Certain features of signal transmission from multiple alpha-axons to multiple Ia fibres via extrafusal muscle tissue were studied by stimulating three ventral root filaments separately and simultaneously and by recording spike trains from three dorsal root filaments. 2. Post-stimulus time histograms (PSTH) revealed widely graded influences of single motor unit contractions on different muscle spindles, thus confirming results of Binder et al. (1976, 1980a). 3. In most instances, the signal transmission from a single motor unit to a single Ia fibre was disturbed to varying degrees by concomitant activity from other motor units. 4. This deterioration of signal transmission can probably be compensated for, at least in part, by correlations between discharge patterns of two or more Ia fibres induced by the "common input" of motor unit activity. 5. These correlations showed a topographical pattern such that a peak in the cross-correlogram (CCH) occurred at zero time for pairs of spindles located virtually in parallel to each other, and was shifted away from zero time as a function of the serial distance between spindles in the longitudinal muscle fibre direction. 6. Fusimotor innervation had complex effects on correlations between Ia fibre discharge patterns. It might de-correlate the latter as well as favour new correlations. 7. The possible role of topographical correlation patterns in the afferent reflex limb for tremor suppression is discussed.
