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Chapter 16

Chapter 16. Drugs That Block Nicotinic Cholinergic Transmission: Neuromuscular Blocking Agents and Ganglionic Blocking Agents. Neuromuscular Blockers. Prevent acetylcholine from activating nicotinic M Cause muscle relaxation (paralysis) No oral forms Cannot cross Blood-brain barrier

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Chapter 16

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  1. Chapter 16 Drugs That Block Nicotinic Cholinergic Transmission: Neuromuscular Blocking Agents and Ganglionic Blocking Agents

  2. Neuromuscular Blockers • Prevent acetylcholine from activating nicotinicM • Cause muscle relaxation (paralysis) • No oral forms • Cannot cross • Blood-brain barrier • No impact on CNS (paralysis, not sedation) • Placenta • Minimal effects on fetus

  3. Control of Muscle Contractions • Basic concepts (see Fig. 16-1) • Polarization • Depolarization • Repolarization • Steps in muscle contraction (see Fig. 16-2)

  4. Classification of Neuromuscular Blocking Agents

  5. Fig. 16-1. The depolarization-repolarization cycle of the motor end-plate and muscle membrane. (ACh = acetylcholine.)

  6. Fig. 16-2. Steps in excitation-contraction coupling. (ACh = acetylcholine.)

  7. Competitive Neuromuscular Blockers I: Tubocurarine • Oldest competitive neuromuscular blocker (NMB) • No longer used in United States • Replaced by newer NMBs • One of active principles found in curare, a poison used by primitive arrow hunters

  8. Competitive Neuromuscular Blockers • Chemistry • Quaternary nitrogen atom • Mechanism of action • Competes with ACh for nicotinicM receptors • Pharmacologic effects • Muscle relaxation: flaccid paralysis • Hypotension • Central nervous system

  9. Competitive Neuromuscular Blockers • Pharmacokinetics • Rapid onset of paralysis • Adverse effects • Respiratory arrest • Cardiovascular effects

  10. Competitive Neuromuscular Blockers I: Tubocurarine • Precautions and contraindications • Myasthenia gravis • Electrolyte disturbances • Drug interactions • General anesthetics • Antibiotics • Cholinesterase inhibitors

  11. Competitive Neuromuscular Blockers I: Tubocurarine • Toxicology • Overdose • Prolonged apnea, massive histamine release, and cardiovascular collapse • Preparations, dosage, and administration • No longer used in the United States

  12. Competitive Neuromuscular Blockers II: Others

  13. Fig. 16-3. Structural formulas of representative neuromuscular blocking agents. Note that all of these agents contain quaternary nitrogen atoms and therefore cross membranes poorly. Consequently, they must be administered parenterally and have little effect on the central nervous system or a developing fetus.

  14. Fig. 16-4. Mechanism of competitive neuromuscular blockade. Tubocurarine competes with acetylcholine (ACh) for binding to nicotinicM receptors on the motor end-plate. Binding of tubocurarine does not depolarize the end-plate and therefore does not cause contraction. At the same time, the presence of tubocurarine prevents ACh from binding to the receptor, hence contraction is prevented.

  15. Depolarizing Neuromuscular Blockers: Succinylcholine • Mechanism of action • Pharmacologic effects • Ultrashort-acting • (peak 1 min, fades 4–10 min) • Muscle relaxation • Central nervous system • Pharmacokinetics • Eliminated by plasma cholinesterases • Therapeutic uses • Muscle relaxation during intubation

  16. Depolarizing Neuromuscular Blockers: Succinylcholine • Adverse effects • Prolonged apnea in patients with low pseudocholinesterase activity • Malignant hyperthermia • Postoperative muscle pain • Hyperkalemia

  17. Depolarizing Neuromuscular Blockers: Succinylcholine • Drug interactions • Cholinesterase inhibitors • Antibiotics • Toxicology • Preparations, dosage, and administration

  18. Therapeutic Uses of Neuromuscular Blockers • Muscle relaxation during surgery • Facilitation of mechanical ventilation • Adjunct to electroconvulsive therapy • Endotracheal intubation • Diagnosis of myasthenia gravis

  19. Ganglionic Blocking Agents • Mechanism of action • Pharmacologic effects • Pharmacokinetics • Therapeutic use • Adverse effects • Antimuscarinic effects • Orthostatic hypotension • CNS effects • Preparations, dosage, and administration

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