Drugs that provide muscle relaxation by an action on the central nervous system or on the muscle itself are not useful for this purpose in surgery; they are insufficiently selective and full relaxation, even if achievable, is accompanied by general cerebral depression. But there is a place for drugs that reduce spasm of the voluntary muscles without impairing voluntary movement. Such drugs can be useful in spastic states, low back syndrome, and rheumatism with muscle spasm.
Baclofen is structurally related to gamma-ami-nobutyric acid (GABA), an inhibitory central nervous system transmitter; it inhibits reflex activity mainly in the spinal cord. Baclofen reduces spasticity and flexor spasms, but as it has no action on voluntary muscle power, function is commonly not improved. Ambulant patients may need their leg spasticity to provide support and reduction of spasticity may expose the weakness of the limb. It benefits some cases of trigeminal neuralgia. Baclofen is given orally ( t '/2 3 h).
Dantrolene acts directly on muscle and prevents the release of calcium from sarcoplasm stores (see Malignant hyperthermia, p. 427).
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