LATERAL RECTUS EMG AND CONTRACTILE RESPONSES ELICITED BY CAT ABDUCENSMOTONEURONS

Stimulation of 41 single, abducens nucleus motoneurons in the cat evok ed electromyographic (EMG) and contractile responses in the ipsilatera l lateral rectus muscle. Separate, bipolar, fine wire EMG recording el ectrodes in the global and orbital muscle layers showed that 22 muscle units were confined to the global layer, 8 to the orbital layer, and 11 units were contained in both (''bilayer'') muscle layers, ''Bilayer '' units demonstrated significantly greater twitch (P less than or equ al to 0.002) and maximum tetanic (P less than or equal to 0.001) tensi ons as well as faster fusion frequencies (P less than or equal to 0.02 2) than either global or orbital units, ''Bilayer'' units also showed the lowest average kt values (the slope of the linear relationship bet ween motoneuron stimulation frequency and isometric tetanic tension), ''Bilayer'' units were predominantly fast fatigable (FF), Global units displayed all muscle unit types including all the nontwitch (NT) unit s, Orbital units were identified as slow fatigable (SF) and fast fatig ue resistant (FR). (C) 1995 John Wiley and Sons, Inc.