Biology I - Brain Structure - Upper Motor Neurons

Q: Where are the somas of upper motor neurons located?

A: In the cerebral cortex.

Q: What does the corticospinal tract control, and where does it cross?

A: Controls body movement; most fibers cross in the medulla.

Q: A spinal cord lesion affects the right corticospinal tract. Which side is weak?

A: Right side—same side as lesion

Q: What is the Babinski sign, and what does it indicate?

A: Upward toe extension; suggests UMN lesion.

Q: Why does hyperreflexia occur with UMN lesions?

A: Loss of descending inhibition on spinal reflex arcs.

Q: What is clonus and what causes it?

A: Rhythmic muscle contractions due to UMN lesion-induced reflex instability.

Q: How does corticobulbar tract damage differ from corticospinal tract damage?

A: It affects cranial nerves; weakness patterns vary due to bilateral innervation.

Q: What does corticospinal tract damage in the brain cause, and why?

A: Contralateral weakness, fibers cross midline at the medulla.