Reflex Action and the Reflex Arc: Stimulus, Receptor, Impulse, Effector and Response (ICSE Class 8)

Understand Reflex Action, Reflex Arc and Voluntary Action

Reflex Action, Reflex Arc and Voluntary Action: key facts

Reflex action

• •Quick
• •Automatic
• •Involuntary
• •No thinking
• •Mostly spinal cord

Protects the body.

Terms

Stimulus	change
Receptor	detects
Impulse	message
Effector	muscle or gland
Response	action

Receptor receives; effector effects.

Reflex arc

• •Receptor
• •Sensory neuron
• •Spinal cord
• •Motor neuron
• •Effector

Brain learns afterwards.

How to answer Reflex Action, Reflex Arc and Voluntary Action questions

1. 1Name what changed (stimulus) and which organ sensed it (receptor).
2. 2Follow the arc in order, naming the neurons and the spinal cord.
3. 3Name the muscle or gland that acts (effector) and the action (response).
4. 4Say whether the brain started the action: reflex needs no thinking.
5. 5Compare with voluntary actions using speed, control and the part of the nervous system.

Reflex Action, Reflex Arc and Voluntary Action in real life

The idea, met in an everyday situation. Answer the question in your own words, then compare with the model answer. The one-line hook is what to remember.

Tricks to remember Reflex Action, Reflex Arc and Voluntary Action

Short memory aids. Say each one aloud once, then use it on a question the same day.

• Receptor receives, effector effects

  The receptor detects; the effector does the act.

  Use it when: Terms.

• SRSSMER

  Stimulus, Receptor, Sensory neuron, Spinal cord, Motor neuron, Effector, Response.

  Use it when: Order of arc.

• Skip the brain

  Reflex goes through the cord, not the cerebrum.

  Use it when: Why fast.

Reflex Action, Reflex Arc and Voluntary Action: solved examples

Ordered from board level to HOTS. Try each one before you open the solution.

Where students lose marks in Reflex Action, Reflex Arc and Voluntary Action

The slips examiners see most often, and what to write instead.

• ✗Mistake: Saying the cerebrum controls reflexes

  Students assume the brain controls everything.

  ✓Do this: Reflexes are mostly controlled by the spinal cord.

  Usually costs: 1 mark.

• ✗Mistake: Confusing receptor and effector

  Both are parts of the arc.

  ✓Do this: Receptor receives the stimulus; effector acts.

  Usually costs: 1 mark.

• ✗Mistake: Arc in the wrong order

  The spinal cord is left out.

  ✓Do this: Receptor, sensory neuron, spinal cord, motor neuron, effector.

  Usually costs: 1 mark.

• ✗Mistake: Calling a reflex voluntary

  The action is purposeful and seems chosen.

  ✓Do this: If it occurs without thinking it is a reflex.

  Usually costs: 1 mark.

Practice questions on Reflex Action, Reflex Arc and Voluntary Action

Write each answer on paper first. Open the hint only if you are stuck, and the answer only when you have finished.

1. Q1 · 1 mark · Very short answerBoard level

   Name the organs that detect the stimulus and carry out the response in a reflex.

   Hint

   Two terms.

   Answer

   The receptor detects the stimulus and the effector (muscle or gland) carries out the response.

2. Q2 · 1 mark · Assertion-reasonHarder board level

   Assertion (A): We move our hand away from a hot object before feeling pain.
   Reason (R): In a reflex the spinal cord, and not the brain, starts the response.
   (a) (A) is true and (R) is false.
   (b) (A) is false and (R) is true.
   (c) Both (A) and (R) are true, and (R) is the correct explanation of (A).
   (d) Both (A) and (R) are true, but (R) is not the correct explanation of (A).

   Hint

   Short route.

   Answer

   (c).

3. Q3 · 3 marks · Short answerHarder board level

   A cricket ball flies towards a boy's face and his eyes shut at once. Identify the stimulus, receptor, effector and response, and say whether the brain had to decide.

   Hint

   Think about the speed.

   Answer

   Stimulus: the ball coming close. Receptor: the eye. Effector: the muscles of the eyelids. Response: the eyelids close. It is a reflex, so no conscious decision by the brain was needed.

4. Q4 · 4 marks · Long answerHarder board level

   Explain why the reflex arc is a faster pathway than a voluntary response, and name the part of the nervous system concerned in each.

   Hint

   Length of path.

   Answer

   In a reflex the impulse goes from the receptor along a sensory neuron to the spinal cord and straight out along a motor neuron to the effector, a short route without thought. A voluntary response needs the impulse to reach the cerebrum, be considered and then sent back, which takes longer. The reflex is controlled by the spinal cord; the voluntary response by the cerebrum.

5. Q5 · 3 marks · Case-basedHOTS / competency

   Read and answer.
   Suppose a doctor taps below the kneecap of a patient and the leg does not jerk, although the patient can move the leg at will.
   (a) Which pathway may be faulty? (1)
   (b) Why can the patient still move the leg voluntarily? (1)
   (c) Name the type of nerve fibres in the tested pathway. (1)

   Hint

   Arc versus voluntary path.

   Answer

   (a) The reflex arc, such as the sensory or motor neuron or the spinal cord centre. (b) Voluntary movement uses the cerebrum and its own pathways, which can be intact. (c) Sensory and motor fibres.

Test your recall

Answer in your head or on paper first, then open the card. Pulling an answer out of memory makes it stick far better than re-reading.

1. 1.What is an impulse?Show answer

   The electrical message carried along neurons.

2. 2.Which part is the centre of most reflexes?Show answer

   The spinal cord.

3. 3.Give an example of a reflex action.Show answer

   Blinking, knee jerk or pulling the hand from a hot object.

4. 4.What is a response?Show answer

   The action produced by the effector.

5. 5.Which part controls sneezing and coughing reflexes?Show answer

   The medulla oblongata.

Reflex Action, Reflex Arc and Voluntary Action: frequently asked questions