Altered nerve excitability properties after stroke are potentially associated with reduced neuromuscular activation

Objective To determine limb differences in motor axon excitability properties in stroke survivors and their relation to maximal electromyographic (EMG) activity. Methods The median nerve was stimulated to record compound muscle action potentials (CMAP) from the abductor pollicis brevis (APB) in 28 stroke subjects (57.3 +/- 7.5 y) and 24 controls (56.7 +/- 9.3 y). Results Paretic limb axons differed significantly from non-paretic limb axons including (1) smaller superexcitability and subexcitability, (2) higher threshold during subthreshold depolarizing currents, (3) greater accommodation (S3) to hyperpolarization, and (4) a larger stimulus-response slope. There were smaller differences between the paretic and control limbs. Responses in the paretic limb were reproduced in a model by a 5.6 mV hyperpolarizing shift in the activation voltage of Ih (the current activated by hyperpolarization), together with an 11.8% decrease in nodal Na+ conductance or a 0.9 mV depolarizing shift in the Na+ activation voltage. Subjects with larger deficits in APB maximal voluntary EMG had larger limb differences in excitability properties. Conclusions Stroke leads to altered modulation of Ih and altered Na+ channel properties that may be partially attributed to a reduction in neuromuscular activation. Significance Plastic changes occur in the axon node and internode that likely influence axon excitability.