Distinct Cortical Areas Participate in Voluntary Movement

The primary motor cortex (MI), Brodmann's area 4, fulfills all three criteria for a motor area (Fig. 5.13). The supplementary motor cortex (MII), which also fulfills all three criteria, is rostral and medial to MI in Brodmann's area 6. Other areas that fulfill some of the criteria include the rest of Brodmann's area 6; areas 1, 2, and 3 of the postcentral

Brodmann Areas Map

^Brodmann's cytoarchitectural map of the human cerebral cortex. Area 4 is the primary motor cortex (MI), area 6 is the premotor cortex and includes the supplementary motor area (MII) on the medial aspect of the hemisphere, area 8 influences voluntary eye movements, areas 1, 2, 3, 5, and 7 have sensory functions but also contribute axons to the corticospinal tract.

gyrus; and areas 5 and 7 of the parietal lobe. All of these areas contribute fibers to the corticospinal tract, the efferent motor pathway from the cortex.

The Primary Motor Cortex (MI). This cortical area corresponds to Brodmann's area 4 in the precentral gyrus. Area 4 is structured in six well-defined layers (I to VI), with layer I being closest to the pial surface. Afferent fibers terminate in layers I to V. Thalamic afferent fibers terminate in two layers; those that carry somatosensory information end in layer IV, and those from nonspecific nuclei end in layer I. Cerebellar afferents terminate in layer IV. Efferent axons arise in layers V and VI to descend as the corticospinal tract. Body areas are represented in an orderly manner, as somatotopic maps, in the motor and sensory cortical areas (Fig 5.14). Those parts of the body that perform fine movements, such as the digits and the facial muscles, are controlled by a greater number of neurons that occupy more cortical territory than the neurons for the body parts only capable of gross movements.

Low-level electrical stimulation of MI produces twitchlike contraction of a few muscles or, less commonly, a single muscle. Slightly stronger stimuli also produce responses in adjacent muscles. Movements elicited from area 4 have the lowest stimulation thresholds and are the most discrete of any movements elicited by stimulation. Stimulation of MI limb areas produces contralateral movement, while cranial cortical areas may produce bilateral motor responses. Destruction of any part of the primary motor cortex leads to immediate paralysis of the muscles controlled by that area. In humans, some function may return weeks to months later, but the movements lack the fine degree muscle control of the normal state. For example, after a lesion in the arm area of MI, the use of the hand recovers, but the capacity for discrete finger movements does not.

Neurons in MI encode the capability to control muscle force, muscle length, joint movement, and position. The area receives somatosensory input, both cutaneous and proprioceptive, via the ventrobasal thalamus. The cerebellum projects to MI via the red nucleus and ventrolateral thalamus. Other afferent projections come from the nonspecific nuclei of the thalamus, the contralateral motor cortex, and many other ipsilateral cortical areas. There are many axons between the precentral (motor) and postcentral (so-matosensory) gyri and many connections to the visual cortical areas. Because of their connections with the so-matosensory cortex, the cortical motor neurons can also respond to sensory stimulation. For example, cells innervating a particular muscle may respond to cutaneous stimuli originating in the area of skin that moves when that muscle is active, and they may respond to proprioceptive stimulation from the muscle to which they are related. Many efferent fibers from the primary motor cortex terminate in brain areas that contribute to ascending somatic sensory pathways. Through these connections, the motor cortex can control the flow of somatosensory information to motor control centers.

The close coupling of sensory and motor functions may play a role in two cortically controlled reflexes that were originally described in experimental animals as being important for maintaining normal body support during locomotion—the placing and hopping reactions. The placing reaction can be demonstrated in a cat by holding it so that its limbs hang freely. Contact of any part of the animal's foot with the edge of a table provokes immediate placement of the foot on the table surface. The hopping reaction is demonstrated by holding an animal so that it stands on one leg. If the body is moved forward, backward, or to the side, the leg hops in the direction of the movement so that the foot is kept directly under the shoulder or hip, stabilizing the body position. Lesions of the contralateral precentral or postcentral gyrus abolish placing. Hopping is abolished by a contralateral lesion of the precentral gyrus.

The Supplementary Motor Cortex (MII). The MII cortical area is located on the medial surface of the hemispheres, above the cingulate sulcus, and rostral to the leg area of the primary motor cortex (see Fig. 5.14). This cortical region within Brodmann's area 6 has no clear cytoarchitectural boundaries; that is, the shapes and sizes of cells and their processes are not obviously compartmentalized, as in the layers of MI.

Electrical stimulation of MII produces movements, but a greater strength of stimulating current is required than for MI. The movements produced by stimulation are also qualitatively different from MI; they last longer, the postures elicited may remain after the stimulation is over, and the movements are less discrete. Bilateral responses are common. MII is reciprocally connected with MI, and receives input from other motor cortical areas. Experimental lesions in MI eliminate the ability of MII stimulation to produce movements.

Current knowledge is insufficient to adequately describe the unique role of MII in higher motor functions. MII is thought to be active in bimanual tasks, in learning and preparing for the execution of skilled movements, and in the control of muscle tone. The mechanisms that underlie

Control Motor Functions

A cortical map of motor functions. Primary motor cortex (MI) and supplementary motor cortex (MII) areas in the monkey brain. MII is on the medial aspect of the hemisphere.

the more complex aspects of movement, such as thinking about and performing skilled movements and using complex sensory information to guide movement, remain incompletely understood.

The Primary Somatosensory Cortex and Superior Parietal Lobe. The primary somatosensory cortex (Brod-mann's areas 1, 2, and 3) lies on the postcentral gyrus (see Fig. 5.13) and has a role in movement. Electrical stimulation here can produce movement, but thresholds are 2 to 3 times higher than in MI. The somatosensory cortex is reciprocally interconnected with MI in a somatotopic pattern—for example, arm areas of sensory cortex project to arm areas of motor cortex. Efferent fibers from areas 1, 2, and 3 travel in the corticospinal tract and terminate in the dorsal horn areas of the spinal cord.

The superior parietal lobe (Brodmann's areas 5 and 7) also has important motor functions. In addition to contributing fibers to the corticospinal tract, it is well connected to the motor areas in the frontal lobe. Lesion studies in animals and humans suggest this area is important for the utilization of complex sensory information in the production of movement.

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