How the body senses pain
> The first step in sensing pain involves transduction, the translation of physical energy (noxious stimuli) into electrical activity at the peripheral nociceptor.
> The second process involves sensory nerves. Also referred to as transmission, this is the movement of impulses through the peripheral nervous system to the spinal cord.
> The third, modulation, is the movement and alteration of impulses through the spinal cord toward the brain.
> Finally, perception results from the successful completion of the other three processes as the brain subsequently recognizes the pain.
Source: Tranquilli WJ, Grimm KA, Lamont LA. Pain management for the small animal practitioner. Jackson, Wyo: Teton Press, 2000;4,7-10,14.