Subdivisions of primary motor cortex based on cortico-motoneuronal cells

Restoring Movement in Paralysis with a Bioelectronic Neural Bypass Approach: Current State and Future Directions

Bioelectronic medicine is a rapidly growing field that explores targeted neuromodulation in new treatment options addressing both disease and injury. New bioelectronic methods are being developed to monitor and modulate neural activity directly. The therapeutic benefit of these approaches has been validated in recent clinical studies in various conditions, including paralysis. By using decoding and modulation strategies together, it is possible to restore lost function to those living with paralysis and other debilitating conditions by interpreting and rerouting signals around the affected portion of the nervous system. This, in effect, creates a bioelectronic "neural bypass" to serve the function of the damaged/degenerated network. By learning the language of neurons and using neural interface technology to tap into critical networks, new approaches to repairing or restoring function in areas impacted by disease or injury may become a reality.

Finger Posture and Finger Load are Perceived Independently

The ability to track the time-varying postures of our hands and the forces they exert plays a key role in our ability to dexterously interact with objects. However, how precisely and accurately we sense hand kinematics and kinetics has not been completely characterized. Furthermore, the dominant source of information about hand postures stems from muscle spindles, whose responses can also signal isometric force and are modulated by fusimotor input. As such, one might expect that changing the state of the muscles – for example, by applying a load – would influence perceived finger posture. To address these questions, we measure the acuity of human hand proprioception, investigate the interplay between kinematic and kinetic signals, and determine the extent to which actively and passively achieved postures are perceived differently. We find that angle and torque perception are highly precise; that loads imposed on the finger do not affect perceived joint angle; that joint angle does not affect perceived load; and that hand postures are perceived similarly whether they are achieved actively or passively. The independence of finger posture and load perception contrasts with their interdependence in the upper arm, likely reflecting the special functional importance of the hand.

Simulation of transcranial magnetic stimulation in head model with morphologically-realistic cortical neurons

BACKGROUND

Transcranial magnetic stimulation (TMS) enables non-invasive modulation of brain activity with both clinical and research applications, but fundamental questions remain about the neural types and elements TMS activates and how stimulation parameters affect the neural response.

OBJECTIVE

To develop a multi-scale computational model to quantify the effect of TMS parameters on the direct response of individual neurons.

METHODS

We integrated morphologically-realistic neuronal models with TMS-induced electric fields computed in a finite element model of a human head to quantify the cortical response to TMS with several combinations of pulse waveforms and current directions.

RESULTS

TMS activated with lowest intensity intracortical axonal terminations in the superficial gyral crown and lip regions. Layer 5 pyramidal cells had the lowest thresholds, but layer 2/3 pyramidal cells and inhibitory basket cells were also activated at most intensities. Direct activation of layers 1 and 6 was unlikely. Neural activation was largely driven by the field magnitude, rather than the field component normal to the cortical surface. Varying the induced current direction caused a waveform-dependent shift in the activation site and provided a potential mechanism for experimentally observed differences in thresholds and latencies of muscle responses.

CONCLUSIONS

This biophysically-based simulation provides a novel method to elucidate mechanisms and inform parameter selection of TMS and other cortical stimulation modalities. It also serves as a foundation for more detailed network models of the response to TMS, which may include endogenous activity, synaptic connectivity, inputs from intrinsic and extrinsic axonal projections, and corticofugal axons in white matter.

Precentral degeneration and cerebellar compensation in amyotrophic lateral sclerosis: A multimodal MRI analysis

Amyotrophic lateral sclerosis (ALS) is a progressive and intractable neurodegenerative disease of human motor system characterized by progressive muscular weakness and atrophy. A considerable body of research has demonstrated significant structural and functional abnormalities of the primary motor cortex in patients with ALS. In contrast, much less attention has been paid to the abnormalities of cerebellum in this disease. Using multimodal magnetic resonance imagining data of 60 patients with ALS and 60 healthy controls, we examined changes in gray matter volume (GMV), white matter (WM) fractional anisotropy (FA), and functional connectivity (FC) in patients with ALS. Compared with healthy controls, patients with ALS showed decreased GMV in the left precentral gyrus and increased GMV in bilateral cerebellum, decreased FA in the left corticospinal tract and body of corpus callosum, and decreased FC in multiple brain regions, involving bilateral postcentral gyrus, precentral gyrus and cerebellum anterior lobe, among others. Meanwhile, we found significant intermodal correlations among GMV of left precentral gyrus, FA of altered WM tracts, and FC of left precentral gyrus, and that WM microstructural alterations seem to play important roles in mediating the relationship between GMV and FC of the precentral gyrus, as well as the relationship between GMVs of the precentral gyrus and cerebellum. These findings provided evidence for the precentral degeneration and cerebellar compensation in ALS, and the involvement of WM alterations in mediating the relationship between pathologies of the primary motor cortex and cerebellum, which may contribute to a better understanding of the pathophysiology of ALS.

Regional motor cortex dysfunction in amyotrophic lateral sclerosis

OBJECTIVE

The pathophysiological processes underlying amyotrophic lateral sclerosis (ALS) need to be better understood, although cortical dysfunction has been implicated. Previous transcranial magnetic stimulation (TMS) studies have assessed cortical dysfunction from the hand. The aim of the present study was to determine whether cortical dysfunction was evident across representations of three body regions, and to relate these changes to clinical features of ALS.

METHODS

In this cross-sectional study, threshold tracking TMS was undertaken in 60 sporadic ALS patients, with motor evoked potential (MEP) responses recorded over the hand (abductor pollicis brevis), lower limb (tibialis anterior), and bulbar (trapezius) regions. The cross-sectional findings were compared to 28 age- and gender-matched controls.

RESULTS

Cortical dysfunction was evident across the representation of the three body regions, although the degree and nature of the dysfunction varied. Cortical hyperexcitability, as heralded by reduced short interval intracortical inhibition (SICI), was evident in all cortical regions (hand, P < 0.01; leg, P < 0.05; bulbar, P < 0.05) in ALS patients when compared with healthy control subjects. Importantly, features of cortical hyperexcitability seemed more prominent in clinically affected body regions and correlated with functional disability and muscle weakness. Cortical inexcitability was more prominent in the leg (P < 0.001) and bulbar regions (P < 0.01) when compared with controls.

INTERPRETATION

The nature of cortical dysfunction varied across the body regions in ALS, with cortical hyperexcitability being more prominent in the upper limbs while cortical inexcitability was more evident in the lower limbs and bulbar regions. The findings suggest a heterogeneity of cortical pathophysiological processes in ALS.

