Comparative Features of Centrally and Peripherally Acting Muscle Relaxants

Centrally Acting vs Peripherally Acting Muscle Relaxants

• Centrally acting muscle relaxants decrease muscle tone without reducing voluntary power, whereas peripherally acting ones cause muscle paralysis, resulting in loss of voluntary movements.
• Centrally acting muscle relaxants selectively inhibit polysynaptic reflexes in the CNS, whereas peripherally acting ones block neuromuscular transmission.
• Centrally acting muscle relaxants cause some CNS depression, whereas peripherally acting ones have no effect on the CNS.
• Centrally acting muscle relaxants are given orally or parenterally, whereas peripherally acting ones are practically always given intravenously.
• Centrally acting muscle relaxants are used in chronic spastic conditions, acute muscle spasms, and tetanus, whereas peripherally acting ones are used for short-term purposes, such as surgical operations.

Thiocolchicoside

• Thiocolchicoside is a GABA mimetic and glycinergic muscle relaxant.
• It is used in combination with NSAIDs for painful muscle spasms, sprains, torticollis, and backache.

Tizanidine

• Tizanidine is a central a2 adrenergic agonist that inhibits the release of excitatory amino acids in spinal interneurones.
• It facilitates the release of inhibitory transmitter glycine.
• It inhibits polysynaptic reflexes, reduces muscle tone, and frequency of muscle spasms without reducing muscle strength.
• It is used in spasticity due to neurological disorders and painful muscle spasms of spinal origin.
• Common side effects of tizanidine include dry mouth, drowsiness, night-time insomnia, hallucinations, and dose-dependent elevation of liver test values.

Classification of Skeletal Muscle Relaxants

• Skeletal muscle relaxants can be classified into two categories: peripherally acting and centrally acting.
• Peripherally acting muscle relaxants can be further classified into neuromuscular blocking agents, which include nondepolarizing blockers (long acting, intermediate acting, and short acting) and depolarizing blockers.
• Centrally acting muscle relaxants can be classified into directly acting agents, which include mephenesin congeners, dantrolene sodium, chlorzoxazone, GABA mimetic, and central a2 agonist.

Neuromuscular Blocking Agents

• Neuromuscular blocking agents act on the endplate of skeletal muscle fibers.
• Competitive blockers, such as curare, have affinity for the nicotinic (NM) cholinergic receptors at the muscle endplate but have no intrinsic activity.
• Depolarizing blockers, such as decamethonium and succinylcholine, have affinity and submaximal intrinsic activity at the NM cholinceptors, and initially produce twitching and fasciculations.

Uses of Skeletal Muscle Relaxants

• Acute muscle spasms, torticollis, lumbago, backache, and neuralgias respond to centrally acting muscle relaxants.
• Spastic neurological diseases, such as hemiplegia, paraplegia, spinal injuries, multiple sclerosis, and cerebral palsy, are treated with centrally acting muscle relaxants.
• Tetanus is treated with diazepam infusion.
• Electroconvulsive therapy and orthopedic manipulations may be performed under the influence of centrally acting muscle relaxants.