It was the aim of the present study to isolate and identify the components underlying the human sensory compound action potential and to study their axonal conduction velocities and refractory periods. For this purpose the technique of percutaneous microneurography was combined with intradermal electrical stimulation of nerve fibre terminals. Sixty-four median and ulnar nerve afferents innervating the glabrous skin of the digits were isolated and type identified. 2. The range of axonal conduction velocities was wide (20-60 m/s), but similar for each afferent category (20-60 m/s). Most afferents conducted slower than expected from the intrafascicularly recorded compound potential (50-60 m/s) and their conduction velocities generally decreased from the base to the tip of the digits. 3. The duration of the absolute axonal refractory periods of all types of afferents ranged from 0.7 to 3.5 ms. The duration of the total refractory periods ranged from 3 to 9 ms. Both absolute and total axonal refractory periods were inversely correlated (r = -0.70 and r = -0.67) with their axonal conduction velocities. 4. The size of individual action potentials was significantly correlated with axonal conduction velocities, although the correlation coefficient was relatively low (r = 0.43), even after correction for variability due to electrode resistance (partial correlation r = 0.44). 5. The results showed that different types of cutaneous afferents cannot be separated on the basis of their axonal conduction properties. The data demonstrate features of neural impulse conduction along the entire axonal tree and which are inaccessible to routine electrodiagnostic procedures. The present approach provides a sensitive means for assessing, in health and disease, nerve conduction in terminal axons.
