OVERVIEW OF THE MOTOR SYSTEM

The motor system can be grossly divided into peripheral and central elements.

The peripheral somatic motor system includes muscles, joints, and their sensory and motor innervations.

The central elements can be divided into three hierarchical levels to assist understanding their organization.

Bear et al provide a practical description of the three hierarchical levels relative to their functional contributions to motor control as follows:


Hierarchical levels:

The highest level, represented by the association areas of the neocortex and basal ganglia of the forebrain, is concerned with strategy: the goal of the movement and the movement strategy that best achieve the goal.

The middle level, represented by the motor cortex and cerebellum, is concerned with tactics: the sequences of muscle contractions, arranged in space and time, required to smoothly and accurately achieve the strategic goal.

The lowest level, represented by the brain stem and spinal cord, is concerned with execution: activation of the motor neuron and interneuron pools that generate the goal-directed movement and make any necessary adjustment.

Parallel system:

The motor system can also be viewed as having a parallel arrangement. For example, information is conveyed not only from the motor cortex to the spinal cord but also directly from premotor areas as well. Although the cerebellum and basal ganglion are involved in movement, they have no direct output to the spinal cord. Instead, their effect on movement is provided via connections to the motor cortex.

Role of Sensory Input:

The integration of sensory input provides an internal representation of the environment that informs and guides motor responses. These sensory representations provide the foundation on which motor programs for purposeful movements are planned, coordinated, and implemented.

Sensory input to the motor system guides selection and adaptation of motor responses as well as shapes motor programs for corrective action.

For example, the somatosensory system provides the needed information to adjust walking when moving from a smooth surface to an uneven terrain; to maintain standing balance on a moving bus; or to make the required adjustments when throwing a ball from a stable sitting surface (chair) versus an unstable one (therapeutic ball).