| Abstract: | A search for postsynaptic elements, excited only by slowly-conducting afferent fibers (Aδ and C), was made by recording from the dorsal horn of cats and monkeys with dye-filled microelectrodes. Units identified by afferent responsiveness were tested for antidromic invasion by electrical stimulation of the opposite ventrolateral funiculus at the midcervical level. Recording points were marked by iontophoretically passing dye from the recording electrode and subsequently located in histologically-prepared material. Evidence for antidromic excitation from spinal cord stimulation was found for five of 21 units in cat and 13 of 31 neurons in monkey. Antidromically excited cells were located within the distribution of the large posteromarginal neurons of the dorsal marginal zone (Lamina I). Although recording loci for a number of the elements studied were unequivocally located within Laminae II (substantia gelatinosa) or III, none could be antidromically excited. All antidromically driven units received a powerful input from Aδ primary afferent units and upon testing with natural stimuli responded specifically to stimuli of the type initiating activity either for high threshold mechanoreceptors or for low threshold thermoreceptors. It is concluded that some Lamina I neurons form part of an ascending projection which follows the spinal pathway of the spinothalamic tract and thereby contribute to the mechanical nociceptive and thermoreceptive features of this pathway. The absence of antidromic response is argued to be uncertain and evidence for a lack of projection is open to alternative explanation. |
