An experimental study on mechanism of functional compensation for file injured extremity with brachial plexus upper trunk avulsion repaired by ipsilateral C_7 nerve root transfer

Objective To investigate the mechanism of no further impairment of the injured upper limb function following ipsilateral C_7 nerve root transection in brachial plexus upper trunk avulsion.Methods Sixty SD rats were divided into 2 groups.In the experimental group ipsilateral C_7 root was transected after C_5-C_6 root avulsion.In the control group only C_5-C_6 root avulsion was created.Ten rats from each group were subject to electrophysiologic,histologic and histomorphometric examination of C_7-innervated muscles including latissimus dorsi,triceps brachii,extensor carpi radialis brevis and extensor digiti communis muscles at post-operative 3,6 and 12 weeks respectively.Results At 3 weeks post-operatively,significant differences were noted between the two groups.Compared to those of the control group,reduced muscle weight and muscle fibre area of the C_7- innervated muscles,decreased amplitude and prolonged latent period of compound muscle action potential (CMAP)and diminished area of postsynaptic membranes were seen in the C_7 root transection group(P0.05 or P0.01).Degraded distribution of motor end plates could be found under fluorescent microscope and immature structure of regenerative motor end plates under electron microscope in those muscles from the experimental group.At postoperative 6 weeks,there were no significant differences in recovery rate of amplitude and latent period of CMAP in triceps brachii muscle and recovery rate of muscle fibre area,amplitude and latent period of CMAP and area of postsynapfic membranes in extensor carpi radialis brevis and extensor digifi communis muscles between the experimental group and control group(P0.05).The motor end plates were more than it had been at 3 weeks post-operatively.Furthermore,the ultrastructure of which also had a tendency to maturation.At 12 weeks after the operation,no significant differences were found in all parameters between the experimental and control groups(P0.05).The motor end plates were populated and their ultrastructure was close to a mature level similar to the control group.Conclusions Following ipsilateral C_7 nerve root transection in brachial plexus upper tnmk avulsion,neuraxons of the lower trunk could compensatively dominate fibres of C_7-innervated muscle by means of motor end plate regeneration,so that there was no further impairment in function of the affected extremity.