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Pain pathways matter because pain is not a fixed signal, it can be turned up or down by the nervous system itself. This is exactly why rubbing a bumped knee eases the pain. Two types of nerve fibre carry pain signals: fast A-delta fibres, which carry sharp, well-localised pain, and slower C fibres, which carry dull, aching, poorly localised pain. For example, imagine stubbing a toe: a sharp, immediate pain arrives first through A-delta fibres, followed by a duller, longer-lasting ache carried by C fibres. In practice, gate control theory explains why rubbing the area afterwards genuinely reduces how much pain is felt.

Pain signals from A-delta and C fibres travel up the spinothalamic tract (a spinal cord pathway) to the thalamusA deep brain structure that relays and filters sensory information on its way to the cortex.Read more → and then the cortex, where pain is consciously perceived. Melzack and Wall's gate control theory proposes that a 'gate' in the spinal cord controls how much pain signal gets through. Non-painful touch signals, carried by different, faster fibres, can close this gate and reduce the pain signal that reaches the brain. Say a bumped knee gets rubbed: the touch signal activates the gate-closing mechanism, reducing the pain signal getting through, even though the injury itself is unchanged. Descending pathways from the brain also modulate the gate, using serotonin, noradrenaline, and the body's own opioids (endorphins) to dampen incoming pain signals from above. In practice, this is why antidepressants that boost serotonin and noradrenaline, such as duloxetine, are also used to treat chronic pain, independent of their effect on mood.

• Why is duloxetine effective in neuropathic pain?
• Why is cardiac pain often felt in the left arm?
• Which process is thought to underlie phantom limb pain after amputation?

• A-delta fibres, carry fast, sharp, localised pain, unlike C fibres which carry slow, dull, poorly localised pain.
• Gate control theory, touch signals close the spinal pain gate, whereas the injury signal itself is unchanged by rubbing the area.
• Descending modulation, serotonin, noradrenaline, and opioids dampen pain from above, rather than pain signals travelling only one direction toward the brain.
• Duloxetine, treats chronic pain via descending serotonin-noradrenaline pathways, unlike a simple analgesic that acts locally at the site of injury.

• The arm is not ischaemic during angina.
• Cardiac nociception travels with sympathetic afferents rather than the vagus.
• Sodium channel blockade describes local anaesthetics and carbamazepine.
• Opioid receptor binding describes opioid analgesics.
• Peripheral anti-inflammatory action describes non-steroidal drugs.

• Localise the clue to the relevant anatomical level, cell type, pathway or molecular process.
• Map that structure or process to its normal function.
• Predict the deficit or clinical sign produced when it is disrupted.
• Reject options that belong to a neighbouring structure, pathway or level of explanation.