8 Necessary Components of Motor Control
Much of the brain and nervous system is devoted to the processing of sensory input, in order to construct detailed representations of the external environment.
Through vision, audition, somatosensation, and the other senses, we perceive the world and our relationship to it. This elaborate processing would be of limited value, however, unless we had a way to act upon the environment that we are sensing, whether that action consist of running away from a predator; seeking shelter against the rain; searching for food when one is hungry; moving one’s lips and vocal cords in order to communicate with others; or performing the countless other varieties of actions that make up our daily lives. In some cases, the relationship between the sensory input and the motor output are simple and direct; for example, touching a hot stove elicits an immediate withdrawal of the hand. Usually, however, our conscious actions require not only sensory input but a host of other cognitive processes that allow us to choose the most appropriate motor output for the given circumstances. In each case, the final output is a set of commands to certain muscles in the body to exert force against some other object or forces (e.g., gravity). This entire process falls under the subject of motor control.
Some of the Necessary Components of Proper Motor Control:
Volition. The motor system must generate movements that are adaptive and that accomplish the goals of the organism. These goals are evaluated and set by high-order areas of the brain. The motor system must transform the goals into the appropriate activations of muscles to perform the desired movements.
Coordination of signals to many muscle groups. Few movements are restricted to the activation of a single muscle. For example, the act of moving your hand from inside your pocket to a position in front of you requires the coordinated activity of the shoulder, elbow, and wrist. Making the same movement while removing a 2-lb weight from your pocket may result in the same trajectory of your hand, but will require different sets of forces on the muscles that make the movement. The task of the motor system is to determine the necessary forces and coordination at each joint in order to produce the final, smooth motion of the arm.
Proprioception. In order to make a desired movement (e.g., raising your hand to ask a question), it is essential for the motor system to know the starting position of the hand. Raising one’s hand from a resting position on a desk, compared to a resting position on top of the head, results in the same final position of the arm, but these two movements require different patterns of muscle activation. The motor system has a set of sensory inputs (called proprioceptors) that inform it of the length of muscles and the forces being applied to them; it uses this information to calculate joint position and other variables necessary to make the appropriate movement.
Postural adjustments. The motor system must constantly produce postural adjustments in order to compensate for changes in the body’s center of mass as we move our limbs, head, and torso. Without these automatic adjustments, the simple act of reaching for a cup would cause us to fall, as the body’s center of mass shifts to a location in front of the body axis.
Sensory feedback. In addition to the use of proprioception to sense the position of the body before a movement, the motor system must use other sensory information in order to perform the movement accurately. By comparing desired activity with actual activity, sensory feedback allows for corrections in movements as they take place, and it also allows modifications to motor programs so that future movements are performed more accurately.
Compensation for the physical characteristics of the body and muscles. To exert a defined force on an object, it is not sufficient to know only the characteristics of the object (e.g., its mass, size, etc.). The motor system must account for the physical characteristics of the body and muscles themselves. The bones and muscles have mass that must be considered when moving a joint, and the muscles themselves have a certain degree of resistance to movement.
Unconscious processing. The motor system must perform many procedures in an automatic fashion, without the need for high-order control. Imagine if walking across the room required thinking about planting the foot at each step, paying attention to the movement of each muscle in the leg and making sure that the appropriate forces and contraction speeds are taking place. It would be hard to do anything else but that one task. Instead, many motor tasks are performed in an automatic fashion that does not require conscious processing. For example, many of the postural adjustments that the body makes during movement are performed without our awareness. These unconscious processes allow higher-order brain areas to concern themselves with broad desires and goals, rather than low-level implementations of movements.
Adaptability. The motor system must adapt to changing circumstances. For example, as a child grows and its body changes, different constraints are placed on the motor system in terms of the size and mass of bones and muscles. The motor commands that work to raise the hand of an infant would fail completely to raise the hand of an adult. The system must adapt over time to change its output to accomplish the same goals. Furthermore, if the system were unable to adapt, we would never be able to acquire motor skills, such as playing a piano, hitting a baseball, or performing microsurgery.
These are some of the many components of the motor system that allow us to perform complex movements in a seemingly effortless way. The brain has evolved exceedingly complex and sophisticated mechanisms to perform these tasks, and researchers have only scratched the surface in understanding the principles that underlie the brain’s control of movement.