Voluntary Movement: The Primary Motor Cortex
Motor Functions Are Localized within the Cerebral Cortex
Many Cortical Areas Contribute to the Control of Voluntary Movements
Voluntary Motor Control Appears to Require Serial Processing
The Primary Motor Cortex Plays an Important Role in the Generation of Motor Commands
The Motor Cortex Encodes Both the Kinematics and Kinetics of Movement
Hand and Finger Movements Are Directly Controlled by the Motor Cortex
“…. The physiology of movements is basically a study of the purposive activity of the nervous system as a whole.”
—Gelfand et al., 1966
ONE OF THE MAIN FUNCTIONS OF THE BRAIN is to direct the body’s purposeful interaction with the environment. Understanding how the brain fulfils this role is one of the great challenges in neural science. Because large areas of the cerebral cortex are implicated in voluntary motor control, the study of the cortical control of voluntary movement provides important insights into the functional organization of the cerebral cortex as a whole.
Evolution has endowed mammals with adaptive neural circuitry that allows them to interact in sophisticated ways with the complex environments in which they live. Adaptive patterning of voluntary movements gives mammals a distinct advantage in locating food, finding mates, and avoiding predators, all of which enhance the survival potential of the individual and a species.
The ability to use fingers, hands, and arms in voluntary actions independent of locomotion further helps primates, and especially humans, exploit their environment. Most animals must search their environment for food when hungry. In contrast, humans can also “forage” by using their hands to cook a meal or simply punch a few buttons on a telephone and order takeout. The central neural circuits responsible for such nonlocomotor behavior emerged from and remain intimately associated with the phylogenetically older circuits that control the forelimb during locomotor behaviors.
In this and the following chapter we focus on the control of voluntary movements of the hand and arm in primates. In this chapter we describe the cortical networks that control voluntary movement, particularly the role of the primary motor cortex in the generation of motor commands. In the next chapter we address broader questions about cortical control of voluntary motor behavior, in particular how the cerebral cortex organizes the stream of incoming sensory information to guide voluntary movement.
Voluntary movements differ from reflexes and basic locomotor rhythms in several important ways. By definition they are intentional—they are initiated by an internal decision to act—whereas reflexes are automatically triggered by external stimuli. Even when a voluntary action is directed toward an object, such as reaching for a cup, the cause of action is not the object but an internal decision to interact with the object. The presence of the object provides only the opportunity for acting. Voluntary actions involve choices between alternatives, including the choice not to act. Furthermore, they are organized to achieve some goal in the near or distant future.
Voluntary movements often have a labile, context-dependent association with sensory inputs. The same object can evoke different voluntary actions or no response at all depending on the context in which it appears. That is, the neural circuits controlling voluntary behavior are able to differentiate between an object’s physical properties and its behavioral salience.
The nature and effectiveness of voluntary movements often improve with experience. The motor system can learn new behavioral strategies or new reactions to familiar stimuli to improve behavioral outcomes, and it can learn new skills to cope with predictable variations and perturbations of the environment.
Thus the neural control of voluntary movement involves far more than simply generating a particular pattern of muscle activity. It also involves processes that are usually considered to be more sensory, perceptual, and cognitive in nature. As we shall see, these processes are not rigidly compartmentalized into different neural structures or neural populations.
Motor Functions Are Localized within the Cerebral Cortex
For centuries it was believed that the human cerebral cortex was responsible for only higher-order, conscious mental functions. In the middle of the 19th century the English neurologist John Hughlings Jackson made the controversial proposal that a specific part of the cerebral cortex anterior to the central sulcus has a causal role in movement. He reached this conclusion from treating patients with epileptic seizures that were characterized by repeated spasmodic involuntary movements that sometimes resembled fragments of purposive voluntary actions.
During each episode the seizures always spread to different body parts in a fixed temporal sequence that varied from patient to patient, a pattern called Jacksonian march. Jackson concluded that paroxysmal neural activity generated by epileptic foci located near the central sulcus caused the involuntary seizures. He speculated that the progression of seizures across the body resulted from the spread of paroxysmal activity across small clusters of neurons lying along the central sulcus, each of which controlled movement of a different body part. Jackson’s proposal that a discrete cortical region is involved in the control of movement was a strong argument for the localization of different functions in distinct parts of the cerebral cortex. His observations, along with contemporaneous studies by Pierre Paul Broca and Karl Wernicke on the language deficits resulting from specific cortical lesions, laid the foundation for the modern scientific study of cortical function.
