ENLARGEMENT OF MOTONEURON PERIPHERAL FIELD FOLLOWING PARTIAL DENERVATION WITH OR WITHOUT DORSAL RHIZOTOMY

In partially denervated skeletal muscle, spared motor fibres sprout, e nlarging motor unit size. Neuritogenesis and sprouting are known to de pend on the synaptic input to the neurons. This suggests that spared m otoneuron reaction to partial muscle denervation might be controlled b y primary sensory neurons which directly or indirectly project to moto neurons. In two groups of rats, different surgical procedures were car ried out: partial denervation of the extensor digitorum longus muscle without or with homolateral dorsal rhizotomy. Spared motoneuron periph eral field was evaluated by nerve-evoked tension measures. Following p artial muscle denervation, spared motoneurons enlarged their projectio n peripheral field five to six times, innervating most of the denervat ed portion of the muscle. When dorsal rhizotomy was carried out togeth er with partial denervation, the enlargement of the motoneuron's perip heral field occurred later; however, the peripheral held size was the same or greater than that found in partially denervated muscles withou t dorsal rhizotomy in the long term. Excitatory postsynaptic potential recordings at neuromuscular junctions consistently showed that innerv ation of denervated muscle cells by spared motoneurons was impaired wh en the dorsal roots were cut. Finally, in both groups of operated rats an increase in motor unit number occurred early after surgery, antici pating a process normally occurring in the same age range. These findi ngs are consistent with the idea that sensory input trans-synaptically controls motoneuron peripheral field size. (C) 1998 IBRO. Published b y Elsevier Science Ltd.