Comparing excitability at 37°C versus at 20°C: Differences between motor and sensory axons.

INTRODUCTION

In some peripheral nervous system disorders, cold induces symptoms of muscle weakness without loss of sensation. To understand this selective effect on motor function, it is first essential to delineate the effects of cooling in motor and sensory axons of healthy subjects.

METHODS

In 17 healthy volunteers, we performed excitability and clinical tests of median nerve motor and sensory axons at 37°C and at 20°C. Clinical tests consisted of assessing thenar muscle strength, 2-point discrimination, and joint position sense of the third finger.

RESULTS

Excitability tests showed that cooling induced opposite changes to hyperpolarizing current in threshold electrotonus (motor, decreased threshold change; sensory, increased threshold change) and current-voltage relation slopes (motor, steepening; sensory, less steep). Clinical tests showed worsening in motor function but no consistent changes in sensory function.

DISCUSSION

Cooling induces changes in motor axons consistent with depolarization and more complicated changes in sensory axons, possibly related to differences in hyperpolarization-activated cyclic nucleotide-gated channel expression. Muscle Nerve 57: 574-580, 2018.