Upper and lower motor neurons

Both somatic motor and branchiomotor nerves supply voluntary muscles. Pathways between motor cortex and muscles may be thought of as being arranged in two neuronal groups: upper motor neurons and lower motor neurons. Axons of upper motor neurons decussate before synapsing with lower motor neurons, so the right motor cortex controls the left side of the body, and vice versa – contralateral control.

Upper motor neurons: cortex to nucleus

For cranial nerves, cell bodies of upper motor neurons are in the head and neck area of the motor cortex. Axons descend, decussating just before synapsing with cell bodies of lower motor neurons which make up the motor nucleus of that cranial nerve. The term upper motor neurons is also used clinically to include fibres from other brain centres (e.g. parietal lobe, basal ganglia, cerebellum, reticular formation, midbrain, etc.) that connect with the lower motor neurons in the cranial nerve nucleus, thus influencing their activity.

Lower motor neurons: nucleus to muscle

Cell bodies of lower motor neurons form the brain stem nucleus. Axons leave the brain stem and pass in the cranial nerve to the destination. Thus, although most of the axon of the lower motor neuron is part of the peripheral nervous system, the cell body and first part of the axon is in the central nervous system.

Corticonuclear and corticobulbar

These terms describe the upper motor neuron pathways described above.