engleski

Evidence for a direct connection between corticobulbar pathways and motoneurons supplying lower facial muscles in man

Corticobulbar influences on the orbicularis muscle of the mouth (OR) were studied using either transcranial anodal or magnetic stimulation, and were compared to corticospinal influences on the first dorsal interosseus muscles (FDI). FDI muscle was chosen, as to believe that distal muscles are directly connected with the corticospinal pathway in humans (Ashby and Zielm, 1982). Discharges of single motor units in the OR (n=24 units) and (FDI=17 units) muscles were recorded in 22 volunteers, using the peristimulus time histogram (PSTH) technique. The PSTH peaks are assumed to coincide with multiple EPSPs induced in motoneurones by corticobulbar input. For each peak a jitter was measured, for both muscles. Also, for the OR motor units the jitter and rise time were used as an indirect measure of the complexity of these connections to spinal motoneurones. The EPSP amplitude was used as a measure of the synaptic transmission effectiveness. There were no significant differences in jitter between the OR (300-450 ms) and the FDI (230-300 ms) muscles, being relatively small for both muscles. The rise time for the OR motor units ranged between 1.0 and 1.2 ms, and the EPSP amplitude was 4.0 mV. Therefore, we suggest that cortical projections are mono- or disynaptically connected to the lower facial muscle and small hand muscle motoneurons.

facial muscles; motoneurons; corticobulbar pathways

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