Motor-Unit Architecture in Long Parallel-Fibered Muscles

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DESCRIPTION (provided by applicant): The long-term goal of this research is to understand the architecture and behavior of motor units in different human muscles. The motor unit is the basic functional unit of the neuromuscular system. The way muscle fibers are organized into motor units and the way motor unit activity is coordinated are important determinants of a muscle's ability to produce force and movement. The focus of this proposal is to investigate motor-unit organization and coordination in long, parallel-fibered human muscles. The fibers in these muscles are often assumed to span the entire length of the muscle. However, preliminary work supports the idea that some muscles, including brachioradialis, may instead have a complex architecture, consisting of arrays of shorter fibers arranged in series over a scaffolding of longer fibers. Some of the longer fibers receive innervation from more than one motoneuron at widely separated endplate zones. The way in which in-series fibers are organized into motor units, and the way these motor units are coordinated to transfer force effectively to the tendon are not known. The proposed study will investigate these issues through the following specific aims: (1) determining the innervation pattern in brachioradialis; (2) determining how motor units in brachioradialis are organized; (3) elucidating the strategy used to coordinate motor units at different proximodistal levels in brachioradialis; and (4) determining whether polyneuronally innervated fibers exist in other long, parallel-fibered muscles, including brachial biceps, sartorius, gracilis, and latissimus dorsi. A novel electrophysiological approach will be used. Electromyographic signals will be recorded during voluntary contractions using multiple fine-wire and needle electrodes. The signals will be decomposed to identify individual motor-unit action potentials. Motor-unit architectural properties, including endplate locations and fiber lengths, will be estimated by analyzing the action-potential waveforms, Motor-unit control properties, including recruitment and synchronization, and the existence of polyneuronally innervated fibers will be determined by analyzing the motor-unit discharge patterns. The proposed work will contribute knowledge about the structure and function of particular human muscles which will be directly relevant to several clinical applications, including tendon-transfer and reconstructive surgery, functional electrical stimulation, neurological and kinesiological electromyography, and therapeutic exercise.