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Peripheral and central delays in the cortical projections from human truncal muscles. Rapid central transmission of proprioceptive input from the hand but not the trunk.

In contrast to the cortical connections to and from the muscles of the hand, the transmission of an afferent volley from the intercostal muscles to the cerebral cortex takes approximately 10 ms longer than it takes a cortical motor volley to reach the muscle. This disparity in afferent and efferent cortical transmission times could be due to a slower peripheral conduction velocity of intercostal muscle afferents or a slower afferent conduction within the central nervous system. The present study derived peripheral and central conduction times for the truncal muscles from the onsets of the mechanically evoked intercostal and abdominal spinal reflexes and the onsets of the cortical sensory potentials. Mean latencies of the ipsilateral intercostal and abdominal reflexes (evoked and recorded in the mid-clavicular line) were 11.9 +/- 0.7 (SEM) ms and 13.7 +/- 0.9 ms, respectively; calculated peripheral conduction velocities were 69.4 +/- 4.1 m/s and 56.2 +/- 2.3 m/s (assuming equal velocities for the sensory and motor axons and an intraspinal delay of 1 ms). Central sensory conduction time (spinal cord to cortex) was calculated by subtracting the peripheral conduction times for the intercostal and abdominal afferents (5.5 +/- 0.3 ms and 6.4 +/- 0.4 ms) from the onsets of the cortical sensory potentials (19.4 +/- 0.8 ms and 25.3 +/- 12.3 ms); central sensory conduction times (14.2 +/- 1.7 ms and 18.6 +/- 2.3 ms) were 8-11 ms longer than central motor conduction times. These results demonstrate that peripheral conduction velocities of intercostal and abdominal afferents are not slow, and that, when compared with the extremities, there is a relatively long central conduction time for proprioceptive information from the trunk to the cerebral cortex.