10.3 Reflex Actions

A free GCSE Biology lesson for AQA (8461) from OBA School: automatic, rapid responses, the reflex arc, and how impulses cross synapses. Watch the video, read the explanation and worked examples, then try the revision questions. You can work through the full lesson, with every video, practice quizzes and printable worksheets with mark schemes, in our free online course.

What you will learn

• Explain what a reflex action is and why reflexes are important
• Describe the pathway of a reflex arc, naming each neurone
• Describe how an impulse crosses a synapse
• Compare reflex actions with conscious (voluntary) actions

Key words

• Reflex action: An automatic and rapid response to a stimulus that does not involve the conscious part of the brain.
• Reflex arc: The pathway taken by impulses in a reflex: receptor → sensory → relay → motor neurone → effector.
• Relay neurone: A short neurone in the CNS that links a sensory neurone to a motor neurone.
• Synapse: The junction (tiny gap) between two neurones.
• Neuro-transmitter: The chemical released at a synapse that carries the signal across the gap.

What is a reflex?

Reflex actions are automatic and rapid. They do not involve the conscious part of the brain, so you do not have to think. Most protect you from harm, but some control body functions.

• Withdrawal; Stimulus Touching a hot or sharp object; Response Arm muscles contract; hand pulls away
• Blinking; Stimulus Object close to the eye; Response Eyelid closes
• Pupil reflex; Stimulus Bright light; Response Pupil gets smaller to protect the retina
• Knee jerk; Stimulus Tap below the kneecap; Response Thigh muscle contracts; leg kicks
• Newborn grasp; Stimulus Object pressed into baby’s palm; Response Fingers grip it

The reflex arc

The impulse travels through the spinal cord (or an unconscious part of the brain) and straight back out. A relay neurone links the sensory and motor neurones. Because the conscious brain is bypassed, the response is very fast.

Reflex arc: stimulus → receptor → sensory neurone → relay neurone → motor neurone → effector → response

Impulses also travel up the spinal cord to the brain, so you feel the pain, but only after your hand has moved.

Synapses

Neurones do not touch. There is a tiny gap between them called a synapse.

• 1. An electrical impulse arrives at the end of the first neurone.
• 2. This triggers the release of a chemical (neurotransmitter).
• 3. The chemical diffuses across the gap.
• 4. It binds to receptors on the next neurone and sets off a new electrical impulse.

Synapses make sure impulses travel in one direction only. Crossing a synapse is slower than travelling along an axon, because diffusion takes time.

Worked example: how many synapses?

In the withdrawal reflex above, how many synapses does the impulse cross between receptor and effector (not counting the muscle)?
Sensory → relay (1), relay → motor (2).

Answer: 2 synapses

Exam tip: the signal crosses the synapse as a chemical by diffusion, not as an electrical impulse. Use both words.

Reflex or voluntary?

• Thinking?; Reflex action No: automatic; Voluntary (conscious) action Yes: you decide
• Pathway; Reflex action Spinal cord (or unconscious brain); Voluntary (conscious) action Conscious brain (cerebral cortex)
• Speed; Reflex action Very fast (often under 0.1 s); Voluntary (conscious) action Slower (often 0.2–0.3 s)
• Example; Reflex action Blinking at a sudden puff of air; Voluntary (conscious) action Pressing a button when a light comes on

Common mistakes

• Saying the brain “decides” in a reflex. The conscious brain is not involved.
• Missing out the relay neurone in a reflex arc.
• Saying the electrical impulse jumps the gap. A chemical diffuses across the synapse.
• Calling the spinal cord the effector. The effector is the muscle (or gland).

Revision questions

Try these, then check your answers below each one.

Question 1. Which describes a reflex action? (Choose from: Slow and requires thought, Automatic and rapid, not involving the conscious brain, Always controlled by hormones, Learned through practice.)

Answer: Automatic and rapid, not involving the conscious brain

Working: Reflexes are automatic and rapid and do not involve the conscious part of the brain.

Question 2. Which type of neurone links the sensory neurone to the motor neurone in a reflex arc?

Answer: relay

Working: A relay neurone in the spinal cord links sensory and motor neurones.

Question 3. How does a signal cross a synapse? (Choose from: An electrical impulse jumps the gap, A chemical diffuses across the gap and binds to receptors on the next neurone, The two neurones touch, Hormones in the blood carry it.)

Answer: A chemical diffuses across the gap and binds to receptors on the next neurone

Working: The impulse triggers release of a chemical, which diffuses across the gap and binds to receptors, starting a new impulse.

Related lessons

• Previous lesson: 10.2 The Structure and Function of the Human Nervous System
• Next lesson: 10.4 The Brain
• In this chapter: 10.1 Principles of Homeostasis
• In this chapter: 10.5 The Eye
• In this chapter: 10.6 Common Problems of the Eye

Continue in the full course

This lesson is section 10.3 (AGCBH1003) of our free GCSE Biology course for AQA. In the full course you get every video for this section, a self-marking practice quiz, a printable worksheet with a mark scheme, chapter tests, and your progress saved. Go to the full GCSE Biology course.