Short-latency afferent inhibition determined by the sensory afferent volley

Abstract

afferent inhibition (SAI) is characterized by the suppression of the transcranial magnetic stimulation motor evoked potential (MEP) by the cortical arrival of a somatosensory afferent volley. It remains unknown whether the magnitude of SAI reflects changes in the sensory afferent volley, similar to that observed for somatosensory evoked potentials (SEPs). The present study investigated stimulus-response relationships between sensory nerve action potentials (SNAPs), SAI, and SEPs and their interrelatedness. Experiment 1 (n ϭ 23, age 23 Ϯ 1.5 yr) investigated the stimulus-response profile for SEPs and SAI in the flexor carpi radialis muscle after stimulation of the mixed median nerve at the wrist using ϳ25%, 50%, 75%, and 100% of the maximum SNAP and at 1.2ϫ and 2.4ϫ motor threshold (the latter equated to 100% of the maximum SNAP). Experiment 2 (n ϭ 20, age 23.1 Ϯ 2 yr) probed SEPs and SAI stimulus-response relationships after stimulation of the cutaneous digital nerve at ϳ25%, 50%, 75%, and 100% of the maximum SNAP recorded at the elbow. Results indicate that, for both nerve types, SAI magnitude is dependent on the volume of the sensory afferent volley and ceases to increase once all afferent fibers within the nerve are recruited. Furthermore, for both nerve types, the magnitudes of SAI and SEPs are related such that an increase in excitation within somatosensory cortex is associated with an increase in the magnitude of afferent-induced MEP inhibition.TMS; short-latency afferent inhibition; somatosensory evoked potentials; sensory nerve action potential; recruitment curve; afferent volley NEW & NOTEWORTHY This is the first investigation of the relationship between short-latency afferent inhibition (SAI) and the sensory afferent volley and the first to examine the relationship between SAI and somatosensory evoked potentials (SEPs).The data indicate that 1) SAI increases with the recruitment of sensory fibers and 2) its stimulus-response profile is correlated with SEPs. These novel data provide practical guidelines and also contribute to our understanding of SAI mechanisms.

Attention modulates specific motor cortical circuits recruited by transcranial magnetic stimulation

“…Relatively more is known about the relationship between afferent input and PA SAI than for AP SAI. PA SAI is directly related to the magnitude of the thalamo-cortical somatosensory afferent relay, as indexed by the N20-P25 SEP component (Bailey et al, 2016). No direct assessment of the relationship between somatosensory afferent volley and AP SAI has been conducted however, models of I-wave generation posit a premotor origin (Di Lazzaro et al, 2012; Volz et al, 2014).…”

“…Somatosensory cortex activation (Meehan and Staines, 2007, 2009) and more specifically, N20-P25 amplitude (Meehan et al, 2009), is reduced in the presence of a strong somatosensory input sharing spatial and temporal characteristics during visuomotor transformations. Given the relationship between N20-P25 SEP amplitude and PA SAI established by Bailey et al (2016), it is possible that stronger spatial and temporal competition between vision and somatosensation could lead to changes in somatosensory afferent processing in both the PA and AP sensitive circuits. Nonetheless, the attentional dissociation observed here is consistent with a differential functional role of afferent input to the PA and AP I-wave circuits.…”

“…Tasks with high perceptual loads draw attention resources away from task-irrelevant sensory processing leading to suppression of the irrelevant afference (Lavie, 2005). In the somatosensory system such attention related suppression can occur as early as the N20-P25 thalamo-cortical afferent projections (Staines et al, 2002; Legon and Staines, 2006; Meehan et al, 2009), the same projections shown to underline SAI magnitude (Bailey et al, 2016). Therefore, we hypothesized that an increasingly attention demanding visual detection task would reduce somatosensory afference and thereby reduce SAI elicited by both PA and AP sensitive networks.…”

“…That at least part of the SAI network is driven by somatosensory afference to motor cortex is evidenced by increasing inhibition as the intensity of the peripheral nerve stimulus is increased (Fischer and Orth, 2011). Further, the positive relationship between the N20-P25 somatosensory evoked potential (SEP) and magnitude of SAI suggests that at least part of the network mediating SAI is dependent upon thalamo-cortical projections to somatosensory cortex (Bailey et al, 2016). Although there is a strong relationship between somatosensory afference and SAI, SAI is a malleable phenomenon modulated by movement timing and the relevance of a muscle to voluntary movement (Voller et al, 2006; Asmussen et al, 2013; Asmussen et al, 2014).…”

Role of cutaneous and proprioceptive inputs in sensorimotor integration and plasticity occurring in the facial primary motor cortex

“…Differences in the responsible afferent input would complement other known differences in the mechanism of SAI and LAI (Chen et al 1999;Sailer et al 2002Sailer et al , 2003Paulus et al 2008;Bailey et al 2016, Turco et al 2017. SAI involves GABA A and cholinergic systems Paulus et al 2008), while GABA B pathways may mediate LAI (Sailer et al 2002(Sailer et al , 2003Paulus et al 2008).…”