Physiology of nociceptive pain transmission
1. Peripheral Transduction: nociceptor stimulation leads to generation of action potentials
2. Transmission from the periphery to the CNS: the action potentials are conducted to the dorsal horn of the spinal cord by sensory afferent nerves (also referred to as primary(1°) afferents or 1st order neurons)
3. Ascending modulation from the spinal cord to the brain: the peripheral afferent synapses onto cell bodies (referred to secondary neurons, or 2nd order neurons) that send axonal projections to the brain
4. Brain awareness/perception: within the brain, the peripheral nociceptic stimuli is perceived as pain
5. Descending modulation: axonal projections from neurons in the brain descend down the spinal cord and synapse onto cell bodies that transmit nociception from the periphery to the brain, thus, modulating the extent of ascending nociceptive input to the brain.