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C2.2.15 (HL)—Pain perception

Pain perception depends on sensory neurons, spinal pathways and brain interpretation, with modulation possible at synapses and higher centres, linking cellular mechanisms to rapid communication in nervous systems.

Syllabus
First assessment 2025
Objective
C2.2.15
Level
HL

Pain Perception Depends on Nociceptor Signalling

HL only

Pain perception can begin at free sensory nerve endings in the skin that act as nociceptors for potentially damaging stimuli.

High temperature, acid or chemicals such as capsaicin open stimulus-sensitive channels for positively charged ions. Ion entry depolarizes the ending; if threshold is reached, action potentials travel along the sensory neuron to the brain, where pain is perceived.

Sequence: damaging stimulus → positive-ion channels open in free nerve ending → depolarization reaches threshold → impulses propagate to brain → conscious pain perception.

Capsaicin in chilli peppers activates ion channels in free nerve endings, producing depolarization and impulses that the brain interprets as burning pain even without high temperature.

Nociceptor activation is the neural input to pain, not pain perception itself. Conscious pain emerges after processing in the brain.

Neural Control And Evidence

HL only
  • At threshold, voltage-gated Na+ channels depolarize the axon; K+ channels then repolarize it. Local currents propagate the impulse and the refractory period enforces direction.
  • Action potentials are all-or-nothing; stimulus intensity is encoded by frequency. Myelin enables fast saltatory conduction between nodes of Ranvier.
  • Synapses integrate excitatory and inhibitory inputs through spatial and temporal summation. Chemicals may mimic, block or prolong neurotransmitter action.
  • Pain receptors use TRP channels that respond to heat, acid, capsaicin or tissue damage.
  • EEG, MRI and fMRI link conscious processing to coordinated activity across brain regions, providing neural correlates rather than a single consciousness centre.

Concept essentials

  • Nociceptors detect damaging or potentially damaging stimuli.
  • Sensory neurons carry pain signals toward the central nervous system.
  • Synapses can modulate pain signalling before conscious perception.
  • Pain perception requires interpretation by the brain.