Neurones, reflexes and the nervous system — Edexcel International A Level Biology
Sensory, relay and motor neurones, myelin and Schwann cells, the CNS and PNS, and the spinal reflex arc with grey and white matter.
Revision notes
3 short notes, in the order of the specification. Each one in short:
Sensory neurones carry impulses from receptors to the central nervous system, relay neurones connect neurones within it, and motor neurones carry impulses from it to effectors. Many axons are wrapped by Schwann cells, which form an insulating myelin sheath that speeds up conduction.
The mammalian nervous system consists of the central nervous system, the brain and spinal cord, and the peripheral nervous system, which is all the neurones outside them. A receptor detects a stimulus, sensory neurones carry impulses to the CNS, and motor neurones then make an effector respond.
A spinal reflex arc is a rapid, automatic, protective response in which a receptor, sensory neurone, relay neurone in the grey matter of the spinal cord, motor neurone and effector are linked. White matter, made of myelinated axons, carries impulses up and down the cord to and from the brain.
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Quick check questions
Where is the cell body of a sensory neurone found?
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In the dorsal root ganglion, just outside the spinal cord.
What is myelin made from?
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Many layers of the plasma membrane of Schwann cells, which are rich in lipid.
Name the two parts of the mammalian nervous system.
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The central nervous system (brain and spinal cord) and the peripheral nervous system.
What does the white matter of the spinal cord contain?
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Myelinated axons carrying impulses up and down the cord.
Frequently asked questions
What is the difference between a sensory, relay and motor neurone?
A sensory neurone carries impulses from a receptor to the CNS, a relay neurone links neurones inside the CNS, and a motor neurone carries impulses from the CNS to an effector. They differ in structure: the sensory cell body lies in the dorsal root ganglion, while relay and motor cell bodies are in the grey matter.
What do Schwann cells do?
Schwann cells wrap around axons in the peripheral nervous system and form the myelin sheath. This sheath is made of layers of lipid-rich membrane, so it insulates the axon and stops ions crossing it except at the nodes of Ranvier. This makes impulses travel much faster. Schwann cells also support and help repair axons.
What is a spinal reflex arc?
A spinal reflex arc is the pathway of a rapid, automatic response that passes through the spinal cord. It runs from a receptor along a sensory neurone, a relay neurone and a motor neurone to an effector. Because the brain is not needed for the response, it is very fast and protects the body.
What is the difference between grey matter and white matter?
Grey matter is the inner region of the spinal cord and contains the cell bodies of relay and motor neurones and many synapses. White matter surrounds it and is made of myelinated axons that carry impulses up and down the cord. The myelin gives white matter its pale colour.
Written and checked against the Edexcel International A Level Biology (YBI11) specification · Updated October 2026