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Arthur W. English, Dario I. Carrasco, and Charles G. Widmer

Intramuscular microstimulation was used to evaluate the mechanical properties of 6 different neuromuscular compartments of four adult rabbit masseter muscles. Single compartment stimulation was verified by simultaneous multichannel electromyo-graphic recordings. A multi-axis force-moment sensor was used to measure evoked reaction forces at the mandibular symphysis, and these records were used to determine the magnitude and directions of the torques produced about the ipsilateral temoromandibular joint. The magnitude of the resultant torque vector differed between compartments. The most superficial compartments of the superficial masseter (MSSla, MSSlp) produced the largest torques, and compartments of the superficial masseter lying more caudal, such as MSS3, produced the smallest torques. The rise time of this vector was shortest for the posterior deep masseter (MPPo) compartment and longest for compartment MSSla. The directions of the torque vectors of different compartments varied considerably. Torque vectors of deep compartments such as MPPo, and the pars reflexa of the superficial masseter (MSPR) produced significant yaw torques toward the working masseter but very small lingual roll torques. Torque vectors from superficial compartments such as MSSI contain larger lingual roll components and smaller working yaw components. These observations are consistent with the hypothesis that neuromuscular compartments of the rabbit masseter muscle form output elements in the neural control of masticatory movements.

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Dario I. Carrasco, John Lawrence III, and Arthur W. English

The primary purpose of this study was to establish whether the neuromuscular compartments of cat lateral gastrocnemius produce different mechanical actions on the skeletal system, by determining the contributions made by these compartments to the torques produced about the ankle joint. It was postulated that neuromuscular compartments might represent output elements of the spinal circuits. If so, they should produce unique mechanical actions. Isometric torques about the center of the ankle joint produced by the neuromuscular compartments of the cat lateral gastrocnemius were measured with a multiaxis force-moment sensor connected to the plantar surface of the foot. Individual compartment torques were elicited by activation of the primary compartment branches of the lateral gastrocnemius nerve. The magnitude of the individual torque components, and thus of the resultant torque, was significantly different between compartments. In three of the four lateral gastrocnemius compartments, significantly different torque trajectories were noted. The results, together with those from previous studies demonstrating that compartments can be activated in a task-dependent manner, suggest that neuromuscular compartments represent anatomical substrates that can be used by the nervous system for regulating movement.

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Alan J. Sokoloff, Timothy C. Cope, T. Richard Nichols, and Arthur W. English

Differences in the directions of torque produced by motor units might be used by the nervous system to coordinate posture and movement. Here we report plantar flexion and abduction isometric torques exerted at the ankle by 158 motor units in the cat medial gastrocnemius (MG) muscle. In five cats, motor unit torque direction differed by an average of 10°. Torque direction was weakly correlated with unit contraction time in 3 of 5 experiments, with tetanic force in 3 of 5 experiments, and with conduction velocity in 1 of 5 experiments. The direction of whole muscle torque, however, was constant at all levels of MG activation produced in sural and crossed extension reflexes. Thus, although there is a range in the direction of torque produced by motor units in the cat MG, we find no evidence for the ordering of motor units according to torque direction during MG activation.