Primary Motor Cortex

M1 occupies a broad band of cortex just anterior to somatosensory cortex (Fig. 1). In primates with a central sulcus, M1 is located just anterior to somato-sensory areas 3a and 3b, on the anterior bank of the central sulcus, and extends onto the surface of most of the precentral gyrus. M1 closely corresponds to the region described by Brodmann as area 4 or the area of giant pyramidal cells (Betz cells). The term area 4 is commonly used as an alternate designation for M1. However, not all portions of M1 contain Betz cells. The lateral part of M1 is characterized by smaller size pyramidal cells. Thus, some definitions of the cortex comprising M1 may have included some of area 6. M1 is one of the thickest portions of cerebral cortex. M1 can be identified by its characteristic representation of body movements, unique histological appearance (most notably, a lack of an obvious layer 4 of granule cells and large layer 5 pyramidal cells), and consistent

Bottom View Brain

Figure 1 Motor areas of primates shown on a macaque monkey brain. (Bottom) A dorsolateral view of the frontal two-thirds of the brain. (Top) A view of cortex of the medial surface of the left cerebral hemisphere of the brain below shown rotated out so that ventral is at the top, allowing the motor areas to remain continuous. Primary motor cortex (M1) is on the precentral gyrus and it extends into the depths of the central sulcus (CS). Premotor cortex includes dorsal (PMD) and ventral (PMV) fields that appear to have rostral (PMDr) and caudal (PMDc) subdivisions (not shown for PMV). The supplementary motor area (SMA) was the first well-defined premotor area. Recently, a presupplementary motor area (pre-SMA) has been defined just rostral to SMA. On the medial surface of the cerebral hemisphere, two rostral (CMAr) and caudal (CMAc) cingulate motor areas have been defined. Some investigators divide CMAc into ventral (CMAv) and dorsal (CMAd) areas. CgS, cingulate sulcus; FEF, frontal eye field; LS, lateral sulcus; SEF, supplementary eye field; SSA, supplementary sensory area.

pattern of connections with somatosensory areas, premotor cortex, and the spinal cord.

The basic topographic organization of Ml has long been known from early studies in which many locations were electrically stimulated with electrodes placed on the surface of the cortex. Recently, the detailed pattern of how Ml represents movements has been determined with microelectrodes advanced into the cortex to allow the electrode tip to be placed close to the layer 5 pyramidal cells that project to the spinal cord. Thus, lower levels of current can be used to stimulate these output cells and cause movements, a technique known as microstimulation.

A long-standing debate has been whether Ml represents muscles or movements. Recordings from muscles, made while the cortex is microstimulated, indicate that single sites in M1 activate a number of pools of motor neurons in the spinal cord and brain stem so that each site of cortical stimulation activates several muscles that seem to be related in some form of movement. Although one muscle may be activated more strongly than others, the cortex seems to be subdivided into clusters or columns of neurons related to different types of movements or the generation of specifically directed forces. Cortical neurons typically activate several muscles that work together to cause a movement, while they inhibit outputs to muscles that produce the opposite movement. However, the specific muscles involved in making the movement are closely reflected in the activities of at least some neurons in Ml.

Overall, movements of different body parts are represented in a systematic pattern across the mediolateral length of the Ml belt of cortex (Fig. 2). The most medial sites cause tail, toe, foot, and leg movements, whereas more lateral sites cause trunk movements, and a large lateral region is devoted to digit, wrist, and arm movements. The most lateral sites contribute to face and tongue movements. Because the projections of Ml via the pyramidal tract cross in the lower brain stem to terminate on motor neurons on the opposite side of the brain stem or spinal cord, these movements are all of muscles of the opposite side of the body. Parts of Ml that represent the trunk and proximal limbs of the two hemispheres are linked by connections through the corpus callosum. However, callosal connections are distinctly sparse or absent in Ml areas that represent the hands and feet. Thus, movements of hands or feet are controlled independently by a single Ml. Although Ml is devoted to movements of the opposite side of the body, some coordination of movements of the two sides may occur via connections of one Ml with the other, especially for trunk and other proximal movements.

