Volitional inspiration is mediated by two independent output channels in the primary motor cortex.

The diaphragm is a multifunctional muscle that mediates both autonomic and volitional inspiration. It is critically involved in vocalization, postural stability, and expulsive core-trunk functions, such as coughing, hiccups, and vomiting. In macaque monkeys, we used retrograde transneuronal transport of rabies virus injected into the left hemidiaphragm to identify cortical neurons that have multisynaptic connections with phrenic motoneurons. Our research demonstrates that representation of the diaphragm in the primary motor cortex (M1) is split into two spatially separate and independent sites. No cortico-cortical connections are known to exist between these two sites. One site is located dorsal to the arm representation within the central sulcus and the second site is lateral to the arm. The dual representation of the diaphragm warrants a revision to the somatotopic map of M1. The dorsal diaphragm representation overlaps with trunk and axial musculature. It is ideally situated to coordinate with these muscles during volitional inspiration and in producing intra-abdominal pressure gradients. The lateral site overlaps the origin of M1 projections to a laryngeal muscle, the cricothyroid. This observation suggests that the coordinated control of laryngeal muscles and the diaphragm during vocalization may be achieved, in part, by co-localization of their representations in M1. The neural organization of the two diaphragm sites underlies a new perspective for interpreting functional imaging studies of respiration and/or vocalization. Furthermore, our results provide novel evidence supporting the concept that overlapping output channels within M1 are a prerequisite for the formation of muscle synergies underlying fine motor control.

The Cortical Motor Areas and the Emergence of Motor Skills: A Neuroanatomical Perspective.

What changes in neural architecture account for the emergence and expansion of dexterity in primates? Dexterity, or skill in performing motor tasks, depends on the ability to generate highly fractionated patterns of muscle activity. It also involves the spatiotemporal coordination of activity in proximal and distal muscles across multiple joints. Many motor skills require the generation of complex movement sequences that are only acquired and refined through extensive practice. Improvements in dexterity have enabled primates to manufacture and use tools and humans to engage in skilled motor behaviors such as typing, dance, musical performance, and sports. Our analysis leads to the following synthesis: The neural substrate that endows primates with their enhanced motor capabilities is due, in part, to (a) major organizational changes in the primary motor cortex and (b) the proliferation of output pathways from other areas of the cerebral cortex, especially from the motor areas on the medial wall of the hemisphere.

The mind-body problem: Circuits that link the cerebral cortex to the adrenal medulla.

Which regions of the cerebral cortex are the origin of descending commands that influence internal organs? We used transneuronal transport of rabies virus in monkeys and rats to identify regions of cerebral cortex that have multisynaptic connections with a major sympathetic effector, the adrenal medulla. In rats, we also examined multisynaptic connections with the kidney. In monkeys, the cortical influence over the adrenal medulla originates from 3 distinct networks that are involved in movement, cognition, and affect. Each of these networks has a human equivalent. The largest influence originates from a motor network that includes all 7 motor areas in the frontal lobe. These motor areas are involved in all aspects of skeletomotor control, from response selection to motor preparation and movement execution. The motor areas provide a link between body movement and the modulation of stress. The cognitive and affective networks are located in regions of cingulate cortex. They provide a link between how we think and feel and the function of the adrenal medulla. Together, the 3 networks can mediate the effects of stress and depression on organ function and provide a concrete neural substrate for some psychosomatic illnesses. In rats, cortical influences over the adrenal medulla and the kidney originate mainly from 2 motor areas and adjacent somatosensory cortex. The cognitive and affective networks, present in monkeys, are largely absent in rats. Thus, nonhuman primate research is essential to understand the neural substrate that links cognition and affect to the function of internal organs.

Functional Anatomy of Basal Ganglia Circuits with the Cerebral Cortex and the Cerebellum.

The neural connections of the basal ganglia provide important insights into their function. Here, we discuss the current perspective on basal ganglia connections with the cerebral cortex and with the cerebellum. We review the evidence that the basal ganglia participate in functionally segregated circuits with motor and non-motor areas of the cerebral cortex. We then discuss the data that the basal ganglia are interconnected with the cerebellum. These results provide the anatomical substrate for basal ganglia contributions not only to the control of movement, but also to a variety of cognitive and affective functions. Furthermore, these findings indicate that abnormal activity in basal ganglia circuits with the cerebral cortex and with the cerebellum may contribute to both motor and non-motor deficits associated with several neurologic and psychiatric conditions.

