Sensory nerve conduction velocity is inversely related to axonal length

It was found that the axonal length was inversely related to motor conduction velocity (CV). However, it is not clear that sensory CV is inversely related to axonal length. The nerve lengths of the median sensory fascicles from the C6 and C7 intervertebral foramen to the digital branches of the thumb and middle finger were compared in ten cadavers. Sixty healthy subjects (24 men, 36 women; mean age 35, range 24-54 years) had median sensory CV testing. The median sensory nerve action potentials were obtained antidromically in the thumb and middle finger with wrist and elbow. The CVs across the forearm for the thumb and the middle finger fascicles were then calculated. It was found that the nerve length of C7 was longer than C6 with a difference of 3.6 +/- 0.6 cm. The mean forearm CV for the median sensory axons innervating the middle finger (60.0 +/- 3.9 m/s) was slower than the CV for the median sensory axons innervating the thumb (61.4 +/- 4.1 m/s,p = 0.0012). These results demonstrate that sensory CV is slowed by 3.9 m/s per 10 cm of axon length. This study confirms that the inverse relation of CV and axonal length reported in motor axons also applies to the sensory nerves.