If you've ever walked into a wall, you know that your motor skills and your senses have to work together in order to prevent a disaster. And by studying the brain regions, scientists now understand how they interact so closely.
The process that initiates a voluntary motor response is just as intricate as the sensory systems that provide the visual and auditory stimuli leading to it. In fact, the brain's motor functions have many points in common with its sensory mechanisms, especially those involved in tactile sensations. Thus, the primary motor cortex in the posterior portion of the frontal lobe is immediately adjacent to the somatosensory cortex in the anterior portion of the parietal lobe.
These two elongated regions face each other, and the nerve fibres leaving and entering them have the same somatotopic organization; they are like maps that reproduce the anatomy of the human body on a small scale. But both in the motor cortex and in the somatosensory cortex, the scale of this map is not constant. In the motor cortex, it varies with the precision of the movements controlled in the body part in question. In the somatosensory cortex, it varies with each body part's sensitivity to sensory information.
Of course, people can learn to make very fine movements of body parts that do not normally make such movements (for example, the wrist, elbow, and shoulder motions that a violinist must master). This suggests that the surface area devoted to such movements on the cortex can grow with practice. Many observations supporting this idea have been made in experimental microstimulation of the motor cortex in rats. For example, when the motor nerves innervating the muscles of a rat's snout are severed, the part of the motor cortex normally involved in controlling movements of the rat's whiskers can trigger movements of its front paws instead.
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CIHR – Institute of Neurosciences, Mental Health and Addiction