The Neuropsychology of Movement and Movement Disorders: Neuroanatomical and Cognitive Considerations.

This paper highlights major developments over the past two to three decades in the neuropsychology of movement and its disorders. We focus on studies in healthy individuals and patients, which have identified cognitive contributions to movement control and animal work that has delineated the neural circuitry that makes these interactions possible. We cover advances in three major areas: (1) the neuroanatomical aspects of the "motor" system with an emphasis on multiple parallel circuits that include cortical, corticostriate, and corticocerebellar connections; (2) behavioral paradigms that have enabled an appreciation of the cognitive influences on the preparation and execution of movement; and (3) hemispheric differences (exemplified by limb praxis, motor sequencing, and motor learning). Finally, we discuss the clinical implications of this work, and make suggestions for future research in this area. (JINS, 2017, 23, 768-777).

Posterior parietal cortex contains a command apparatus for hand movements.

Mountcastle and colleagues proposed that the posterior parietal cortex contains a "command apparatus" for the operation of the hand in immediate extrapersonal space [Mountcastle et al. (1975) J Neurophysiol 38(4):871-908]. Here we provide three lines of converging evidence that a lateral region within area 5 has corticospinal neurons that are directly linked to the control of hand movements. First, electrical stimulation in a lateral region of area 5 evokes finger and wrist movements. Second, corticospinal neurons in the same region of area 5 terminate at spinal locations that contain last-order interneurons that innervate hand motoneurons. Third, this lateral region of area 5 contains many neurons that make disynaptic connections with hand motoneurons. The disynaptic input to motoneurons from this portion of area 5 is as direct and prominent as that from any of the premotor areas in the frontal lobe. Thus, our results establish that a region within area 5 contains a motor area with corticospinal neurons that could function as a command apparatus for operation of the hand.

Motor, cognitive, and affective areas of the cerebral cortex influence the adrenal medulla.

Modern medicine has generally viewed the concept of "psychosomatic" disease with suspicion. This view arose partly because no neural networks were known for the mind, conceptually associated with the cerebral cortex, to influence autonomic and endocrine systems that control internal organs. Here, we used transneuronal transport of rabies virus to identify the areas of the primate cerebral cortex that communicate through multisynaptic connections with a major sympathetic effector, the adrenal medulla. We demonstrate that two broad networks in the cerebral cortex have access to the adrenal medulla. The larger network includes all of the cortical motor areas in the frontal lobe and portions of somatosensory cortex. A major component of this network originates from the supplementary motor area and the cingulate motor areas on the medial wall of the hemisphere. These cortical areas are involved in all aspects of skeletomotor control from response selection to motor preparation and movement execution. The second, smaller network originates in regions of medial prefrontal cortex, including a major contribution from pregenual and subgenual regions of anterior cingulate cortex. These cortical areas are involved in higher-order aspects of cognition and affect. These results indicate that specific multisynaptic circuits exist to link movement, cognition, and affect to the function of the adrenal medulla. This circuitry may mediate the effects of internal states like chronic stress and depression on organ function and, thus, provide a concrete neural substrate for some psychosomatic illness.

Cerebellar networks with the cerebral cortex and basal ganglia.

The dominant view of cerebellar function has been that it is exclusively concerned with motor control and coordination. Recent findings from neuroanatomical, behavioral, and imaging studies have profoundly changed this view. Neuroanatomical studies using virus transneuronal tracers have demonstrated that cerebellar output reaches vast areas of the neocortex, including regions of prefrontal and posterior parietal cortex. Furthermore, it has recently become clear that the cerebellum is reciprocally connected with the basal ganglia, which suggests that the two subcortical structures are part of a densely interconnected network. Taken together, these findings elucidate the neuroanatomical substrate for cerebellar involvement in non-motor functions mediated by the prefrontal and posterior parietal cortex, as well as in processes traditionally associated with the basal ganglia.

