The Activation Sequence for the Motor Areas
- Published4 Jan 2012
- Reviewed4 Jan 2012
- Source CIHR – Institute of Neurosciences, Mental Health and Addiction
The basic function of the brain is to produce behaviors which are, first and foremost, movements. Several different regions of the cerebral cortex are involved in controlling the body's movements.
Any voluntary movement can be accurately described as an intentional effort undertaken jointly by the motor cortex and numerous other neural systems acting in a "consulting capacity." This effort is organized hierarchically. First, the top level of the hierarchy takes care of defining the motor strategies: the objectives of the movement and the behaviors to be applied to achieve these objectives. When you decide to take an elevator, for example, which will involve walking over to the Up button and pressing it, your prefrontal cortex prepares the plans for this movement. Meanwhile, your frontal cortex is receiving information from a large number of axons projecting from the parietal cortex, which is involved in spatial perception. Its analysis of the position of your body and its various members in space will accordingly be essential to preparing for the movement. The basal ganglia are another set of brain structures involved in this part of the process.
Second, the secondary motor areas (PMA and SMA) work with the cerebellum to specify the precise sequence of contractions of the various muscles that will be required to carry out the selected motor action, in this case, raising your arm and stretching your index finger out to the elevator button. But to do this, your brain will need to convert the elevator button's location in the external environment into a set of intrinsic co-ordinates that will let you adjust the angles of the various joints that will be involved in the movement.
Third, the primary motor cortex, the brainstem, and the spinal cord come into play to produce the contractions of all the muscles needed for the chosen movement. The primary motor cortex determines how much force each muscle group must exert, and then sends this information to the spinal motor neurons and interneurons that generate the movement itself, as well as the postural adjustments that accompany it.
For another example, here is how these three levels work together when you throw a baseball. First, using the visual, auditory, somatic, and proprioreceptive information provided by your sensory organs, your cerebral cortex determines your body's position in space. The cortex exchanges information with the basal ganglia about your goal in throwing the ball (for instance, whether you want to throw it as high, as far, or as hard as possible) and the strategy to adopt to achieve this goal, based on such things as your past experience in throwing balls. Next, the secondary motor areas in your cerebral cortex and cerebellum make the appropriate decisions concerning the amplitude, direction, and force of the movements to make with your arm. These areas send these instructions to your brainstem and cervical spinal cord, which trigger a coordinated movement of your shoulder, elbow, wrist, and fingers. Simultaneously, commands sent to the thoracic and lumbar spinal cord from the brainstem determine the postural adjustments that will let you keep your balance while optimizing your movement as you throw the ball. The motor neurons in your brainstem will also be activated to keep your eye on the target that you are throwing at.
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CIHR – Institute of Neurosciences, Mental Health and Addiction