What are neurons located in the brain and brainstem whose axons descend to synapse in the spinal cord called?

• A. Upper motor neurons
• B. Lower motor neurons
• C. Interneurons
• D. Motor units

Answer: (A)

Neurons located in the brain and brainstem whose axons descend to synapse in the spinal cord are upper motor neurons. They originate in motor areas of the cortex and in brainstem motor nuclei and project down the corticospinal and corticobulbar tracts to connect with lower motor neurons in the spinal cord or brainstem. This descending control shapes voluntary movement and helps regulate reflexes and muscle tone. When upper motor neurons are damaged, you see increased tone and exaggerated reflexes (spasticity) and signs like an upgoing Babinski due to loss of inhibitory input on spinal circuits. In contrast, lower motor neurons lie in the spinal cord or brainstem and send axons out to muscles; their injury causes flaccid paralysis, atrophy, fasciculations, and reduced reflexes. Interneurons stay within the CNS to connect neurons locally, and a motor unit refers to a single motor neuron and all the muscle fibers it innervates.