Central terminations of muscle afferents on motoneurones in the cat spinal cord.

Abstract

Intraspinal branches of primary afferent axons in the cat lumbar cord were revealed by filling them with Co, followed by precipitation as CoS and Ag intensification. Primary afferent collaterals which reached the ventral horn were myelinated and had axon diameters of around 2 .mu.m. Up to 4 (mean 1) side branches occurred at nodes within the ventral horn. The finest branches were less than 1 .mu.m in diameter. The finest branches formed synaptic boutons on nerve cells. The total observed association between 1 afferent and a motoneuron (synaptic structure) consisted of up to 6 boutons (mean 1.85). The boutons constituting a synaptic structure were usually located close together on the soma or dendrites, but sometimes were spread along more than 60 .mu.m of dendrite. These monosynaptic structures on motoneurons were probably formed by muscle spindle afferents. There was no correlation between the total contact area of a synaptic structure and its location on a motoneuron. A computer model of the motoneuron was used to investigate the effect of multiple synaptic contact on electrophysiological estimates of synaptic location. The observed spread of boutons making up a synaptic structure probably would not significantly affect distance allocation of distal synapses but could lead to some proximal dendritic structures being incorrectly classified as somatic.