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Chapter 3: Motor Cortex

James Knierim, Ph.D., Department of Neuroscience, The Johns Hopkins University

Last Review 20 Oct 2020

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3.1 Introduction

The previous chapters discussed the lower levels of the motor hierarchy (the spinal cord and brainstem), which are involved in the low-level, “nuts and bolts” processing that controls the activity of individual muscles. Individual alpha motor neurons control the force exerted by a particular muscle, and spinal circuits can control sophisticated and complex behaviors such as walking and reflex actions. The types of movements controlled by these circuits are not initiated consciously, however. Voluntary movements require the participation of the third and fourth levels of the hierarchy: the motor cortex and the association cortex. These areas of the cerebral cortex plan voluntary actions, coordinate sequences of movements, make decisions about proper behavioral strategies and choices, evaluate the appropriateness of a particular action given the current behavioral or environmental context, and relay commands to the appropriate sets of lower motor neurons to execute the desired actions.

3.2 Motor Cortex Comprises the Primary Motor Cortex, Premotor Cortex, and Supplementary Motor Area

Figure 3.1 Motor cortex areas (lateral, dorsal, and medial views). The primary motor cortex is located immediately anterior to the central sulcus. Select from the boxes in the center to see larger views.

The motor cortex comprises three different areas of the frontal lobe, immediately anterior to the central sulcus. These areas are the primary motor cortex (Brodmann’s area 4), the premotor cortex, and the supplementary motor area (Figure 3.1). Electrical stimulation of these areas elicits movements of particular body parts. The primary motor cortex, or M1, is located on the precentral gyrus and on the anterior paracentral lobule on the medial surface of the brain. Of the three motor cortex areas, stimulation of the primary motor cortex requires the least amount of electrical current to elicit a movement. Low levels of brief stimulation typically elicit simple movements of individual body parts. Stimulation of premotor cortex or the supplementary motor area requires higher levels of current to elicit movements, and often results in more complex movements than stimulation of primary motor cortex. Stimulation for longer time periods (500 msec) in monkeys results in the movement of a particular body part to a stereotyped posture or position, regardless of the initial starting point of the body part (Figure 3.2). Thus, the premotor cortex and supplementary motor areas appear to be higher level areas that encode complex patterns of motor output and that select appropriate motor plans to achieve desired end results.

 

 

 

 

 

Figure 3.2 Electrical stimulation of premotor cortex of a monkey for 500 msec produces movement to stereotyped postures depending on the location of the stimulating electrode. Stimulation of site one (click STIMULATE 1) causes the monkey to bring its arm in front of its eyes, regardless of the starting location of the arm, as if the monkey were producing a defensive posture. Stimulation of site two (click STIMULATE 2) causes the monkey to bring its arm to its mouth and open the mouth, regardless of the starting location of the arm, as if it were bringing a piece of food to its mouth (Graziano et al., 2002).

Like the somatosensory cortex of the postcentral gyrus, the primary motor cortex is somatotopically organized (Figure 3.3). Stimulation of the anterior paracentral lobule elicits movements of the contralateral leg. As the stimulating electrode is moved across the precentral gyrus from dorsomedial to ventrolateral, movements are elicited progressively from the torso, arm, hand, and face (most laterally). The representations of body parts that perform precise, delicate movements, such as the hands and face, are disproportionately large compared to the representations of body parts that perform only coarse, unrefined movements, such as the trunk or legs. The premotor cortex and supplementary motor area also contain somatotopic maps.

Figure 3.3 Somatotopic representation of motor outputs in motor cortex.

One might predict that the motor cortex “homunculus” arises because neurons that control individual muscles are clustered together in the cortex. That is, all of the neurons that control the biceps muscle may be located together, and all of the neurons that control the triceps may be clustered nearby, and the neurons that control the soleus muscle may be clustered in a region further removed. Electrophysiological recordings have shown that this is not the case, however. Movements of individual muscles are correlated with activity from widespread parts of the primary motor cortex. Similarly, stimulation of small regions of primary motor cortex elicits movements that require the activity of numerous muscles. Thus, the primary motor cortex homunculus does not represent the activity of individual muscles. Rather, it apparently represents the movements of individual body parts, which often require the coordinated activity of large groups of muscles throughout the body.

3.3 Cortical Afferents and Efferents

The motor cortex exerts its influence over muscles by a variety of descending routes (Figure 3.4). Some of the descending pathways reviewed in the last chapter can be influenced by motor cortex output. Thus, in addition to the direct cortical innervation of alpha motor neurons via the corticospinal tract, the following cortical efferent pathways influence the remaining descending tracts:

1. the corticorubral tract allows cortex to modulate the rubrospinal tract
   
2. the corticotectal tract allows cortex to modulate the tectospinal tract
   
3. the corticoreticular tract allows cortex to modulate the reticulospinal tracts
   

Figure 3.4 Parallel pathways from the motor cortex allow the cortical motor areas to influence the processing of all descending motor tracts and side loops of the motor system. Mouse over the pathways for more information.

