Physiology of Neuromuscular Blocking Agents (NMBAs) - ADHD Friendly Guide
NMBAs work by blocking the connection between nerves and muscles, preventing muscles from contracting when your brain sends signals.
How Muscles Normally Work
- Your nerves release a chemical called acetylcholine
- Acetylcholine travels across a tiny gap (20 nanometers) to muscle
- It binds to special receptors (nicotinic acetylcholine receptors) on the muscle
- When two acetylcholine molecules bind, they open ion channels
- This creates an electrical signal that makes the muscle contract 1
How NMBAs Stop This Process
There are two main types:
1. Nondepolarizing NMBAs (Most Common)
- These are competitive blockers - they sit on the receptor but don't activate it
- They prevent acetylcholine from binding to receptors
- They're like putting the wrong key in a lock - it fits but doesn't turn 1
- Examples: vecuronium, rocuronium, pancuronium
2. Depolarizing NMBAs
- These look like acetylcholine and initially activate the receptor
- But they don't leave quickly - they keep the receptor stuck "on"
- This causes initial muscle twitches (fasciculations) then paralysis
- Example: succinylcholine (only used for short procedures like intubation) 1
How We Monitor NMBA Effects
- We use a "Train of Four" (TOF) test
- Four small electrical pulses are sent to a nerve
- We watch how the muscle responds
- As the NMBA takes effect, twitches disappear in order: 4th, 3rd, 2nd, then 1st
- As it wears off, they return in reverse order: 1st, 2nd, 3rd, then 4th 1
Things That Change How NMBAs Work
- Many medications can make NMBAs stronger: antibiotics, heart meds, magnesium
- Medical conditions affect sensitivity:
- Myasthenia gravis: more sensitive to nondepolarizing NMBAs
- Burns/trauma: more resistant to NMBAs (need higher doses)
- Temperature: cold patients need less NMBA
- Acid-base balance: acidosis increases NMBA potency 1, 2
How We Reverse NMBAs
- Medications like neostigmine block the enzyme that breaks down acetylcholine
- This increases acetylcholine levels at the junction
- More acetylcholine can then outcompete the NMBA
- Newer reversal agent (sugammadex) actually grabs and removes certain NMBAs 3, 4
Safety Note
NMBAs only affect muscles - they don't affect consciousness or pain! Patients still need sedation and pain medication when paralyzed 3.
Remember: NMBAs are powerful medications that require careful monitoring and should only be used by trained professionals with proper equipment for breathing support.