It was not until later in the 19th century, however, when improved anesthesia and aseptic surgical techniques allowed direct experimental study of the cerebral cortex in live subjects, that conclusive experimental evidence for a discrete region of the cerebral cortex devoted to motor function was possible. Gustav Fritsch and Eduard Hitzig in Berlin and David Ferrier in England showed that electrical stimulation of the surface of a limited area of cortex of different surgically anesthetized mammals evoked movements of parts of the contralateral body. The electric currents needed to evoke movements were lowest in a narrow strip along the rostral bank of the central sulcus.
Their experiments demonstrated that, even within this strip of tissue, discrete sites contained neurons with distinctive functions. Stimulation of adjacent sites evoked movements in adjacent body parts, starting with the foot, leg, and tail medially, and proceeding to the trunk, arm, hand, face, mouth, and tongue more laterally. When they lesioned a cortical site at which stimulation had evoked movements of a part of the body, motor control of that body part was perturbed or lost after the animal recovered from surgery. These early experiments showed that the motor strip contains an orderly motor map of the contralateral body and that the integrity of the motor map is necessary for voluntary control of the corresponding body parts.
In the first half of the 20th century more focal electrical stimulation allowed the motor map to be defined in greater detail. Clinton Woolsey and his colleagues tested the functional organization of the motor cortex in several species of mammals, whereas Wilder Pen-field and co-workers tested discrete sites in human neurosurgical patients (Figure 37–1). Their findings demonstrated that the same general topographic organization is conserved across many species. One important discovery was that the motor map is not a point-to-point representation of the body. Instead, the most finely controlled body parts, such as the fingers, face, and mouth, are represented in the motor map by disproportionately large areas, reflecting the larger number of neurons needed for fine motor control.
Figure 37–1 The motor cortex contains a topographic map of motor output to different parts of the body.
A. Studies by Clinton Woolsey and colleagues confirmed that the representation of different body parts in the monkey follows an orderly plan: Motor output to the foot and leg is medial, whereas the arm, face, and mouth areas are more lateral. The areas of cortex controlling the foot, hand, and mouth are much larger than the regions controlling other parts of the body.
B. Wilder Penfield and colleagues showed that the human motor cortex motor map has the same general mediolateral organization as in the monkey. However, the areas controlling the hand and mouth are even larger than in monkeys, whereas the area controlling the foot is much smaller. Penfield emphasized that this cartoon illustrated the relative size of the representation of each body part in the motor map; he did not claim that each body part was controlled by a single separate part of the motor map.
Woolsey and Penfield both recognized, however, that their simple motor map masked a deeper complexity. Today the best-studied regions of the map are those parts controlling the arm and hand. Recent mapping studies have revealed that the neurons controlling the muscles of the digits, hand, and distal arm tend to be concentrated within a central zone, whereas those controlling more proximal arm muscles are located in a horseshoe-shaped ring around the central core (Figure 37–2A). Furthermore, across the concentrically organized areas of the arm motor map there is extensive overlap of stimulation sites that causes contractions of muscles acting across different joints; conversely, each muscle can be activated by stimulating many widely dispersed sites (Figure 37–2B). Moreover, different combinations of muscle contractions and joint motions can be evoked by stimulating different sites. Finally, local horizontal axonal connections link different sites, allowing neural activity at multiple output sites in the map to be coordinated during the formation of motor commands.
Figure 37–2 Internal organization of the motor map of the arm in the motor cortex.
A. The arm motor map in monkeys has a concentric, horseshoe-shaped organization: Neurons that control the distal arm (digits and wrist) are concentrated in a central core (yellow) surrounded by neurons that control the proximal arm (elbow and shoulder; blue). The neuron populations that control the distal and proximal parts of the arm overlap extensively in a zone of proximal-distal cofacilitation (green). The arm motor representation is seen in its normal anatomical location in the anterior bank of the central sulcus (left), and also after flattening and rotation to bring it into approximate alignment with the microstimulation maps in part B. (Reproduced, with permission, from Park et al. 2001.)
B. Microstimulation of several sites in the arm motor map can produce rotations of the same joint. Neurons that control wrist movements are concentrated in the central core whereas those that regulate shoulder movements are distributed around the core, with some overlap between the two populations. In these maps, the height of each peak is scaled to the inverse of the stimulation current: the higher the peak, the lower the current necessary to produce a response. The distribution and overlap of stimulation sites that evoke contractions of muscles in the shoulder (deltoid) and wrist (extensor carpi radialis) are even more extensive than that of sites for joint rotations. The yellow, green, and blue color zones on these maps correspond only approximately to the functional zones identified in the motor map of part A. (Reproduced, with permission, from Humphrey and Tanji 1991.)
