Motor Functions and Controls
Each time a part of the body—no matter how small—moves, it is the result of a contraction of a muscle. Such a contraction is caused by binding of the neurotransmitter acetylcholine (ACh) to receptors on the muscle fibers. The ACh causes an action potential in the muscle fiber, admitting sodium and calcium ions and causing the actin filaments to slide across the myosin filaments, thus shortening the muscle. The neurons that deliver ACh to the muscle are called motor neurons, and the point at which a motor neuron and a muscle fiber meet is called a neuromuscular junction.
The axon of each motor neuron divides so that one neuron may innervate a large number of muscle fibers. The more fibers innervated, the stronger the contraction. Although the basic mechanism appears to be simple, there are many levels of control of movement arranged in a hierarchy.
Certain movements can occur even if motor neurons are disconnected from the brain. The neurons of the spinal cord are sufficient to cause these simple behaviors. More complex behaviors, however, are the result of the integration of information from several areas of the brain.
The right side of the brain directs movements in the left side of the body, and vice versa. Each side of the primary motor cortex contains the cell bodies of the neurons that eventually communicate with the motor neurons on the opposite side of the spinal cord, those that actually contact the muscle. If only this one synapse occurred, however, it would not be possible to "custom design" movements to fit the needs of the organism in its particular situation. Therefore, there are sub-cortical modulating systems that fine-tune commands coming from the primary motor cortex. When the motor neurons finally leave the brain and enter the spinal cord, crossing to the side opposite that from which they originated, their information has been adjusted by the modulating systems.
One of the modulating systems is called the basal ganglia, which comprises the caudate nucleus, putamen, globus pallidus, subthalamus, and substantia nigra. Many of the synapses in the basal ganglia are inhibitory, so certain commands from the primary motor cortex will be suppressed. The other modulating system is the cerebellum, which, also using inhibitory neurotransmitters, allows for the precise timing of muscle contractions in a sequence, resulting in coordinated rather than jerky movement. The lack of coordination associated with overindulgence in alcohol is the result of the depression of the cerebellar circuits by alcohol.
Both modulating systems receive sensory information from other parts of the brain, including input from proprioceptors. This information is used to tailor motions appropriately to fit the situation. Information from the modulating systems is then sent back to the cortex, where it affects the primary motor neuron's decision to fire, sending information to the spinal cord, or not.
The cortex is the site of still another level of control. The premotor cortex, which receives information from the cerebellum, and the supplementary motor cortex, which receives information from the basal ganglia, also influence neurons in the primary motor cortex. These areas are involved in planning of motions. PET scans reveal that thinking about raising the right arm causes increased blood flow to the left supplementary motor cortex.
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