Motor Unit Number Estimation in Normal and Parkinsonism Model of Medial Gastrocnemius Muscle in Rats

Motor units (MUs) reflect the function of the central nervous motor system. Thus, the estimated MU number is a good option to investigate the functional movement disorder in the Parkinson's disease (PD). The purpose of this study was to compare the estimated MUs number in the medial gastrocnemius (MG) muscle of the normal rats and those with the parkinsonism. The MG muscle of two age-matched group (normal and parkinsonism) of anesthetized male, adult (154-304 days old) Wistar rats were studied after the insertion of electromyography (EMG) needles. The insertion activity and the MU recruitment (MUR), the strengths of mechanical involuntary contractions and the evoked spike potentials, were recorded. The means initial and the maximal amplitudes of the motor unit potentials (MUPs) were calculated for the estimated MUs number. The spinal cord at the L4-L6 was removed for pathological study. The parkinsonism MUPs trace showed irregular and low threshold discharge rate. The normal spikes trace, however, was different. Increased age was not associated with any increase in the MU number in the two groups. However, there was a significant correlation between the mentioned parameters and the insertion activity (r= -0.25, r= -0.177) and the MUR (r= 0.86, r= 0.248) in the normal and the parkinsonism groups, respectively. There was a correlation between the ages and mean MUP amplitude in the normal and the parkinsonism insertion activity (r =0.766, r =0.659) and the MUR (r =0.89, r = 0.4), respectively. Similarly, there was a correlation between the ages and maximal amplitudes in the normal and parkinsonism groups (r =0.53, r =0.42; r =0.86, r =0.248), respectively, (p<0.001). In the parkinsonism group, there was no significant correlation between the MUs number and the mean MUPs amplitudes in the insertion activity (r= 0.074, p= 0.088) and the MUR (r= 0.226, p=0.762). The spinal cord in the parkinsonism group showed degenerated nerve fibers and apoptosis in the degenerative nerve fibers and in the medium and large motor neurons with Lewy bodies and neurofibrillary tangles. The small ones, however, remained intact. The parkinsonism MUPs, compared to normal ones, have lower threshold and recruit less MUs. The apoptotic medium and large motor neurons with Lewy bodies contribute to the disuse of the relative MUs, while small ones remain intact.