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Skeletal Muscle Relaxants

Skeletal Muscle Relaxants. By S. Bohlooli, PhD. Drugs affecting skeletal muscle function. Neuromuscular blockers Used during surgical procedures and ICU Spasmolytics to reduce spacticity in various neurologic conditions. Neuromuscular blocking drugs. History

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Skeletal Muscle Relaxants

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  1. Skeletal Muscle Relaxants By S. Bohlooli, PhD

  2. Drugs affecting skeletal muscle function • Neuromuscular blockers Used during surgical procedures and ICU • Spasmolytics to reduce spacticity in various neurologic conditions

  3. Neuromuscular blocking drugs • History • Raw material in arrow poison called curare • d-tubucurarine • Mechanism of blocked • Depolarinzing agoinst • Non depolarizing antagonist • Normal neuromuscular function

  4. Molecular structure of the nicotinic cholinergic receptor.

  5. Basic pharmacology of neuromuscular blocking drugs • Chemistry • Structurally resemble to acetylcholine • conserving double acetylcholine structure • Most of them have two quaternary nitrogens

  6. Mechanism of action • Nondepolarizing blocking drugs • Prototype is tubocurarine • Surmountable blockade • Low doses  act at nicotinic receptor site • High doses  blockade of ion channel pore

  7. Mechanism of action • Depolarizing blocking drugs • Phase I block ( depolarizing) • Depolarization of the end plate • Causing generalized disorganized contraction of muscle motor unit • Finally flaccid paralysis occur • Augmented by cholinesterase inhibitors • Phase II block ( desensitizing ) • Membrane become repolarized • Desensitized • Mechanism is unclear  channel blocking is important • Resemble to that of nondepolarizing drugs • Surmountable by acetyl cholinesterase inhibitors

  8. Clinical pharmacology • Skeletal muscle paralysis • Nondepolarizing drugs • Flaccid paralysis • Larger muscles are more resistant and recover more rapidly • Duration of action • Time to onset of effect • Depolarizing drugs • Transient fasciculations followed by flaccid paralysis • Rapid onset and short duration of action • Control of ventilation • Treatment of convulsions

  9. Effect of neuromuscular blocking drugs on other tissues

  10. Effect seen only with depolarizing blockades • Hyperkalemia • Increased intraocular pressure • Increased intragastric pressure • Muscle pain

  11. Interaction with other drugs • Anesthetics Augmentation of effect with Isoflurane, sevoflurane, desfulrane, and enflurane > halothane > nitrous oxide-barbiturate-benzodiazepine-opioid anesthesia • Antibiotics Especially aminoglycosides • Local anesthetics and antiarrhythmic drugs • Other neuromuscular blocking drugs

  12. Spasmolytic drugs • What is spasticity? • Spasticity is characterized by an increase in tonic stretch reflexes and flexor Muscle spasms together with muscle weakness. Often associated with cerebral palsy, multiple sclerosis , and stroke. • It appear to involve not the stretch reflex arc itself but higher centers (“ upper motor neuron lesion”) • Drugs may ameliorate some symptoms by: • Acting at CNS level • Acting at stretch reflex arc • Acting directly with skeletal muscle excitation-contraction coupling

  13. Diazepam • Facilitating the action of -aminobutyric acid (GABA) • Acts at all GABAA synapses • Useful in muscle spasms of any origin • Baclofen • GABAB agonist • Induce hyperpolarization  serve as presynaptic inhibitory function • Toxicity: drowsiness, seizure activity • Intrathecal administration effective in sever spasticity.

  14. Tizanidine • It is congener of clonidine • 2-adrenoceptor agonist • Reinforces both presynaptic and postsynaptic inhibition in the cord and inhibition of nociceptive transmission • Toxicity: drowsiness, hypotension, dry mouth, asthenia • Dose requirement is varies markedly among patient

  15. Postulated sites of spasmolytic action

  16. Other drugs that act in the CNS • Gabapentin • Progabide • Glycine • Idrocilamide • Riluzole

  17. Dantrolene • Chemically is a hydantoin derivative • It reduces skeletal muscle strength by interfering with excitation-coupling in the muscle fiber. • In detail, dantrolene bind to ryanodine receptor and blocks calcium release from sarcoplasmic reticulum .

  18. Pharmacokinetics • Only one-third of an oral dose of dantrolene is absorbed. • Half life is about 8 hours • Major adverse effects are generalized muscle weakness, sedation, and occasionally hepatitis • Special application is in the treatment of malignant hyperthermia

  19. Botulinum toxin Carisoprodol Chlorophenesin Chlorzoxone Cyclobezaprine Metaxalone Methocarbamol orphenadrine Other drugs used for local muscle spasm • Most of them act as sedative or at level of the spinal cord or brain stem • The main therapeutic use is in relief of acute temporary muscle spasm cause by Local trauma or strain

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