Characteristics of background and evoked discharges of multireceptive neurons in lumbar spinal cord of cat.

1. Intracellular recordings were made in laminae III-V of the dorsal horn of the cat spinal cord from a group of multireceptive neurons that exhibited similar physiological properties. The background discharge contained irregular and occasional clusters of action potentials, each arising from a complex excitatory postsynaptic potential (EPSP); brushing elicited more frequent clusters containing more action potentials, whereas noxious heating elicited more regular firing, with action potentials arising singly. The distribution of interspike intervals (ISIs) was measured for the background and heat-evoked discharges and revealed characteristic differences in pattern between the two states. 2. Statistical tests were applied to the point process of times of occurrence of spikes within a discharge to establish stationary, to identify renewal instants, and to describe the process between renewal instants. In each case, the statistical description of the discharge was related to the physiological state of the neuron as revealed by recording of synaptic potentials and analysis of the different activating inputs. 3. Background discharge was stationary, and spikes occurred singly or occasionally in clusters. The data were therefore analyzed as a series of "bursts," where a burst could be made up of one or more spikes. Examination of the pattern of serial dependence led to the conclusion that the point process renewed itself after each burst. This, together with the distribution of the intervals between bursts, suggested that bursts were triggered by single, distinct events, which occurred randomly at a slightly and randomly varying average rate. The variation in the number of spikes in a burst suggested random variation in the strength of the physiological trigger. 4. Clusters of action potentials arising from a complex EPSP could be produced by simultaneous stimulation of a number of fast-conducting A beta fibers in the periphery and gentle mechanical stimulation, whereas stimulation of slow C fibers and noxious heat evoked discrete EPSPs from which action potentials arose singly. It was, therefore, concluded that background activity was, at least in part, the result of random activity in a randomly varying number of A beta primary afferent fibers, which could arise from operative procedures. 5. The discharge evoked by heat was stationary, and the absence of serial dependence established that the point process renewed itself after every spike and was, therefore, a simple renewal process. The distribution of the ISIs suggested that each spike was triggered by a randomly occurring physiological event.(ABSTRACT TRUNCATED AT 400 WORDS)