QS Study

Pain is defined as an unpleasant sensation due to tissue damage. Pain is a protective mechanism. It is not just a bodily feeling. You may feel pain in one area of your body, such as your back, abdomen, chest, pelvis, or you may feel pain all over.

Pain Receptor – Free nerve endings. It is any one of the many free nerve endings throughout the body that warn of potentially harmful changes in the environment, such as excessive pressure or temperature. Pain stimulates pain receptors, and this stimulus is transferred via specialized nerves to the spinal cord and from there up to the brain.


The pain stimulus is processed in the brain, which then sends an impulse down the spinal cord and via appropriate nerves which command the body to react, for instance by withdrawing the hand from a very hot object. Pain receptor is stimulated by any mechanical, thermal, or chemical changes of the tissue:

Mechanical: e.g., Muscle spasm, Stretch of hollow viscera.

Thermal: > 45°C.

Chemical: Chemical that stimulates pain fibers –

  • Bradykinin.
  • Serotonin.
  • Histamine.
  • K+.
  • Prostaglandin.
  • Substance-P.

Fig: Pathway of pain from a periphery to the brain

Pathway of pain:

  • Stimulation of pain receptors (free nerve endings),
  • Afferent nerve,
  • Dorsal root mocha (1st order neuron),
  • Synapse within spinal cord in the dorsal horn,
  • Substantia gelatinosa of Rolando (2nd order neuron begins),
  • Most fibers cross the midline,
  • Ascend as a lateral spinothalamic tract,
  • Spinal lemniscus of the medulla,
  • Pons,
  • Midbrain,
  • The ventral posterolateral nucleus of the thalamus (3rd order neuron begins),
  • Posterior limb of the Internal capsule,
  • Corona radiate
  • Postcentral gyrus of cerebral certex.
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