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

Intraspinal branches of primary afferent axons in the cat lumbar cord have been revealed by filling then with cobalt, followed by precipitation as cobalt sulphide and silver intensification. 2. Primary afferent collaterals which reached the ventral horn were myelinated and had axon diameters of around 2 mum. Up to four (mean 1) side branches occurred at nodes within the ventral horn. The finest branches were less than 1 mum in diameter. 3. The finest branches formed synaptic boutons on nerve cells. The total observed association between one afferent and a motoneurone (synaptic structure) consisted of up to six 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 mum of dendrite. It was assumed that these monosynaptic structures on moto-neurones were formed by muscle spindle afferents. 4. There was no correlation between the total contact area of a synaptic structure and its location on a motoneurone. 5. A computer model of the motoneurone was used to investigate the effect of multiple synaptic contact on electrophysiological estimates of synaptic location. It was concluded that the observed spread of boutons making up a synaptic structure would not significantly affect distance allocation of distal synapses but could lead to some proximal dendrite structures being incorrectly classified as somatic.