Electron microscopic observations on the synaptic contacts of group Ia muscle spindle afferents in the cat lumbosacral spinal cord.

After intra-axonal injection of horseradish peroxidase (HRP) into afferent fibers originating from muscle spindle primary endings of the cat gastrocnemius, group Ia boutons located in the ventral horn of the spinal cord were identified and studied electron microscopically. The Ia boutons were invariably found to contain spherical synaptic vesicles (S-type boutons), and a number of them were also postsynaptic to smaller P-type boutons (large S-type boutons with axo-axonic contacts). None of the present Ia-boutons belonged to the previously described M-type. The vast majority of the studied boutons were considered to be located at less than 500 microns distance from the alpha-motoneuron soma. The results are discussed in relation to previous light and electron microscopic data.