De Luca CJ, Gonzalez-Cueto JA, Bonato P, Adam A. Motor unit recruitment and proprioceptive feedback decrease the common drive. J Neurophysiol 101: 1620 -1628, 2009. First published June 18, 2008 doi:10.1152/jn.90245.2008. It has been documented that concurrently active motor units fire under the control of a common drive. That is, the firing rates show high correlation with near-zero time lag. This degree of correlation has been found to vary among muscles and among contractions performed at different force levels in the same muscle. This study provides an explanation indicating that motor units recruited during a contraction cause an increase in the variation (SD) and a decrease in the degree (amplitude) of the correlation of the firing rates. The degree of correlation is lower in muscles having greater spindle density. This effect appears to be mediated by the proprioceptive feedback from the spindles and possibly the Golgi tendon organs. Muscle spindles in particular respond to the mechanical excitation of the nonfused muscle fibers and provide a discordant excitation to the homonymous motoneurons, resulting in a decrease in the correlation of the firing rates of motor units. The implication of this work is that the decreased correlation of the firing rates in some muscles is not necessarily an indication of a decreased common drive from the CNS, but rather an inhibitory influence of the proprioceptive feedback from the peripheral nervous system. This explanation is useful for understanding various manifestations of the common drive reported in the literature.

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A conduction velocity distribution (CVD) estimator that incorporates volume conductor modeling of the nerve-evoked response is introduced in this paper. The CVD estimates are obtained from two compound nerve action potentials (CNAP) recorded at the skin surface. A third channel is introduced in order to assess the estimator performance in the experimental case. The relevance of using an accurate signal model is shown by comparing the performance of the proposed estimator with a previous approach based on a different CNAP model. The performance of the proposed estimator is evaluated for simulated and experimental data. The study assesses signal-to-noise ratio immunity and sensitivity to errors in the model parameters.

This study examines the ability to selectively activate different nerve fibers in a finger by using different sinusoidal stimulation frequencies. Specifically, the stimulation of A-beta, A-delta, and C-fibers is looked into, and responses from each of three different stimuli (5 Hz, 250 Hz, and 2000 Hz) are compared. Action potential (AP) responses from the different nerve fibers are simulated. Activation thresholds are determined for each fiber type. The resulting firing frequencies are compared with thresholds found in the literature to determine the stimulating signal amplitude at which sensations begin to be perceived for each stimulation frequency. Results indicate that while selective stimulation of C-fibers and A-beta fibers appears to be possible with 5 Hz and 2000 Hz, respectively, selective stimulation of A-delta fibers may not be possible due to them requiring a higher stimulating signal amplitude to cause the nerve to reach the physiological threshold than A-beta fibers for 250 Hz. Thus, selective stimulation of the three types of nerve fibers may not be possible for all three examined frequencies.