TEMPERATURE AND DENDRITIC RESPONSE OF SPINAL MOTONEURONS

Well-established are the facts that the dendrites of spinal motoneurons carry impulses, at least in the cellulifugal sense, and that they do so with a decrement. 1-4 Whether or not they do these things in the cellulipetal sense is not a matter of present, concern. The experiments to be discussed relate to "antidromic" conduction in dendrites of spinal motoneurons of the cat and the influence upon it of temperature change, an essentially unexplored problem. Utilizing the fact that the dendrites of those motoneurons that supply the small plantar muscles extend laterally to the surface of the cord,' one can by chilling the cord surface produce a temperature change confined essentially to the dendrites themselves. In practice the warmed oil routinely employed to cover the exposed spinal cord was replaced by chilled oil which produced a rapid fall in temperature. During the ensuing 45 or so minutes, with the aid of an infrared lamp, temperature at the cord surface returned over a roughly exponential course to normal. Throughout this time dendritic responses were elicited and recorded from the point of maximal response' on the cord surface, which is to say at the periphery of the motoneuron pool, by means of an electrode pitted against another at a distance. A thermocouple, by means of which temperature was recorded, was placed immediately adjacent to the electrode on the cord surface. Figure 1 illustrates the manner in which dendritic response grows as temperature is lowered from 370 to 320C. Since the superimposed recordings are identical until some time (indicated by an arrow) after the region ceases to be a source of current flow to impulse sinks in the more central parts of the motoneurons, it follows that there has been no influence upon conduction through the axons, the cell body, and the proximal dendrites. If, by way of contrast, body temperature generally is lowered, not only is the negative phase of dendritic response increased but so too is the prodromal positive phase, and the onset of each phase is later in time, all of which indicate the more widespread field of effect. Thus it would seem that the procedure employed reveals change in the more peripheral stretches of the dendrites and, furthermore, change that is primarily there rather than secondarily the result of change elsewhere. In Figure 2 is plotted amplitude of the dendritic responses as a function of temperature in the range between 200 and 380C. A ceiling of amplitude exists between 21.50 and 25.50C above which temperature range there is a linear decrease in amplitude with increase in temperature. On the average in these experiments the maximum, and ceiling of, dendritic response occurred between 20.5° and 24.50C. This result is in sharp contrast with that of Chang6 who found the cortical "dendritic potential" to be maximal in the range of normal body temperature and to fall off in the amplitude to practical extinction. at 200C. No simple explanation for this discrepancy presents itself, but the fact suggests that it is unwise to generalize concerning the activity of dendrites.