Motor Control Hierarchy
Throughout the central Body Function" href="/body-function/figure-125.html">nervous system, the neurons involved in controlling the motor neurons to skeletal muscles can be thought of as being organized in a hierarchical fashion, each level of the hierarchy having a certain task in motor control (Figure 12-1). To begin a movement, a general "intention" such as "pick up sweater" or "write signature" or "answer telephone" is generated at the highest level of the motor control hierarchy. This highest level encompasses many regions of the brain, including those involved in memory, emotions, and motivation. Very little is known, however, as to exactly where intentions for movements are formed in the brain.
Information is relayed from these highest hierarchical neurons, referred to as the "command" neurons, to parts of the brain that make up the middle level of the motor control hierarchy. The middle-level structures specify the postures and movements needed to carry out the intended action. In our example of picking up a sweater, structures of the middle hierarchical level coordinate the commands that tilt the body and extend the arm and hand toward the sweater and shift the body's weight to maintain balance. The middle hierarchical structures are located in parts of the cerebral cortex (termed, as we shall see, the sensorimotor cortex) and in the cerebellum, subcortical nuclei, and brainstem (Figures 12-1 and 12-2a and b). These structures have extensive interconnections, as indicated by the arrows in Figure 12-1.
Vander et al.: Human Physiology: The Mechanism of Body Function, Eighth Edition
Control of Body Movement CHAPTER TWELVE
Control of Body Movement CHAPTER TWELVE
FIGURE 12-1
The conceptual hierarchical organization of the neural systems controlling body movement. All the skeletal muscles of the body are controlled by motor neurons. Sensorimotor cortex includes those parts of the cerebral cortex that act together to control skeletal-muscle activity. The middle level of the hierarchy also receives input from the vestibular apparatus and eyes (not shown in the figure).
FIGURE 12-1
The conceptual hierarchical organization of the neural systems controlling body movement. All the skeletal muscles of the body are controlled by motor neurons. Sensorimotor cortex includes those parts of the cerebral cortex that act together to control skeletal-muscle activity. The middle level of the hierarchy also receives input from the vestibular apparatus and eyes (not shown in the figure).
As the neurons in the middle level of the hierarchy receive input from the command neurons, they simultaneously receive afferent information (from receptors in the muscles, tendons, joints, and skin, as well as from the vestibular apparatus and eyes) about the starting position of the body parts that are to be "commanded" to move. They also receive information about the nature of the space just outside the body into which that movement will take place. Neurons of the middle level of the hierarchy integrate all this afferent information with the signals from the command neurons to create a motor program—that is, the pattern of neural activity required to perform the desired movement. People can perform many slow, voluntary movements without sensory feedback, but the movements are abnormal.
The information determined by the motor program is then transmitted via descending pathways to the lowest level of the motor control hierarchy, the local level, at which the motor neurons to the muscles exit the brainstem or spinal cord. The local level of the hierarchy includes the motor neurons and the in-terneurons whose function is related to them; it is the final determinant of exactly which motor neurons will be activated to achieve the desired action and when this will happen. Note in Figure 12-1 that the descending pathways to the local level arise only in the sensorimotor cortex and brainstem; the basal ganglia, thalamus, and cerebellum exert their effects on the local level only indirectly, via the descending pathways from the cerebral cortex and brainstem.
Cerebral cortex
»ensorimotor cortex
- Cerebellum
Basal ganglia
Basal ganglia
Thalamus
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