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The key difference
Sensory neurones carry impulses from receptors to the CNS. Relay neurones connect sensory and motor neurones within the CNS. Motor neurones carry impulses from the CNS to effectors (muscles or glands).
The nervous system uses three types of neurone to coordinate rapid responses. Each type has a distinct structure adapted for its specific role in transmitting electrical impulses. Understanding the pathway from receptor to effector: and which neurone carries the signal at each stage: is essential for reflex arc questions.
Feature
Sensory Neurone
Motor Neurone
Relay Neurone
Direction of impulse
From receptor → CNS (brain or spinal cord)
From CNS → effector (muscle or gland)
Within the CNS, connecting sensory to motor neurone
Cell body position
In the middle of the axon (off to one side)
At one end of the neurone (in the CNS)
Within the CNS (brain or spinal cord)
Axon length
Long: from receptor to spinal cord
Long: from spinal cord to effector
Short: entirely within the CNS
Function
Detects stimuli via receptors and transmits impulses to CNS
Transmits impulses from CNS to cause a response
Links sensory and motor neurones; processes information
Role in reflex arc
First neurone: carries impulse from receptor to spinal cord
Last neurone: carries impulse from spinal cord to effector
Middle neurone: connects sensory to motor in the spinal cord
Distinguishing structural feature
Cell body located along the middle of the fibre
Many short dendrites at the cell body end
Many short connections to other neurones
The Reflex Arc Pathway
In a reflex arc, the three neurones work in sequence:
Stimulus detected by a receptor (e.g. Pain receptor in the skin)
Sensory neurone carries the electrical impulse from the receptor to the spinal cord (CNS)
Relay neurone in the spinal cord receives the impulse and passes it on
Motor neurone carries the impulse from the spinal cord to the effector
Effector (muscle or gland) carries out the response (e.g. Muscle contracts to pull hand away)
The connections between neurones are called synapses. At each synapse, the electrical impulse is converted into a chemical signal (neurotransmitter) that diffuses across the gap and triggers a new electrical impulse in the next neurone.
Why Reflexes Are Fast
Reflex actions are rapid and involuntary because:
The pathway is short: impulses travel through only three neurones (or sometimes just two: some reflex arcs bypass the relay neurone)
The response is automatic: no conscious decision-making is involved, so the brain does not need to process the information before a response occurs
The pathway is pre-wired: the synaptic connections are fixed, so the same stimulus always produces the same response
Try a comparison question (5 marks)
Describe the pathway of a nerve impulse in a reflex arc when a person touches a hot object. [5]
Show the answer and marking guidance
Heat/pain receptor in the skin detects the stimulus (high temperature)
Sensory neurone transmits the electrical impulse from the receptor to the spinal cord (CNS)
Relay neurone in the spinal cord receives the impulse and passes it to a motor neurone
Motor neurone carries the impulse from the spinal cord to the effector (muscle in the arm)
Effector (muscle) contracts, pulling the hand away from the hot object
Original Cambridge-style practice written for this site.
Common comparison mistakes
When answering a compare question, write matched pairs rather than two separate descriptions.
Putting the neurones in the wrong order: it is always sensory → relay → motor
Saying the impulse goes to the brain in a reflex: most reflexes are coordinated by the spinal cord, not the brain
Confusing the position of the cell body: sensory neurone has its cell body in the middle, motor neurone has it at one end
Forgetting that synapses use chemical transmission (neurotransmitters) across the gap between neurones