Amplitude modulation of EPSPs in motoneurons in response to a frequency-modulated train in single la afferent fibers.

In anesthetized cats, single group Ia fibers were activated with a train of 52 stimuli whose interspike intervals were obtained from the discharge of a group Ia fiber in a walking cat (courtesy of Dr. G. E. Loeb, NIH). The EPSPs recorded in motoneurons to which the afferents projected were averaged in register (EPSP1, EPSP2, ..., EPSP52) in response to multiple presentations of the train at a rate (0.77 Hz) similar to the stepping rate of the cat. Amplitudes of these averaged EPSPs were found to be highly variable, depending both on afferent discharge rate and history of activation. The initial EPSP was generally potentiated, and EPSPs in the high-frequency (greater than 200 Hz) middle portion of the train were generally reduced in amplitude. Connections at which the first EPSP was most potentiated tended to be those which showed the most depression of EPSP amplitude during the high-frequency portion of the train. Thus, at some connections, modulation of EPSP amplitude during the frequency-modulated train was much greater than at other connections. In general, the extent of modulation was greater on low-rheobase motoneurons than on high-rheobase motoneurons. We suggest that these differences in transmission permit type S (i.e., low-rheobase) motoneurons, which on average generate the largest EPSPs, to reach threshold at low levels of input (size principle) but prevent excessive depolarization due to temporal summation during high-frequency stimulation.