To date, studies have not revealed any repeating functional elements in the fine details of the motor map for the arm and hand analogous to the ocular-dominance bands and orientation pinwheels in the visual cortex. However, the complex, extensively overlapped organization of the arm motor map and the network of local horizontal connections likely provide a mechanism to coordinate whole-limb actions such as reaching to grasp and manipulate an object.
Many Cortical Areas Contribute to the Control of Voluntary Movements
Voluntary Motor Control Appears to Require Serial Processing
Much of what we do in everyday life involves a sequence of actions. One normally does not take a shower after getting dressed or put cake ingredients into the oven to bake before blending them into a batter. It seems logical that most brain functions are also serial.
Largely on the basis of indirect psychological studies, the neural processes by which the brain controls voluntary behavior are commonly divided into three sequential stages. First, perceptual mechanisms generate a unified sensory representation of the external world and the individual within it. Next, cognitive processes use this internal replica of the world to decide on a course of action. Finally, the selected motor plan is relayed to action systems for implementation (Figure 37–3A).
Figure 37–3 Cortical control of voluntary behavior appears to be organized in a hierarchical series of operations.
A. The brain’s control of voluntary behavior has often been divided into three main operational stages, in which perception generates an internal neuronal image of the world, cognition analyzes and reflects on this image to decide what to do, and the final decision is relayed to action systems for execution. However, this three-stage serial organization was largely based on introspective psychological studies rather than on direct neurophysiological study of neural mechanisms.
B. Each of the three main operational stages is presumed to involve its own serial processes. For example, the “action” stage that converts an intention into a physical movement is often presumed to involve a hierarchy of operations that transform a general plan into progressively more detailed instructions about its implementation. The model shown here, inspired by early controller designs for multijoint robots, suggests that the brain plans a chosen reaching movement by first calculating the extrinsic kinematics of the movement (eg, target location, trajectory of hand displacement from the starting location to the target location), then calculating the required intrinsic kinematics (eg, joint rotations) and finally the causal kinetics or dynamics of movement (eg, forces, torques, and muscle activity). (See also Figure 33–2.)
The final stage, execution of the chosen motor plan, also appears to be serial in nature. It has often been modeled as a series of sensorimotor transformations of representations of a movement into different coordinate frameworks, progressing from a general description of the overall form of the movement to increasingly specific details, culminating in patterns of muscle activity (Figure 37–3B).
According to this serial scheme, each sequential operation is encoded by a different neuronal population. Each population encodes specific features or parameters of the intended movement in a particular coordinate system, such as the direction of movement of the hand through space or the patterns of muscle contractions and forces. These several populations are connected serially and only the last population in the chain projects to the spinal cord.
As we shall see in this and the next chapter, this model has some heuristic value for describing how the brain is organized to control voluntary movement, but direct neurophysiological studies of neural mechanisms show that a strict adherence to serial processing is simplistic and incorrect. We know now for instance that the brain does not have a single, unified perceptual representation of the world (see Chapter 38). The serial scheme also wrongly implies that the only role of the motor system is to determine which muscles to contract, when, and by how much. We now know that several cortical motor areas also play a critical role in the actual choice of what action to take, a process that is usually considered more “cognitive” than “motor.” This is described in more detail in Chapter 38.
The Functional Anatomy of Precentral Motor Areas is Complex
In the early 20th century Alfred Campbell and Korbinian Brodmann divided the human cerebral cortex into a large number of cytoarchitectonic areas with distinct anatomical features. They noted that the precentral cortex in the gyri immediately rostral to the central sulcus lacks the six layers characteristic of most cerebral cortex. It lacks a distinct internal granule cell layer and thus is often called agranular cortex. Campbell and Brodmann subdivided the precentral cortex into caudal and rostral parts, which Brodmann designated cytoarchitectonic areas 4 and 6 (Figure 37–4).
Figure 37–4 Multiple areas of the cerebral cortex are devoted to motor control and many are somatotopically organized.
