Which statement about the mechanism of action of the two muscle-relaxant subclasses is correct?

• A. They share the same mechanism
• B. Each subclass has a mechanism unique to itself
• C. They have no mechanism
• D. They are identical to other CNS depressants

Answer: (B)

Mechanisms differ between muscle-relaxant subclasses, and that difference is why each group has its own distinct way of producing lower muscle tone. One class works by boosting inhibitory signaling in the CNS—agents that enhance GABA activity at GABA-A receptors, increasing chloride flow into neurons and dampening motor neuron firing, which relaxes muscles. Another class acts through a separate pathway, such as activating GABA-B receptors in the spinal cord (baclofen) to reduce excitatory neurotransmitter release and motor reflex activity, or using alpha-2 adrenergic mechanisms (like tizanidine) to lessen spasticity. Because these actions occur at different receptors and places in the nervous system, the statement that each subclass has a mechanism unique to itself is the best fit. They are not all identical to other CNS depressants, and they do have defined mechanisms even though they share the goal of reducing muscle tone.