Transneuronal tracing with neurotropic viruses reveals network macroarchitecture.

A major challenge in systems neuroscience is to unravel the complex matrix of connections that characterize functional circuits within the central nervous system. Retrograde transneuronal transport of rabies virus has proven to be especially useful for this purpose. Here we provide specific examples in which transneuronal transport of rabies virus has been used to unravel multi-synaptic pathways within motor, cognitive and autonomic circuits. Tracing with rabies virus defined: first, the closed-loop organization of cerebellar and basal ganglia circuits with the cerebral cortex; second, the presence of bidirectional communication between the cerebellum and basal ganglia; third, the specific cortical areas that have monosynaptic and/or disynaptic connections to spinal motoneurons in non-human primates; and fourth, the areas in the cerebral cortex with the most direct influence on the sympathetic innervation of the kidney. These examples demonstrate the power of transneuronal tracing with rabies virus to identify the macroarchitecture of complex neural circuits.

Cerebellar vermis is a target of projections from the motor areas in the cerebral cortex.

The cerebellum has a medial, cortico-nuclear zone consisting of the cerebellar vermis and the fastigial nucleus. Functionally, this zone is concerned with whole-body posture and locomotion. The vermis classically is thought to be included within the "spinocerebellum" and to receive somatic sensory input from ascending spinal pathways. In contrast, the lateral zone of the cerebellum is included in the "cerebro-cerebellum" because it is densely interconnected with the cerebral cortex. Here we report the surprising result that a portion of the vermis receives dense input from the cerebral cortex. We injected rabies virus into lobules VB-VIIIB of the vermis and used retrograde transneuronal transport of the virus to define disynaptic inputs to it. We found that large numbers of neurons in the primary motor cortex and in several motor areas on the medial wall of the hemisphere project to the vermis. Thus, our results challenge the classical view of the vermis and indicate that it no longer should be considered as entirely isolated from the cerebral cortex. Instead, lobules VB-VIIIB represent a site where the cortical motor areas can influence descending control systems involved in the regulation of whole-body posture and locomotion. We argue that the projection from the cerebral cortex to the vermis is part of the neural substrate for anticipatory postural adjustments and speculate that dysfunction of this system may underlie some forms of dystonia.

The basal ganglia communicate with the cerebellum.

The basal ganglia and cerebellum are major subcortical structures that influence not only movement, but putatively also cognition and affect. Both structures receive input from and send output to the cerebral cortex. Thus, the basal ganglia and cerebellum form multisynaptic loops with the cerebral cortex. Basal ganglia and cerebellar loops have been assumed to be anatomically separate and to perform distinct functional operations. We investigated whether there is any direct route for basal ganglia output to influence cerebellar function that is independent of the cerebral cortex. We injected rabies virus (RV) into selected regions of the cerebellar cortex in cebus monkeys and used retrograde transneuronal transport of the virus to determine the origin of multisynaptic inputs to the injection sites. We found that the subthalamic nucleus of the basal ganglia has a substantial disynaptic projection to the cerebellar cortex. This pathway provides a means for both normal and abnormal signals from the basal ganglia to influence cerebellar function. We previously showed that the dentate nucleus of the cerebellum has a disynaptic projection to an input stage of basal ganglia processing, the striatum. Taken together these results provide the anatomical substrate for substantial two-way communication between the basal ganglia and cerebellum. Thus, the two subcortical structures may be linked together to form an integrated functional network.

The spinothalamic system targets motor and sensory areas in the cerebral cortex of monkeys.

