The neural circuits responsible for the control of movement can be divided into four distinct but highly interactive subsystems, each of which makes a unique contribution to motor control (Figure 16.1). The first of these subsystems is the local circuitry within the gray matter of the spinal cord and the analogous circuitry in the brainstem. The relevant cells include the lower motor neurons (which send their axons out of the brainstem and spinal cord to innervate the skeletal muscles of the head and body, respectively) and the local circuit neurons (which are the major source of synaptic input to the lower motor neurons). All commands for movement, whether reflexive or voluntary, are ultimately conveyed to the muscles by the activity of the lower motor neurons; thus they comprise, in the words of the great British neurophysiologist Charles Sherrington, the “final common path” for movement. The local circuit neurons receive sensory inputs as well as descending projections from higher centers. Thus, the circuits they form provide much of the coordination between different muscle groups that is essential for organized movement. Even after the spinal cord is disconnected from the brain in an experimental animal such as a cat, appropriate stimulation of local spinal circuits elicits involuntary but highly coordinated movements of the four limbs that resemble walking.

Figure 16.1

Overall organization of neural structures involved in the control of movement. Four systems—local spinal cord and brainstem circuits, descending modulatory pathways, the basal ganglia, and the cerebellum—make essential and distinct contributions (more...)

The second motor subsystem consists of neurons whose cell bodies lie in the brainstem or cerebral cortex. The axons of these higher-order or upper motor neurons descend to synapse with the local circuit neurons or, more rarely, with the lower motor neurons directly. The upper motor neuron pathways that arise in the cortex are essential for the initiation of voluntary movements and for complex temporal sequences of movement. In particular, descending projections from cortical areas in the frontal lobe, including Brodmann's area 4 (the primary motor cortex), the lateral part of area 6 (the lateral premotor cortex), and the medial part of area 6 (the medial premotor cortex) are essential for planning, initiating, and directing temporal sequences of voluntary movements. Upper motor neurons originating in the brainstem are responsible for regulating muscle tone and for orienting the eyes, head, and body with respect to vestibular, somatic, auditory, and visual sensory information. Their contributions are thus critical for basic navigational movements of the body, and in the control of posture.

The third and fourth subsystems are structures (or groups of structures) that have no direct access to either the local circuit neurons or the lower motor neurons; instead, they control movement by regulating the activity of the upper motor neurons. The third and larger of these subsystems, the cerebellum, is located on the dorsal surface of the pons (see Chapter 1). The cerebellum acts via its efferent pathways to the upper motor neurons as a servomechanism, detecting the difference, or “motor error,” between an intended movement and the movement actually performed (see Chapter 19). The cerebellum uses this information about discrepancies to mediate both real-time and long-term reductions in these motor errors (the latter being a form of motor learning). As might be expected from this account, patients with cerebellar damage exhibit persistent errors in movement. The fourth subsystem, embedded in the depths of the forebrain, consists of a group of structures collectively referred to as the basal ganglia (see Chapter 1). The basal ganglia suppress unwanted movements and prepare (or “prime”) upper motor neuron circuits for the initiation of movements. The problems associated with disorders of basal ganglia, such as Parkinson's disease and Huntington's disease, attest to the importance of this complex in the initiation of voluntary movements (see Chapter 18).

Despite much effort, the sequence of events that leads from thought to movement is still poorly understood. The picture is clearest, however, at the level of control of the muscles themselves. It therefore makes sense to begin an account of motor behavior by considering the anatomical and physiological relationships between lower motor neurons and the muscle fibers they innervate.