Functional relationships between neurons of marginal and substantia gelatinosa layers of primate dorsal horn

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Abstract
This study examined the functional role of substantia gelatinosa neurons in the transmission of nociceptive and nonnociceptive input to spinothalamic neurons in the marginal layer of the spinal cord dorsal horn. Neurons (130) in layer I (marginal layer) and Rexed''s layer II (substantia gelatinosa, subdivided into an outer zone, a, and an inner zone, b) of L[lumbar]7-S[sacral]1 were studied in rhesus monkeys [M. mulatta] lightly anesthetized with sodium pentobarbital. The response properties of 51 pairs of neurons recorded successively in the same microelectrode track were compared. The dorsal neuron of each pair was a layer I spinothalamic or non-projection neuron and the ventral neuron, located within 200 .mu.m, was a layer II neuron. The location of all neurons in layer II was determined histologically from small lesions made at the sites of maximum spike amplitudes. Several classes of neurons were distinguished by the range of their responses to stimuli applied to their receptive fields. The properties of most neurons in layer II were similar to those previously reported for neurons in layer I and deeper layers of the dorsal horn. Nociceptive-specific neurons that were maximally excited by noxious mechanical or thermal stimuli did not respond to low-threshold mechanical stimuli and were located mainly in layers I and IIa. Data obtained from 51 pairs of neurons revealed that neurons within a pair often received convergent input from the same types of primary afferent fibers. The paired neurons had overlapping receptive fields, with layer II neurons having smaller receptive fields than layer I neurons and layer II neurons had shorter first-spike latencies than layer I neurons. Some layer II neurons may function as excitatory interneurons and relay primary afferent input to spinothalamic neurons in layer I, where some additional convergence occurs. This conclusion is consistent with observations from Golgi anatomical studies. Layer II neurons responded to noxious heat stimuli with long-latency response components that showed temporal summation and prolonged discharge with repeated stimuli. Central facilitatory mechanisms are generated in layer II neurons that appear to receive direct monosynaptic input from primary nociceptive afferent fibers.

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