Differential Changes in the Lateralized Activity of Identified Projection Neurons of Motor Cortex in Hemiparkinsonian Rats

In the parkinsonian state, the motor cortex and basal ganglia (BG) undergo dynamic remodeling of movement representation. One such change is the loss of the normal contralateral lateralized activity pattern. The increase in the number of movement-related neurons responding to ipsilateral or bilateral limb movements may cause motor problems, including impaired balance, reduced bimanual coordination, and abnormal mirror movements. However, it remains unknown how individual types of motor cortical neurons organize this reconstruction. To explore the effect of dopamine depletion on lateralized activity in the parkinsonian state, we used a partial hemiparkinsonian model [6-hydroxydopamine (6-OHDA) lesion] in Long-Evans rats performing unilateral movements in a right-left pedal task, while recording from primary (M1) and secondary motor cortex (M2). The lesion decreased contralateral preferred activity in both M1 and M2. In addition, this change differed among identified intratelencephalic (IT) and pyramidal tract (PT) cortical projection neurons, depending on the cortical area. We detected a decrease in lateralized activity only in PT neurons in M1, whereas in M2, this change was observed in IT neurons, with no change in the PT population. Our results suggest a differential effect of dopamine depletion in the lateralized activity of the motor cortex, and suggest possible compensatory changes in the contralateral hemisphere.

Motor Neuron Susceptibility in ALS/FTD

Amyotrophic lateral sclerosis (ALS) is a neurodegenerative disease characterized by the death of both upper and lower motor neurons (MNs) in the brain, brainstem and spinal cord. The neurodegenerative mechanisms leading to MN loss in ALS are not fully understood. Importantly, the reasons why MNs are specifically targeted in this disorder are unclear, when the proteins associated genetically or pathologically with ALS are expressed ubiquitously. Furthermore, MNs themselves are not affected equally; specific MNs subpopulations are more susceptible than others in both animal models and human patients. Corticospinal MNs and lower somatic MNs, which innervate voluntary muscles, degenerate more readily than specific subgroups of lower MNs, which remain resistant to degeneration, reflecting the clinical manifestations of ALS. In this review, we discuss the possible factors intrinsic to MNs that render them uniquely susceptible to neurodegeneration in ALS. We also speculate why some MN subpopulations are more vulnerable than others, focusing on both their molecular and physiological properties. Finally, we review the anatomical network and neuronal microenvironment as determinants of MN subtype vulnerability and hence the progression of ALS.

Corticocortical Systems Underlying High-Order Motor Control

Cortical networks are characterized by the origin, destination, and reciprocity of their connections, as well as by the diameter, conduction velocity, and synaptic efficacy of their axons. The network formed by parietal and frontal areas lies at the core of cognitive-motor control because the outflow of parietofrontal signaling is conveyed to the subcortical centers and spinal cord through different parallel pathways, whose orchestration determines, not only when and how movements will be generated, but also the nature of forthcoming actions. Despite intensive studies over the last 50 years, the role of corticocortical connections in motor control and the principles whereby selected cortical networks are recruited by different task demands remain elusive. Furthermore, the synaptic integration of different cortical signals, their modulation by transthalamic loops, and the effects of conduction delays remain challenging questions that must be tackled to understand the dynamical aspects of parietofrontal operations. In this article, we evaluate results from nonhuman primate and selected rodent experiments to offer a viewpoint on how corticocortical systems contribute to learning and producing skilled actions. Addressing this subject is not only of scientific interest but also essential for interpreting the devastating consequences for motor control of lesions at different nodes of this integrated circuit. In humans, the study of corticocortical motor networks is currently based on MRI-related methods, such as resting-state connectivity and diffusion tract-tracing, which both need to be contrasted with histological studies in nonhuman primates.

Putting the "Sensory" Into Sensorimotor Control: The Role of Sensorimotor Integration in Goal-Directed Hand Movements After Stroke

Integration of sensory and motor information is one-step, among others, that underlies the successful production of goal-directed hand movements necessary for interacting with our environment. Disruption of sensorimotor integration is prevalent in many neurologic disorders, including stroke. In most stroke survivors, persistent paresis of the hand reduces function and overall quality of life. Current rehabilitative methods are based on neuroplastic principles to promote motor learning that focuses on regaining motor function lost due to paresis, but the sensory contributions to motor control and learning are often overlooked and currently understudied. There is a need to evaluate and understand the contribution of both sensory and motor function in the rehabilitation of skilled hand movements after stroke. Here, we will highlight the importance of integration of sensory and motor information to produce skilled hand movements in healthy individuals and individuals after stroke. We will then discuss how compromised sensorimotor integration influences relearning of skilled hand movements after stroke. Finally, we will propose an approach to target sensorimotor integration through manipulation of sensory input and motor output that may have therapeutic implications.

An experimental and computational framework for modeling multi-muscle responses to transcranial magnetic stimulation of the human motor cortex

Current knowledge of coordinated motor control of multiple muscles is derived primarily from invasive stimulation-recording techniques in animal models. Similar studies are not generally feasible in humans, so a modeling framework is needed to facilitate knowledge transfer from animal studies. We describe such a framework that uses a deep neural network model to map finite element simulation of transcranial magnetic stimulation induced electric fields (E-fields) in motor cortex to recordings of multi-muscle activation. Critically, we show that model generalization is improved when we incorporate empirically derived physiological models for E-field to neuron firing rate and low-dimensional control via muscle synergies.

The effect of paired corticospinal-motoneuronal stimulation on maximal voluntary elbow flexion in cervical spinal cord injury: an experimental study

STUDY DESIGN

Randomised, controlled, crossover study.

OBJECTIVES

Paired corticospinal-motoneuronal stimulation (PCMS) involves repeatedly pairing stimuli to corticospinal neurones and motoneurones to induce changes in corticospinal transmission. Here, we examined whether PCMS could enhance maximal voluntary elbow flexion in people with cervical spinal cord injury.

SETTING

Neuroscience Research Australia, Sydney, Australia.

METHODS

PCMS comprised 100 pairs of transcranial magnetic and electrical peripheral nerve stimulation (0.1 Hz), timed so corticospinal potentials arrived at corticospinal-motoneuronal synapses 1.5 ms before antidromic motoneuronal potentials. On two separate days, sets of five maximal elbow flexions were performed by 11 individuals with weak elbow flexors post C4 or C5 spinal cord injury before and after PCMS or control (100 peripheral nerve stimuli) conditioning. During contractions, supramaximal biceps brachii stimulation elicited superimposed twitches, which were expressed as a proportion of resting twitches to give maximal voluntary activation. Maximal torque and electromyographic activity were also assessed.

