Which drug is a direct-acting skeletal muscle relaxant?

• A. Dantrolene
• B. Baclofen
• C. Cyclobenzaprine
• D. Chlorzoxazone

Answer: (A)

Direct-acting skeletal muscle relaxants work by acting on the muscle itself to reduce contraction, rather than dampening the CNS. Dantrolene does this by directly inhibiting calcium release from the sarcoplasmic reticulum through the ryanodine receptor. With less calcium available, the interaction between actin and myosin is diminished, so muscle contraction weakens. This direct interference with the muscle’s calcium handling sets it apart from the others listed, which are mainly CNS-acting: baclofen works through GABA-B receptors in the spinal cord to lower motor output, while cyclobenzaprine and chlorzoxazone primarily act in the brainstem or other CNS sites to reduce muscle spasm with sedative effects. Clinically, dantrolene is notable for malignant hyperthermia management and can be used for severe spasticity, but it requires liver function monitoring due to potential hepatotoxicity.