Motor Control Overview

Motor control occurs in a three-level hierarchy. The spinal cord is the most primitive, controlling reflex and stereotypical actions. Integration of sensory and vestibular information occurs in the brain stem, which controls posture, eye, and head motions. The apex of the motor hierarchy is the cerebral cortex, in particular the primary motor cortex, the lateral premotor area, and the supplementary motor area.

It appears that the supplementary motor area is concerned with programming sequences of movements, whereas the the premotor area is concerned with the selection of movement plans based on external cues [6].

Motor activity is also modified by the cerebellum and basal ganglia.

Both the cerebellum and the basal ganglia receive input from the cerebral cortex and send output to the cortex via the thalamus. According to Kandel et al. [32], the three major differences between the connections of the cerebellum and the basal ganglia are:

• The cerebellum receives input only from the sensorimotor parts of the cortex, whereas the basal ganglia receive input from the entire cerebral cortex.
• The cerebellum output goes only to the premotor and motor cortex, while the basal ganglia also project to the prefrontal association cortex.
• The cerebellum has direct ties with the spinal cord and the brain stem. The basal ganglia have little connection to the brainstem, and no direct connections with the spinal cord.

The cerebellum apparently modifies the output of motor systems to bring intention into line with performance; it acts as a comparator. It has been suggested that the cerebellum is a very large associative memory [33].

It is thought that the cerebellum regulates motion directly, whereas the basal ganglia have to do with the planning and execution of complicated motor strategies.