control. The cortex is the location where all new learn- ing takes place. Its surface area is highly differentiated, with different parts taking on different functions.
Figure 2–10shows the basic cortical layout. The pre- motor (including the supplemental) and motor areas are most important for muscle control. The premotor and supplemental area is just anterior to the motor area. It is here that the gross motor plan for movement is formed. This information is then passed to the motor area, where fine motor coordination is set forth. The motor area is laid out in a highly differentiated fashion, as seen in the anatomical map shown in Figure 2–11. In this figure, the size of the icon for a particular body part represents the amount of cortical space allocated to that region. It is clear that the face and hand consume most of the neurons. From the motor area, nearly 60%
of the fibers travel in a finely differentiated fashion di- rectly to the lower motor neurons via the pyramidal tracts. The other 40% synapse along the way in the basal ganglia and the brain stem to receive fine-tuning from the cerebellum and to integrate some of the higher-order reflexes.
visual, auditory, vestibular, proprioceptive). It receives input from all of the muscle spindles and Golgi tendon or- gans as well as a large portion of the extrapyramidal output from the cortex and gamma motor output from the basal ganglia. No alpha or gamma motor responses originate from the cerebellum, but almost all of them pass through its influence. This part of the brain is highly organized and mapped, just like the cortex, with its own sensory and motor homunculus. It sends out in- formation to the reticular formation, the basal ganglia, the thalamus, the motor cortex, and the spine. Its basic job is to take the cortical plan for movement and com- pare the incoming sensory information with the de- scending alpha and gamma motor information and fine-tune the final output so that the movement is smooth and coordinated.
The Cortex
Control of the neuromuscular system is quite complex and involves three major divisions—cortex, brain-stem, and spinal cord—which operate in a hierarchical arrangement as well as in parallel. Figure 2–9illustrates a dynamic systems approach in which there is a very strong interrelation between the various levels. Thus, the motor areas of the cortex can influence the spinal cord both directly or through systems descending through the brain stem. All three levels of the motor sys-
Cerebral cortex Motor areas
Limbic Basal ganglia Cerebellum Thalamus
Brainstem
Spinal cord
Sensory input of movement Sensory receptors Emotions
Posture Movement
Figure 2–9 A highly schematized flow diagram of the hierarchical and parallel aspects of the motor system.
The extrapyramidal system has three distinct de- scending tracts, each with its own function:
• The vestibulospinal tract primarily has to do with righting reflexes, postural stabilization, and the facilitation of extensors and inhibition of flexors that assist in maintaining the upright posture
• The reticulospinal tract primarily mediates the facilitation or inhibition of the gamma motor system
• The rubrospinal tract carries the fine-tuning information coming out of the cerebellum All of these tracts carry information that ultimately modifies the inherited reflexes, usually by inhibiting the obligatory responses in favor of some learned pattern.
It would appear that we have two very different sen- sorimotor systems: the alpha and gamma motor sys- tems. Each has its own muscles, motor neurons, and principles of organization. The gamma motor system, whose origins lie primarily in the basal ganglia, car- ries out the “ancient knowledge.” It controls the resting length of the stretch receptors, thereby passing on our species-specific postures and behaviors through con-
trol of our reflexes. These reflexes have been selected and passed down through thousands of generations;
they are fixed and obligatory and there is no danger of forgetting them. Yet, they are modifiable with enough repetitions of a new posture or movement. In con- trast, the alpha motor system, whose origins lie in the cortex, provides us with the opportunity to acquire and use new knowledge. It primarily modulates or in- hibits the obligatory reflexes we inherited. While the gamma motor system provides us with the wisdom of our ancestors, the alpha motor system allows us to adapt to the ever-changing aspects of our world.
Together, these two systems provide us with the best of both worlds.
It is important to recognize that the neuromuscular system is extremely complex and that it involves more than just these two motor systems. The neuro- muscular system is, in fact, a very interactive and dy- namic, multilayered system that operates in both hierarchical and parallel modes. No one part of the system is more important than any other. In fact, for the system to survive successfully as a whole, all of the various parts are linked together through feedback and feedforward loops that must be integrated at any given movement.
Elaboration of thought
Speech Supplemental motor synergies
Voluntary motor
Somatic sensory
Hand skills
Memory
Hearing
Speech
Vision
Cerebellum
Spinal cord
Figure 2–10 Functional areas of the human cerebral cortex.
Figure 2–11 The pyramidal system is highly differentiated from the cortex through the spinal cord. Note the degree of cortical representation of the various body parts.
Source: Netter Anatomy Illustration Collection, ©Elsevier, Inc. All Rights Reserved.