The cortex can also influence the processing of the side loops of the motor hierarchy. The corticostriate tract innervates the caudate nucleus and putamen of the basal ganglia. The corticopontine tract and cortico-olivary tract innervate important inputs to the cerebellum. Finally, cortical areas can influence other cortical areas, directly via corticocortical pathways and indirectly via the corticothalamic pathways (Figure 3.5). Most of these pathways are bi-directional. Thus, motor cortex receives input from other cortical areas, directly and indirectly through the thalamus, and it receives input from the cerebellum and basal ganglia, always through the thalamus.

Figure 3.5 Major connections of motor cortex. The cross-section on the left is a schematic version of an idealized brain section that contains the major structures of the motor system hierarchy for illustrative purposes; no actual brain section would contain all of these structures. Move the cursor over each box on the right to highlight the inputs (blue) and outputs (red) of each region.

3.4 Motor Cortex Cytoarchitecture

Like all parts of the neocortex, the primary motor cortex is made of six layers (Figure 3.6). Unlike primary sensory areas, primary motor cortex is agranular cortex; that is, it does not have a cell-packed granular layer (layer 4). Instead, the most distinctive layer of primary motor cortex is its descending output layer (Layer 5), which contains the giant Betz cells. These pyramidal cells and other projection neurons of the primary motor cortex make up ~30% of the fibers in the corticospinal tract. The rest of the fibers come from the premotor cortex and the supplementary motor area (~30%), the somatosensory cortex (~30%), and the posterior parietal cortex (~10%).

Figure 3.6 Pyramidal and non-pyramidal neurons in motor cortex. The cerebral cortex is organized into six layers. These layers contain different proportions of the two main classes of cortical neurons, pyramidal and non-pyramidal cells. Pyramidal cells send long axons down the spinal cord and are the major output neurons. They are abundant in layer 5. Non-pyramidal cells have axons which terminate locally.

3.5 Encoding of Movement by Motor Cortex

Primary Motor Cortex

As discussed above, the primary motor cortex does not generally control individual muscles directly, but rather appears to control individual movements or sequences of movements that require the activity of multiple muscle groups. Alpha motor neurons in the spinal cord, in turn, encode the force of contraction of groups of muscle fibers using the rate code and the size principle. Thus, in accordance with the concept of hierarchical organization of the motor system, the information represented by motor cortex is a higher level of abstraction than the information represented by spinal motor neurons.

What is encoded by the neurons in primary motor cortex? Clues have come from recording the activity of these neurons as experimental animals perform different motor tasks. In general, primary motor cortex encodes the parameters that define individual movements or simple movement sequences.

1. Primary motor cortex neurons fire 5-100 msec before the onset of a movement. Thus, rather than firing as the result of muscle activity, these neurons are involved in relaying motor commands to the alpha motor neurons that eventually cause the appropriate muscles to contract.
   
2. Primary motor cortex encodes the force of a movement. The amount of force required to raise the arm from one location to another is much greater if one is holding a bowling ball than if one is holding a balloon. Many neurons in primary motor cortex encode the amount of force that is necessary to make such a movement (Figure 3.7). Note the distinction between movement force and muscle force. Whereas a minority of primary motor cortex neurons encodes individual muscle force, a larger number encodes the amount of force necessary for a particular movement, regardless of which individual muscles are used. Alpha motor neurons, in turn, translate the commands of the motor cortex neurons and control the amount of force generated by individual muscles to accomplish that movement, under the principles of the rate code and the size principle.
   
   

   Figures 3.7A, 3.7B, and 3.7C Motor cortex encodes the force necessary to make a movement. (Evarts 1968)
   	Figure 3.7A. When there is little load, a motor neuron in primary motor cortex that controls an extension of the wrist fires when the wrist extends. A motor neuron that controls wrist flexion does not change its low rate of activity. Note that the extension motor neuron begins to fire spikes before the onset of the movement. text goes here
   	Figure 3.7B. When a 5 lb. load is placed on the left pulley, more force must be used to initially hold the weight steady and then lift it. The extension motor neuron in primary motor cortex fires more strongly to produce the greater force. text goes here
   	Figure 3.7C. When a 5 lb. load is placed on the right pulley, the load is on the flexor. Thus, primary motor cortex neurons for flexion are activated to keep the weight stable. When the wrist extends, the neurons are quieter, as the force of the movement is actually produced by the weight itself. (Note that motor cortex encodes the force of a movement, such as wrist extension or more complicated, multi-joint movements. The force of individual muscles is encoded by alpha motor neurons in the spinal cord and brain stem.) text goes here

3. Primary motor cortex encodes the direction of movement. Many neurons in the primary motor cortex are selective for a particular direction of movement. For example, one cell may fire strongly when the hand is moved to the left, whereas it will be inhibited when the hand is moved to the right (Figure 3.8).
   
