Genesis of sleep bruxism: motor and autonomic-cardiac interactions

This is a short review paper presenting hypothesis to explain the mechanism that may be involved in the genesis of sleep bruxism (SB). In humans, SB is a repetitive sleep movement disorder mainly characterized by rhythmic masticatory muscle activity (RMMA) at a frequency of 1Hz and by occasional tooth grinding. Until recently, the mechanism by which RMMA and SB episodes are triggered has been poorly understood. It is reported that during light sleep, most SB episodes are observed in relation to brief cardiac and brain reactivations (3-15s) termed "micro-arousals". We showed that RMMA are secondary to a sequence of events in relation to sleep micro-arousals: the heart (increase in autonomic sympathetic activity) and brain are activated in the minutes and seconds, respectively, before the onset of activity in suprahyoid muscles and finally by RMMA in jaw closing masseter or temporalis muscles. In non-human primate study, we have shown that the excitability of cortico-bulbar pathways is depressed during sleep; no rhythmic jaw movements (RJM) are observed following intracortical microstimulation (ICMS) of cortical masticatory area (CMA) during sleep compared to the quiet awake state. The above results suggest that the onset of RMMA and SB episodes during sleep are under the influences of brief and transient activity of the brainstem arousal-reticular ascending system contributing to the increase of activity in autonomic-cardiac and motor modulatory networks.     

Identification of motor areas of the cat cerebral cortex based on studies of cortical stimulation and corticospinal connections

The location and topography of motor areas in the cat cerebral cortex were studied by electrical stimulation of the cortex in five animals, and by the injection of retrograde tracers into the spinal cord of four animals. Movements evoked by intracortical microstimulation (ICMS) of the anterior, posterior and lateral sigmoid gyri, both banks of the cruciate sulcus and the dorsal bank of the presylvian sulcus were observed in anaesthetized cats. Fluorescent tracers (Fast Blue and/or Diamadino Yellow) were injected into the lateral funiculus in the second cervical segment, into the gray matter of cervical segments C3-T1 and/or into the gray matter of lumbar segments L2-S1. Contraction of the contralateral forelimb, hindlimb or facial muscles was observed following electrical stimulation of several cytoarchitectonic areas: 4γ, 4δ, 6aα, 6aγ, and 3a. These findings suggested representations of contralateral forelimb and hindlimb movements in areas 4γ and 4δ, and of the contralateral forelimb muscles in areas 6aα, and 6aγ. Corticospinal neurons were located in all the above cytoarchitectonic areas as well as in areas 3b, 1, 2, 2pri, and 5. Large numbers of neurons were labeled in areas 4γ and 4δ, and moderate labeling was observed in areas 6aγ and 6aα. Corticospinal neurons projecting to cervical and lumbar segments were located in areas 4γ and 4δ, while those projecting only to cervical segments were detected in areas 6aα, and 6aγ. Based on these findings it is proposed that within the motor cortex of the cat there are representations of limb movements in several cytoarchitectonic subdivisions. Many of these representations may be candidate secondary motor areas. J. Comp. Neurol. 380:191–214, 1997. © 1997 Wiley-Liss, Inc.     

Calcium activation of cortical neurons by continuous electrical stimulation: Frequency dependence,temporal fidelity,and activation density

Electrical stimulation of the brain has become a mainstay of fundamental neuroscience research and an increasingly prevalent clinical therapy. Despite decades of use in basic neuroscience research and the growing prevalence of neuromodulation therapies, gaps in knowledge regarding activation or inactivation of neural elements over time have limited its ability to adequately interpret evoked downstream responses or fine-tune stimulation parameters to focus on desired responses. In this work, in vivo two-photon microscopy was used to image neuronal calcium activity in layer 2/3 neurons of somatosensory cortex (S1) in male C57BL/6J-Tg(Thy1-GCaMP6s)GP4.3Dkim/J mice during 30 s of continuous electrical stimulation at varying frequencies. We show frequency–dependent differences in spatial and temporal somatic responses during continuous stimulation. Our results elucidate conflicting results from prior studies reporting either dense spherical activation of somas biased toward those near the electrode, or sparse activation of somas at a distance via axons near the electrode. These findings indicate that the neural element specific temporal response local to the stimulating electrode changes as a function of applied charge density and frequency. These temporal responses need to be considered to properly interpret downstream circuit responses or determining mechanisms of action in basic science experiments or clinical therapeutic applications.     

The Neurophysiological Representation of Imagined Somatosensory Percepts in Human Cortex

Intracortical microstimulation (ICMS) in human primary somatosensory cortex (S1) has been used to successfully evoke naturalistic sensations. However, the neurophysiological mechanisms underlying the evoked sensations remain unknown. To understand how specific stimulation parameters elicit certain sensations we must first understand the representation of those sensations in the brain. In this study we record from intracortical microelectrode arrays implanted in S1, premotor cortex, and posterior parietal cortex of a male human participant performing a somatosensory imagery task. The sensations imagined were those previously elicited by ICMS of S1, in the same array of the same participant. In both spike and local field potential recordings, features of the neural signal can be used to classify different imagined sensations. These features are shown to be stable over time. The sensorimotor cortices only encode the imagined sensation during the imagery task, while posterior parietal cortex encodes the sensations starting with cue presentation. These findings demonstrate that different aspects of the sensory experience can be individually decoded from intracortically recorded human neural signals across the cortical sensory network. Activity underlying these unique sensory representations may inform the stimulation parameters for precisely eliciting specific sensations via ICMS in future work.SIGNIFICANCE STATEMENT Electrical stimulation of human cortex is increasingly more common for providing feedback in neural devices. Understanding the relationship between naturally evoked and artificially evoked neurophysiology for the same sensations will be important in advancing such devices. Here, we investigate the neural activity in human primary somatosensory, premotor, and parietal cortices during somatosensory imagery. The sensations imagined were those previously elicited during intracortical microstimulation (ICMS) of the same somatosensory electrode array. We elucidate the neural features during somatosensory imagery that significantly encode different aspects of individual sensations and demonstrate feature stability over almost a year. The correspondence between neurophysiology elicited with or without stimulation for the same sensations will inform methods to deliver more precise feedback through stimulation in the future.     

