A Behavioral Animal Model for the Study of Pain Mechanisms in Primates

Abstract

Recent neurophysiological studies in primates have increased significantly our knowledge of neural pathways that play a role in pain sensation and reaction. In the peripheral nervous system, neural populations have been identified which respond exclusively to intense or noxious stimuli applied to the skin (Burgess & Perl, 1973). Similarly, some neurons in the spinal cord dorsal horn with axon projections to the thalamus respond only to tissue-threatening or tissue-damaging stimuli (Willis, Trevino, Coulter, and Maunz, 1974; Price and Mayer, 1975). However, the situation is complicated by the finding that many spinothalamic neurons have a wide dynamic response range to innocuous and noxious mechanical stimuli (Willis et al, 1974; Price and Mayer, 1975). What role do such neurons play in pain? Furthermore, all of these studies have been performed under general anesthesia or after surgical brain lesions, both of which alter ongoing and evoked central neuronal activity. For example, dorsal horn neurons activated by noxious mechanical stimulation in a spinalized cat do not respond to similar stimuli in a decerebrated animal (Brown, 1971). Other studies have shown that cold block of the cervical spinal cord in anesthetized cats results in a significant increase in the sensitivity of lumbar spinal cord dorsal horn neurons to noxious heat stimuli (Zimmermann and Handwerker, 1974). Since the responsivity of these neurons is dependent upon the waking state of the animal, only tentative conclusions can be drawn from such experiments about the functional role of different spinal cord populations in pain.