RESULTS

Baseline median (range) maximal torque was 11 Nm (6-41 Nm) and voluntary activation was 92% (62-99%). Linear mixed modelling revealed no significant differences between PCMS and control protocols after conditioning (maximal torque: p = 0.87, superimposed twitch: p = 0.87, resting twitch: p = 0.44, voluntary activation: p = 0.36, biceps EMG: p = 0.25, brachioradialis EMG: 0.67).

CONCLUSIONS

Possible explanations for the lack of effect include a potential ceiling effect for voluntary activation, or that PCMS may be less effective for elbow flexors than distal muscles. Despite results, previous studies suggest that PCMS is worthy of further investigation.

Terminal organization of the corticospinal projection from the lateral premotor cortex to the cervical enlargement (C5-T1) in rhesus monkey

High-resolution tract tracing and stereology were used to study the terminal organization of the corticospinal projection (CSP) from the ventral (v) and dorsal (d) regions of the lateral premotor cortex (LPMC) to spinal levels C5-T1. The LPMCv CSP originated from the postarcuate sulcus region, was bilateral, sparse, and primarily targeted the dorsolateral and ventromedial sectors of contralateral lamina VII. The convexity/lateral part of LPMCv did not project below C2. Thus, very little LPMCv corticospinal output reaches the cervical enlargement. In contrast, the LPMCd CSP was 5× more prominent in terminal density. Bilateral terminal labeling occurred in the medial sectors of lamina VII and adjacent lamina VIII, where propriospinal neurons with long-range bilateral axon projections reside. Notably, lamina VIII also harbors axial motoneurons. Contralateral labeling occurred in the lateral sectors of lamina VII and the dorsomedial quadrant of lamina IX, noted for harboring proximal upper limb flexor motoneurons. Segmentally, the CSP to contralateral laminae VII and IX preferentially innervated C5-C7, which supplies shoulder, elbow, and wrist musculature. In contrast, terminations in axial-related lamina VIII were distributed bilaterally throughout all cervical enlargement levels, including C8 and T1. These findings demonstrate the LPMCd CSP is structured to influence axial and proximal upper limb movements, supporting Kuypers conceptual view of the LPMCd CSP being a major component of the medial motor control system. Thus, distal upper extremity control influenced by LPMC, including grasping and manipulation, must occur through indirect neural network connections such as corticocortical, subcortical, or intrinsic spinal circuits.

An acute application of transcranial random noise stimulation does not enhance motor skill acquisition or retention in a golf putting task

Transcranial random noise stimulation (tRNS) is a brain stimulation technique that has been shown to increase motor performance in simple motor tasks. The purpose was to determine the influence of tRNS on motor skill acquisition and retention in a complex golf putting task. Thirty-four young adults were randomly assigned to a tRNS group or a SHAM stimulation group. Each subject completed a practice session followed by a retention session. In the practice session, subjects performed golf putting trials in a baseline test block, four practice blocks, and a post test block. Twenty-four hours later subjects completed the retention test block. The golf putting task involved performing putts to a small target located 3 m away. tRNS or SHAM was applied during the practice blocks concurrently with the golf putting task. tRNS was applied over the first dorsal interosseus muscle representation area of the motor cortex for 20 min at a current strength of 2 mA. Endpoint error and endpoint variance were reduced across the both the practice blocks and the test blocks, but these reductions were not different between groups. These findings suggest that an acute application of tRNS failed to enhance skill acquisition or retention in a golf putting task.

Dexterous Hand Movements and Their Recovery After Central Nervous System Injury

Hand dexterity has uniquely developed in higher primates and is thought to rely on the direct corticomotoneuronal (CM) pathway. Recent studies have shown that rodents and carnivores lack the direct CM pathway but can control certain levels of dexterous hand movements through various indirect CM pathways. Some homologous pathways also exist in higher primates, and among them, propriospinal (PrS) neurons in the mid-cervical segments (C3-C4) are significantly involved in hand dexterity. When the direct CM pathway was lesioned caudal to the PrS and transmission of cortical commands to hand motoneurons via the PrS neurons remained intact, dexterous hand movements could be significantly recovered. This recovery model was intensively studied, and it was found that, in addition to the compensation by the PrS neurons, a large-scale reorganization in the bilateral cortical motor-related areas and mesolimbic structures contributed to recovery. Future therapeutic strategies should target these multihierarchical areas.

Starting and stopping movement by the primate brain

We review the current knowledge about the part that motor cortex plays in the preparation and generation of movement, and we discuss the idea that corticospinal neurons, and particularly those with cortico-motoneuronal connections, act as ‘command’ neurons for skilled reach-to-grasp movements in the primate. We also review the increasing evidence that it is active during processes such as action observation and motor imagery. This leads to a discussion about how movement is inhibited and stopped, and the role in these for disfacilitation of the corticospinal output. We highlight the importance of the non-human primate as a model for the human motor system. Finally, we discuss the insights that recent research into the monkey motor system has provided for translational approaches to neurological diseases such as stroke, spinal injury and motor neuron disease.

Recent advances in our understanding of the primate corticospinal system

The last few years have seen major advances in our understanding of the organisation and function of the corticospinal tract (CST). These have included studies highlighting important species-specific variations in the different functions mediated by the CST. In the primate, the most characteristic feature is direct cortico-motoneuronal (CM) control of muscles, particularly of hand and finger muscles. This system, which is unique to dexterous primates, is probably at its most advanced level in humans. We now know much more about the origin of the CM system within the cortical motor network, and its connectivity within the spinal cord has been quantified. We have learnt much more about how the CM system works in parallel with other spinal circuits receiving input from the CST and how the CST functions alongside other brainstem motor pathways. New work in the mouse has provided fascinating insights into the contribution of the CM system to dexterity. Finally, accumulating evidence for the involvement of CM projections in motor neuron disease has highlighted the importance of advances in basic neuroscience for our understanding and possible treatment of a devastating neurological disease.

Motor-imagery ability and function of hemiplegic upper limb in stroke patients

Objectives

We quantitatively examined the motor‐imagery ability in stroke patients using a bimanual circle‐line coordination task (BCT) and clarified the relationship between motor‐imagery ability and motor function of hemiplegic upper limbs and the level of use of paralyzed limbs.

