Sensorimotor integration to cutaneous afferents in humans: the effect of the size of the receptive field

Abstract

Transcranial magnetic stimulation (TMS) can be used to study sensorimotor integration in humans non-invasively. Motor excitability has been found to be inhibited when afferent stimuli are given to a peripheral nerve and precede TMS at interstimulus intervals (ISIs) of 20-50 ms. This phenomenon has been referred to as short-latency afferent inhibition (SAI). To better understand the functional meaning of these phenomena, we examined the effect of the size of the receptive field on SAI to cutaneous afferents in upper-limb sensorimotor areas in humans. We examined the effect of the stimulation of the isolated right first (D1), second (D2) and third finger (D3), the right second and third finger together (D23) and the right first three fingers together (D123) on the amplitude of motor evoked potentials (MEPs) to TMS in hand and forearm muscles. We examined the right abductor pollicis brevis (APB), first dorsal interosseous (FDI), extensor carpi radialis (ECR) and flexor carpi radialis (FCR) muscles. Digital stimulation preceded TMS at ISIs of 20-50 ms. The effect of D2 stimulation was MEP inhibition (SAI), which was more marked and consistent in APB and FDI muscles than in ECR and FCR muscles. Similarly, D1 and D3 stimulation caused MEP reduction, while no MEP enhancement could be found to single finger stimulation. In contrast, D123 stimulation induced less effective SAI in upper-limb muscles. MEP potentiation was recorded in some muscles to D123 stimulation. A significant difference between D2 and D123 stimulation was found in APB (ISIs = 30-50 ms) and FDI (ISIs = 40-50 ms) muscles, but not in forearm muscles. The effect to D23stimulation on MEP amplitude was intermediate between those to D2 and D123 stimulation. Our data suggest that motor excitability to cutaneous afferents may be influenced by the size of the receptive fields, this effect being the result of increasing convergence between hand afferents in the somatosensory system. These phenomena appear to be topographically arranged across the representation of upper-limb muscles. These findings may help to understand the functional significance of SAI in normal physiology and pathophysiology.

Sensorimotor integration is abnormal in asymptomatic Parkin mutation carriers

“…Regarding potential genetic differences, these are likely to be negligible since Family LA belongs to the German-speaking minority in South Tyrol and is of German descent. Furthermore, inspection of dSAI measurements of healthy subjects in previous studies in Northern Italy, 23 England, 15 and Germany 24,25 where the same dSAI protocol was used revealed that the level of dSAI was comparable to our control group. In particular, at an ISI of 25 msec mean dSAI in previous studies was 61% which is nearly identical to results in the present study (59%).…”

Surround inhibition in the motor system

“…Surround inhibition can also be demonstrated in a sensorimotor interaction. For example, the inhibitory effect of second finger stimulation on a motor evoked potential (MEP) was reduced by stimulating the first and third fingers at the same time as the second (Tamburin et al 2005). …”

Change in EMG with skin friction at different frequencies during elbow flexion

“…Aimonetti et al (2000) observed that Ia presynaptic inhibition of the wrist extensors with mechanical cutaneous stimulation modulates activation of motoneurons and in previous studies examining the H-reflex, brushing stimulation was shown to produce inhibition of alpha-MNs (Mason 1985;Wood et al 1998). It has also been suggested that muscle activation may be modulated based upon the size of the cutaneous receptive field (Tamburin et al 2005). All of the studies above point toward possible clinical benefits from cutaneous stimulation, therefore, we feel the effect of skin friction deserves further investigation.…”