Abstract
1. Intracellular injections with horseradish peroxidasewere performed in cat alpha-motoneurones supplying various hind-limb muscles. 2. Ten alpha-motoneurones from each of the quadriceps, posterior biceps, gastrocnemius-soleus and anterior tibial pools as well as from the pool supplying the short plantar muscles were collected for morphological analysis of the intramedullary axonal systems including the recurrent axon collaterals. 3. The diameter of the alpha-motor axons showed considerable variation within each motoneurone pool, the total range being from 4.6 to 9.0 micrometer. No significant difference in mean axon diameter was obtained between the different pools. 4. All alpha-motoneurones supplying the short plantar muscles and one single alpha-motoneurone supplying the quadriceps muscle lacked collaterals completely, while the remaining motoneurones gave off one to five collaterals. 5. The number of axon collateral outbulgings, interpreted as synaptic boutons, wihch originated from a single alpha-motoneurone showed large variation within each pool that possessed axon collaterals, the total range being from seventeen of 158. The mean number varied from forty-four (quadriceps) to eighty-two (anterior tibial). 6. The axon collateral outbulgings were distributed not only in the Renshaw cell area ventromedial to the main motor nuclei but also in those parts of the motor nuclei which were located in the vicinity of the parent cell bodies. In the rostrocaudal direction, the outbulgings were distributed within a distances of less than 1 mm around the position of the parent cell bodies. 7. Some physiological implications of the lack of axon collaterals from alpha-motoneurones supplying the short plantar muscles were discussed in relation to the functional characteristics of plantar muscles and motor units.