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Chapter 17: Muscle Relaxants

How does succinylcholine cause neuromuscular blockade at the neuromuscular junction?

(A) competitive inhibition of acetylcholine

(B) sensitization of the nicotinic acetylcholine receptor (nAChR)

(C) inactivation of voltage gated sodium channels

(D) decreased potassium permeability

(E) decreased depolarization of the end-plate

The answer is C. Succinylcholine's mechanism of action remains incompletely understood. Succinylcholine binds to the nAChR and causes prolonged depolarization of the end-plate. This leads to:

  • inactivation of voltage gated sodium channels

  • desensitization of the nAChR

  • increased potassium permeability

Nondepolarizing muscle relaxants are competitive inhibitors of acetylcholine.

Ref: Miller RD. Miller's Anesthesia. 8th ed. Philadelphia, PA: Elsevier; 2015.

A nondepolarizing muscle relaxant (NDMR) binds to which subunit of the nicotinic acetylcholine receptor (nAChR)?

(A) alpha

(B) beta

(C) delta

(D) epsilon

(E) gamma

The answer is A. The nAChR is made up of 2 alpha-subunits, 1 beta-subunit, 1 delta-subunit, and either 1 gamma-subunit or 1 epsilon-subunit. A NDMR binds to the alpha-subunit of the nAChR. The adult nAChR contains the gamma-subunit and the fetal nAChR contains the epsilon-subunit.

Ref: Miller RD. Miller's Anesthesia. 8th ed. Philadelphia, PA: Elsevier; 2015.

Concurrent disease states can alter the response to nondepolarizing muscle relaxants (NDMRs). Which disease state results in a hypersensitive response to NDMRs?

(A) burn injury

(B) cerebral palsy

(C) peripheral nerve injury

(D) hemiplegia

(E) myasthenia gravis

The answer is E. Myasthenia gravis results in a hypersensitive response to nondepolarizing muscle relaxants (NDMRs). Burn injury, cerebral palsy, peripheral nerve injury, and hemiplegia cause resistance to NDMRs.

Ref: Butterworth JF IV, Mackey DC, Wasnick JD. Morgan & Mikhail's Clinical Anesthesiology. 5th ed. New York, NY: McGraw Hill; 2013.

Liver failure will have the most significant effect on the duration of action of which nondepolarizing muscle relaxant (NDMR)?

(A) vecuronium

(B) cisatracurium

(C) rocuronium

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