A. Based on their histological studies at the beginning of the 20th century, Korbinian Brodmann and Alfred Campbell each divided the precentral cortex in humans into two anatomically distinct cytoarchitectonic areas: the primary motor cortex (Brodmann’s area 4) and premotor cortex (Brodmann’s area 6). Subsequent studies by Woolsey and colleagues led to subdivision of the premotor cortex into medial and lateral halves, the supplementary motor area and lateral premotor cortex, respectively. Since those pioneering studies the human premotor cortex and supplementary motor area have been subdivided into several smaller functional areas whose homologs can be seen in nonhuman primates. The medial surface of the hemisphere is shown in this and other similar figures as if reflected in a mirror.
B. More recent studies have subdivided the premotor cortex of macaque monkeys into several more functional zones with different patterns of cortical and subcortical anatomical connections and different neuronal responses during various motor tasks. A similarly detailed functional subdivision of the parietal cortex has also been made (not illustrated). (M1, primary motor cortex; Pre-SMA, pre-supplementary motor area; PMd, dorsal premotor cortex; Pre-PMd, pre-dorsal premotor cortex; PMv, ventral premotor cortex.)
Campbell proposed that these two regions were functionally distinct motor areas. He thought that the caudal region, or primary motor cortex, controlled the motor apparatus in the spinal cord and generated simple movements. The rostral region, he argued, was specialized for higher-order aspects of motor control and for movements that are more complex, conditional, and voluntary in nature. He thought that these areas influenced movement indirectly by projecting to the primary motor cortex and so he named them the premotor cortex.
Some years later, while mapping motor areas of cortex with electrical stimuli, Clinton Woolsey and his colleagues discovered that movements of the contralateral body can be evoked not only by electrical stimulation of the primary motor cortex, but also by stimulating a second region in a part of the premotor cortex on the medial surface of the cerebral hemisphere now known as the supplementary motor area (Figure 37–4B). The motor map of different body parts evoked by stimulation of the supplementary motor area is less detailed than that of the primary motor cortex and lacks the enlarged distal arm and hand representation seen in the primary motor cortex. Stimulation of the supplementary motor area can evoke movements on both sides of the body or halt ongoing voluntary movements, effects that rarely result from stimulation of the primary motor cortex.
Anatomical and functional studies in humans and nonhuman primates over the past 25 years have radically changed the view of how the precentral cortex is organized functionally. First, architectonic studies demonstrated that Brodmann’s area 6 is not homogeneous but consists of several distinct subareas. Second, these subareas have specific connections among themselves and with the rest of the cerebral cortex. Third, functional studies found that each subarea separately controls movements of some or all parts of the body and that the properties of neurons in each subarea differ in important ways. These areas are identified by two different nomenclatures in the literature.
As a result, in current maps of the precentral cortex Brodmann’s area 6 is usually divided into five or six functional areas in addition to the primary motor cortex (or area F1) in Brodmann’s area 4 (Figure 37–4B). The classical supplementary motor area originally identified by Woolsey on the medial cortical surface is now split into two functional regions. The more caudal part is called the supplementary motor area proper (area F3), whereas the more rostral part is the pre-supplementary motor area (F6). The caudal and rostral parts of the dorsal convexity of area 6 are called the dorsal premotor cortex (F2) and predorsal premotor cortex (F7), respectively. The ventral convexity of Brodmann’s area 6 has also been identified as a separate functional area called the ventral premotor cortex, and has been further subdivided into two subareas called F4 and F5 (Figure 37–4B). Finally, three additional motor areas outside Brodmann’s area 6, in the rostral cingulate cortex, have been delineated recently.
The multiplicity of cortical motor areas would seem redundant if their only role was to initiate or coordinate muscle activity. However, we now know that neurons in these areas have unique properties and interact to perform diverse operations that select, plan, and generate actions appropriate to external and internal needs and context.
The Anatomical Connections of the Precentral Motor Areas Do Not Validate a Strictly Serial Organization
To understand the roles of these multiple precentral motor areas in voluntary motor control, it is important to know their connections with one another, their connections with other cortical areas, and their descending projections.
The cortical motor areas are interconnected by complex patterns of reciprocal, convergent, and divergent projections rather than simple serial pathways. The supplementary motor area, dorsal premotor cortex, and ventral premotor cortex have somatotopically organized reciprocal connections not only with the primary motor cortex but also with each other. The primary motor cortex and supplementary motor area receive somatotopically organized input from the primary somatosensory cortex and the rostral parietal cortex, whereas the dorsal and ventral premotor areas are reciprocally connected with progressively more caudal, medial, and lateral parts of the parietal cortex. These somatosensory and parietal inputs provide the primary motor cortex and caudal premotor regions with sensory information to organize and guide motor acts.
