06 - NMB Agents and Reversal Flashcards
what is the neuromuscular junction?
the narrow gap between neuron and muscle fiber
when the nerve depolarizes, which ions and neurotransmitters are involved?
Ca ions enter neuron and ACh is released from vesicles
where does ACh bind?
ACh binds nicotinic cholinergic receptors on motor end-place of muscle fiber
how does ACh work?
ACh receptor opens ion channel to generate end-plate potential
potential propogates along muscle membrane leading to contraction
amt of ACh released far exceeds (by 10-fold) amount needed for depolarization
how is ACh hydrolyzed? what are the products?
ACh is hydrolyzed into acetate and choline by acetylcholinesterase (AChE)
embedded in motor end-plate membrane
what happens to the muscle when ACh receptors close and end-plate repolarizes?
muscle begins to relax
describe NMB agents and reversal
quaternary ammounium compounds with affinity for nicotinic ACh receptors
what happens with depolarizing NMBs?
cloely resemble ACh and bind to ACh receptors (agonist)
generate muscle action potential
T/F depolarizing NMBs are metabolized by AChE
False
what is a Phase I block?
end plate cannot repolarize, muscle becomes flaccid.
peripheral nerve stimulation: constant but diminished twitch, no fade, no post-tetanic potentiation
what is a Phase II block?
prolonged depolarization causes abnormal response to ACh
resembles non-depolarizing block
what happens with non-depolarizing NMBs?
bind ACh receptor but cannot induce ion channel opening (competitive agonist)
where do non-depolarizing NMBs bind?
bind to a subunit of ACh receptor so ACh cannot bind, but receptor is not activated
T/F non depolarizing NMBs do not generate end-plate potential
true
what percentage of receptors are blocked before fade is observed?
> 70%
what percentage of receptors are blocked before complete twitch suppression is observed?
> 90%
what is peripheral nerve stimulation?
fade effect during prolonged/repeated stimulation
why would there be fade of twitches?
less ACh available in nerve terminal
when is fade most obvious?
sustained tetanus or double-burst stimulation
how are depolarizing agents reversed?
diffuse away from NMJ -> hydrolysis by plasma pseudocholinesterase
Phase I block: no reversal agent exists
what happens if you try to reverse a depolarizing agent with AChE inhibitor?
reversal of Phase I block with AChE inihbitor can lead to prolonged depolarization
reversal of Phase II block with AChE inhibitor may be appropriate
how are non-depolarizing agents eliminated?
redistribution, metabolism, excretion
how will the amount of ACh be affected if you inhibit AChE?
increase amount of ACh in NMJ
increased AChe will compete with the NMB drug
what is the only available depolarizing muscle relaxant?
succinylcholine