   

   Figure 3.8 Directional tuning of motor cortex neurons. The cell fires maximally when the hand is moved in the 135º or 180º directions, moderately when the hand moves in the 90º and 225º directions, and is silent when the hand moves in the opposite directions (0º, 45º, 270º, and 315º) (Georgopoulos et al., 1982).

   
   
4. Primary motor cortex encodes the extent of movement. The firing of some neurons is correlated with the distance of a movement. A monkey was trained to move its arm to different target locations that varied in direction and distance from the center. The firing of many neurons was correlated with the direction of movement (as in Point 3), whereas the firing of other neurons was correlated with the distance of the movement. Interestingly, some neurons were correlated with the interaction of a particular distance and direction; that is, they were correlated with a particular target position.
5. Primary motor cortex neurons encode the speed of movement. Almost all targeted movements follow a typical bell-shaped curve of velocity as a function of distance (Figure 3.9). For example, when the hand moves an object such as a coffee cup from one location to another (the target), the hand accelerates during the first half of the movement, reaches a peak velocity approximately halfway to the target, and then decelerates until it reaches the target. The firing rate of some primary motor cortex neurons in monkeys correlates with this bell-shaped speed profile, demonstrating that information about movement speed is contained in the spike trains of these neurons.

Figure 3.9 Velocity profile of targeted movements.

Premotor Cortex

The premotor cortex sends axons to the primary motor cortex as well as to the spinal cord directly. It performs more complex, task-related processing than primary motor cortex. Stimulation of premotor areas in the monkey at a high level of current produces more complex postures than stimulation of the primary motor cortex. The premotor cortex appears to be involved in the selection of appropriate motor plans for voluntary movements, whereas the primary motor cortex is involved in the execution of these voluntary movements.

1. Premotor cortex neurons signal the preparation for movement. Monkeys were trained to make a particular movement in response to a visual signal, with a variable delay between the onset of the signal and the onset of the movement (Figure 3.10). Recordings from premotor cortex have shown that many neurons fire selectively in the delay interval, for many seconds before the onset of the movement. A particular neuron will fire when the monkey is preparing to make a movement to the left, for example, but will be silent when the monkey is preparing to make a movement to the right. Thus, the firing of this type of neuron does not cause the movement itself, but appears to be involved in preparing the monkey to make the correct movement when the “Go” signal is given. This type of neuron is called a motor-set neuron, as it fires when the monkey is preparing, or getting set, to make a movement.
   
   

   Figure 3.10 Premotor cortex neurons signal preparation for movement. A monkey is trained to prepare to make a movement to the right or left depending on a cue instruction, but to delay the movement until a “Move” signal is given (Weinrich and Wise 1982). Some neurons will fire selectively when the animal is preparing to make a movement to the right (Play Prepare right cell). Other neurons will fire selectively when the animal is preparing to make a movement to the left (Play Prepare left cell). Note that the cells fire in the interval between the Prepare instruction and the Move instruction, but they do not fire during the movement itself.

   
   
2. Premotor cortex neurons signal various sensory aspects associated with particular motor acts. Some premotor neurons fire when the animal is performing a particular action, such as breaking a peanut (Figure 3.11). Interestingly, the same neuron fires selectively when the animal sees another monkey or person breaking the peanut. It also fires selectively to the sound of a peanut shell being broke, even without any visual or motor activity. These neurons are called “mirror” neurons, because they respond not only to a particular action of the monkey but also to the sight (or sound) of another individual performing the same action. (For an interesting PBS video on mirror neurons, go to http://www.pbs.org/wgbh/nova/sciencenow/3204/01.html.)
   
   

   Figure 3.11 Mirror neuron in premotor cortex fires to the monkey’s action as well as the monkey’s perception of a person performing the same action (Kohler et al., 2002).

   
   
3. Premotor cortex is sensitive to the behavioral context of a particular movement. The premotor cortex of human subjects was imaged with functional MRI as they observed video of a hand grasping a cup (Figure 3.12). In one condition, the cup was full and surrounded by full plates of food; the implication was that the person was grasping the cup to take a drink. In the other condition, the cup was empty and surrounded by dirty dishes; the implication was that the person was grasping the cup to clear the table. In this experiment, the premotor cortex was more active when subjects viewed the former video than the latter, even though the movements were the same. Thus, premotor cortex neurons are sensitive to the inferred intentions of a movement, not just the movement itself, as deduce from the behavioral context in which the movement occurred.
   
