Conduction velocity in motor, cutaneous afferent, and muscle afferent fibers within the same mixed nerve

Mammalian axons subserving different functions have different conduction velocities (CV); motor fibers conduct more slowly than cutaneous fibers, which conduct slower than muscle afferents. However, human studies have yielded conflicting results. We studied isolated fiber populations in human sciatic nerve to examine further this question. Motor studies were performed in standard fashion, stimulating at gluteal fold (GF) and popliteal fossa (PF) and recording soleus. In addition, conduction velocity of a pure motor nerve volley was calculated for 3 subjects. Stimulating and recording electrodes were needles placed close to the nerve. Cutaneous afferents were studied by stimulating the sural nerve at the ankle and recording at PF and GF. Muscle afferent velocity was assessed by comparing soleus H reflex latency with stimulation at PF and GF. Results in 10 subjects showed muscle afferent CV of 57.6, cutaneous afferent CV of 55.1, motor CV of 52.4, and mixed nerve CV of 56.3 m/s. Although statistically significant, these differences are much smaller than in animal studies. These results have implications for understanding what fibers contribute to spinal reflexes.