Methods

We enrolled 31 stroke patients. Tasks included unimanual‐line (U‐L)—drawing straight lines on the nonparalyzed side; bimanual circle‐line (B‐CL)—drawing straight lines with the nonparalyzed limb while drawing circles with the paralyzed limb; and imagery circle‐line (I‐CL)—drawing straight lines on the nonparalyzed side during imagery drawing on the paralyzed side, using a tablet personal computer. We calculated the ovalization index (OI) and motor‐imagery ability (image OI). We used the Fugl–Meyer motor assessment (FMA), amount of use (AOU), and quality of motion (QOM) of the motor activity log (MAL) as the three variables for cluster analysis and performed mediation analysis.

Results

Clusters 1 (FMA <26 points) and 2 (FMA ≥26 points) were formed. In cluster 2, we found significant associations between image OI and FMA, AOU, and QOM. When AOU and QOM were mediated between image OI and FMA, we observed no significant direct association between image OI and FMA, and a significant indirect effect of AOU and QOM.

Interpretation

In stroke patients with moderate‐to‐mild movement disorder, image OI directly affects AOU of hemiplegic upper limbs and their QOM in daily life and indirectly influences the motor functions via those parameters.

Motor primitives in space and time via targeted gain modulation in cortical networks

Motor cortex (M1) exhibits a rich repertoire of neuronal activities to support the generation of complex movements. Although recent neuronal-network models capture many qualitative aspects of M1 dynamics, they can generate only a few distinct movements. Additionally, it is unclear how M1 efficiently controls movements over a wide range of shapes and speeds. We demonstrate that modulation of neuronal input-output gains in recurrent neuronal-network models with a fixed architecture can dramatically reorganize neuronal activity and thus downstream muscle outputs. Consistent with the observation of diffuse neuromodulatory projections to M1, a relatively small number of modulatory control units provide sufficient flexibility to adjust high-dimensional network activity using a simple reward-based learning rule. Furthermore, it is possible to assemble novel movements from previously learned primitives, and one can separately change movement speed while preserving movement shape. Our results provide a new perspective on the role of modulatory systems in controlling recurrent cortical activity.

Higher primate-like direct corticomotoneuronal connections are transiently formed in a juvenile subprimate mammal

The corticospinal (CS) tract emerged and evolved in mammals, and is essentially involved in voluntary movement. Over its phylogenesis, CS innervation gradually invaded to the ventral spinal cord, eventually making direct connections with spinal motoneurons (MNs) in higher primates. Despite its importance, our knowledge of the origin of the direct CS-MN connections is limited; in fact, there is controversy as to whether these connections occur in subprimate mammals, such as rodents. Here we studied the retrograde transsynaptic connection between cortical neurons and MNs in mice by labeling the cells with recombinant rabies virus. On postnatal day 14 (P14), we found that CS neurons make direct connections with cervical MNs innervating the forearm muscles. Direct connections were also detected electrophysiologically in whole cell recordings from identified MNs retrogradely-labeled from their target muscles and optogenetic CS stimulation. In contrast, few, if any, lumbar MNs innervating hindlimbs showed direct connections on P18. Moreover, the direct CS-MN connections observed on P14 were later eliminated. The transient CS-MN cells were distributed predominantly in the M1 and S1 areas. These findings provide insight into the ontogeny and phylogeny of the CS projection and appear to settle the controversy about direct CS-MN connections in subprimate mammals.

From hand to mouth: monkeys require greater effort in motor preparation for voluntary control of vocalization than for manual actions

Voluntary control of vocal production is an essential component of the language faculty, which is thought to distinguish humans from other primates. Recent experiments have begun to reveal the capability of non-human primates to perform vocal control; however, the mechanisms underlying this ability remain unclear. Here, we revealed that Japanese macaque monkeys can learn to vocalize voluntarily through a different mechanism than that used for manual actions. The monkeys rapidly learned to touch a computer monitor when a visual stimulus was presented and showed a capacity for flexible adaptation, such that they reacted when the visual stimulus was shown at an unexpected time. By contrast, successful vocal training required additional time, and the monkeys exhibited difficulty with vocal execution when the visual stimulus appeared earlier than expected; this occurred regardless of extensive training. Thus, motor preparation before execution of an action may be a key factor in distinguishing vocalization from manual actions in monkeys; they do not exhibit a similar ability to perform motor preparation in the vocal domains. By performing direct comparisons, this study provides novel evidence regarding differences in motor control abilities between vocal and manual actions. Our findings support the suggestion that the functional expansion from hand to mouth might be a critical evolutionary event for the acquisition of voluntary control of vocalizations.

Implicit Grasp Force Representation in Human Motor Cortical Recordings

In order for brain-computer interface (BCI) systems to maximize functionality, users will need to be able to accurately modulate grasp force to avoid dropping heavy objects while also being able to handle fragile items. We present a case-study consisting of two experiments designed to identify whether intracortical recordings from the motor cortex of a person with tetraplegia could predict intended grasp force. In the first task, we were able classify neural responses to attempted grasps of four objects, each of which required similar grasp kinematics but different implicit grasp force targets, with 69% accuracy. In the second task, the subject attempted to move a virtual robotic arm in space to grasp a simple virtual object. For each trial, the subject was asked to grasp the virtual object with the force appropriate for one of the four objects from the first experiment, with the goal of measuring an implicit representation of grasp force. While the subject knew the grasp force during all phases of the trial, accurate classification was only achieved during active grasping, not while the hand moved to, transported, or released the object. In both tasks, misclassifications were most often to the object with an adjacent force requirement. In addition to the implications for understanding the representation of grasp force in motor cortex, these results are a first step toward creating intelligent algorithms to help BCI users grasp and manipulate a variety of objects that will be encountered in daily life.

Clinical Trial Identifier: NCT01894802 https://clinicaltrials.gov/ct2/show/NCT01894802.

A Neural Population Mechanism for Rapid Learning

Long-term learning of language, mathematics, and motor skills likely requires cortical plasticity, but behavior often requires much faster changes, sometimes even after single errors. Here, we propose one neural mechanism to rapidly develop new motor output without altering the functional connectivity within or between cortical areas. We tested cortico-cortical models relating the activity of hundreds of neurons in the premotor (PMd) and primary motor (M1) cortices throughout adaptation to reaching movement perturbations. We found a signature of learning in the "output-null" subspace of PMd with respect to M1 reflecting the ability of premotor cortex to alter preparatory activity without directly influencing M1. The output-null subspace planning activity evolved with adaptation, yet the "output-potent" mapping that captures information sent to M1 was preserved. Our results illustrate a population-level cortical mechanism to progressively adjust the output from one brain area to its downstream structures that could be exploited for rapid behavioral adaptation.

