Principal Investigator: Kevin C. McGill, PhD Project Staff: Zoia C. Lateva, PhD and M. Elise Johanson, MS, PT Project Category: Spinal Cord Injury - 2004 Objective: 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 units are coordinated are important determinants of a muscle's ability to produce force and movement. The focus of this project is to investigate motor-unit organization and coordination in long, parallel-fibered human muscles. Research Plan: We will investigate these issues by: (1) determining the innervation pattern in the brachioradialis muscle; (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. The protocol involves recording electromyographic signals during voluntary contractions using multiple fine-wire and needle electrodes. The signals are decomposed to identify individual motor-unit action potentials. Motor-unit architectural properties, including endplate locations and fiber lengths, are estimated by analyzing the potential waveforms. Motor-unit coordination properties, including recruitment and synchronization, and the existence of polyneuronally innervated fibers are determined by analyzing the motor-unit discharge patterns. Work Accomplished: We have studied the architectural organization of the brachioradialis muscle in six normal subjects. This forearm muscle is a common candidate for tendon transfer in individuals with spinal cord injuries at the C5-C6 levels. It is a long, parallel-fibered muscle that is widely assumed to have a simple architecture in which all the fibers extend from origin to insertion and receive innervation in a single, centrally located endplate zone. However, our study suggests that in many, although not all, normal subjects, brachioradialis actually consists of arrays of shorter fibers connected in series and has multiple innervation zones. Moreover, some of the muscle fibers are innervated by two different motoneurons at widely separated endplates. Expected Outcome: We believe that other long, parallel-fibered human muscles may also have series-fibered architectures, including several muscles of the thigh and back. A better understanding of the architecture of these muscles is important in tendon transfer and reconstructive surgeries, both for predicting biomechanical outcomes as well as for understanding issues related to the survivability of muscle fibers and intramuscular nerve branches at different proximodistal levels. The existence of doubly innervated fibers also raises a number of important questions about the development, role, and control of these fibers. Funding Source: NIH Funding Status: Active |
