A morphological study of the axons and recurrent axon collaterals of cat alpha-motoneurones supplying different functional types of muscle unit.

Abstract

Cat .alpha.-motoneurons supplying muscle units of the soleus S type and gastrocnemius S, FR and FF types were injected intracellularly with horseradish peroxidase. Ten .alpha.-motoneurons of each motor unit type were collected for morphological analysis of the intramedullary parts of their axonal systems including the recurrent axon collaterals. The .alpha.-motor axon diameters in the spinal cord white matter were significantly smaller for the soleus motor units (mean = 5.2 .mu.m) than for the gastrocnemius S units (mean = 6.4 .mu.m), which in turn differed significantly from those of the gastrocnemius FR (mean 7.4 .mu.m) and FF (mean = 7.3 .mu.m) types. The number of axon collateral outbulgings, interpreted as synaptic terminals, which originated from the soleus motor axons (mean = 27.8) was significantly smaller while that of the gastrocnemius FF motor axons (mean = 98.1) was significantly larger than the number of outbulgings from the gastrocnemius S (mean = 44.4) and FR (mean = 53.8) motor axons. The latter 2 types were not significantly different. In transverse sections the axon collateral outbulgings were found not only in the classical Renshaw cell area ventromedial to the main motor nuclei but also within the homonymous motor nucleus. Only some of the neurons projected to the latter area. Rostrocaudally, the outbulgings were distributed within less than 1 mm around the position of the parent cell bodies. Some physiological implications of the observed differences between axon collateral systems of various functional types of .alpha.-motoneurons were discussed in relation to the present knowledge of the effects of the recurrent axon collaterals during different types of involuntary and voluntary activation of the .alpha.-motoneurons.