Threshold tracking as a tool to study activity-dependent axonal plasticity

Moldovan, Mihai

doi:10.1016/j.clinph.2020.03.005

Cited by 1 publication

(1 citation statement)

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“…Thus, activation capacity, and presumably residual daily axon impulse traffic, may not be primary determinants of bilateral axon plasticity revealed here. The correlation of fanning out of threshold electrotonus and FIM may indicate that some of the post-stroke axon plasticity is related more to deficits in bilateral central nervous system processing (i.e., reduced ability to use compensatory strategies to complete certain FIM tasks) rather than deficits in maximal activation ( 7 , 57 , 58 ). Interestingly, fanning out in APB axons was associated with disability after severe acute cerebellar stroke ( 32 ) but not after cortical or subcortical stroke ( 33 ).…”

Section: Discussionmentioning

confidence: 99%

Altered flexor carpi radialis motor axon excitability properties after cerebrovascular stroke

Klein

Zhao

et al. 2023

Front. Neurol.

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BackgroundSpinal motoneurons may become hyperexcitable after a stroke. Knowledge about motoneuron hyperexcitability remains clinically important as it may contribute to a number of phenomena including spasticity, flexion synergies, and abnormal limb postures. Hyperexcitability seems to occur more often in muscles that flex the wrist and fingers (forearm flexors) compared to other upper limb muscles. The cause of hyperexcitability remains uncertain but may involve plastic changes in motoneurons and their axons.AimTo characterize intrinsic membrane properties of flexor carpi radialis (FCR) motor axons after stroke using nerve excitability testing.MethodsNerve excitability testing using threshold tracking techniques was applied to characterize FCR motor axon properties in persons who suffered a first-time unilateral cortical/subcortical stroke 23 to 308 days earlier. The median nerve was stimulated at the elbow bilaterally in 16 male stroke subjects (51.4 ± 2.9 y) with compound muscle action potentials recorded from the FCR. Nineteen age-matched males (52.7 ± 2.4 y) were also tested to serve as controls.ResultsAxon parameters after stroke were consistent with bilateral hyperpolarization of the resting potential. Nonparetic and paretic side axons were modeled by a 2.6-fold increase in pump currents (IPumpNI) together with an increase (38%–33%) in internodal leak conductance (GLkI) and a decrease (23%–29%) in internodal H conductance (Ih) relative to control axons. A decrease (14%) in Na+ channel inactivation rate (Aah) was also needed to fit the paretic axon recovery cycle. “Fanning out” of threshold electrotonus and the resting I/V slope (stroke limbs combined) correlated with blood potassium [K+] (R = −0.61 to 0.62, p< 0.01) and disability (R = −0.58 to 0.55, p < 0.05), but not with spasticity, grip strength, or maximal FCR activity.ConclusionIn contrast to our expectations, FCR axons were not hyperexcitable after stroke. Rather, FCR axons were found to be hyperpolarized bilaterally post stroke, and this was associated with disability and [K+]. Reduced FCR axon excitability may represent a kind of bilateral trans-synaptic homeostatic mechanism that acts to minimize motoneuron hyperexcitability.

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