C S Klein1, W Z Rymer2, M A Fisher3. 1. Guangdong Work Injury Rehabilitation Center, Guangzhou 510440, China; Sensory Motor Performance Program, Rehabilitation Institute of Chicago, Chicago, IL 60611, USA. Electronic address: c-klein@northwestern.edu. 2. Sensory Motor Performance Program, Rehabilitation Institute of Chicago, Chicago, IL 60611, USA; Department of Physical Medicine and Rehabilitation, Feinberg School of Medicine, Northwestern University, Chicago, IL, USA. 3. Department of Neurology, Hines VAH, Hines, IL 60141, USA; Loyola University Chicago Medical Center, 2160 S. First Ave., Maywood, IL 60153, USA.
Abstract
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.
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.