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An upper motor neuron is a neuron, or nerve cell, that exists in the brain and sends higher-level motor information to the medulla, located in the brain, or to the correct spinal cord level outside of the brain. From the medulla or the spinal cord, lower motor neurons carry motor information to muscle fibers, making them much more directly responsible for movement than upper motor neurons. One can find an upper motor neuron in a couple of different parts the the brain. Upper motor neurons are generally concentrated in the motor region of the brain stem or cerebral cortex; they send motor information downward from there.
There are many different pathways down which an upper motor neuron can send electrical impulses that eventually translate to movement. The rubrospinal tract, for example, is heavily involved in involuntary movements to improve and maintain the body's balance. Most of the movements relating to this tract occur in the arms. The tectospinal tract is related to the movements of the muscles in the neck, and the reticulospinal tract has an important role in the control of autonomous actions in the body. The corticospinal tract, or pyramid tract, is largely in control of conscious movements and muscle action.
In general, an upper motor neuron in the pyramid tract will have a role in controlling conscious movement while an upper motor neuron in an extrapyramidal tract, or any pathway outside of the pyramid tract, will probably be involved in a subconscious motor process such as balance or posture. The pyramid tract begins in the cerebral cortex, specifically in the section of the frontal lobe that is known as the motor strip. In general, the nerve impulses travel from this motor strip to the spinal cord.
The lower motor neurons receive the information from the upper motor neurons and transmit it to the actual muscle fibers in both conscious and autonomous movements. As such, both the upper and lower neuron tracts need to be working properly to facilitate the proper transfer of nerve impulses from the brain to the muscles. A disorder in either part can cause motor problems of varying severity.