DIFFERENCES IN EXCITABILITY PROPERTIES BETWEEN MEDIAL GASTROCNEMIUS , TIBIALIS ANTERIOR , AND ABDUCTOR POLLICIS BREVIS MOTOR AXONS

Introduction: Excitability properties of motor nerves to different muscles are different, but the explanation is uncertain. We characterized motor axon excitability properties to the medial gastrocnemius (MG) in 27 adults, and made comparisons with the peroneal nerve to the tibialis anterior (TA) and median nerve to the abductor pollicis brevis (APB) in 10 subjects. Methods: Recordings of multiple excitability properties were made using threshold tracking, stimulating the nerves at the wrist or knee. Results: Threshold electrotonus and superexcitability differed between nerves (APB>MG>TA axons) that may reflect differences in fast K conductance. APB axons had larger S2 accommodation and undershoot than TA and MG axons, indicating greater slow K conductance. TA axons demonstrated greater accommodation during hyperpolarizing currents than MG and APB axons, suggestive of greater inwardly rectifying current. Discussion: Inherent differences in several conductances underlie nerve differences in excitability, presumably related to muscle or motoneuron properties. Muscle Nerve 57: E60–E69, 2018 In motor axons, excitability is modulated by passive and active properties of the nodes and internodes. Motor axon excitability properties, including the resting membrane potential and ion channel conductances, can be estimated in vivo using nerve excitability testing. Investigators found significant differences in excitability properties between sensory and motor axons, between proximal and distal axons of the same nerve, between different motor nerves, and even between motor axons in the same nerve innervating different muscles. The explanation for these differences remains uncertain. There is evidence that peroneal motor axons (stimulated at the fibular head) to the tibialis anterior (TA) are depolarized relative to tibial motor axons (stimulated at the ankle) to the abductor hallucis (AH); TA axons had a lower rheobase, “fanning in” of threshold electrotonus, longer relative refractory period (RRP), and a smaller superexcitability. The authors proposed that the differences may reflect a length-dependent gradient in excitability. However, differences between motor axons to the TA and AH may also be related to the muscle (i.e., a muscle-dependent effect). To more rigorously assess whether excitability differences are muscle-dependent, the peroneal and tibial nerves should be stimulated at a similar site (i.e., knee). Tibial nerve excitability properties have been studied at the ankle and knee, but none have made a comprehensive study of axon properties to the human gastrocnemius. There may be inherent differences in excitability properties between motor axons to the gastrocnemius and TA due to differences in motor unit activity. The prospect of muscle-dependent differences in motor axon excitability is clinically relevant because they may make a nerve more or less susceptible to dysfunction. Excitability testing may also provide novel insights into motor impairment. Motor disorders often lead to changes in axon properties, and this plasticity may vary between different nerves. For example, changes in motor axon properties following a cervical spinal cord injury are greater in the peroneal compared to the median nerve. In contrast, aging and motor disorders often lead to pronounced atrophy and contractile property changes in the gastrocnemius but preservation of the TA. There is also evidence that old age causes larger changes in motor unit properties in lower than upper limb muscles. Although the mechanisms underlying these apparent musclespecific or limb-specific changes are uncertain, clues about the underlying processes may be provided by studying motor axon properties in different nerves. The purpose of this study was to determine the excitability properties of three different nerves to functionally distinct muscles. MATERIALS AND METHODS Subjects. Tibial nerve excitability properties nerve were recorded in 17 men (22–31 years; mean 25.2 6 0.7 years) and 10 women (20–27 years; mean 22.5 6 0.6 years) Additional supporting information may be found in the online version of this article. Abbreviations: AH, abductor hallucis; APB, abductor pollicis brevis; CMAP, compound muscle action potential; EDB, extensor digitorum brevis; EMG, electromyography; I/V slope, current-threshold slope; MG, medial gastrocnemius; MVC, maximal voluntary contraction; RRP, relative refractory period; SDTC, strength-duration time constant; TA, tibialis anterior