Finally, we will summarize as best we can what is known about overall control of movement. To do this, let us first give a synopsis of the different levels of control.
SPINAL LEVEL
Programmed in the spinal cord are local patterns of movement for all muscle areas of the body—for instance, programmed withdrawal reflexes that pull any part of the body away from a source of pain. The cord is the locus also of complex patterns of rhythmical motions such as to-and-fro movement of the limbs for walking, plus reciprocal motions on opposite sides of the body or of the hindlimbs versus the forelimbs in four legged animals.
All these programs of the cord can be commanded into action by higher levels of motor control, or they can be inhibited while the higher levels take over control.
HINDBRAIN LEVEL
The hindbrain provides two major functions for general motor control of the body: (1) maintenance of axial tone of the body for the purpose of standing and (2) continuous modification of the degrees of tone in the different muscles in response to information from the vestibular apparatuses for the purpose of maintaining body equilibrium.
MOTOR CORTEX LEVEL
The motor cortex system provides most of the activating motor signals to the spinal cord. It functions partly by issuing sequential and parallel commands that set into motion various cord patterns of motor action. It can also change the intensities of the different patterns or modify their timing or other characteristics. When needed, the corticospinal system can bypass the cord pat terns, replacing them with higher level patterns from the brain stem or cerebral cortex. The cortical patterns are usually complex; also, they can be “learned,” whereas cord patterns are mainly determined by heredity and are said to be “hard wired.”
Associated Functions of the Cerebellum. The cerebellum functions with all levels of muscle control. It functions with the spinal cord especially to enhance the stretch reflex, so when a contracting muscle encounters an unexpectedly heavy load, a long stretch reflex signal transmit ted all the way through the cerebellum and back again to the cord strongly enhances the load-resisting effect of the basic stretch reflex.
At the brain stem level, the cerebellum functions to make the postural movements of the body, especially the rapid movements required by the equilibrium system, smooth and continuous and without abnormal oscillations.
At the cerebral cortex level, the cerebellum operates in association with the cortex to provide many accessory motor functions, especially to provide extra motor force for turning on muscle contraction rapidly at the start of a movement. Near the end of each movement, the cerebellum turns on antagonist muscles at exactly the right time and with proper force to stop the movement at the intended point. Furthermore, there is good physiological evidence that all aspects of this turn-on/turn off patterning by the cerebellum can be learned with experience.
The cerebellum functions with the cerebral cortex at still another level of motor control: it helps to program in advance muscle contractions that are required for smooth progression from a present rapid movement in one direction to the next rapid movement in another direction, with all this occurring in a fraction of a second. The neural circuit for this passes from the cerebral cortex to the large lateral zones of the cerebellar hemispheres and then back to the cerebral cortex.
The cerebellum functions mainly when rapid muscle movements are required. Without the cerebellum, slow and calculated movements can still occur, but it is difficult for the corticospinal system to achieve rapid and changing intended movements to execute a particular goal or especially to progress smoothly from one rapid movement to the next.
Associated Functions of the Basal Ganglia. The basal ganglia are essential to motor control in ways entirely different from those of the cerebellum. Their most important functions are (1) to help the cortex execute subconscious but learned patterns of movement and (2) to help plan multiple parallel and sequential patterns of movement that the mind must put together to accomplish a purposeful task.
The types of motor patterns that require the basal ganglia include those for writing all the different letters of the alphabet, for throwing a ball, and for typing. Also, the basal ganglia are required to modify these patterns for writing small or writing very large, thus controlling dimensions of the patterns.
At a still higher level of control is another combined cerebral and basal ganglia circuit, beginning in the thinking processes of the cerebrum to provide overall sequential steps of action for responding to each new situation, such as planning one’s immediate motor response to an assailant or one’s sequential response to an unexpectedly fond embrace.
WHAT DRIVES US TO ACTION?
What is it that arouses us from inactivity and sets into play our trains of movement? We are beginning to learn about the motivational systems of the brain. Basically, the brain has an older core located beneath, anterior, and lateral to the thalamus—including the hypothalamus, amygdala, hippocampus, septal region anterior to the hypothalamus and thalamus, and even old regions of the thalamus and cerebral cortex—all of which function together to initiate most motor and other functional activities of the brain. These areas are collectively called the limbic system of the brain. We discuss this system in detail in Chapter 59.
