PHARMACOLOGY-neuromuscular blockers AND reversal agents Flashcards
(166 cards)
What components make up the neuromuscular junction
Axon terminal (presynapse) Synaptic cleft (space between) Motor end plate (postsynapse)
What are the 2 types of nicotinic ACh receptors at the neuromuscular junction
- Prejunctional Nn, on the presynaptic nerve
2. Postsynaptic Nm on motor endplate
Describe the structure of a postsynaptic nicotinic receptor
Pentameric
Ligand-gated ion channel
5 subunits around ion-conducting pore
Subunits = 2 alpha, 1 beta, 1 delta, 1 epsilon
How does a nicotinic ACh receptor become active
1 ACh molecule binds to each alpha subunit
The pore channel opens when both subunits are occupied
Na+ and Ca++ enter the cell and K+ exits
Which ions are conducted through the nicotinic ACh channel
Na+ and Ca++ in
K+ out
Once the nicotinic receptor is activated by ACh, what happens to the charge of the neuron
When ions pass through the pore, the inside of the neuron becomes positive and activates an action potential
How do nicotinic receptors initiate action potentials
Once the pore opens and conducts Na+ and Ca++ inside, the positive charge activates voltage-gated Na+ channels causing depolarization and an action potential
After depolarization of the myocyte occurs, what happens next
Depol instructs the sarcoplasmic reticulum to release Ca++ into the cytoplasm
Ca++ engages with myofilaments and initiate muscle contraction
How are nicotinic receptors turned off
Acetylcholinesterase is positioned around the receptors and hydrolyzes ACh into choline and acetate. This occurs almost immediately after ACh binds the nicotinic receptor
Describe 2 pathologic variants of nicotinic receptors
- has a gamma subunit instead of an epsilon
2. Has 7 alpha subunits
What factors increase the presence of extra-junctional nicotinic receptors
Denervation
Prolonged immobility
Where are extrajunctional nicotinic receptors located
NOT at the NMJ site
What are patients with extrajunctional receptors at risk for with succinylcholine administration
Hyperkalemia
Serum K+ can increase by 0.5 - 1.0 mEq/L
In what conditions is succinylcholine contraindicated (9)
- Upper or lower motor neuron injury
- Spinal cord injury
- Burns
- Skeletal muscle trauma
- CVA
- Tetanus
- Severe sepsis
- Muscular dystrophy
- Prolonged chemical denervation (Mg, long term NMB)
How do extrajunctional nicotinic receptors respond differently to succinylcholine
They remain open for a longer time, allowing more Na+ to enter the cell
Why can extrajunctional nicotinic receptors increase K+
The receptors are open for longer time allowing more K+ to leak out
What can depolarize the a7 type of nicotinic receptor
Succinylcholine AND choline (succ metabolite)
How is succinylcholine-induced hyperkalemia treated (4)
- IV calcium chloride
- Hyperventilation
- Sodium bicarb
- glucose + insulin
How do patients with upregulated extrajunctional receptors respond to nondepolarizers
The receptors are resistant to ND-NMB
The nondepolarizers have decreased potency
More receptors = more drug needed
What causes fade during train-of-four stimulation
Antagonism of the presynaptic nerve nicotinic receptor (nondepolarizer)
This blocks the mobilization of ACh for potential release with stimulation
What action occurs when ACh binds to receptors of the presynaptic nerve
It mobilizes more ACh in the nerve terminal
Why doesn’t succinylcholine produce fade
Because it agonizes the presynaptic nicotinic receptor, mobilizing ACh vesicles for immediate release
What triggers ACh vesicles to release ACh into the synaptic cleft
An action potential opens VG gated Ca++ channels allowing Ca++ into the nerve terminal
Increased Ca++ destabilizes the proteins holding ACh vesicles
The vesicles exit the nerve terminal via exocytosis
What action does ACh take in the synaptic cleft
- Binds to postynaptic Nm receptors, opening ion channels
2. Binds to presynaptic Nn receptors, mobilizing more ACh vesicles for immediate release