Topographic distribution of terminals of Ia and group II fibers in spinal cord, as revealed by postsynaptic population potentials

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Abstract
Postsynaptic population potentials (PSPP) evoked in large numbers of motoneurons by impulses in single Ia or spindle group II fibers, were recorded simultaneously from 2 adjacent ventral roots perfused with isotonic sucrose. By comparing the amplitudes and rise times of these PSPP with the entry points of the afferent fibers, the distribution of their terminals was inferred. Ia impulses entering a spinal segment evoked larger PSPP in its motoneurons than in those of the adjacent segment. The size of the PSPP was correlated with the exact entry level of the afferent fiber, indicating that Ia-fibers give off more endings to motoneurons located near the entry point. The sizes of the 2 PSPP were also related directly to the conduction velocity of the afferent impulses eliciting them, as reported recently. The slope of this relationship was greater when the Ia impulses entered the segment from which the recordings were taken. In a single experiment with appropriate data, the relative influence of the entry point and conduction velocity of afferent impulses on the PSPP they evoked were plotted in a 3-dimensional diagram. The entry level of a spindle group II impulse has a relatively small effect on the size of the PSPP it elicits, suggesting that group II terminals are distributed quite evenly over the whole extent of the motoneuron pools they supply. The rise times of Ia PSPP were briefer than those of group II PSPP. This finding may be due to the more distal location of group II endings on motoneuron dendrites, rather than to the slower intraspinal conduction in group II fibers. Somatotopic factors may be of much greater importance than generally believed in the development of Ia monosynaptic connections. The somatotopic distribution of Ia terminals described here provides a possible explanation for the topographically localized stretch reflexes within a single muscle, and the focusing of signal transmission in the multichannel muscle stretch system.