There have been various attempts to characterize the detailed local organization of Ml. There does not seem to be a simple topographic pattern. Instead, the same or similar movements can be evoked from several nearby sites in the same region of Ml, whereas

Somatosensory Cortex Topography

Figure 2 Some of the ipsilateral cortical connections of primary motor cortex, M1, in New World owl monkey. Most of these connections have also been reported for other species of monkeys. The organization of the sensory and motor representations in each area is indicated by shading. Major interconnections are indicated by thick arrows. Less dense connections are indicated by dashed arrows. Motor areas include M1 with rostral (M1r) and caudal (M1c) subdivisions, ventral (PMV) and dorsal (PMD) premotor cortex, supplementary motor area (SMA), presupplementary motor area (pre-SMA), and rostral (CMAr) and caudal (CMAc) divisions of cingulate motor cortex. Somatosensory areas include areas 3a, 3b, 1, and 2 of anterior parietal cortex; the second somatosensory area (S2); and the parietal ventral (PV) and the parietal rostral (PR) areas of lateral parietal cortex and posterior parietal cortex (PP).

Figure 2 Some of the ipsilateral cortical connections of primary motor cortex, M1, in New World owl monkey. Most of these connections have also been reported for other species of monkeys. The organization of the sensory and motor representations in each area is indicated by shading. Major interconnections are indicated by thick arrows. Less dense connections are indicated by dashed arrows. Motor areas include M1 with rostral (M1r) and caudal (M1c) subdivisions, ventral (PMV) and dorsal (PMD) premotor cortex, supplementary motor area (SMA), presupplementary motor area (pre-SMA), and rostral (CMAr) and caudal (CMAc) divisions of cingulate motor cortex. Somatosensory areas include areas 3a, 3b, 1, and 2 of anterior parietal cortex; the second somatosensory area (S2); and the parietal ventral (PV) and the parietal rostral (PR) areas of lateral parietal cortex and posterior parietal cortex (PP).

adjoining sites evoke different but related movements. Thus, M1 appears to consist of a mosaic of small efferent zones or modules, with each module evoking a specific movement. Modules related to a given body region are grouped; within a region a specific type of module often occurs more than once. For example, modules moving different digits tend to be grouped; while bordering each other in different ways. These, in turn, border on modules for wrist, elbow, and even shoulder movements. Since neighboring modules might easily facilitate each other through local interconnections, this arrangement might allow different combinations of useful movements to be programmed in M1.

Stimulation of sites in M1 evokes small or restricted movements only when low levels of current are used. At higher levels of current, additional body parts become involved in more complex movement. For example, wrist movements may be added to digit movements. Thus, cortical sites may be involved in the control of a number of related muscles, provided that the recruitment of additional movements is not due to current spread. In addition, the local circuits in motor cortex are modifiable so that the consequences of electrical stimulation depend somewhat on ongoing sensory events or on recent activity patterns in M1. Conditioning by periods of electrical stimulation of M1, for example, can alter the precise nature of the response evoked by a subsequent stimulation. These effects are likely mediated by local connections within and between cortical modules in M1, allowing some flexibility in performance.

The details of the representations of movements in M1 can be modified by experience and training. The organization of M1 in humans can be evaluated in a noninvasive manner by stimulating motor cortex through the scalp by placing a magnetic coil over various parts of motor cortex and inducing current flow in cortex (transcranial magnetic stimulation). Finger movements can be evoked over a larger region of cortex in highly skilled musicians who use their fingers to play instruments, suggesting that years of practice have changed the organization of their motor cortex so that more neurons in M1 are used to control finger movements. Changes in M1 with practice may also play a role in the recovery of motor control after partial lesions of motor cortex accompanying stroke. Monkeys with damage to parts of M1 that represent finger movements may reorganize so that remaining parts of M1 reinstate control over finger movements. Possibly as a result of cortical reorganization, the monkeys improve in their ability to skillfully use their fingers. Thus, the reinstatement of cortical representations may be responsible for the recovery of motor skills. Other motor areas may also be modified by experience as new motor skills are acquired.

