Animation 14.4: From Input to Output
Consider the sequence and timing of events that are necessary to produce a voluntary movement, such as reaching for an apple that is sitting on a table. Your nervous system must identify the objects in the field of view (the table, the apple, the surroundings), determine the spatial relationships between the objects, and then generate a plan of action—a motor plan. Once a motor plan is developed, the appropriate motoneurons and muscles must be activated in the correct sequence so that an accurate and coordinated movement can be carried out.
What enables us to perform the movements and acts that make up such a behavior? Clearly, voluntary movements depend upon a broad range of cortical inputs and outputs including sensory (visual and somatosensory), cognitive (identification of objects and planning of behaviors), and motor (initiation and coordination of movement).
In this tutorial, we have shown how various regions of the brain work together to carry out a relatively simple voluntary movement. Our understanding of the brain regions that contribute to voluntary motor behavior continues to improve, made possible, in part, by the application of biological imaging techniques such as positron emission tomography (PET) and functional magnetic resonance imaging (fMRI).
While the model presented here suggests a rather linear process, such behavior is, in reality, much more complicated. For example, even after the instructions for carrying out a given behavior are generated, a feedback system constantly adjusts the movement of limbs as the behavior is being carried out. As the motoneurons signal the muscles to lengthen or shorten, sensory feedback from the muscles, tendons, and joints provides information about the state of the muscles so that the CNS can compensate for the weight and size of the object, etc. Furthermore, while the motor cortex may act as an “executive” motor control center, input from the basal ganglia and cerebellum helps to adjust the force, timing, and accuracy of the movements before and during their actual execution.
Textbook Reference: Attention May Be Endogenous or Exogenous, p. 414