Presynaptic gating of monkey proprioceptive signals for proper motor action

Pharmacological blocking of spinal GABAA receptors in monkeys reduces sensory transmission to the spinal cord, thalamus, and cortex

A century of research established that GABA inhibits proprioceptive inputs presynaptically to sculpt spinal neural inputs into skilled motor output. Recent results in mice challenged this theory by showing that GABA can also facilitate action potential conduction in proprioceptive afferents. Here, we tackle this controversy in monkeys, the most human-relevant animal model, and show that GABAA receptors (GABAARs) indeed facilitate sensory inputs to spinal motoneurons and interneurons and that this mechanism also influences sensory transmission to supraspinal centers. We performed causal manipulations of GABAARs with intrathecal pharmacology in anesthetized monkeys while recording electrical signals in the muscles, spinal cord, thalamus, and cortex. We show that blocking GABAARs suppresses spinal reflexes to hand muscles, sensory-evoked single-unit firing in the spinal cord, and sensory-evoked potentials in the thalamus and somatosensory cortex. Our results portray a sophisticated picture of presynaptic modulation of sensory inputs by GABA in the spinal cord.

Future spinal reflex is embedded in primary motor cortex output

Mammals can execute intended limb movements despite the fact that spinal reflexes involuntarily modulate muscle activity. To generate appropriate muscle activity, the cortical descending motor output must coordinate with spinal reflexes, yet the underlying neural mechanism remains unclear. We simultaneously recorded activities in motor-related cortical areas, afferent neurons, and forelimb muscles of monkeys performing reaching movements. Motor-related cortical areas, predominantly primary motor cortex (M1), encode subsequent afferent activities attributed to forelimb movement. M1 also encodes a subcomponent of muscle activity evoked by these afferent activities, corresponding to spinal reflexes. Furthermore, selective disruption of the afferent pathway specifically reduced this subcomponent of muscle activity, suggesting that M1 output drives muscle activity not only through direct descending pathways but also through the "transafferent" pathway composed of descending plus subsequent spinal reflex pathways. Thus, M1 provides optimal motor output based on an internal forward model that prospectively computes future spinal reflexes.

Topographical and cell type-specific connectivity of rostral and caudal forelimb corticospinal neuron populations

Corticospinal neurons (CSNs) synapse directly on spinal neurons, a diverse assortment of cells with unique structural and functional properties necessary for body movements. CSNs modulating forelimb behavior fractionate into caudal forelimb area (CFA) and rostral forelimb area (RFA) motor cortical populations. Despite their prominence, the full diversity of spinal neurons targeted by CFA and RFA CSNs is uncharted. Here, we use anatomical and RNA sequencing methods to show that CSNs synapse onto a remarkably selective group of spinal cell types, favoring inhibitory populations that regulate motoneuron activity and gate sensory feedback. CFA and RFA CSNs target similar spinal neuron types, with notable exceptions that suggest that these populations differ in how they influence behavior. Finally, axon collaterals of CFA and RFA CSNs target similar brain regions yet receive highly divergent inputs. These results detail the rules of CSN connectivity throughout the brain and spinal cord for two regions critical for forelimb behavior.

Modulation of somatosensory signal transmission in the primate cuneate nucleus during voluntary hand movement

Primate hands house an array of mechanoreceptors and proprioceptors, which are essential for tactile and kinematic information crucial for daily motor action. While the regulation of these somatosensory signals is essential for hand movements, the specific central nervous system (CNS) location and mechanism remain unclear. Our study demonstrates the attenuation of somatosensory signals in the cuneate nucleus during voluntary movement, suggesting significant modulation at this initial relay station in the CNS. The attenuation is comparable to the cerebral cortex but more pronounced than in the spinal cord, indicating the cuneate nuclei's role in somatosensory perception modulation during movement. Moreover, our findings suggest that the descending motor tract may regulate somatosensory transmission in the cuneate nucleus, enhancing relevant signals and suppressing unnecessary ones for the regulation of movement. This process of recurrent somatosensory modulation between cortical and subcortical areas could be a basic mechanism for modulating somatosensory signals to achieve active perception.

GABA Increases Sensory Transmission In Monkeys

Sensory input flow is central to voluntary movements. For almost a century, GABA was believed to modulate this flow by inhibiting sensory axons in the spinal cord to sculpt neural inputs into skilled motor output. Instead, here we show that GABA can also facilitate sensory transmission in monkeys and consequently increase spinal and cortical neural responses to sensory inputs challenging our understanding of generation and perception of movement.

Topographical and cell type-specific connectivity of rostral and caudal forelimb corticospinal neuron populations

Corticospinal neurons (CSNs) synapse directly on spinal neurons, a diverse group of neurons with unique structural and functional properties necessary for body movements. CSNs modulating forelimb behavior fractionate into caudal forelimb area (CFA) and rostral forelimb area (RFA) motor cortical populations. Despite their prominence, no studies have mapped the diversity of spinal cell types targeted by CSNs, let alone compare CFA and RFA populations. Here we use anatomical and RNA-sequencing methods to show that CSNs synapse onto a remarkably selective group of spinal cell types, favoring inhibitory populations that regulate motoneuron activity and gate sensory feedback. CFA and RFA CSNs target similar spinal cell types, with notable exceptions that suggest these populations differ in how they influence behavior. Finally, axon collaterals of CFA and RFA CSNs target similar brain regions yet receive surprisingly divergent inputs. These results detail the rules of CSN connectivity throughout the brain and spinal cord for two regions critical for forelimb behavior.