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Neurones and nerve impulsesEdexcel A-Level Biology A: Revision notes

Section 1

Sensory, relay and motor neurones

Neurones carry electrical impulses. Three types work together in a reflex arc.

  • Sensory neurone: carries impulses from a receptor to the CNS. Long dendron, cell body in a side branch just outside the CNS, short axon.
  • Relay neurone: lies entirely within the CNS and links sensory and motor neurones. Short, with many dendrites.
  • Motor neurone: carries impulses from the CNS to an effector (muscle or gland). Cell body in the CNS with many dendrites and a long axon.
Key termssensory neuronerelay neuronemotor neuronedendronaxon

Section 2

Schwann cells and myelination

Schwann cells wrap around the axons of many neurones many times, forming a fatty myelin sheath. Myelin is an electrical insulator: ions cannot cross the membrane where it is present.

Between neighbouring Schwann cells are short unmyelinated gaps called nodes of Ranvier, where ions can cross the membrane.

Key termsSchwann cellmyelin sheathnode of Ranvier
Common mistake

Myelin does not make the impulse 'travel faster' by itself. It insulates, so depolarisation happens only at the nodes.

Section 3

Resting potential

At rest the inside of an axon is negative relative to the outside, about -70 mV. This resting potential is maintained by:

  • the sodium-potassium pump, which actively transports 3 Na⁺ out and 2 K⁺ in using ATP;
  • the membrane being more permeable to K⁺ (through leak channels) than to Na⁺, so K⁺ diffuses out.
Key termsresting potentialsodium-potassium pump

Section 4

Generating an action potential

  1. A stimulus opens some voltage-gated Na⁺ channels; Na⁺ diffuses in and the membrane depolarises.
  2. If the threshold (about -55 mV) is reached, many more Na⁺ channels open (positive feedback) and the potential rises to about +40 mV.
  3. Na⁺ channels close and voltage-gated K⁺ channels open; K⁺ diffuses out and the membrane repolarises, briefly overshooting (hyperpolarisation).
  4. The pump and K⁺ leak channels restore the resting potential.

The response is all-or-nothing: a stimulus below threshold produces no action potential.

Key termsdepolarisationthreshold potentialrepolarisationall-or-nothing

Section 5

Conduction and the refractory period

Depolarisation at one point causes local circuits of ion movement that depolarise the next region, so the action potential travels along the axon.

After an action potential, Na⁺ channels cannot open for a short time: the refractory period. This ensures that impulses are discrete and travel in one direction only.

Key termslocal circuitrefractory period

Section 6

Saltatory conduction

In myelinated axons, depolarisation occurs only at the nodes of Ranvier, and local circuits carry the signal between nodes. The impulse seems to jump from node to node: saltatory conduction.

This is much faster (up to about 100 m s⁻¹, compared with about 1 m s⁻¹ in unmyelinated axons) and uses less ATP as fewer ions need to be pumped back.

Worked example: 0.84 m at 70 m s⁻¹ takes 0.84 ÷ 70 = 0.012 s = 12 ms.

Loss of myelin, as in Guillain-Barré syndrome, slows or blocks conduction.

Key termssaltatory conduction
Exam tip

Link the three ideas: insulation, depolarisation only at nodes, so faster conduction.

That's the notes covered.

Carry on to the next subtopic.

Exam questions on Neurones and nerve impulses

  1. 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
  2. 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
  3. 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).