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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.

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.