In contrast, the pre-supplementary and pre-dorsal premotor areas do not project to the primary motor cortex and are only weakly connected with the parietal lobe. They receive higher-order cognitive information through reciprocal connections with the pre-frontal cortex and so may impose more arbitrary context-dependent control over voluntary behavior.
Several cortical motor regions project in multiple parallel tracts to subcortical areas of the brain as well as the spinal cord. The best studied output path is the pyramidal tract, which originates in cortical layer V in a number of precentral and parietal cortical areas. Precentral areas include not only primary motor cortex but also the supplementary motor and dorsal and ventral premotor areas. The pre-supplementary motor and pre-dorsal premotor areas do not send axons to the spinal cord; their descending output reaches the spinal cord indirectly through projections to other subcortical structures. Parietal areas that contribute descending axons to the pyramidal tract include the primary somatosensory cortex and adjacent rostral parts of the superior and inferior parietal lobules.
Many pyramidal tract axons decussate at the pyramid and project to the spinal cord itself, forming the corticospinal tract (Figure 37–5A). Because several cortical areas contribute axons to the corticospinal tract, the traditional view that the primary motor cortex is the spinal cord is incorrect. Instead, several premotor and parietal areas of cortex can also influence spinal motor function through their own corticospinal projections.
Figure 37–5 Cortical origins of the corticospinal tract. (Reproduced, with permission, from Dum and Strick 2002.)
A. Neurons that modulate muscle activity in the contralateral arm and hand originate in the primary motor cortex (M1) and many subdivisions of the premotor cortex (PMd, PMv, SMA) and project their axons into the spinal cord cervical enlargement. Corticospinal fibers projecting to the leg, trunk, and other somatotopic parts of the brain stem and spinal motor system originate in the other parts of the motor and premotor cortex. (M1, primary motor cortex; SMA, supplementary motor area; PMd, dorsal premotor cortex; PMv, ventral premotor cortex; CMAd, dorsal cingulate motor area; CMAv, ventral cingulate motor area; CMAr, rostral cingulate motor area.)
B. The axons of corticospinal fibers from the primary motor cortex, supplementary motor area, and cingulate motor areas terminate on interneuronal networks in the intermediate laminae (VI, VII, and VIII) of the spinal cord. Only the primary motor cortex contains neurons whose axons terminate directly on spinal motor neurons in the most ventral and lateral part of the spinal ventral horn. Rexed’s laminae I to IX of the dorsal and ventral horns are shown in faint outline. The dense cluster of labeled axons adjacent to the dorsal horn (upper left) in each section are the corticospinal axons descending in the dorsolateral funiculus, before entering the spinal intermediate and ventral laminae.
Many corticospinal axons from the primary motor cortex and premotor areas in primates, and virtually all corticospinal axons in other mammals, terminate on spinal interneurons in the intermediate region of the spinal cord (Figure 37–5B). These interneurons are components of reflex and pattern-generating circuits that produce stereotypical motor synergies and locomotor rhythms (see Chapter 36). In primates much of the control exerted by the primary motor cortex on spinal motor circuits and all of the control from premotor areas is mediated indirectly through these descending cortical projections to spinal interneurons.
In primates the terminals of some corticospinal axons also extend into the ventral horn of the spinal cord (lamina IX) where they arborize and contact the dendrites of spinal motor neurons (Figure 37–6B; Figure 37–5B). These monosynaptically projecting cortical neurons are called corticomotoneurons. The axons of these neurons become a progressively larger component of the corticospinal tract in primate phylogeny from prosimians to monkeys, great apes, and humans.
Figure 37–6 Corticomotoneurons activate complex muscle patterns through divergent connections with spinal motor neurons that innervate different arm muscles.
A. Corticomotoneurons, which project monosynaptically to spinal motor neurons, are located almost exclusively in the caudal part of the primary motor cortex (M1), within the anterior bank of the central sulcus. The corticomotoneurons that control a single hand muscle are widely distributed throughout the arm motor map, and there is extensive overlap of the distribution of neurons projecting to different hand muscles. The distributions of the cell bodies of corticomotoneurons that project to the spinal motor neuron pools that innervate the adductor pollicis, abductor pollicis longus, and extensor digitorum communis (shown on the right), illustrate this pattern. (R, rostral; M medial.) (Reproduced, with permission, from Rathelot and Strick 2006.)
