Skeletal Muscle Relaxants (Peripheral)

Peripheral skeletal muscle relaxants reduce muscle spasms and tone by blocking neuromuscular activity.

• These are drugs that act peripherally on skeletal muscle to reduce muscle tone or cause muscle paralysis.
• They do not have central nervous system (CNS) effects, unlike centrally acting muscle relaxants.

Classification of Peripheral Skeletal Muscle Relaxants

1. Non-Depolarizing Neuromuscular Blockers (Competitive antagonists)

   • Mechanism of Action:
     • These agents compete with acetylcholine (ACh) for nicotinic receptors at the NMJ.
     • By blocking ACh binding, they prevent depolarization of the motor endplate, resulting in muscle relaxation without initial stimulation.
   • Reversal:
     • The blockade can be reversed by acetylcholinesterase inhibitors (e.g., neostigmine), which increase the concentration of ACh at the synapse.
   • Classification Based on Duration of Action:
     1. Long-acting Agents
        • d-Tubocurarine
        • Pancuronium
        • Doxacurium
        • Gallamine
     2. Intermediate-acting Agents
        • Atracurium
        • Vecuronium
        • Rocuronium
     3. Short-acting Agents
        • Mivacurium

2. Depolarizing Neuromuscular Blockers

   • Mechanism of Action:
     • These drugs mimic ACh and bind to nicotinic receptors, causing initial depolarization of the motor endplate.
     • This is followed by persistent depolarization and desensitization, leading to paralysis.
   • Reversal:
     • Not reversed by acetylcholinesterase inhibitors.
     • The effect diminishes as the drug is metabolized, primarily by plasma cholinesterase.
   • Examples:
     • Succinylcholine (SCh)
     • Decamethonium

Pharmacodynamics and Pharmacokinetics

• Onset and Duration:

  • Succinylcholine: Rapid onset (less than 1 minute), short duration (5–10 minutes)
  • Non-depolarizing agents vary:
    • Rocuronium: rapid onset, intermediate duration
    • Pancuronium: slower onset, long duration

Metabolism and Excretion:

• Succinylcholine: Rapidly hydrolyzed by plasma pseudocholinesterase
• Atracurium: Undergoes Hofmann elimination (organ-independent metabolism)
• Others are metabolized hepatically or renally excreted.

Clinical Uses of Skeletal Muscle Relaxants:

1. Facilitate endotracheal intubation
2. Muscle relaxation during surgery
3. Ventilation control in ICU
4. Adjunct to anesthesia

Adverse Effects

• Non-depolarizing:

  • Histamine release → hypotension, bronchospasm (e.g., atracurium)
  • Prolonged paralysis (especially in hepatic/renal impairment)

• Depolarizing (Succinylcholine):

  • Hyperkalemia (dangerous in burns, trauma)
  • Malignant hyperthermia (especially with halothane)
  • Bradycardia
  • Fasciculations (initial muscle twitching)
  • Increased intraocular/intragastric pressure

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