   

   Figure 3.12 Premotor cortex activity distinguishes the same movement based on the behavioral context of the movement (Iacoboni et al., 2005). When the subject viewed an arm moving to pick up a cup to drink (PLAY top), the activity in premotor cortex was greater than when the subject viewed an arm moving to pick up a cup to clear the table after a meal (PLAY bottom). Note that the strength of activity in the cortex (denoted by the brightness of the activated cortical region) is greater in the top than in the bottom animations.

   
   
4. Premotor cortex signals correct and incorrect actions. Human subjects were studied in an fMRI experiment as they observed video clips of various correct and incorrect motor acts. A correct action was one in which the movement and the associated object was performed correctly, such as setting the time on a clock. An object error was one in which the action was correct, but the object was incorrect, such as polishing a brown shoe with black shoe polish. A movement error was one in which the object was correct, but the movement was incorrect, such as attempting to put a coin into a piggy bank when the coin was oriented perpendicular to the coin slot. In this experiment, the premotor cortex was activated bilaterally during the correct actions trials and the movement error trials; for the object error trials, only the premotor cortex of the left hemisphere was activated preferentially.

Supplementary Motor Area

The supplementary motor area (SMA) is involved in programming complex sequences of movements and coordinating bilateral movements. Whereas the premotor cortex appears to be involved in selecting motor programs based on visual stimuli or on abstract associations, the supplementary motor area appears to be involved in selecting movements based on remembered sequences of movements.

1. SMA responds to sequences of movements and to mental rehearsal of sequences of movements (Figure 3.13). Brain activity was measured in a PET scanner while subjects made simple and complex sequences of movement. When the movements were simple, such as a repetitive movement of a single digit, the primary motor cortex and the primary somatosensory cortex were activated on the contralateral hemisphere. When the subject was asked to perform a complex sequence of finger movements, the SMA was activated bilaterally, in addition to the contralateral primary motor and somatosensory cortex activation. Finally, when the subject was asked to remain still but to mentally rehearse the complex sequence of activity, the SMA was still active, even though the primary motor and somatosensory cortex areas were silent. Thus, the SMA appears to be involved in bilateral movements and in the mental rehearsal of these movements.
   
   

   Figure 3.13 Positron emission tomography (PET) study of simple vs. complex finger movements (Roland et al.,1980). The SMA is activated bilaterally when subjects perform complex movements, and even when they only imagine performing the movements.

   
   
2. SMA is involved in the transformation of kinematic to dynamic information. Movements can be defined in terms of dynamics (the amount of force necessary to make a movement) and kinematics (the distance and angles that define a particular movement in space). Many movement plans are represented in kinematic terms (e.g., Move the hand to the left). However, the motor system must eventually translate this to a representation based on dynamics, in order to instruct the appropriate muscles to contract with the appropriate force. Recordings from monkeys have shown that during the preparatory delay before a monkey makes an instructed movement, some SMA neurons change their firing correlates from a kinematic-based representation to a dynamics-based representation, suggesting that SMA plays a vital role in this transformation.

Association Cortex

The fourth level of the motor hierarchy is the association cortex, in particular the prefrontal cortex and the posterior parietal cortex (Figure 3.14). These brain areas are not motor areas in the strict sense. Their activity does not correlate precisely with individual motor acts, and stimulation of these areas does not result in motor output. However, these areas are necessary to ensure that movements are adaptive to the needs of the organism and appropriate to the behavioral context.

Figure 3.14 Association cortex. The prefrontal cortex is highlighted on the left, and the posterior parietal cortex is highlighted on the right.

1. Posterior parietal cortex is involved in ensuring that movements are targeted accurately to objects in external space. This area is involved in processing spatial relationships of objects in the world and in constructing a representation of external space that is independent of the observer’s eye position or body position. Such representations allow a stable percept of the world that is independent of viewer orientation, as well as the representation of desired trajectories in space that are independent of body position. Damage to the posterior parietal cortex can result in a number of apraxias, that is, the inability to make complex, coordinated movements. For example, a patient with constructional apraxia is unable to replicate the configuration of a set of blocks in the proper sequence, even though the patient is able to maneuver each block individually with dexterity.
   
2. Prefrontal cortex is involved in the selection of appropriate actions for a particular behavioral context. It is also involved in the evaluation of the consequences of a particular course of action. Patients with damage to the prefrontal cortex have problems in executive processing. They make inappropriate behavioral decisions, and often cannot anticipate the likely consequences of their actions. They display impulsive behavior, often showing an inability to delay instant gratification for a long-term larger reward.

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