In recumbent animals, the clinician may apply a noxious stimulus to a digit. This generates afferent impulses that enter the spinal cord via peripheral nerves and associated dorsal nerve roots that are relayed to bilateral tracts in the lateral funiculi of the spinal cord. These tracts ascend the spinal cord and continue through the medulla, pons, and midbrain to specific nuclei in the thalamus for relay to somatic sensory areas of the cerebral cortex. A positive response is evidenced by vocalization, turning of the head, or dilation of the pupils when the impulses reach the thalamus or cerebrum.
Although several grading schemes for spinal cord injury differentiate between superficial and deep pain perception, this can be extremely challenging and is not typically necessary. However, it is critical to differentiate nociception from the withdrawal component of the withdrawal-flexor reflex (see earlier). Animals with transverse spinal cord lesions maintain the withdrawal reflex and this should not be mistaken for intact nociception.