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C3.1.9—Pain reflex arcs

Pain reflex arcs links named stimuli, coordinating structures and effectors so body systems or plant tissues produce an integrated biological response.

Syllabus
First assessment 2025
Objective
C3.1.9
Level
SL

Exam analysis

Chance of appearing9%of analysed past papers
Latest appearanceNovember 2025
Most common paperPaper3
Typical marks1–3

Common command terms

  • Define
  • Label
  • Explain
  • Draw
  • Identify
  • Outline
  • Describe
  • Annotate

Scoring notes

Common mistake
Putting the brain before the spinal relay in a rapid pain reflex.

Recent exam appearances

November 2025Paper1A ["SL"] · TZ116[ 1 ]C3.1.9—Pain reflex arcs
May 2025Paper1A ["SL"] · TZ117[ 1 ]C3.1.9—Pain reflex arcs
November 2015Paper3 ["SL"] · TZ014(a)[ 4 ]C3.1.9—Pain reflex arcs
May 2015Paper3 ["SL"] · TZ214(b)[ 2 ]C3.1.9—Pain reflex arcs
November 2014Paper3 ["SL"] · TZ014(a)[ 3 ]C3.1.9—Pain reflex arcs
Practice this objective

Coverage 2010–2025 · Updated 16 Jul 2026

A reflex arc trades deliberation for speed

A pain withdrawal reflex is an involuntary response integrated in the spinal cord, with skeletal muscle as the effector.

A free sensory nerve ending in the hand detects a damaging stimulus. The sensory neuron enters the spinal cord and synapses with one interneuron in grey matter, which synapses with a motor neuron; the motor neuron stimulates a skeletal muscle to withdraw the hand.

Free nerve ending → sensory neuron → single interneuron in spinal grey matter → motor neuron → skeletal-muscle contraction. The short spinal route starts withdrawal before conscious pain processing is complete.

Touching a hot object activates the pain receptor; flexor muscle contraction withdraws the hand while impulses also travel to the brain and pain becomes conscious.

The reflex does not bypass the CNS—the spinal cord is part of the CNS. The brain can be informed in parallel but is not required to initiate this immediate response.

Pain reflex arcs

Assessment in practice

1–3 marks
How it is assessed

This objective is assessed through experimental design, commonly using Define / Label / Explain.

Command terms

Define / Label / Explain / Draw / Identify / Outline / Describe / Annotate

What earns marks

Mark schemes reward named structures, correct direction of information flow and explanations that keep the sequence causal.

Watch for

Putting the brain before the spinal relay in a rapid pain reflex.

Representative question

Question 1

[Maximum number: 4]

Draw a labelled diagram of a reflex arc for a pain withdrawal reflex.

Body-System Integration

  • Nervous signals, hormones and blood transport integrate organs into coordinated systems; emergent functions arise from their interactions.
  • Sensory neurons carry receptor input to the CNS; motor neurons carry output to effectors. Mixed nerves contain both fibre types.
  • Reflex arcs provide rapid involuntary responses through sensory, relay and motor neurons. The spinal cord also links the brain and peripheral nerves.
  • Cerebral hemispheres support conscious processing; the cerebellum coordinates movement and balance; the medulla adjusts ventilation and heart activity.
  • The hypothalamus links nervous and endocrine control through the pituitary. Pineal melatonin helps time sleep; adrenal epinephrine supports acute stress responses.
  • Baroreceptors and chemoreceptors provide feedback about pressure, CO2, pH and O2.
  • The CNS controls voluntary swallowing and egestion, while the enteric nervous system coordinates gut peristalsis.

Concept essentials

  • Pain reflex arcs depends on linking the stimulus to the coordinating structure or signal.
  • The pathway for pain reflex arcs must keep information flow and response direction clear.
  • Named tissues, hormones, neurons or effectors give pain reflex arcs its biological specificity.
  • Evidence about pain reflex arcs is strongest when structure, mechanism and outcome are connected.