A morphological study of the axons and recurrent axon collaterals of cat alpha‐motoneurones supplying different hind‐limb muscles.

Abstract

Intracellular injections with horseradish peroxidase were performed in cat .alpha.-motoneurons supplying various hind limb muscles. Ten .alpha.-motoneurons 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. The diameter of the .alpha.-motor axons showed considerable variation within each motoneuron pool; the total range was 4.6-9.0 .mu.m. No significant difference in mean axon diameter was obtained between the different pools. All .alpha.-motoneurons supplying the short plantar muscles and one single .alpha.-motoneuron supplying the quadriceps muscle lacked collaterals completely; the remaining motoneurons gave off 1-5 collaterals. The number of axon collateral outbulgings interpreted as synaptic boutons, which originated from a single .alpha.-motoneuron showed large variation within each pool that possessed axon collaterals; the total range was 17-158. The mean number varied from 44 (quadriceps) to 82 (anterior tibial). 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 less than 1 mm around the position of the parent cell bodies. Some physiological implications of the lack of axon collaterals from .alpha.-motoneurons supplying the short plantar muscles were discussed in relation to the functional characteristics of plantar muscles and motor units.