B. A single corticomotoneuron axon terminal is shown arborized in the ventral horn of one segment of the spinal cord.
It forms synapses with the spinal motor neuron pools of four different intrinsic hand muscles (yellow and blue zones) as well as with surrounding interneuronal networks. Each axon has several such terminal arborizations distributed along several spinal segments. (Reproduced, with permission, from Shinoda, Yokata, and Futami 1981.)
C. Different colonies of corticomotoneurons in the primary motor cortex terminate on different combinations of spinal interneuron networks and spinal motor neuron pools, thus activating different combinations of agonist and antagonist muscles. Many other corticospinal axons terminate only on spinal interneurons (not shown). The figure shows corticomotoneuronal projections largely onto extensor motor neuron pools. Flexor motor pools receive similar complex projections (not shown). (Modified, with permission, from Cheney, Fetz, and Palmer 1985.)
In monkeys corticomotoneurons are found only in the most caudal part of the primary motor cortex that lies within the anterior bank of the central sulcus. There is extensive overlap in the distribution of the corticomotoneurons that project to the spinal motor neuron pools innervating different muscles (Figure 37–6A). In monkeys more corticomotoneurons project to the motor neuron pools for muscles of the digits, hand, and wrist than to those for more proximal parts of the arm.
The terminal of a single corticomotoneuron axon often branches and terminates on spinal motor neurons for several different agonist muscles, and can also influence the contractile activity of still more muscles through synapses on spinal interneurons (Figure 37–6B, C). This termination pattern is functionally organized to produce coordinated patterns of activity in a muscle field of agonist and antagonist muscles. Most frequently, a single corticomotoneuron axon directly excites the spinal motor neurons for several agonist muscles and indirectly suppresses the activity of some antagonist muscles through local inhibitory interneurons (Figure 37–6C). The fact that corticomotoneurons are more prominent in humans than in monkeys may be one of the reasons why lesions of the primary motor cortex have such a devastating effect on motor control in humans compared to lower mammals (Box 37–1).
Although neurons in several motor-cortical areas send axons into the corticospinal tract, the primary motor cortex has the most direct access to spinal motor neurons, including the monosynaptic projections of corticomotoneurons. However, the corticospinal tract is not the only pathway for descending control signals to spinal motor circuits. The spinal cord also receives inputs from the rubrospinal, reticulospinal, and vestibulospinal tracts. These pathways influence movement through monosynaptic terminations onto spinal interneurons and spinal motor neurons.
In summary, a strictly serial organization of voluntary movement would require a pattern of serial connections between cortical areas, ending at the primary motor cortex, which then projects to the spinal cord. In reality, however, the multiple precentral and parietal cortical motor areas are interconnected by a complex network of reciprocal, divergent, and convergent axonal projections. Moreover, several cortical areas project to the spinal cord in parallel with projections from the primary motor cortex. Finally, the spinal motor circuits receive inputs from several subcortical motor centers in addition to those from the cerebral cortex.
The Primary Motor Cortex Plays an Important Role in the Generation of Motor Commands
In the 1950s Herbert Jasper and colleagues pioneered chronic microelectrode recordings from alert animals engaged in natural behaviors. This approach, which allows researchers to study the activity of single neurons while animals perform a controlled behavioral task, has made enormous contributions to our knowledge of the neuronal mechanisms underlying many brain functions. A microelectrode can also be used to deliver weak electrical currents to a small volume of tissue around its tip. When used in the cerebral cortex, this technique is called intracortical microstimulation.
These methods have been complemented more recently by techniques that can be used in human subjects, such as functional imaging and transcranial magnetic stimulation. Nearly every insight that will be described in the rest of this chapter and in Chapter 38 has been derived from these techniques.
Edward Evarts, the first to use chronic microelectrode recordings to study the primary motor cortex in behaving monkeys, made several discoveries of fundamental importance. He found that single neurons in this area discharge during movements of a limited part of the contralateral body, such as one or two adjacent joints in the hand, arm, or leg (Figure 37–9). Some neurons discharge during flexion of a particular joint and are reciprocally suppressed during extension, whereas other cells display the opposite pattern. This movement-related activity typically begins 50 to 150 ms before the onset of agonist muscle activity. These pioneering studies suggested that single neurons in primary motor cortex generate signals that provide specific information about movements of specific parts of the body before those movements are executed.
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