Neuromodulation by combined sensory and motor stimulation in the peripheral nerve: tendon organ afferent activity.

OBJECTIVE: Neuromuscular stimulation is a therapeutic approach to treat impairments such as stroke or pain, although the resulting inputs to the nervous system due to the stimulation are not well quantified. Stimulation activates both afferents and efferents, and the resulting neural activity is a mix of the effects of both: the changes in afferent activity due to efferent physiological actions plus the alterations due to afferent stimulation. This study quantitatively describes the resulting Golgi tendon organ Ib afferent activity in response to mixed afferent and efferent stimulation.

APPROACH: Neural and stimulated action potentials interact by means of collision, neural resetting, and refractory block. We simulated the action potential patterns of Ib afferents in the human first dorsal interosseous during constant voluntary, stimulated, and combined contractions, varying both stimulation rate and location, and both with and without simultaneous Ib stimulation.

MAIN RESULTS: Efferent stimulation affects the firing rate versus force relationships of individual tendon organs, but the effects on the population mean are very small. In contrast, afferent stimulation changes the firing rate versus force relationship of individual afferents by increasing the firing rate, decreasing the force resolution, broadening the distribution of action potential rates, and temporally phase locking a portion of the action potentials. Force resolution is retained in the population mean. The effects of afferent stimulation change with the stimulation rate and location, and with receptor firing rates.

SIGNIFICANCE: This is the first quantitative description of the changes in afferent feedback during combined efferent and afferent nerve stimulation. The small effects of efferent stimulation on the population response implies that tendon organs could provide accurate force feedback during stimulated contractions. The effects of afferent stimulation can be altered by choice of stimulus rate and site and are generalizable to other afferents.