Stochastic spatiotemporal characterizations of mouse spinal substantial gelatinosa

The spatial functional organization of the mouse spinal cord substantia gelatinosa was studied for afferent sensory barrages produced by sciatic nerve stimulation. Since many concepts of coding in the spinal cord depend upon the differential excitation of morphologically distinct populations of large and small cells, the ease with which such populations are functionally identified is of considerable interest. The spatial temporal characteristics of multiple unit activity were analyzed stochastically for various types of afferent input. A functional characterization of mouse spinal substantia gelatinosa was achieved and a strategy for single electrode identification of statistically different populations of cells was developed. The spinal cord of anesthetized (80 mg/kg, pentobarbital sodium) mice was exposed by laminectomy using a dissecting microscope. The sciatic nerve was also exposed and both exposures were covered with warm mineral oil. Spinal responses to sciatic nerve stimulation were recorded by two extracellular platinum microelectrodes (tip equal to or approx. equal to 25μ). The electrode tips were separated by 150μ and each electrode was referenced to an indifferent electrode lodged in surrounding muscle. Two channels of unit responses were recorded simultaneously during stimulation. During the course of the observations, both electrodes were lowered systematically in 25μ steps. The data were analyzed into amplitude discriminated, post stimulus time histograms. These were used to construct cross correlation coefficients for activity from both electrodes recorded throughout several penetrations of the substantia gelatinosa. Large cross correlation values over large electrode displacements identified large cells. The strategy can be repeated using a single electrode at two different depths in the spinal cord.