Explain the structure and function of a reflex arc including sensory,…

The idea

A reflex arc is the fixed pathway that carries a stimulus to a response without the conscious brain deciding first, which makes the reaction fast and protective. The receptor detects the stimulus and creates an impulse. A sensory neurone carries the impulse to the spinal cord, where a relay neurone passes it across a synapse to a motor neurone. The motor neurone carries the impulse to an effector, a muscle or gland, which contracts or secretes to produce the response. Each neurone is a cell with a cell body and long fibres; the sensory neurone has a dendron and the motor neurone has dendrites, and the relay neurone is short and stays inside the cord. Synapses between neurones use chemical transmitters, so the route is one-way. Example: touching something hot triggers withdrawal of the hand before the brain registers pain.

What earns the marks

• Identifies the receptor as the structure that detects the stimulus and starts the impulse.
• States that the sensory neurone carries the impulse from the receptor to the spinal cord (central nervous system).
• States that the relay neurone lies in the spinal cord and passes the impulse from sensory to motor neurone across a synapse.
• States that the motor neurone carries the impulse from the spinal cord to an effector.
• Identifies the effector as a muscle or gland and links it to the response (muscle contraction or secretion).
• Explains that the pathway does not require conscious decision by the brain, giving a fast protective response.
• Recognises that synapses make transmission one-way and involve chemical transmitters.

Common mistakes and how to put them right

• Reversing the order and saying the motor neurone carries the impulse towards the cord.

  Put it right by fixing the sequence receptor → sensory neurone → relay neurone → motor neurone → effector.

• Claiming the relay neurone is in the brain.

  Put it right by placing the relay neurone in the spinal cord for this pathway.

• Saying the brain must decide before the response happens.

  Put it right by explaining the reflex bypasses conscious decision, though the brain is informed afterwards.

• Treating the effector as a neurone.

  Put it right by identifying the effector as a muscle or gland.

Exam tips

• Learn the pathway as an ordered chain and practise writing it in sequence before answering.
• Link each named neurone to the direction it carries the impulse, not just its name.
• When asked about function, always connect the arc to speed and protection from harm.