Synergies in coordination: a comprehensive overview of neural, computational, and behavioral approaches

At face value, the term "synergy" provides a unifying concept within a fractured field that encompasses complementary neural, computational, and behavioral approaches. However, the term is not used synonymously by different researchers but has substantially different meanings depending on the research approach. With so many operational definitions for the one term, it becomes difficult to use as either a descriptive or explanatory concept, yet it remains pervasive and apparently indispensable. Here we provide a summary of different approaches that invoke synergies in a descriptive or explanatory context, summarizing progress, not within the one approach, but across the theoretical landscape. Bernstein's framework of flexible hierarchical control may provide a unifying framework here, since it can incorporate divergent ideas about synergies. In the current motor control literature, synergy may refer to conceptually different processes that could potentially operate in parallel, across different levels within the same hierarchical control scheme. There is evidence for the concurrent existence of synergies with different features, both "hard-wired" and "soft-wired," and task independent and task dependent. By providing a comprehensive overview of the multifaceted ideas about synergies, our goal is to move away from the compartmentalization and narrow the focus on one level and promote a broader perspective on the control and coordination of movement.

Context-dependent tactile texture-sensitivity in monkey M1 and S1 cortex

Caudal primary motor cortex (M1, area 4) is sensitive to cutaneous inputs, but the extent to which the physical details of complex stimuli are encoded is not known. We investigated the sensitivity of M1 neurons (4 Macaca mulatta monkeys) to textured stimuli (smooth/rough or rough/rougher) during the performance of a texture discrimination task and, for some cells, during a no-task condition (same surfaces; no response). The recordings were made from the hemisphere contralateral to the stimulated digits; the motor response (sensory decision) was made with the nonstimulated arm. Most M1 cells were modulated during surface scanning in the task (88%), but few of these were texture-related (24%). In contrast, 44% of M1 neurons were texture related in the no-task condition. Recordings from the neighboring primary somatosensory cortex (S1), the potential source of texture-related signals to M1, showed that S1 neurons were significantly more likely to be texture related during the task (57 vs 24%) than M1. No difference was observed in the no-task condition (52 vs. 44%). In these recordings, the details about surface texture were relevant for S1 but not for M1. We suggest that tactile inputs to M1 were selectively suppressed when the animals were engaged in the task. S1 was spared these controls, as the same inputs were task-relevant. Taken together, we suggest that the suppressive effects are most likely exerted directly at the level of M1, possibly through the activation of a top-down gating mechanism specific to motor set/intention. NEW & NOTEWORTHY Sensory feedback is important for motor control, but we have little knowledge of the contribution of sensory inputs to M1 discharge during behavior. We showed that M1 neurons signal changes in tactile texture, but mainly outside the context of a texture discrimination task. Tactile inputs to M1 were selectively suppressed during the task because this input was not relevant for the recorded hemisphere, which played no role in generating the discrimination response.

Topographic precision in sensory and motor corticostriatal projections varies across cell type and cortical area

The striatum shows general topographic organization and regional differences in behavioral functions. How corticostriatal topography differs across cortical areas and cell types to support these distinct functions is unclear. This study contrasted corticostriatal projections from two layer 5 cell types, intratelencephalic (IT-type) and pyramidal tract (PT-type) neurons, using viral vectors expressing fluorescent reporters in Cre-driver mice. Corticostriatal projections from sensory and motor cortex are somatotopic, with a decreasing topographic specificity as injection sites move from sensory to motor and frontal areas. Topographic organization differs between IT-type and PT-type neurons, including injections in the same site, with IT-type neurons having higher topographic stereotypy than PT-type neurons. Furthermore, IT-type projections from interconnected cortical areas have stronger correlations in corticostriatal targeting than PT-type projections do. As predicted by a longstanding model, corticostriatal projections of interconnected cortical areas form parallel circuits in the basal ganglia.

Changes of motor corticobulbar projections following different lesion types affecting the central nervous system in adult macaque monkeys

Functional recovery from central nervous system injury is likely to be partly due to a rearrangement of neural circuits. In this context, the corticobulbar (corticoreticular) motor projections onto different nuclei of the ponto-medullary reticular formation (PMRF) were investigated in 13 adult macaque monkeys after either, primary motor cortex injury (MCI) in the hand area, or spinal cord injury (SCI) or Parkinson's disease-like lesions of the nigro-striatal dopaminergic system (PD). A subgroup of animals in both MCI and SCI groups was treated with neurite growth promoting anti-Nogo-A antibodies, whereas all PD animals were treated with autologous neural cell ecosystems (ANCE). The anterograde tracer BDA was injected either in the premotor cortex (PM) or in the primary motor cortex (M1) to label and quantify corticobulbar axonal boutons terminaux and en passant in PMRF. As compared to intact animals, after MCI the density of corticobulbar projections from PM was strongly reduced but maintained their laterality dominance (ipsilateral), both in the presence or absence of anti-Nogo-A antibody treatment. In contrast, the density of corticobulbar projections from M1 was increased following opposite hemi-section of the cervical cord (at C7 level) and anti-Nogo-A antibody treatment, with maintenance of contralateral laterality bias. In PD monkeys, the density of corticobulbar projections from PM was strongly reduced, as well as that from M1, but to a lesser extent. In conclusion, the densities of corticobulbar projections from PM or M1 were affected in a variable manner, depending on the type of lesion/pathology and the treatment aimed to enhance functional recovery.

How does human motor cortex regulate vocal pitch in singers?

Vocal pitch is used as an important communicative device by humans, as found in the melodic dimension of both speech and song. Vocal pitch is determined by the degree of tension in the vocal folds of the larynx, which itself is influenced by complex and nonlinear interactions among the laryngeal muscles. The relationship between these muscles and vocal pitch has been described by a mathematical model in the form of a set of 'control rules'. We searched for the biological implementation of these control rules in the larynx motor cortex of the human brain. We scanned choral singers with functional magnetic resonance imaging as they produced discrete pitches at four different levels across their vocal range. While the locations of the larynx motor activations varied across singers, the activation peaks for the four pitch levels were highly consistent within each individual singer. This result was corroborated using multi-voxel pattern analysis, which demonstrated an absence of patterned activations differentiating any pairing of pitch levels. The complex and nonlinear relationships between the multiple laryngeal muscles that control vocal pitch may obscure the neural encoding of vocal pitch in the brain.

