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8.1.3Unit 6: Practical Skills in Biology II

The Reflex Arc — IAL Edexcel Linear Revision Notes & Topical Questions

5 min read 4 parts Lesson 37 of 48 Unit quiz

📚 IAL Edexcel Linear Hub / Topical Revision
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Part 1 of 4Reflexes

8.1.3 The Reflex Arc
Definition
  • A reflex is a rapid, automatic response to a stimulus that does not involve conscious decision by the brain.
Advantage
  • Because few neurones and synapses are involved, the response is fast and protects the body from harm.
Examples
  • Blinking, coughing, the knee-jerk and the pupil reflex are reflexes.
Reflex arc
  • The pathway from receptor to effector, which usually includes a sensory, relay and motor neurone.
Awareness
  • Conscious awareness comes after the action, as information takes longer to reach the conscious regions of the brain.

Part 2 of 4A Spinal Reflex: Withdrawal From Pain

Grey matter holds cell bodies of relay and motor neurones, while white matter holds long myelinated axons.

1
A receptor in the skin of the foot detects a sharp object.
2
A sensory neurone carries impulses into the spinal cord.
3
The sensory neurone synapses with a relay neurone in the grey matter of the spinal cord.
4
The relay neurone synapses with a motor neurone, and the impulse crosses each synapse by a neurotransmitter.
5
The motor neurone carries impulses to a leg muscle, which contracts and pulls the foot away.

Part 3 of 4A Cranial Reflex: The Pupil Reflex

A Cranial Reflex: The Pupil Reflex
Receptors
  • Photoreceptors in the retina detect the light intensity.
Pathway
  • Impulses pass along a sensory neurone to the brain, to a relay neurone, then to a motor neurone.
Bright light
  • Circular muscles in the iris contract and the pupil constricts, protecting the retina.
Dim light
  • Radial muscles contract and the pupil dilates, letting in more light.
Antagonism
  • The two sets of muscles work antagonistically, so one relaxes when the other contracts.
FeatureSpinal reflexCranial reflex
Relay neurone inSpinal cordBrain
ExampleWithdrawal from painPupil reflex

Part 4 of 4Exam Checklist & Tips

8.1.3 – Key Points for the Exam

  • Reflex arc The pathway of neurones from receptor to effector in a reflex.
  • Grey matter Spinal cord tissue containing cell bodies of relay and motor neurones.
  • White matter Spinal cord tissue containing myelinated axons.
  • Cranial reflex A reflex whose relay neurone is in the brain.

Common Mistakes

  • Saying the brain controls a spinal reflex; the response is coordinated in the spinal cord.
  • Stating the circular iris muscles dilate the pupil; they constrict it.
  • Forgetting the synapses between the neurones.
  • Calling a reflex a voluntary response.

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