Why do the CNS and PNS axes change in humans?

• A. Because the CNS and PNS are not in a straight line
• B. Because the brain changes orientation while the spinal cord does not
• C. Because directions are fixed
• D. Because the PNS is always straight

Answer: (A)

The main idea is that the brain’s growth and rotation change how its internal directions line up with the body’s directions, so the paths of the central nervous system and the peripheral nerves don’t form a single straight line. During development, the brain expands and folds, with bends like the cephalic and other flexures that rotate the cerebrum relative to the brainstem and spinal cord. At the same time, peripheral nerves sprout from the spinal cord and travel out to the limbs and trunk along routes that diverge from that CNS axis. The result is a CNS axis that isn’t simply extended into the PNS; the CNS and PNS axes are not a straight line in humans.