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Coordination and Response: Question 1

Syllabus 14.1

Multiple choice Core 1 mark

A chef checks on a roast by opening a very hot oven. As the chef closes the oven door, the back of one hand briefly brushes against the hot metal door frame. The hand jerks away from the door before the chef consciously feels any pain.

This rapid, automatic response is a simple reflex action, coordinated through a reflex arc.

Which row shows the correct order in which structures are involved, from the stimulus to the response?

Choose an answer to check it, then compare with the worked solution below.
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Worked solution

Step 1: Identify the structures in a reflex arc

A simple reflex arc always has the same five components, always occurring in the same fixed order:

  • a receptor, which detects the stimulus
  • a sensory neurone, which carries the impulse towards the central nervous system
  • a relay neurone, found in the spinal cord, which connects the sensory neurone to the motor neurone
  • a motor neurone, which carries the impulse to the effector
  • an effector (a muscle or a gland), which produces the response

Step 2: Apply this to the chef’s reflex

  • Receptor: heat receptors in the skin of the hand detect the hot door frame.
  • Sensory neurone: carries the impulse from the receptor to the spinal cord.
  • Relay neurone: sits within the spinal cord and passes the impulse on to a motor neurone. Because this connection happens in the spinal cord, the response can happen automatically and rapidly, without first needing to involve the brain.
  • Motor neurone: carries the impulse from the relay neurone to the effector.
  • Effector: the muscles in the arm, which contract to pull the hand away from the hot door.

So the impulse travels: receptor → sensory neurone → relay neurone → motor neurone → effector.

Why the other options are wrong

  • A: swaps the sensory and relay neurones, but the impulse must reach the spinal cord via the sensory neurone before it can reach the relay neurone.
  • C: places the motor neurone before the relay neurone, but the relay neurone is what connects the sensory input to the motor output.
  • D: reverses the entire pathway, starting at the effector rather than ending there.

Final answer

  • The correct order is receptor → sensory neurone → relay neurone → motor neurone → effector, option B.