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Upper motor neurons are any neurons that carry motor information down to the final common pathway, that is, any neurons that are not directly responsible for stimulating the target muscle. The main effector neurons for voluntary movement lie within layer V of the primary motor cortex and are called Betz cells. The cell bodies of these neurons are some of the largest in the brain, approaching nearly 100μm in diameter. These neurons connect the brain to the spinal cord, from which point nerve signals continue to the muscles by means of the lower motor neurons
Upper motor neurons travel in several pathways through the CNS:
- corticospinal tract: the direct pathway from the motor cortex to lower motor neurons in the ventral horn of the spinal cord. The major function of this pathway is fine voluntary motor control of the limbs. The pathway also controls voluntary body posture adjustments.
- corticobulbar tract: pathway from the motor cortex to several nuclei in the pons and medulla, which are involved in involuntary maintenance of body posture.
- tectospinal tract/colliculospinal tract: pathway from the superior colliculus to lower motor neurons, involved in involuntary adjustment of head position in response to visual information.
- rubrospinal tract: pathway from red nucleus to lower motor neurons, involved in involuntary adjustment of arm position in response to balance information.
- vestibulospinal tract: pathway from vestibular nuclei, which processes stimuli from semicircular canals. It is responsible for adjusting posture to maintain balance.
- reticulospinal tract: pathway from reticular formation, regulates various involuntary motor activities and assists in balance.