Figure 122

(a) Side view of the brain showing three of the four components of the middle level of the motor control hierarchy.

(b) Cross section of the brain showing the basal ganglia—part of the subcortical nuclei, the fourth component of the hierarchy's middle level. %

PART TWO Biological Control Systems

Vander et al.: Human Physiology: The Mechanism of Body Function, Eighth Edition

PART TWO Biological Control Systems

TABLE 12-1 Conceptual Motor Control

Hierarchy for Voluntary Movements

I. The highest level a. Function: forms complex plans according to individual's intention and communicates with the middle level via "command neurons."

b. Structures: areas involved with memory and emotions; supplementary motor area; and association cortex. All these structures receive and correlate input from many other brain structures.

II. The middle level a. Function: converts plans received from the highest level to a number of smaller motor programs, which determine the pattern of neural activation required to perform the movement. These programs are broken down into subprograms that determine the movements of individual joints. The programs and subprograms are transmitted, often via the cerebral cortex, through descending pathways to the lowest control level.

b. Structures: sensorimotor cortex, cerebellum, parts of basal ganglia, some brainstem nuclei.

III. The lowest level (the local level)

a. Function: specifies tension of particular muscles and angle of specific joints at specific times necessary to carry out the programs and subprograms transmitted from the middle control levels.

b. Structures: levels of brainstem or spinal cord from which motor neurons exit.

The motor programs are continuously adjusted during the course of most movements. As the initial motor program is implemented and the action gets underway, brain regions at the middle level of the hierarchy continue to receive a constant stream of updated afferent information about the movements taking place. Say, for example, that the sweater being picked up is wet and heavier than expected so that the initially determined amount of muscle contraction is not sufficient to lift it. Any discrepancies between the intended and actual movements are detected, program corrections are determined, and the corrections are relayed via the lowest level of the hierarchy to the motor neurons.

If a complex movement is repeated frequently, learning takes place and the movement becomes skilled. Then, the initial information from the middle hierarchical level is more accurate and fewer corrections need to be made. Movements performed at high speed without concern for fine control are made solely according to the initial motor program.

The structures and functions of the motor control hierarchy are summarized in Table 12-1.

We must emphasize that this hierarchical model, widely used by physiologists who work on the motor system, is only a guide, one that requires qualification. The different areas of the brain, and neurons within each area, have so many reciprocal connections that it is often impossible to assign a specific function to a given area or group of neurons. In addition, different neurons in different areas of the brain are often active simultaneously, and neurons with similar properties are widely distributed over different regions of the brain. Nevertheless, just as researchers have found it useful to retain the notion of a motor control hierarchy despite its flaws, you the reader should also find the hierarchical model conceptually helpful.

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