Curriculum
Course: SQA National 5 Biology
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Curriculum

SQA National 5 Biology

1. Unit 1 — Cell Biology

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2. Unit 2 — Multicellular Organisms

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3. Unit 3 — Life on Earth

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Text lesson

2.2.2 The Reflex Arc

2.2.2 The Reflex Arc: Describe the pathway of a reflex arc, explain the role of the synapse and explain why reflexes are important.

2.2.2 The Reflex Arc

A reflex is a rapid, automatic response to a stimulus.

1

A stimulus, such as touching something hot, is detected by a receptor in the skin.
2

A sensory neurone carries the impulse to the spinal cord, part of the CNS.
3

A relay neurone in the spinal cord passes the impulse across to a motor neurone.
4

The motor neurone carries the impulse to an effector, in this case a muscle in the arm.
5

The muscle contracts and the hand is pulled away before the brain is even aware of the pain.
Gap
  • Neurones do not touch; a tiny gap called a synapse separates them.
Neurotransmitter
  • When an impulse arrives, chemical neurotransmitters are released from the end of the first neurone.
Diffusion
  • The chemical diffuses across the gap and binds to receptors on the next neurone.
New impulse
  • Binding causes an electrical impulse to start in the next neurone, so the signal continues.
Why Reflexes Matter
Fast
  • Fewer neurones are involved and the brain is not needed, so the response is very quick.
Automatic
  • No conscious thought is needed, so the response occurs without deliberate decision.
Protective
  • Reflexes help avoid injury, such as pulling a hand away from heat or blinking when something approaches the eye.
Examples
  • Pupil constriction in bright light and blinking are reflexes.
Reflex vs Voluntary
Voluntary
  • Involves the conscious part of the brain and is slower, as in choosing to pick up a pen.
Reflex
  • Does not need the brain to make a decision before the response starts.
Example
  • Choosing to kick a ball is voluntary, while the knee jerk caused by a tap on the tendon is a reflex.

2.2.2 – Key Points for the Exam

  • Reflex: A rapid, automatic response to a stimulus.
  • Synapse: The gap between two neurones.
  • Neurotransmitter: A chemical that carries the signal across a synapse.
  • Relay neurone: A neurone in the CNS linking sensory and motor neurones.
  • Spinal cord: The part of the CNS running through the backbone.

Common Mistakes

  • Saying that the signal crosses the synapse as an electrical impulse; it crosses chemically.
  • Putting the pathway in the wrong order, such as effector before motor neurone.
  • Stating that reflexes go through the brain before the response.
  • Forgetting the relay neurone between sensory and motor neurones.
YOUR STUDY PATH

2.2.2 The Reflex Arc

SQA National 5 Biology / 2. Unit 2 — Multicellular Organisms

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