Cuneate nucleus: The somatosensory gateway to the brain

The dorsal column nuclei scale mechanical sensitivity in naive and neuropathic pain states

During pathological conditions, tactile stimuli can aberrantly engage nociceptive pathways leading to the perception of touch as pain, known as mechanical allodynia. The brain stem dorsal column nuclei integrate tactile inputs, yet their role in mediating tactile sensitivity and allodynia remains understudied. We found that gracile nucleus (Gr) inhibitory interneurons and thalamus-projecting neurons are differentially innervated by primary afferents and spinal inputs. Functional manipulations of these distinct Gr neuronal populations bidirectionally shifted tactile sensitivity but did not affect noxious mechanical or thermal sensitivity. During neuropathic pain, Gr neurons exhibited increased sensory-evoked activity and asynchronous excitatory drive from primary afferents. Silencing Gr projection neurons or activating Gr inhibitory neurons in neuropathic mice reduced tactile hypersensitivity, and enhancing inhibition ameliorated paw-withdrawal signatures of neuropathic pain and induced conditioned place preference. These results suggest that Gr activity contributes to tactile sensitivity and affective, pain-associated phenotypes of mechanical allodynia.

Haptic Perception and Its Relation to Action

Haptic perception uses signals from touch receptors to detect, locate, and mentally represent objects and surfaces. Research from behavioral science, neuroscience, and computational modeling advances understanding of these essential functions. Haptic perception is grounded in neural circuitry that transmits external contact to the brain via increasingly abstracted representations. Computational models of mechanical interactions at the skin predict peripheral neural firing rates that initiate the processing chain. Behavioral phenomena and associated neural processes illustrate the reciprocal relationship by which perception supports action and action gates experience. The interaction of sensation and action is evident in how features of surfaces and objects such as softness and curvature are encoded. By incorporating touch sensations in conjunction with motor control, biologically embedded prosthetics enhance user capabilities and may elicit feelings of ownership. Efforts to create virtual haptic experience with advanced technologies underscore the complexity of this fundamental perceptual channel and its relation to action.

The Dorsal Column Nuclei Scale Mechanical Sensitivity in Naive and Neuropathic Pain States

Tactile perception relies on reliable transmission and modulation of low-threshold information as it travels from the periphery to the brain. During pathological conditions, tactile stimuli can aberrantly engage nociceptive pathways leading to the perception of touch as pain, known as mechanical allodynia. Two main drivers of peripheral tactile information, low-threshold mechanoreceptors (LTMRs) and postsynaptic dorsal column neurons (PSDCs), terminate in the brainstem dorsal column nuclei (DCN). Activity within the DRG, spinal cord, and DCN have all been implicated in mediating allodynia, yet the DCN remains understudied at the cellular, circuit, and functional levels compared to the other two. Here, we show that the gracile nucleus (Gr) of the DCN mediates tactile sensitivity for low-threshold stimuli and contributes to mechanical allodynia during neuropathic pain in mice. We found that the Gr contains local inhibitory interneurons in addition to thalamus-projecting neurons, which are differentially innervated by primary afferents and spinal inputs. Functional manipulations of these distinct Gr neuronal populations resulted in bidirectional changes to tactile sensitivity, but did not affect noxious mechanical or thermal sensitivity. During neuropathic pain, silencing Gr projection neurons or activating Gr inhibitory neurons was able to reduce tactile hypersensitivity, and enhancing inhibition was able to ameliorate paw withdrawal signatures of neuropathic pain, like shaking. Collectively, these results suggest that the Gr plays a specific role in mediating hypersensitivity to low-threshold, innocuous mechanical stimuli during neuropathic pain, and that Gr activity contributes to affective, pain-associated phenotypes of mechanical allodynia. Therefore, these brainstem circuits work in tandem with traditional spinal circuits underlying allodynia, resulting in enhanced signaling of tactile stimuli in the brain during neuropathic pain.

Task-driven neural network models predict neural dynamics of proprioception

Proprioception tells the brain the state of the body based on distributed sensory neurons. Yet, the principles that govern proprioceptive processing are poorly understood. Here, we employ a task-driven modeling approach to investigate the neural code of proprioceptive neurons in cuneate nucleus (CN) and somatosensory cortex area 2 (S1). We simulated muscle spindle signals through musculoskeletal modeling and generated a large-scale movement repertoire to train neural networks based on 16 hypotheses, each representing different computational goals. We found that the emerging, task-optimized internal representations generalize from synthetic data to predict neural dynamics in CN and S1 of primates. Computational tasks that aim to predict the limb position and velocity were the best at predicting the neural activity in both areas. Since task optimization develops representations that better predict neural activity during active than passive movements, we postulate that neural activity in the CN and S1 is top-down modulated during goal-directed movements.

