Neuromuscular Blockers

Muscle relaxation is part of a balanced anaesthetic technique. Most anaesthetic agents produce a mild-moderate amount of muscle relaxation and often this is not sufficient. Increased muscle relaxation can be produced by increasing anaesthetic depth, the use of local anaesthetic techniques, or the use of centrally or peripherally acting muscle relaxants.

Neuromuscular blocking agents are peripherally acting muscle relaxants.

Neuromuscular Transmission

To understand tranmission of an action potential at the neuromuscular junction it is important to understand the anatomy of the junction. At the nerve terminal, there are a huge number of vesicles containing acetylcholine (ACh), a neurotransmitter. On the muscle membrane there are a number of nicotinic ACh recpetors. As an action potential reaches the nerve terminal, the ACh contain vesicles fuse with the prejunctional membrane releasing it into the junctional cleft. They diffuse across the cleft and bind to the post-junctional receptors. After binding of two ACh molecules to the two binding sites on the repector the activation of ion channel opening leading to an end-plate potential. If enough channels open, the muscle membrane depolarises and an action potential is generated. This causes release of calcuim ions from the sarcoplasmic reticulum leading to muscle contraction. Binding of ACh is extremely short before it is released and hydrolysed, causing the end of the action potential and muscle contraction. If only one of the two sites is occupied, then ion channel opening does not occur and no action potential is produced.