Saccadic dysmetria induced by transient functional decoration of the cerebellar vermis

Summary Saccadic dysmetria is seen in patients with cerebellar diseases as well as in monkeys whose vermis and fastigial nucleus are experimentally lesioned. We investigated the oculomotor signs of the vermis by blocking cerebellar impulses with bicuculline injections into the fastigial nucleus. The oculomotor abnormalities associated with the bicuculline treatment (in effect, functional as well as reversible, unilateral decortication of the vermis) were hypometric saccades toward the injection side and gaze deviation toward the opposite side. These oculomotor signs disappeared with the withdrawal of the bicuculline effect.     

Long-range horizontal connections between supragranular pyramidal cells in the extrastriate visual cortex of the rat

In this study, we examined the morphological structure and synaptic physiology of long-range axon projections among supragranular pyramidal cells in the extrastriate visual cortex of the rat. Intra-and extracellular recordings form layer II/III pyramidal cells were performed in brain slices of area 18a following extracellular stimulation of either the underlying white matter or within layer II/III. Neurons were injected with biocytin for two-dimensional reconstruction of their axon arborizations. The conduction velocity of afferent fibers (0.58 m/s) was twice as high as that of intracortical tangential fibers (0.28 m/s). Layer II/III cells were mainly di-or polysynaptically driven by afferent activation, but predominantly monosynaptically driven from intracortical stimulation sites. The afferent as well as intracortically as well as intracortically evoked postsynaptic potentials showed a very similar time course and shape. From both stimulation sites, suprathreshold action potentials could be elicited. The current threshold for a postsynaptic response and the slope and width of excitatory postsynaptic potentials (EPSPS) increased with the distance of lateral stimulation. The morphological properties of layer II/III pyramidal cell axon collaterals colsely corresponded to the electrophysiological results. Long-range intraareal axon collaterals could be followed up to 1 mm within the supragranular layers. Their length-distance distribution showed an inverse relationship to the threshold currents of EPSPs. Pyramidal cells exhibited regularly spaced patches of horizontal axon collaterals with an interpatch distance of about 250 μm. We concluded that the supragranular horizontal network in the extrastriate visual cortex of the rat is qualitatively very similar to that of cats and monkeys. However, quantitative differences exist in its spatial extent and physiological characteristics. © 1994 Wiley-Liss, Inc.     

Electrophysiological analysis of motor cortical plasticity after cortical lesions in newborn rats

Intracortical microstimulation of the motor cortex in normal adult rats evoked low threshold contralateral forelimb movements and high threshold ipsilateral movements. Ablation of the opposite sensorimotor cortex in adult animals did not alter these thresholds. However, stimulation of the unablated hemisphere in adult rats that sustained unilateral sensorimotor cortical lesions as neonates elicited low threshold ipsilateral forelimb movements that were similar to contralateral movements. These low threshold ipsilateral movements may be mediated via aberrant corticofugal pathways which are known to develop following neonatal cortical lesions.     

Stimulus-dependent,reciprocal up- and downregulation of glutamic acid decarboxylase and Ca2+/calmodulin-dependent protein kinase II gene expression in rat cerebral cortex

Long-train tetanic stimulation of the cerebral cortex induces long-term changes in the excitability of cortical neurons, while short-train electrical stimulation does not. In the present study, we show that both forms of stimulation when applied to rat motor cortex for 4 h enhance c-fos expression, but only tetanic stimulation, when imposed upon short-train stimulation, modulates gene expression for 67-kDa glutamic acid decarboxylase (GAD) and alpha Ca²⁺/calmodulin-dependent protein kinase II (CaMKII). Gene expression for beta Ca²⁺/calmodulin-dependent protein kinase II is not affected by either stimulation mode. GAD messenger RNA (mRNA) is increased from 1 h after the end of tetanization to the longest poststimulus survival time investigated (14 h). CaMKII mRNA is decreased 1–3 h after the end of tetanization but thereafter returns to prestimulus levels. These results imply not only that mechanisms underlying neocortical plasticity are stimulus-dependent but also that they involve reciprocal changes in molecules regulating the balance of excitation and inhibition.     

正中神经损伤修复后大脑运动皮层可塑性变化的实验研究

目的 研究正中神经损伤修复后运动皮层可塑性的变化过程.方法 SD大鼠35只,分为对照组和手术组,手术组又分为术后1 d和1周、4周、8周、12周、16周,每组5只,左侧肢体为损伤修复侧,将正中神经在内侧束分支以远2.0 cm处切断后缝合.通过皮层内微电极刺激技术,定量评价正中神经损伤恢复过程中运动皮层的可塑性变化.结果 正中神经屈指屈腕区运动皮层面积[(0.85±0.1)mm2,-x±s,下同].术后1 d、1周,对侧皮层正中神经屈指屈腕区被桡神经、肌皮神经、腋神经位点占据,没有无反应位点出现;术后4周、8周和16周,运动皮层出现无反应位点;术后8周、12周,对侧皮层出现屈指屈腕位点,差异有统计学意义(P＜0.05);术后16周,与健康组相比,差异无统计学意义(P＞0.05),运动皮层内没有无反应位点出现.结论 成年大鼠正中神经损伤后其对侧运动皮层发生可塑性改变,是一个动态的过程,但其机制还需进一步的研究.