Classically, the spinothalamic (ST) system has been viewed as the major pathway for transmitting nociceptive and thermoceptive information to the cerebral cortex. There is a long-standing controversy about the cortical targets of this system. We used anterograde transneuronal transport of the H129 strain of herpes simplex virus type 1 in the Cebus monkey to label the cortical areas that receive ST input. We found that the ST system reaches multiple cortical areas located in the contralateral hemisphere. The major targets are granular insular cortex, secondary somatosensory cortex and several cortical areas in the cingulate sulcus. It is noteworthy that comparable cortical regions in humans consistently display activation when subjects are acutely exposed to painful stimuli. We next combined anterograde transneuronal transport of virus with injections of a conventional tracer into the ventral premotor area (PMv). We used the PMv injection to identify the cingulate motor areas on the medial wall of the hemisphere. This combined approach demonstrated that each of the cingulate motor areas receives ST input. Our meta-analysis of imaging studies indicates that the human equivalents of the three cingulate motor areas also correspond to sites of pain-related activation. The cingulate motor areas in the monkey project directly to the primary motor cortex and to the spinal cord. Thus, the substrate exists for the ST system to have an important influence on the cortical control of movement.

Cerebellum and nonmotor function.

Does the cerebellum influence nonmotor behavior? Recent anatomical studies demonstrate that the output of the cerebellum targets multiple nonmotor areas in the prefrontal and posterior parietal cortex, as well as the cortical motor areas. The projections to different cortical areas originate from distinct output channels within the cerebellar nuclei. The cerebral cortical area that is the main target of each output channel is a major source of input to the channel. Thus, a closed-loop circuit represents the major architectural unit of cerebro-cerebellar interactions. The outputs of these loops provide the cerebellum with the anatomical substrate to influence the control of movement and cognition. Neuroimaging and neuropsychological data supply compelling support for this view. The range of tasks associated with cerebellar activation is remarkable and includes tasks designed to assess attention, executive control, language, working memory, learning, pain, emotion, and addiction. These data, along with the revelations about cerebro-cerebellar circuitry, provide a new framework for exploring the contribution of the cerebellum to diverse aspects of behavior.

Supplementary motor area and presupplementary motor area: targets of basal ganglia and cerebellar output.

We used retrograde transneuronal transport of neurotropic viruses in Cebus monkeys to examine the organization of basal ganglia and cerebellar projections to two cortical areas on the medial wall of the hemisphere, the supplementary motor area (SMA) and the pre-SMA. We found that both of these cortical areas are the targets of disynaptic projections from the dentate nucleus of the cerebellum and from the internal segment of the globus pallidus (GPi). On average, the number of pallidal neurons that project to the SMA and pre-SMA is approximately three to four times greater than the number of dentate neurons that project to these cortical areas. GPi neurons that project to the pre-SMA are located in a rostral, "associative" territory of the nucleus, whereas GPi neurons that project to the SMA are located in a more caudal and ventral "sensorimotor" territory. Similarly, dentate neurons that project to the pre-SMA are located in a ventral, "nonmotor" domain of the nucleus, whereas dentate neurons that project to the SMA are located in a more dorsal, "motor" domain. The differential origin of subcortical projections to the SMA and pre-SMA suggests that these cortical areas are nodes in distinct neural systems. Although both systems are the target of outputs from the basal ganglia and the cerebellum, these two cortical areas seem to be dominated by basal ganglia input.

Frontal lobe inputs to the digit representations of the motor areas on the lateral surface of the hemisphere.

We examined the frontal lobe connections of the digit representations in the primary motor cortex (M1), the dorsal premotor area (PMd), and the ventral premotor area (PMv) of cebus monkeys. All of these digit representations lie on the lateral surface of the hemisphere. We used intracortical stimulation to identify the digit representations physiologically, and then we injected different tracers into two of the three cortical areas. This approach enabled us to compare the inputs to two digit representations in the same animal. We found that the densest inputs from the premotor areas to the digit representation in M1 originate from the PMd and the PMv. Both of these premotor areas contain a distinct digit representation, and the two digit representations are densely interconnected. Surprisingly, the projections from the digit representation in the supplementary motor area (SMA) to the PMd and PMv are stronger than the SMA projections to M1. The projections from other premotor areas to M1, the PMd, and the PMv are more modest. Of the three digit areas on the lateral surface, only the PMv receives dense input from the prefrontal cortex. Based on these results, we believe that M1, the PMd, and the PMv form a densely interconnected network of cortical areas that is concerned with the generation and control of hand movements. Overall, the laminar origins of neurons that interconnect the three cortical areas are typical of "lateral" interactions. Thus, from an anatomical perspective, this cortical network lacks a clear hierarchical organization.

