Overview of Voluntary Motor Function

The motor systems are responsible for initiating and coordinating all movements that are part of the normal and abnormal behavioral repertoires. The motor systems generate three general types of movements. Reflex responses are rapid, stereotyped involuntary movements elicited by a stimulus that requires a quick reaction at an involuntary level--for example, withdrawal of a bare foot that has touched a sharp or hot object. Rhythmical movements such as walking and running require a stereotyped sequence of muscle activation (see ChapteLl..8 ). Voluntary movements are the most complex. They are goal-directed and initially require conscious direction. Through practice, skilled movements such as playing a piano become more automatic and require less conscious direction.

The three types of movements correlate with the overall organization of the motor system, which is hierarchical and contains three major components: local reflex networks in the spinal cord; motor integration areas of the brain stem; and motor integration areas of the cerebral cortex. The spinal cord contains the circuitry for the reflex responses and some rhythmical motor patterns, whereas the brain stem system contains circuits for more complex patterns of motor movements including rhythm generators. The cortex is the command center that plans and initiates movements and uses the reflex and patterned responses of the brain stem and spinal cord to generate the details of the movement.y

The motor systems do not function in isolation and require a constant flow of sensory information to produce the appropriate movement. The sensory system is also organized in a hierarchical fashion and provides appropriate sensory information to each level of the motor system. In addition to these three hierarchical levels, two other parts of the motor system regulate movement, the cerebellum (see Chaptern17 ) and the basal ganglia (see Chapter l§ ). The cerebellum improves the coordination and accuracy of movement by comparing sensory feedback from the periphery with the descending motor commands, and the basal ganglia receive inputs from both motor and sensory cortical regions and project principally to areas of the frontal cortex that are involved in motor planning.

In addition to this hierarchical organization, many pathways operate in parallel and can act independently to control motor output. Thus, higher centers can adjust the operation of different spinal circuits independently to produce combinations of movements required for complex tasks. For example, postural reflexes residing in the brain stem maintain the ballet dancer's body position and balance while the cortex initiates a reaching movement of the arms.

Ultimately, all of these descending hierarchical and parallel pathways converge on a set of motor neurons in the spinal cord that give rise to the motor nerves that innervate skeletal muscle and produce movement. These are the lower motor neurons of the ventral gray matter of the spinal cord. Sometimes they are referred to as the final common pathway. In contrast, the motor neurons that generate voluntary, rhythmical, and reflex responses are the upper motor neurons. Most of the control signals coming from the upper motor neuron do not act directly on the lower motor neuron but use networks of interposed neurons known as interneurons, or the internuncial pool of neurons. These neurons are very important in integrating the reflex and rhythmical patterns of movement. The final effector of movement, skeletal muscle, responds in two modes, tonic and phasic contractions. For example, muscles involved in posture contract tonically, whereas those involved in voluntary limb movements contract in a phasic pull and push of joints. Phasic movements require coordinated contractions of agonists and antagonists. In the sections that follow, these elements of the motor system are be discussed in more detail, starting with the muscle and the neuromuscular junction and then proceeding with the lowest level of hierarchical organization, the spinal cord, and progressing through the brain stem to the cortex. The parallel processing of motor control is discussed for each of these levels.

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