Upper motor neurons are motor neurons that originate either in the motor region of the cerebral cortex or in the brain stem and carry motor information down to the lower motor neurons. The main effector or motor neurons (efferent neurons) for voluntary movement lie within layer V of the primary motor cortex and are a type of giant pyramidal cell called Betz cells. The cell bodies of these neurons are the largest in the brain, approaching nearly 0.1mm in diameter.