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Key diagram
A reflex arc is the pathway of a reflex action: stimulus, receptor, sensory neurone, relay neurone (in the spinal cord), motor neurone, effector, response. Synapses connect the neurones using chemical transmission.
The reflex arc diagram is a high-frequency exam question that tests whether students can sequence the components correctly and explain the role of synapses. The key is learning the fixed order and using the correct terms.
Drawing the pathway
Draw the reflex arc as a pathway from left to right (or from the stimulus source to the response). Start with a receptor (e.g. A pain receptor in the skin). Connect it to a sensory neurone that carries the impulse toward the spinal cord. Inside the spinal cord (draw a cross-section of the spinal cord with grey matter in a butterfly shape), the sensory neurone synapses with a relay neurone. The relay neurone synapses with a motor neurone, which carries the impulse out to an effector (a muscle or gland). Label each synapse.
The role of synapses
A synapse is the junction between two neurones. There is a small gap called the synaptic cleft. When an electrical impulse arrives at the end of the pre-synaptic neurone, it triggers the release of neurotransmitter chemicals into the cleft. These chemicals diffuse across and bind to receptors on the post-synaptic neurone, triggering a new electrical impulse. Synapses ensure impulses travel in one direction only.
Why reflexes are fast and involuntary
Reflex actions are rapid because the pathway is short and does not involve conscious processing by the brain. They are involuntary: they happen without thinking. This protects the body from harm, for example pulling the hand away from a hot surface. Although the brain may become aware of the stimulus, the response occurs before conscious thought.
An example: the withdrawal reflex
Stimulus: touching a hot object. Receptor: temperature/pain receptor in the fingertip. Sensory neurone carries the impulse to the spinal cord. Relay neurone in the spinal cord passes the impulse on. Motor neurone carries the impulse to the biceps muscle (effector). Response: the arm is pulled away. The entire sequence happens in a fraction of a second.
Label checklist
Check these labels are present and correctly positioned before finishing your drawing.
✓Receptor
✓Sensory neurone
✓Relay neurone
✓Motor neurone
✓Effector (muscle or gland)
✓Synapse (between sensory and relay)
✓Synapse (between relay and motor)
✓Spinal cord (grey and white matter)
Try a diagram question (5 marks)
Describe the pathway of a reflex arc when a person touches a hot plate. [5]
Show the answer and marking guidance
The heat is detected by a receptor (temperature/pain receptor) in the skin.
An electrical impulse travels along the sensory neurone to the spinal cord.
The impulse crosses a synapse to a relay neurone in the spinal cord.
The impulse crosses another synapse to a motor neurone.
The motor neurone carries the impulse to the effector (muscle), which contracts to pull the hand away.
Original Cambridge-style practice written for this site.
Common diagram mistakes
Check these against your own drawing before moving on.
Putting the neurones in the wrong order: it must be sensory, relay, motor.
Forgetting the relay neurone and connecting sensory directly to motor.
Saying impulses "jump" across the synapse: chemicals diffuse across.
Calling the effector a "reactor" or "responder" instead of "effector".
Not drawing the relay neurone inside the spinal cord.