Induction of plasticity in the dominant and non-dominant motor cortices of humans.

There are clear hemispheric differences in the human motor system. Studies using magnetic resonance morphometry have shown that representation of hand muscles is larger in the dominant hemisphere than the non-dominant hemisphere. There is some limited evidence of electrophysiological differences between hemispheres. For example, it has been reported recently that there is less intracortical inhibition in the dominant hemisphere than the non-dominant hemisphere, and it has been hypothesised that this reduction in inhibition may facilitate use-dependent plasticity in the dominant motor cortex. In the present study we examined this hypothesis in human subjects by examining plasticity induction in both dominant and non-dominant hemispheres using an experimental paradigm known to induce motor cortical plasticity, namely paired associative stimulation (PAS). Additionally, we investigated changes in dominant and non-dominant hand performance on a simple ballistic training task. Short-interval intracortical inhibition (SICI) was also measured for both dominant and non-dominant hands at a range of conditioning intensities. There was significantly less SICI in the dominant motor cortical hand area than in the non-dominant hand area. PAS induced a significant, and similar, increase in motor cortical excitability in both the dominant and non-dominant hemispheres. Motor training resulted in significant performance improvement in both dominant and non-dominant hands. However, there was significantly more improvement in the non-dominant hand. The results from these studies provide some further evidence of electrophysiological differences between the motor cortices of the two hemispheres. Additionally, these findings offer no support for the hypothesis that the dominant hemisphere is positioned more favourably, due to decreased inhibitory tone, than the non-dominant hemisphere for use-dependent plasticity.

Modulation of intracortical excitability in human hand motor areas. The effect of cutaneous stimulation and its topographical arrangement.

Changes in afferent input can alter the excitability of intracortical inhibitory systems. For example, using paired transcranial magnetic stimulation (TMS), both electrical digital stimulation and muscle vibration have been shown to reduce short-interval intracortical inhibition (SICI). The effects following muscle vibration are confined to the corticospinal projection to the vibrated muscles. The results following digital stimulation are less clear and the relative timing of the cutaneous stimulation and TMS is critical. Here we investigated further whether changes in SICI following digit stimulation exhibit topographic specificity. Eleven normal subjects were investigated (age 28.2+/-7.5 years, mean+/-SD). Electromyographic recordings were made from the right first dorsal interosseous (FDI), abductor digiti minimi (ADM) and abductor pollicis brevis (APB) muscles. SICI was measured, with and without preceding electrical digit II or digit V cutaneous stimulation. The interval between the digital nerve stimulus and test magnetic stimulus was independently set for each subject and established by subtracting the onset latency of the motor evoked potential (MEP) from the latency of the E2 component of the cutaneomuscular reflex. Therefore, measures of intracortical excitability were made at a time at which it is known that cutaneous input is capable of modulating cortical excitability. Single digital nerve stimuli applied to digit II significantly reduced SICI in FDI but not in ADM. Single digital nerve stimuli applied to digit V significantly reduced SICI in ADM but not in FDI or APB. There was a more generalised effect on intracortical facilitation (ICF) with both digit II and digit V stimulation significantly increasing ICF in FDI and ADM. Digital stimulation (either DII or DV) did not significantly affect SICI/ICF in APB. These findings show that appropriately timed cutaneous stimuli are capable of modulating SICI in a topographically specific manner. We suggest that the selective decrease in SICI seen with cutaneous stimulation may be important for focusing of muscle activation during motor tasks.

Stretch reflexes in the human masticatory muscles: a brief review and a new functional role.

Stretch reflexes play a vital role in fine-tuning movements and in automatically maintaining posture. This article briefly reviews the operation of the stretch reflex in the human masticatory system. The conventional approach of stretching muscles in an open-loop manner has yielded much valuable information on the operation of this reflex. In particular, it has revealed that stretching the jaw-closing muscles evokes a reflex response with two major components. The short-latency reflex is favoured when stretches are brisk, but slower stretches evoke an additional long-latency component. In the hand muscles, the long-latency response is transcortical: in the masticatory muscles, it is not. In addition to its role in servo-control of muscle length during chewing, the stretch reflex in the jaw-closing muscles maintains the vertical position of the mandible during vigorous head movements such as those that occur during running, jumping, hopping and other vigorous whole-body movements in which the head moves briskly up and down. This is an interesting model system in which to investigate stretch reflexes with natural stimuli under unrestrained, physiological conditions.

An apparatus for controlled stretch of human jaw-closing muscles.

The design of a special-purpose muscle stretcher for reflex studies of the human jaw-closing muscles is described. The device is based on a servo-controlled electromagnetic vibrator which imposes controlled displacements on the lower jaw. The mechanics of the device keep jaw movements coaxial with the temporomandibular joint during the stretches. The design incorporates important safety features including mechanical stops and electronic cut-outs to protect the jaws from excessive stretches.