Which factor most strongly limits axon regrowth in the CNS?

• A. Schwann cell presence.
• B. Myelin-derived inhibitors and glial scar.
• C. Abundant growth factors.
• D. High metabolic activity.

Answer: (D)

Axon regrowth in the CNS is mainly thwarted by the environment around the injured neurons. Myelin-derived inhibitors present on CNS myelin, such as Nogo-A, MAG, and OMgp, bind to receptors on regenerating axons and actively suppress growth signals, making extension difficult. After injury, astrocytes also form a glial scar rich in molecules like chondroitin sulfate proteoglycans that create both a chemical barrier and a physical obstacle to growth. This inhibitory milieu is a stronger brake on regeneration than anything related to the neuron's own energy demands. In contrast, the peripheral nervous system supports regrowth more readily because Schwann cells create a more permissive substrate and secrete favorable growth factors. So, the strongest limiting factor in the CNS is the inhibitory environment created by myelin-derived inhibitors and the glial scar.