ channels open to trigger an action potential.
What causes repolarisation?: Na⁺ channels close and voltage-gated K⁺ channels open, so K⁺ diffuses out.
What does all-or-nothing mean?: A stimulus above threshold gives a full-size action potential; one below gives none.
Why is the refractory period important?: Na⁺ channels cannot open, so impulses are discrete and travel in one direction only.
What is saltatory conduction?: The impulse jumps from node of Ranvier to node, since depolarisation occurs only at the nodes.
Why is conduction faster in myelinated axons?: Insulation means depolarisation only at nodes, so local circuits bridge long distances and fewer regions depolarise.

Exam questions on Neurones and nerve impulses

A student touches a hot kettle and pulls her hand away before she feels any pain. The withdrawal reflex involves three types of neurone linking the receptors in her skin to the muscles in her arm.
Describe the role of Schwann cells in the myelination of an axon.2 marks
A student studies the giant axon of a squid using microelectrodes. The potential difference across the axon membrane is -70 mV at rest. When the axon is stimulated, the potential rises rapidly to +40 mV and then falls back, dipping below -70 mV briefly before returning to its resting value.
A weak stimulus fails to produce an action potential. Explain why.2 marks
Motor neurones carry impulses from the spinal cord to the muscles of the foot, a distance of 0.84 m. In a myelinated motor neurone, the action potential travels at 70 m s⁻¹. In an unmyelinated neurone of the same length the action potential travels at 1.4 m s⁻¹.
Calculate the time taken, in milliseconds, for an action potential to travel along the myelinated neurone, and state how many times faster this is than in the unmyelinated neurone.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).