The characteristic movement of a musician's hand is captured in a painting by Giacomo Balla from 1912, "The Hand of the Violinist". Balla studied violin as a boy and, like his contemporaries in the Futurist movement, was interested in depicting motion and speed. The rhythmic brushstrokes evoke the energy of the performer and the vibrations of the music. (Reproduced, with permission, from the Copyright 2011 Artists Rights Society (ARS), New York/SIAE, Rome; and the Bridgeman Art Library International, NY.)
The capacity for movement, as many dictionaries remind us, is a defining feature of animal life. As Sherrington, who pioneered the study of the motor system pointed out, "to move things is all that mankind can do, for such the sole executant is muscle, whether in whispering a syllable or in felling a forest."*
The immense repertoire of motions that humans are capable of stems from the activity of some 640 skeletal muscles—all under the control of the central nervous system. After processing sensory information about the body and its surroundings, the motor centers of the brain and spinal cord issue neural commands that effect coordinated, purposeful movements.
The task of the motor systems is the reverse of the task of the sensory systems. Sensory processing generates an internal representation in the brain of the outside world or of the state of the body. Motor processing begins with an internal representation: the desired purpose of movement. Critically, however, this internal representation needs to be continuously updated by internal (efference copy) and external sensory information to maintain accuracy as the movement unfolds.
Just as psychophysical analysis of sensory processing tells us about the capabilities and limitations of the sensory systems, psychophysical analyses of motor performance reveal regularities and invariances in the control rules used by the motor system.
Because many of the motor acts of daily life are unconscious, we are often unaware of their complexity. Simply standing upright, for example, requires continual adjustments of numerous postural muscles in response to the vestibular signals evoked by miniscule swaying. Walking, running, and other forms of locomotion involve the combined action of central pattern generators, gated sensory information, and descending commands, which together generate the complex patterns of alternating excitation and inhibition to the appropriate sets of muscles. Many actions, such as serving a tennis ball or executing an arpeggio on a piano, occur far too quickly to be shaped by sensory feedback. Instead, centers, such as the cerebellum, make use of predictive models that simulate the consequences of the outgoing commands and allow very short latency corrections. Motor learning provides one of the most fruitful subjects for studies of neural plasticity.
Motor systems are organized in a functional hierarchy, with each level concerned with a different decision. The highest and most abstract level, likely requiring the prefrontal cortex, deals with the purpose of a movement. The next level, which is concerned with the formation of a motor plan, involves interactions between the posterior parietal and premotor areas of the cerebral cortex. The premotor cortex specifies the spatial characteristics of a movement based on sensory information from the posterior parietal cortex about the environment and about the position of the body in space. The lowest level of the hierarchy coordinates the spatiotemporal details of the muscle contractions needed to execute the planned movement. This coordination is executed by the primary motor cortex, brain stem, and spinal cord. This serial view has heuristic value, but evidence suggests that many of these processes can occur in parallel.
Some functions of the motor systems and their disturbance by disease have now been described at the level of the biochemistry of specific transmitter systems. In fact, the discovery that neurons in the basal ganglia of parkinsonian patients are deficient in dopamine was the first important clue that neurological disorders can result from altered chemical transmission. Imaging techniques can provide information as to how local transmitter abnormalities can lead to widespread changes in the networks involved in the selection and control of movements.
Understanding the functional properties of the motor system is not only fundamental in its own right, but it is of further importance in helping us to understand disorders of this system and explore the possibilities for recovery. As would be expected for such a complex apparatus, the motor system is subject to various malfunctions. Lesions at different levels in the motor hierarchy produce distinctive symptoms, including the movement-slowing characteristic of disorders of the basal ganglia, such as parkinsonism, the incoordination seen with cerebellar disease, and the spasticity and weakness typical of spinal damage. For this reason, the neurological examination of a patient inevitably includes tests of reflexes, gait, and dexterity, all of which provide information about the status of the nervous system. In addition to pharmacological therapies, the treatment of neurological disease has recently been augmented by two new approaches. First, focal stimulation of the basal ganglia has been discovered to restore motility to certain patients with Parkinson disease; such deep-brain stimulation is also being tested in the context of other neurological conditions. And second, the motor systems have become a target for the application of neural prosthetics; neural signals are decoded and used to drive mechanical devices that aid patients with paralysis caused by spinal cord injury and stroke.