There are two subclasses of muscle relaxants; their mechanism of action is:

• A. Mechanisms identical to other CNS agents
• B. No mechanism of action
• C. Mechanisms shared with antidepressants
• D. Mechanisms unique to this subclass

Answer: (D)

Muscle relaxants work in two broad ways, and each approach targets muscle activity in a different place. Central acting relaxants decrease muscle tone by dampening the brain and spinal cord circuits that control movement, which reduces the motor signals reaching the muscles. Direct-acting relaxants, on the other hand, affect the muscle itself—things like reducing calcium release inside muscle cells or blocking acetylcholine at the neuromuscular junction—so the muscle contracts less strongly.

Because these mechanisms are specific to muscle-relaxant biology and aren’t the same as how typical CNS depressants or antidepressants work, the best choice is that the mechanisms are unique to this subclass.