B10.3 Reflex Actions

A free GCSE Combined Science: Trilogy lesson for AQA (8464) from OBA School: automatic, rapid responses, the reflex arc, and how signals 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 protect the body
• Describe the reflex arc, naming each neurone and where it is found
• Describe how a signal crosses a synapse
• Explain how the structures in a reflex arc relate to their function
• 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 of neurones in a reflex: receptor → sensory → relay → motor neurone → effector.
• Relay neurone: A neurone inside the CNS that links a sensory neurone to a motor neurone.
• Synapse: The small 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 no thinking time is needed. Most protect you from harm; others control body functions such as heart rate or how much light enters the eye.

• Withdrawal; Stimulus Pain from a hot or sharp object; Response Arm muscle contracts; hand pulls away
• Blinking; Stimulus Something moving close to the eye; Response Eyelid closes
• Pupil reflex; Stimulus Bright light; Response Pupil gets smaller, protecting the retina
• Knee jerk; Stimulus Tap just below the kneecap; Response Thigh muscle contracts; lower leg kicks
• Coughing; Stimulus Irritation in the airway; Response Air forced out of the lungs

The reflex arc

A receptor detects the stimulus and starts an impulse. It travels along a sensory neurone to the spinal cord. A relay neurone in the spinal cord passes it straight to a motor neurone, which carries it to the effector. Because the conscious brain is bypassed, the response is very fast.

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

Other neurones carry impulses from the relay neurone up 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 reaches the end of the axon.
• 2. This triggers the release of a chemical, a neurotransmitter.
• 3. The neurotransmitter diffuses across the gap.
• 4. It binds to receptors on the next neurone and starts a new electrical impulse.

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

Worked example: time for a reflex

In a reflex the impulse travels 0.90 m along axons at 90 m/s and crosses 2 synapses, each taking 0.5 ms. Calculate the total time.
Axons: 0.90 ÷ 90 = 0.010 s = 10 ms. Synapses: 2 × 0.5 = 1.0 ms.

Answer: 10 + 1.0 = 11 ms

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

Reflex or voluntary?

For a conscious (voluntary) action, impulses travel to the brain, which makes a decision before impulses go to the effectors. This takes longer.

• Decision?; Reflex action No: automatic; Voluntary action Yes: you decide
• Pathway; Reflex action Spinal cord (or an unconscious part of the brain); Voluntary action Conscious part of the brain
• Speed; Reflex action Very fast; Voluntary action Slower

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 impulse jumps the gap. A chemical diffuses across it.
• Calling the spinal cord the effector. The effector is a muscle or gland.

Revision questions

Try these, then check your answers below each one.

Question 1. Which statement describes a reflex action? (Choose from: A slow response that you decide to make, An automatic and rapid response that does not involve the conscious part of the brain, A response controlled only by hormones, A response that you have to learn.)

Answer: An automatic and rapid response that does not involve the conscious part of the brain

Working: Reflexes are automatic and rapid. They do not involve the conscious brain, so they protect the body quickly.

Question 2. The diagram shows a reflex arc. Which number labels the relay neurone? (Choose from: 1, 2, 3, 4, 5.)

Answer: 3

Working: 3 is the relay neurone inside the spinal cord. 1 = receptor, 2 = sensory neurone, 4 = motor neurone, 5 = effector (muscle).

Question 3. A knee-jerk reflex takes 0.04 s. Pressing a button when a light comes on takes 0.26 s. How many times longer is the voluntary response?

Answer: 6.5

Working: 0.26 ÷ 0.04 = 6.5 times longer. The voluntary response goes through the conscious brain.

Related lessons

• Previous lesson: B10.2 The Structure and Function of the Human Nervous System
• Next lesson: B10 RP Required Practical: Reaction Time
• In this chapter: B10.1 Principles of Homeostasis

Continue in the full course

This lesson is section B10.3 (AGCSC1003) of our free GCSE Combined Science 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 Combined Science course.