A patient suffers damage to the nerve that carries visual signals from the eye toward the brain. Which classification best describes this nerve, and what is its functional role?
ACNS structure; efferent — carries motor commands to the eye muscles
BPNS structure; afferent — carries sensory information toward the CNS
CCNS structure; afferent — integrates visual signals within the brain
DPNS structure; efferent — carries motor commands from the brain to the eye
The optic nerve is a cranial nerve, making it PNS, not CNS — despite emerging from the brainstem. It carries sensory (visual) information toward the brain, making it afferent. Afferent means 'toward the CNS'; efferent means 'away from the CNS.' Cranial nerves are a classic misconception target — they are PNS structures.
Question 2 True / False
After a spinal cord injury (CNS damage), patients typically have permanent deficits, whereas after a peripheral nerve injury (PNS damage), some recovery is possible. This is because CNS neurons have a fundamentally greater ability to regenerate than PNS neurons.
TTrue
FFalse
Answer: False
This is false — the direction is reversed. PNS neurons can regenerate because Schwann cells form a supportive environment (Bands of Büngner) that guides regrowth. CNS neurons fail to regenerate not because they lack intrinsic capacity, but because oligodendrocytes and reactive astrocytes create an inhibitory environment. The asymmetry is primarily glial, not neuronal.
Question 3 Short Answer
Trace a spinal reflex arc and identify at each step whether the structure is part of the CNS or PNS, and whether each neuron is afferent or efferent.
Think about your answer, then reveal below.
Model answer: Sensory receptor (PNS) → afferent neuron (PNS, carrying signal toward CNS) → interneuron in spinal cord dorsal horn (CNS, integration) → efferent motor neuron (PNS, carrying command away from CNS) → skeletal muscle effector (PNS).
The spinal cord is CNS; everything peripheral to it is PNS. The reflex arc elegantly demonstrates the full CNS/PNS division and both directions of signal flow in a single minimal circuit.