Strength-duration properties and their voltage dependence as measures of athreshold conductance at the node of ranvier of single motor axons

In a number of clinical studies, measurement of axonal strength-duration pr operties has been used to provide indirect insight into conductances at the node of Ranvier, particularly persistent Na+ conductance. However, the spe cificity of any changes is limited because other factors can affect strengt h-duration behavior. The present study was undertaken to define the relatio nship between different strength-duration measures at rest and at different membrane potentials, and also to determine the limits within which strengt h-duration behavior can be used as a measure of nodal conductances. The str ength-duration time constant (tau (SD)) and rheobase of 20 single motor uni ts in the flexor carpi ulnaris were calculated from thresholds defined usin g threshold tracking.'True" rheobase and rheobasic latencies were measured using test stimuli of 100 -ms duration. For ten units, the technique of lat ent addition was used to measure threshold changes directly attributable to nodal conductances, and for six units these were compared with strength-du ration properties at different membrane potentials. The data indicate that measurements of tau (SD) and rheobase can provide sensitive indicators of c onductances present at the node of Ranvier when membrane potential changes. There is a reciprocal relationship between tau (SD) and rheobase for singl e motor units at different membrane potentials, and this relationship may a llow changes in tau (SD) due to depolarization and demyelination to be diff erentiated. (C) 2000 John Wiley & Sons, Inc.