Hi!

The reflex arc involves both aspects of the nervous system. You may remember that the central nervous system (CNS) is comprised of the brain and spinal cord, while the peripheral nervous system (PNS) is comprised of the nerves outside of the brain and spinal cord.

In a simple reflex arc that uses an afferent (sensory neuron), an interneuron, and an efferent (motor neuron) both aspects of the nervous system are involved. The interneuron belongs to the CNS, while the sensory and motor neurons belong to the PNS.

Reflexes can be classified many ways:
- amount of synapse in the CNS (monosynaptic vs polysynaptic)
- type of nerve fiber (somatic vs autonomic)
- origin of nerve fiber (cranial vs spinal)
but they aren't really based on if its CNS or PNS

Good luck with your MCAT preparation!