![]() ![]() 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. 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. 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. 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. 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. 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. 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. 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. ![]() 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. Few movements are restricted to the activation of a single muscle. Coordination of signals to many muscle groups.The motor system must transform the goals into the appropriate activations of muscles to perform the desired movements. These goals are evaluated and set by high-order areas of the brain. The motor system must generate movements that are adaptive and that accomplish the goals of the organism. This entire process falls under the subject of motor control.ġ.2 Some Necessary Components of Proper Motor Control ![]() 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). 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 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 (Figure 1.1). 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. Through vision, audition, somatosensation, and the other senses, we perceive the world and our relationship to it. 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. In most cases, however, cognitive processing occurs to make actions adaptive and appropriate for the particular situation. Sometimes the pathway from sensation to action is direct, as in a reflex. Sensory receptors provide information about the environment, which is then used to produce action to change the environment. ![]()
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