M1 is considered to be a major area for initiating and controlling voluntary movements. Damage to M1 in humans may severely impair the ability to initiate movements, although much recovery occurs over time. Typically, lesions result in decreased use of the impaired contralateral limb or limbs and deficits in manual skill and speed of movement. Partial recovery of lost abilities may depend on the reorganization of remaining parts of M1 to devote more neurons to the needed functions as well as increased involvement of other intact parts of the motor system.

The discharge patterns of neurons in M1 reflect the important role that M1 plays in evoking movements. Neurons subserving a body part activate prior to the movement of that part, as one would expect if they function to initiate and control movements. For a particular movement, many neurons may be active, some much more than others. For a similar but slightly different movement, other neurons will be most active, but many of the same neurons will respond to a lesser degree. Neurons tend to be activated preferentially with some movements. A population of activated neurons has a different activity profile for each type of movement. The type and strength of a given movement depend on what neurons are active and on their levels ofactivity. In addition, some neurons become active in advance of intended movements. A sensory signal indicating that a movement should start soon initiates activity changes in such neurons.

The functions of M1 depend on its connections with other parts of the brain (Fig. 2). Motor behavior may be modified by sensory information, especially from receptors in the skin and muscles. Some of this information reaches M1 from areas of somatosensory cortex, especially areas 3a and 2 of anterior parietal cortex. These areas are activated by inputs from muscle spindle receptors. Feedback from muscle receptors during movements is very important in motor control. Other inputs come from higher order areas of somatosensory cortex such as the second somatosensory area, S2. In addition, many of the premotor areas are densely interconnected with M1. These premotor areas may influence motor behavior partly or largely through altering activity patterns in M1. Another source of sensory information is less direct. The cerebellum receives sensory inputs from muscle receptors and other receptors, integrates and adjusts this information, and sends projections to the specific region of thalamus, which in turn projects to M1 and several premotor areas. M1 sends feedback connections to the thalamus, somatosensory, and premotor cortical areas. The feedback connections from M1 to somatosensory areas of cortex alter the responses of neurons to sensory stimuli so that inputs related to self-initiated movements may be ignored. This feedback may produce corollary discharge in neurons leading to a sensation of movement. M1 also receives the broad cortical input indirectly by way of the internal segment of the globus pallidus, the portion of basal ganglia of the extrapyramidal system that projects to the motor thalamus. Thus, the processing loops in the motor network are complex. A major output of the M1 is via the pyramidal tract to motor neurons distributed in the brain stem and spinal cord. Most of the axons in the descending pyramidal tract cross from their side of origin to the other side in the lower brain stem as the pyramidal decussates. A small portion of these axons continue to descend in the ipsilateral spinal cord, where they may later cross to the opposite side or remain on the same side to influence ipsilateral motor neurons and ipsilateral body movements.

Recent anatomical and physiological data suggest that M1 has two functionally distinct subregions (Fig. 2). Cytoarchitectonic evidence for rostral (M1r) and caudal (M1c) subdivisions was found in both monkey and human brains. On the basis of differences in the cortical and subcortical connectivity of these two regions, as well as differences in the response properties of neurons, it has been proposed that these two divisions are specialized for the control of different stages of movement. M1r, which is less excitable than M1c and receives stronger projections from premotor cortex, may preferentially be involved in early stages of movements, including the postural adjustments required to maintain balance during reaching for an object. These movements could be programmed by nonprimary motor areas and carried out with the aid of kinesthetic information reaching M1r from somatosensory areas. M1c, which is more excitable and receives diverse somatosensory inputs, may be more involved in the control of later stages of movement, such as grasping and holding objects, in which cutaneous feedback is especially important.

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