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.