Two important capabilities of the brain in controlling movement are (1) to determine how rapidly the movement is to be performed and (2) to control how large the movement will be. For instance, a person may write the letter "a" slowly or rapidly. Also, he or she may write a small "a" on a piece of paper or a large "a" on a chalkboard. Regardless of the choice, the proportional characteristics of the letter remain nearly the same.
In patients with severe lesions of the basal ganglia, these timing and scaling functions are poor; in fact, sometimes they are nonexistent. Here again, the basal ganglia do not function alone; they function in close association with the cerebral cortex. One especially important cortical area is the posterior parietal cortex, which is the locus of the spatial coordinates for motor control of all parts of the body as well as for the relation of the body and its parts to all its surroundings. Figure 56-13 shows the way in which a person lacking a left posterior parietal cortex might draw the face of another human being, providing proper proportions for the right side of the face but almost ignoring the left side (which is in his or her right field of vision). Also, such a person will try always to avoid using his or her right arm, right hand, or other portions of his or her right body for the performance of tasks, almost not knowing that these parts of his or her body exist.
Because the caudate circuit of the basal ganglial system functions mainly with association areas of the cerebral cortex such as the posterior parietal cortex, presumably the timing and scaling of movements are functions of this caudate cognitive motor control circuit. However, our understanding of function in the basal ganglia is still so imprecise that much of what is
Typical drawing that might be made by a person who has severe damage in his or her left parietal cortex where the spatial coordinates of the right field of vision are stored.
conjectured in the last few sections is analytical deduction rather than proven fact.
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This ebook provides an introductory explanation of the workings of the human body, with an effort to draw connections between the body systems and explain their interdependencies. A framework for the book is homeostasis and how the body maintains balance within each system. This is intended as a first introduction to physiology for a college-level course.