Neuroprotective Effects of Mitochondria-Targeted Plastoquinone in a Rat Model of Neonatal Hypoxic⁻Ischemic Brain Injury

Neonatal hypoxia⁻ischemia is one of the main causes of mortality and disability of newborns. To study the mechanisms of neonatal brain cell damage, we used a model of neonatal hypoxia⁻ischemia in seven-day-old rats, by annealing of the common carotid artery with subsequent hypoxia of 8% oxygen. We demonstrate that neonatal hypoxia⁻ischemia causes mitochondrial dysfunction associated with high production of reactive oxygen species, which leads to oxidative stress. Targeted delivery of antioxidants to the mitochondria can be an effective therapeutic approach to treat the deleterious effects of brain hypoxia⁻ischemia. We explored the neuroprotective properties of the mitochondria-targeted antioxidant SkQR1, which is the conjugate of a plant plastoquinone and a penetrating cation, rhodamine 19. Being introduced before or immediately after hypoxia⁻ischemia, SkQR1 affords neuroprotection as judged by the diminished brain damage and recovery of long-term neurological functions. Using vital sections of the brain, SkQR1 has been shown to reduce the development of oxidative stress. Thus, the mitochondrial-targeted antioxidant derived from plant plastoquinone can effectively protect the brain of newborns both in pre-ischemic and post-stroke conditions, making it a promising candidate for further clinical studies.

Cortical excitability varies across different muscles

The aim of the present study was to determine whether significant differences in cortical excitability were evident across different body regions in healthy humans. Threshold tracking transcranial magnetic stimulation (TMS) was undertaken in 28 healthy controls. Short-interval intracortical inhibition [SICI between interstimulus intervals (ISI) 1-7 ms], intracortical facilitation (ICF, between ISI 10-30 ms), resting motor threshold (RMT), cortical silent period (CSP) duration (generated at stimulus intensity 150% RMT), and motor evoked potential amplitude were recorded from the abductor pollicis brevis (APB), tibialis anterior (TA), and trapezius muscles. These muscles were selected as they are frequently affected in neurodegenerative diseases, such as amyotrophic lateral sclerosis. SICI and ICF are measured as a percentage difference between conditioned and an unconditioned test response. SICI was significantly greater when recorded over the APB (9.9 ± 1.5%) and TA (8.6 ± 1.4%) muscles compared with the trapezius (4.5 ± 1.9%, P < 0.05). The CSP duration was significantly shorter (CSP_trapezius, 131.0 ± 6.3 ms; CSP_TA, 175.7 ± 9.9 ms; CSP_APB, 188.3 ± 4.0 ms; P < 0.001) and ICF greater ( P < 0.01) in the trapezius muscle. There were no significant correlations between inhibitory and facilitatory processes recorded across the three muscles. The present study established significant differences in cortical excitability across three body regions, with evidence of more prominent inhibition and less facilitation in the limb muscles. NEW & NOTEWORTHY Cortical excitability of muscles with differing motor functions was assessed using threshold tracking transcranial magnetic stimulation. Significantly greater intracortical inhibition and less facilitation were evident over the limb muscles. These findings could relate to differences in the functional organization of the corticomotoneuronal system innervating different muscle regions.

The Control of Vocal Pitch in Human Laryngeal Motor Cortex

In speech, the highly flexible modulation of vocal pitch creates intonation patterns that speakers use to convey linguistic meaning. This human ability is unique among primates. Here, we used high-density cortical recordings directly from the human brain to determine the encoding of vocal pitch during natural speech. We found neural populations in bilateral dorsal laryngeal motor cortex (dLMC) that selectively encoded produced pitch but not non-laryngeal articulatory movements. This neural population controlled short pitch accents to express prosodic emphasis on a word in a sentence. Other larynx cortical representations controlling voicing and longer pitch phrase contours were found at separate sites. dLMC sites also encoded vocal pitch during a non-speech singing task. Finally, direct focal stimulation of dLMC evoked laryngeal movements and involuntary vocalization, confirming its causal role in feedforward control. Together, these results reveal the neural basis for the voluntary control of vocal pitch in human speech. VIDEO ABSTRACT.

Detailed somatotopy in primary motor and somatosensory cortex revealed by Gaussian population receptive fields

The relevance of human primary motor cortex (M1) for motor actions has long been established. However, it is still unknown how motor actions are represented, and whether M1 contains an ordered somatotopy at the mesoscopic level. In the current study we show that a detailed within-limb somatotopy can be obtained in M1 during finger movements using Gaussian population Receptive Field (pRF) models. Similar organizations were also obtained for primary somatosensory cortex (S1), showing that individual finger representations are interconnected throughout sensorimotor cortex. The current study additionally estimates receptive field sizes of neuronal populations, showing differences between finger digit representations, between M1 and S1, and additionally between finger digit flexion and extension. Using the Gaussian pRF approach, the detailed somatotopic organization of M1 can be obtained including underlying characteristics, allowing for the in-depth investigation of cortical motor representation and sensorimotor integration.

Where does TMS Stimulate the Motor Cortex? Combining Electrophysiological Measurements and Realistic Field Estimates to Reveal the Affected Cortex Position

Much of our knowledge on the physiological mechanisms of transcranial magnetic stimulation (TMS) stems from studies which targeted the human motor cortex. However, it is still unclear which part of the motor cortex is predominantly affected by TMS. Considering that the motor cortex consists of functionally and histologically distinct subareas, this also renders the hypotheses on the physiological TMS effects uncertain. We use the finite element method (FEM) and magnetic resonance image-based individual head models to get realistic estimates of the electric field induced by TMS. The field changes in different subparts of the motor cortex are compared with electrophysiological threshold changes of 2 hand muscles when systematically varying the coil orientation in measurements. We demonstrate that TMS stimulates the region around the gyral crown and that the maximal electric field strength in this region is significantly related to the electrophysiological response. Our study is one of the most extensive comparisons between FEM-based field calculations and physiological TMS effects so far, being based on data for 2 hand muscles in 9 subjects. The results help to improve our understanding of the basic mechanisms of TMS. They also pave the way for a systematic exploration of realistic field estimates for dosage control in TMS.

Neurophysiological Mechanisms Underpinning Stretch-Induced Force Loss

It is well known that prolonged passive muscle stretch reduces maximal muscle force production. There is a growing body of evidence suggesting that adaptations occurring within the nervous system play a major role in this stretch-induced force reduction. This article reviews the existing literature, and some new evidence, regarding acute neurophysiological changes in response to passive muscle stretching. We discuss the possible contribution of supra-spinal and spinal structures to the force reduction after passive muscle stretch. In summary, based on the recent evidence reviewed we propose a new hypothesis that a disfacilitation occurring at the motoneuronal level after passive muscle stretch is a major factor affecting the neural efferent drive to the muscle and, subsequently, its ability to produce maximal force.

