Integration of sensory input in laminae I, II and III of the cat's spinal cord.

There is a growing body of evidence supporting the concept of pain as a specific sense. There are afferent fibers from the skin that respond only to noxious stimuli, the nociceptors. Some nociceptor afferents have small myelinated axons (Adelta) while the rest have unmyelinated axons (C). Among the neurons in the superficial layers of the spinal cord (laminae I, II and III) in the cat is a population that responds exclusively to noxious cutaneous stimuli. This input arrives over the nociceptor afferents. Another population responds best to mild temperature changes, reflecting an input from the thermoreceptive afferents. Many units in these two populations of spinal neurons have been shown to project to the brainstem. Therefore, they comprise a relay to the brain for nociceptive and thermal information. Another population of superficial neurons in the spinal cord responds to gentle mechanical stimuli, particularly to slow movement. They reflect an input from the C mechanoreceptor. This population is characterized by spatal and modality convergence. They do not project to the brain and their functional role is still unknown.