Definitions

Upper motor neuron

Upper motor neuron

Upper motor neurons are motor neurons that originate in motor region of the cerebral cortex or the brain stem and carry motor information down to the final common pathway, that is, any motor 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 appropriate level in the spinal cord, from which point nerve signals continue to the muscles by means of the lower motor neurons. The neurotransmitter glutamate transmits the nerve impulses from upper to lower motor neurons where it is detected by glutamatergic receptors.

Pathways

Upper motor neurons travel in several pathways through the CNS:

Tract Pathway Function
corticospinal tract 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 from the motor cortex to several nuclei in the pons and medulla Involved in control of facial and jaw musculature, swallowing and tongue movements.
tectospinal tract/colliculospinal tract from the superior colliculus to lower motor neurons Involved in involuntary adjustment of head position in response to visual information.
rubrospinal tract from red nucleus to lower motor neurons Involved in involuntary adjustment of arm position in response to balance information.
vestibulospinal tract from vestibular nuclei, which processes stimuli from semicircular canals It is responsible for adjusting posture to maintain balance.
reticulospinal tract from reticular formation Regulates various involuntary motor activities and assists in balance.

Lesions

Upper motor neuron lesions are indicated by spasticity, muscle weakness, exaggerated reflexes, and an extensor plantar response known as the Babinski sign.

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