Subcortical contributions to the sense of body ownership

The sense of body ownership (i.e. the feeling that our body or its parts belong to us) plays a key role in bodily self-consciousness and is believed to stem from multisensory integration. Experimental paradigms such as the rubber hand illusion have been developed to allow the controlled manipulation of body ownership in laboratory settings, providing effective tools for investigating malleability in the sense of body ownership and the boundaries that distinguish self from other. Neuroimaging studies of body ownership converge on the involvement of several cortical regions, including the premotor cortex and posterior parietal cortex. However, relatively less attention has been paid to subcortical structures that may also contribute to body ownership perception, such as the cerebellum and putamen. Here, on the basis of neuroimaging and neuropsychological observations, we provide an overview of relevant subcortical regions and consider their potential role in generating and maintaining a sense of ownership over the body. We also suggest novel avenues for future research targeting the role of subcortical regions in making sense of the body as our own.

Proprioception: A New Era Set in Motion by Emerging Genetic and Bionic Strategies?

The generation of an internal body model and its continuous update is essential in sensorimotor control. Although known to rely on proprioceptive sensory feedback, the underlying mechanism that transforms this sensory feedback into a dynamic body percept remains poorly understood. However, advances in the development of genetic tools for proprioceptive circuit elements, including the sensory receptors, are beginning to offer new and unprecedented leverage to dissect the central pathways responsible for proprioceptive encoding. Simultaneously, new data derived through emerging bionic neural machine-interface technologies reveal clues regarding the relative importance of kinesthetic sensory feedback and insights into the functional proprioceptive substrates that underlie natural motor behaviors.

Differential changes in somatosensory evoked potentials and motor performance: pursuit movement task versus force matching tracking task

Force modulation relies on accurate proprioception, and force-matching tasks alter corticocerebellar connectivity. Corticocerebellar (N24) and corticomotor pathways are impacted following the acquisition of a motor tracing task (MTT), measured using both somatosensory evoked potentials (SEPs) and transcranial magnetic stimulation. This study compared changes in early SEP peak amplitudes and motor performance following a force-matching tracking task (FMTT) to an MTT. Thirty (18 females) right-handed participants, aged 21.4 ± 2.76, were electrically stimulated over the right-median nerve at 2.47 Hz and 4.98 Hz (averaged 1,000 sweeps/rate) to elicit SEPs, recorded via a 64-channel electroencephalography cap, before, and after task acquisition using the right abductor pollicis brevis muscle. Retention was measured 24 h later. Significant time-by-group interactions occurred for the N20 SEP: 6.3% decrease post-FMTT versus 5.5% increase post-MTT (P = 0.013); P25 SEP: 4.0% decrease post-FMTT versus 10.3% increase post-MTT (P = 0.006); and N18 SEP: 113.4% increase post-FMTT versus 4.4% decrease post-MTT (P = 0.006). N18 and N30 showed significant effect of time (both P < 0.001). Motor performance: significant time-by-group interactions-postacquisition: FMTT improved 15.3% versus 24.3% for MTT (P = 0.025), retention: FMTT improved 17.4% and MTT by 30.1% (P = 0.004). Task-dependent differences occurred in SEP peaks associated with cortical somatosensory processing (N20 and P25), and cerebellar input (N18), with similar changes in sensorimotor integration (N30), with differential improvements in motor performance, indicating important differences in cerebellar and sensory processing for tasks reliant on proprioception.NEW & NOTEWORTHY This study demonstrates neurophysiological differences in cerebellar and somatosensory cortex pathways when learning a motor task requiring visuomotor tracking versus a task that requires force-matching modulation, in healthy individuals. The clear neurophysiological differences in early somatosensory evoked potentials associated with cortical somatosensory processing, cerebellar input, and sensorimotor integration between these two tasks demonstrate some of the neural correlates of force modulation and validate the force-matching task for use in future work.

Engaging Spinal Networks to Mitigate Supraspinal Dysfunction After CP

Although children with cerebral palsy seem to have the neural networks necessary to generate most movements, they are markedly dysfunctional, largely attributable to abnormal patterns of muscle activation, often characterized as spasticity, largely reflecting a functionally abnormal spinal-supraspinal connectivity. While it is generally assumed that the etiologies of the disruptive functions associated with cerebral palsy can be attributed primarily to supraspinal networks, we propose that the more normal connectivity that persists between peripheral proprioception-cutaneous input to the spinal networks can be used to guide the reorganization of a more normal spinal-supraspinal connectivity. The level of plasticity necessary to achieve the required reorganization within and among different neural networks can be achieved with a combination of spinal neuromodulation and specific activity-dependent mechanisms. By engaging these two concepts, we hypothesize that bidirectional reorganization of proprioception-spinal cord-brain connectivity to higher levels of functionality can be achieved without invasive surgery.