Hierarchical Arrangement Of Motor Cortical Areas

Firing of MI upper motor neurons is associated with relatively simple motor commands; more complex movements are linked to upper motor neurons in area MII (Fig. 2). Two separate somatotopic maps are present in MII: The supplementary motor area (SMA) is located near the superior medial region of the cortex, and the premotor area (PMA) occupies a more lateral position. Pyramidal cells in these areas contribute to corticofugal pathways and are also heavily interconnected to MI. Both areas of MII elicit complex motor responses, but they appear to be involved in somewhat different aspects of generating the motor command, primarily in integrating specific strategies. It has been shown experimentally that the SMA is required for bilaterally coordinated movements. For example, specific cells in the SMA fire in accord with movements in either hand. Individuals with cortical lesions in the SMA area suffer from a condition called apraxia. They retain the ability to make simple movements but have a selective inability to perform complex tasks requiring the coordinated actions of two hands, such as buttoning a shirt.

The PMA area is also involved with planning complex movements; however, its control over lower motor neuron activity is indirect. In addition to its reciprocal connections with MI, its major descending projection is to the reticular formation, which in turn projects to spinal neurons controlling axial muscles. This pathway

Hierarchical Arrangement

FIGURE 2 Schematic representation of the hierarchical arrangement of the cortical motor pathways. Formulation of a motor plan consists of five basic steps, each of which is associated with a specific cortical region. Three other regions—the thalamus, basal ganglia, and cerebellum— modify the primary motor plan through connecting loops involving these cortical regions. Areas shown in blue provide descending motor fibers that constitute corticobulbar and corticospinal tracts.

FIGURE 2 Schematic representation of the hierarchical arrangement of the cortical motor pathways. Formulation of a motor plan consists of five basic steps, each of which is associated with a specific cortical region. Three other regions—the thalamus, basal ganglia, and cerebellum— modify the primary motor plan through connecting loops involving these cortical regions. Areas shown in blue provide descending motor fibers that constitute corticobulbar and corticospinal tracts.

acts to orient the body for upcoming movement. Firing of PMA pyramidal cells occurs prior to firing of command neurons in MI. The pattern of firing in PMA is correlated to the direction of the upcoming movement and is maintained until the movement is completed.

Both elements of MII act as higher order motor areas, projecting to MI and providing complex motor commands to lower motor neurons through direct and indirect projections. The highest level in the motor hierarchy is represented by two other regions: sensory association areas in SII and the prefrontal region which lies anterior to MII. Area SII represents a highly important functional component in the planning of movement, providing focused attention on the parts of the body and the elements in the environment that will likely be involved in the intended motor action. In experiments that visualize neuronal activity in brain areas using positron emission tomography (PET) scanning techniques, the SII region on the lateral aspect of the parietal lobe appears to be most active when human subjects are asked to think about a specific movement, whether or not the movement is subsequently initiated. MI neurons were not activated under these conditions. Lesions in this area of the brain produce peculiar symptoms, collectively called the neglect syndrome, in which the affected individual neglects specific parts of the body corresponding to the areas of the lesioned somato-sensory association area. These individuals appear to lose awareness of the affected body regions, and complex motor activity involving this musculature is impaired.

The prefrontal area represents the second major component of the highest motor level. Of the variety of higher cortical functions that have been ascribed to this region, perhaps the best studied involves the role it plays in delayed-response tasks. Behavior tests in experimental animals suggest that the prefrontal area plays a pivotal role in temporarily storing information used to guide future action, namely, the formation of a working memory. The ability to hold key information in a working memory for a period of time is required in order to weigh the consequences of future actions and to plan accordingly. It is interesting to note that the prefrontal area of primates has a particularly prominent dopaminergic innervation and that depletion of dopamine is thought to contribute to human cognitive disorders, such as schizophrenia. Schizophrenics have smaller frontal lobes than normal individuals, and they show altered activation patterns in PET scans of the frontal cortex, particularly when involved in delayed-response motor tasks.

In summary, the primary motor cortex initiates volitional movement not as the first step but as the third step in the hierarchical motor scheme. The prefrontal cortex formulates a working memory that serves in the initial planning of motor events to be completed later; the SII areas of the parietal lobe help focus attention on the event. This information filters to the next step in the scheme, triggering neurons in area MII to plan the coordination of motor commands to accomplish the intended movement. With successful completion of these preliminary steps, MI neurons, through their powerful connections with alpha motor neurons in the cord, set the selected program in motion.

Suggested Readings

Mardsen CD, Rothwell JC, Day BL. The use of peripheral feedback in the control of movement. Trends Neurosci 1984; 7:253-257.

Miles FA, Evarts E. Concepts of motor organization. Annu Rev Psychol 1979; 30:327-362.

Polit A, Bizzi E. Processes controlling arm movements in monkeys. Science 1978; 201:1235-1237.

Stuart DG, Enoka RM. Motoneurons, motor units and the size principle, in: Rosenberg RN, Ed. The clinical neurosciences, Vol. 5, Neurobiology. New York: Churchill Livingstone, 1983; 471-517.

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