Networks of spinal neurons also participate in controlling the alternating action of the legs during normal walking, maintaining posture, and, to a large degree, in all movements.
In fact, the basic patterns of muscle activation that produce coordinated walking can be generated not only in four-footed animals, but also in humans, within the spinal cord itself. These spinal mechanisms, which evolved in primitive vertebrates, are being studied to determine the degree to which spinal circuitry can be used to recover basic postural and locomotor function after severe paralysis.
The most complex movements that we perform, including voluntary ones that require conscious planning, involve control of these basic spinal mechanisms by the brain. Scientists are only beginning to understand the complex interactions that take place among different brain regions during voluntary movements, mostly through careful experiments on animals.
One important brain area that is responsible for voluntary movement is the motor cortex, which exerts powerful control over the spinal cord, in part through direct control of its alpha motor neurons. Some neurons in the motor cortex appear to specify the coordinated action of many muscles to produce the organized movement of a limb to a particular point in space. Others appear to control only two or three functionally related muscles, such as those of the hand or arm, that are important for finely tuned, skilled movement.
In addition to the motor cortex, movement control involves the interaction of many other brain regions, including the basal ganglia, thalamus, cerebellum, and a large number of neuron groups located within the midbrain and brainstem — regions that send axons to the spinal cord. Scientists know that the basal ganglia and thalamus have widespread connections with motor and sensory areas of the cerebral cortex.
Dysfunction of the basal ganglia can lead to serious movement disorders. The neurotransmitter dopamine, which helps control movement, is supplied to the basal ganglia by the axons of neurons located in the substantia nigra, a midbrain cell group. People with Parkinson’s disease experience degeneration of the nigral neurons. The supply of dopamine is depleted, resulting in the hallmark symptoms of Parkinson’s — tremor, rigidity, and akinesia, the inability to move.
Another brain region that is crucial for coordinating and adjusting skilled movement is the cerebellum. A disturbance of cerebellar function leads to poor coordination of muscle control, disorders of balance and reaching, and even difficulties in speech, one of the most intricate forms of movement control.
The cerebellum receives direct information from all the sensory receptors in the head and the limbs and from most areas of the cerebral cortex. The cerebellum apparently acts to integrate all this information to ensure smooth coordination of muscle action, enabling us to perform skilled movements more or less automatically. Considerable evidence indicates that the cerebellum helps us adjust motor output to deal with changing conditions, such as growth, disability, changes in weight, and aging. It tunes motor output to be appropriate to the specific requirements of each new task: Our ability to adjust when picking up a cup of coffee that is empty or full depends on the cerebellum. Evidence suggests that as we learn to walk, speak, or play a musical instrument, the necessary, detailed control information is stored within the cerebellum, where it can be called upon by commands from the cerebral cortex.
Just as the brain controls movement, it also is responsible for one of the body’s most important functions — sleep. The brain switches back and forth between different stages of sleep all night long.