Driven to decay: Excitability and synaptic abnormalities in amyotrophic lateral sclerosis

Amyotrophic lateral sclerosis (ALS) is the most common motor neuron (MN) disease and is clinically characterised by the death of corticospinal motor neurons (CSMNs), spinal and brainstem MNs and the degeneration of the corticospinal tract. Degeneration of CSMNs and MNs leads inexorably to muscle wastage and weakness, progressing to eventual death within 3-5 years of diagnosis. The CSMNs, located within layer V of the primary motor cortex, project axons constituting the corticospinal tract, forming synaptic connections with brainstem and spinal cord interneurons and MNs. Clinical ALS may be divided into familial (∼10% of cases) or sporadic (∼90% of cases), based on apparent random incidence. The emergence of transgenic murine models, expressing different ALS-associated mutations has accelerated our understanding of ALS pathogenesis, although precise mechanisms remain elusive. Multiple avenues of investigation suggest that cortical electrical abnormalities have pre-eminence in the pathophysiology of ALS. In addition, glutamate-mediated functional and structural alterations in both CSMNs and MNs are present in both sporadic and familial forms of ALS. This review aims to promulgate debate in the field with regard to the common aetiology of sporadic and familial ALS. A specific focus on a nexus point in ALS pathogenesis, namely, the synaptic and intrinsic hyperexcitability of CSMNs and MNs and alterations to their structure are comprehensively detailed. The association of extramotor dysfunction with neuronal structural/functional alterations will be discussed. Finally, the implications of the latest research on the dying-forward and dying-back controversy are considered.

Where did language come from? Precursor mechanisms in nonhuman primates

At first glance, the monkey brain looks like a smaller version of the human brain. Indeed, the anatomical and functional architecture of the cortical auditory system in monkeys is very similar to that of humans, with dual pathways segregated into a ventral and a dorsal processing stream. Yet, monkeys do not speak. Repeated attempts to pin this inability on one particular cause have failed. A closer look at the necessary components of language, according to Darwin, reveals that all of them got a significant boost during evolution from nonhuman to human primates. The vocal-articulatory system, in particular, has developed into the most sophisticated of all human sensorimotor systems with about a dozen effectors that, in combination with each other, result in an auditory communication system like no other. This sensorimotor network possesses all the ingredients of an internal model system that permits the emergence of sequence processing, as required for phonology and syntax in modern languages.

Deciphering the functional role of spatial and temporal muscle synergies in whole-body movements

Voluntary movement is hypothesized to rely on a limited number of muscle synergies, the recruitment of which translates task goals into effective muscle activity. In this study, we investigated how to analytically characterize the functional role of different types of muscle synergies in task performance. To this end, we recorded a comprehensive dataset of muscle activity during a variety of whole-body pointing movements. We decomposed the electromyographic (EMG) signals using a space-by-time modularity model which encompasses the main types of synergies. We then used a task decoding and information theoretic analysis to probe the role of each synergy by mapping it to specific task features. We found that the temporal and spatial aspects of the movements were encoded by different temporal and spatial muscle synergies, respectively, consistent with the intuition that there should a correspondence between major attributes of movement and major features of synergies. This approach led to the development of a novel computational method for comparing muscle synergies from different participants according to their functional role. This functional similarity analysis yielded a small set of temporal and spatial synergies that describes the main features of whole-body reaching movements.

Distributed representation of pelvic floor muscles in human motor cortex

Human motor cortex can activate pelvic floor muscles (PFM), but the motor cortical representation of the PFM is not well characterized. PFM representation is thought to be focused in the supplementary motor area (SMA). Here we examine the degree to which PFM representation is distributed between SMA and the primary motor cortex (M1), and how this representation is utilized to activate the PFM in different coordination patterns. We show that two types of coordination patterns involving PFM can be voluntarily accessed: one activates PFM independently of synergists and a second activates PFM prior to and in proportion with synergists (in this study, the gluteus maximus muscle - GMM). Functional magnetic resonance imaging (fMRI) showed that both coordination patterns involve overlapping activation in SMA and M1, suggesting the presence of intermingled but independent neural populations that access the different patterns. Transcranial magnetic stimulation (TMS) confirmed SMA and M1 representation for the PFM. TMS also showed that, equally for SMA and M1, PFM can be activated during rest but GMM can only be activated after voluntary drive to GMM, suggesting that these populations are distinguished by activation threshold. We conclude that PFM representation is broadly distributed in SMA and M1 in humans.

The Skinny on Brains: Size Matters

This article is the second of two that addresses the development of the human brain. Last month's article, "The Evolution of Human Capabilities and Abilities," focused on neurons, the basic information-processing units of the nervous system. This month's article examines the evolution of the neocortex, a part of the cerebral cortex concerned with sight and hearing in mammals, regarded as the most developed part of the cortex.

Hierarchical Organization Within the Ventral Premotor Cortex of the Macaque Monkey

Recent studies have revealed that the ventral premotor cortex (PMv) of nonhuman primates plays a pivotal role in various behaviors that require the transformation of sensory cues to appropriate actions. Examples include decision-making based on various sensory cues, preparation for upcoming motor behavior, adaptive sensorimotor transformation, and the generation of motor commands using rapid sensory feedback. Although the PMv has frequently been regarded as a single entity, it can be divided into at least five functionally distinct regions: F4, a dorsal convexity region immediately rostral to the primary motor cortex (M1); F5p, a cortical region immediately rostral to F4, lying within the arcuate sulcus; F5c, a ventral convexity region rostral to F4; and F5a, located in the caudal bank of the arcuate sulcus inferior limb lateral to F5p. Among these, F4 can be further divided into dorsal and ventral subregions (F4d and F4v), which are involved in forelimb and orofacial movements, respectively. F5p contains "mirror neurons" to understand others' actions based on visual and other types of information, and F4d and F5p work together as a functional complex involved in controlling forelimb and eye movements, most efficiently in the execution and completion of coordinated eye-hand movements for reaching and grasping under visual guidance. In contrast, F5c and F5a are hierarchically higher than the F4d, F5p, and F5v complexes, and play a role in decision-making based on various sensory discriminations. Hence, the PMv subregions form a hierarchically organized integral system from decision-making to eye-hand coordination under various behavioral circumstances.