Basal ganglia and cerebellar inputs to 'AIP'.

The anterior intraparietal area (AIP) is a subregion of area 7b in posterior parietal cortex. AIP neurons respond to the sight of objects, as well as to the act of grasping them. We used retrograde transneuronal transport of rabies virus to examine subcortical inputs to AIP in the monkey. Virus transport labeled substantial numbers of neurons in the substantia nigra pars reticulata (SNpr), as well as in the dentate nucleus of the cerebellum. The hotspots of labeled neurons in SNpr and in dentate after AIP injections were separate from those created by virus injections into several other parietal or frontal regions. These observations provide the first evidence that a major output nucleus of the basal ganglia, the SNpr, projects to a region of posterior parietal cortex. In addition, our findings provide further support for the concept that posterior parietal cortex is a target of cerebellar output.

An unfolded map of the cerebellar dentate nucleus and its projections to the cerebral cortex.

We have used retrograde transneuronal transport of neurotropic viruses to examine the organization of the projections from the dentate nucleus of the cerebellum to "motor" and "nonmotor" areas of the cerebral cortex. To perform this analysis we created an unfolded map of the dentate. Plotting the results from current and prior experiments on this unfolded map revealed important features about the topography of function in the dentate. We found that the projections to the primary motor and premotor areas of the cerebral cortex originated from dorsal portions of the dentate. In contrast, projections to prefrontal and posterior parietal areas of cortex originated from ventral portions of the dentate. Thus the dentate contains anatomically separate and functionally distinct motor and nonmotor domains.

Motor and nonmotor domains in the monkey dentate.

Our concepts about the organization and functions of the cerebellum have changed substantially in the last 10 years. In recent studies, we used transneuronal virus tracing techniques to demonstrate that the output of the cerebellum of primates projects via the thalamus not only to its classical motor target, the primary motor cortex, but also to "nonmotor" cortical areas in the prefrontal and posterior parietal cortex. We found that dentate neurons projecting to different cortical areas originated from localized regions of the nucleus which we termed "output channels." To compare the locations of the output channels projecting to different cortical targets, we have created an unfolded map of the dentate. This unfolded map revealed that dentate output channels were segregated into spatially separate "motor" and "nonmotor" domains. The output channels in the motor domain exclusively targeted primary motor and premotor areas of the cerebral cortex. These channels were localized in the dorsal portion of the dentate. The output channels in the nonmotor domain projected to prefrontal and posterior parietal cortical areas. The nonmotor domain was confined to the ventral portion of the dentate. In recent studies, we defined a unique molecular marker, monoclonal antibody 8B3, which appears to differentially "recognize" these two domains. Taken together, our results suggest that dentate output is organized according to the functional capabilities of its cortical targets. This organization provides the dentate nucleus with the anatomical substrate to influence not only the control of movement, but also cognitive, higher-order executive and visuospatial functions.

Motor areas in the frontal lobe of the primate.

There has been a substantial change in our concepts about the cortical motor areas. It is now clear that the frontal lobe of primates contains at least six premotor areas that project directly to the primary motor cortex (M1). Two premotor areas, the ventral premotor area (PMv) and the dorsal premotor area (PMd), are located on the lateral surface of the hemisphere. Four premotor areas are located on the medial wall of the hemisphere and include the supplementary motor area (SMA) and three cingulate motor areas. Each of these premotor areas has substantial direct projections to the spinal cord. Corticospinal axons from the premotor areas terminate in the intermediate zone of the spinal cord, and some also terminate in the ventral horn around motoneurons. In this respect, the premotor areas are like M1 and appear to have direct connections with spinal motoneurons, particularly those innervating hand muscles. Furthermore, it is possible to evoke movements of the distal and proximal forelimb using intracortical stimulation at relatively low currents in the premotor areas. Thus, the premotor areas appear to have the potential to influence the control of movement not only at the level of M1, but also more directly at the level of the spinal cord. For these reasons, we have suggested that the premotor areas may operate at a hierarchical level comparable to M1. We propose that each premotor area is a functionally distinct efferent system that differentially generates and/or controls specific aspects of motor behavior.