Plasticity of human motoneurone properties

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Motor control research using animal models has helped establish relations between properties of the motoneurone and the muscle fibers that it innervates.  In humans, Matthews (1996) developed a model for estimating one of the motoneurone properties (afterhyperpolarization (AHP) time-course).  This model represents one of the only windows to view the properties of motoneurones in humans.  The time-course of the AHP influences the capacity of the motoneurone to discharge action potentials and, thereby, the force produced by the muscle.  The adaptability of the AHP has not been thoroughly investigated in animal models and is virtually unexplored in humans.  The proposed research will determine the capacity of synaptic input to modify the time-course of the motoneurone AHP in humans.  We will study three synaptic inputs in separate experiments: cutaneous (skin) input, muscle spindle input, and input from group III-IV afferents activated by muscle fatigue.  Low-force contractions with the ankle dorsiflexor muscles (tibialis anterior) will be held for 30 min: 10 min before, during, and after the addition of synaptic input, with the motoneurone AHP being calculated in each phase from a train of motor unit potentials collected with a fine wire electrode.  Either a non- painful cutaneous stimulus will be applied to the skin overlying tibialis anterior, or vibration will be applied to the tibialis anterior tendon, or the muscle will be fatigued by performing a low-force contraction under ischemic conditions.  Healthy adult volunteers and volunteers with unilateral motor paresis after stroke will participate in the study.  This research program will advance our understanding of human motoneurone properties when they are activated voluntarily with both descending commands and sensory inputs.  This research will have far-reaching implications for motor control mechanisms that have not been investigated to date, primarily because the corresponding animal experiments do not involve voluntary contractions.  The acquired knowledge will contribute to our understanding of motor control disorders and whether sensory inputs may be useful in modifying motoneurone properties after a neurological insult, such as a stroke.