Neural and genetic basis of dexterous hand movements

An ability to control dexterous hand movements is considered to parallel the evolutionary development of the corticospinal tract and the appearance of direct connections between corticospinal neurons and motoneurons (the corticomotoneuronal (CM) pathway), which developed uniquely in higher primates. However, recent studies have revealed that some non-primate animal species have higher levels of dexterity than previously supposed, and in higher primates, various indirect non-CM descending pathways have been shown to participate in the control of dexterous movements. More recently, the CM pathway was shown to exist in rodents during early development, suggesting that rodents and primates diverged in their reliance on the CM pathway at some point in evolution, thus challenging the traditional view of the sequential development of hand control from rodents to primates.

A Systematic Review on Muscle Synergies: From Building Blocks of Motor Behavior to a Neurorehabilitation Tool

The central nervous system (CNS) is believed to utilize specific predefined modules, called muscle synergies (MS), to accomplish a motor task. Yet questions persist about how the CNS combines these primitives in different ways to suit the task conditions. The MS hypothesis has been a subject of debate as to whether they originate from neural origins or nonneural constraints. In this review article, we present three aspects related to the MS hypothesis: (1) the experimental and computational evidence in support of the existence of MS, (2) algorithmic approaches for extracting them from surface electromyography (EMG) signals, and (3) the possible role of MS as a neurorehabilitation tool. We note that recent advances in computational neuroscience have utilized the MS hypothesis in motor control and learning. Prospective advances in clinical, medical, and engineering sciences and in fields such as robotics and rehabilitation stand to benefit from a more thorough understanding of MS.

Challenging human locomotion: stability and modular organisation in unsteady conditions

The need to move over uneven terrain is a daily challenge. In order to face unexpected perturbations due to changes in the morphology of the terrain, the central nervous system must flexibly modify its control strategies. We analysed the local dynamic stability and the modular organisation of muscle activation (muscle synergies) during walking and running on an even- and an uneven-surface treadmill. We hypothesized a reduced stability during uneven-surface locomotion and a reorganisation of the modular control. We found a decreased stability when switching from even- to uneven-surface locomotion (p < 0.001 in walking, p = 0.001 in running). Moreover, we observed a substantial modification of the time-dependent muscle activation patterns (motor primitives) despite a general conservation of the time-independent coefficients (motor modules). The motor primitives were considerably wider in the uneven-surface condition. Specifically, the widening was significant in both the early (+40.5%, p < 0.001) and late swing (+7.7%, p = 0.040) phase in walking and in the weight acceptance (+13.6%, p = 0.006) and propulsion (+6.0%, p = 0.041) phase in running. This widening highlighted an increased motor output’s robustness (i.e. ability to cope with errors) when dealing with the unexpected perturbations. Our results confirmed the hypothesis that humans adjust their motor control strategies’ timing to deal with unsteady locomotion.

Motor Cortex Embeds Muscle-like Commands in an Untangled Population Response

Primate motor cortex projects to spinal interneurons and motoneurons, suggesting that motor cortex activity may be dominated by muscle-like commands. Observations during reaching lend support to this view, but evidence remains ambiguous and much debated. To provide a different perspective, we employed a novel behavioral paradigm that facilitates comparison between time-evolving neural and muscle activity. We found that single motor cortex neurons displayed many muscle-like properties, but the structure of population activity was not muscle-like. Unlike muscle activity, neural activity was structured to avoid "tangling": moments where similar activity patterns led to dissimilar future patterns. Avoidance of tangling was present across tasks and species. Network models revealed a potential reason for this consistent feature: low tangling confers noise robustness. Finally, we were able to predict motor cortex activity from muscle activity by leveraging the hypothesis that muscle-like commands are embedded in additional structure that yields low tangling.

Characterization of corticospinal activation of finger motor neurons during precision and power grip in humans

Direct and indirect corticospinal pathways to finger muscles may play a different role in control of the upper extremity. We used transcranial magnetic stimulation (TMS) and coherence analysis to characterize the corticospinal drive to the first dorsal interosseous (FDI) and abductor pollicis brevis (APB) when active during a precision and power grip task. In experiment 1, single motor units were recorded during precision grip and power grip in 20 adults (25.2 ± 7.1 years). Post-stimulus time histograms (PSTH) were obtained following TMS. In experiment 2, coherence and cross-correlation analysis of the FDI and APB surface EMG were used to investigate the temporal organization of corticospinal drive during precision grip and power grip in 15 adults (27.4 ± 8.1 years). We found no significant differences in PSTH peak onset (26.6 ± 1.9 vs. 26.7 ± 2.0 ms, p = 0.75), maximal peak (27.4 ± 1.9 vs. 27.4 ± 1.9 ms, p = 1.0) or peak duration (2.3 ± 1.1 vs. 2.3 ± 1.0 ms, p = 0.75) for the 11 recovered motor units during precision grip and power grip. Also, no significant difference in coherence or the width of the synchronization peaks during precision grip (7.2 ± 3.7 ms) and power grip (7.9 ± 3.1 ms) could be observed (p = 0.59). The short duration of peaks elicited in the PSTH of single motor units following TMS and central synchronization peaks of voluntarily activated motor units during precision and power grip suggests that the direct corticospinal pathway (the corticomotoneuronal system) is equally involved in the control of both tasks. The data do not support that indirect pathways would make a larger contribution to power grip.

Cortical and subcortical connections of parietal and premotor nodes of the monkey hand mirror neuron network

Mirror neurons (MNs) are a class of cells originally discovered in the monkey ventral premotor cortex (PMv) and inferior parietal lobule (IPL). They discharge during both action execution and action observation and appear to play a crucial role in understanding others' actions. It has been proposed that the mirror mechanism is based on a match between the visual description of actions, encoded in temporal cortical regions, and their motor representation, provided by PMv and IPL. However, neurons responding to action observation have been recently found in other cortical regions, suggesting that the mirror mechanism relies on a wider network. Here we provide the first description of this network by injecting neural tracers into physiologically identified IPL and PMv sectors containing hand MNs. Our results show that these sectors are reciprocally connected, in line with the current view, but IPL MN sectors showed virtually no direct connection with temporal visual areas. In addition, we found that PMv and IPL MN sectors share connections with several cortical regions, including the dorsal and mesial premotor cortex, the primary motor cortex, the secondary somatosensory cortex, the mid-dorsal insula and the ventrolateral prefrontal cortex, as well as subcortical structures, such as motor and polysensory thalamic nuclei and the mid-dorsal claustrum. We propose that each of these regions constitutes a node of an "extended network", through which information relative to ongoing movements, social context, environmental contingencies, abstract rules, and internal states can influence MN activity and contribute to several socio-cognitive functions.