Distortions in the synaptic organization of motor commands to proximal and distal muscles following hemiparetic stroke
Physiology, Physical Medicine and Rehabilitation, Physical Therapy and Human Movement Sciences, Northwestern University, Chicago USA
Motor unit firing patterns are highly sensitive indicators of the motor commands that produce them. Recent advances in electrode technology allow recording of up to 30 motor units simultaneously from a flexible electrode array place on the surface of human muscles(1). Our systematic studies using these arrays to assess motor unit firing patterns in both proximal and distal muscles of the arm in normal subjects have revealed that motor commands to proximal muscles have much stronger input from the reticulospinal system than do distal muscles. This difference is especially striking for the component of the reticulospinal system that utilizes serotonin as a neurotransmitter and constitutes the primary controller of the intrinsic excitability of motoneurons releases serotonin onto spinal motoneurons. Our studies in subjects with hemiparetic stroke however suggest that excessive reliance on the reticulospinal system to generate motor commands results in distorted motor unit firing patterns that lack rate modulation. We interpret these data as revealing inappropriate coupling of excitation and inhibition that lacks the normal reciprocal pattern. We will discuss the possibilities for pharmacological interventions to ameliorate these deficits.
1. Johnson MD, Thompson CK, Tysseling VM, Powers RK, Heckman CJ. The potential for understanding the synaptic organization of human motor commands via the firing patterns of motoneurons. J Neurophysiol. 2017;118(1):520-31.