Chapter 22 – Neuromuscular Physiology and Pharmacology

The physiology of neuromuscular transmission could be analyzed and understood at the most simple level by using the classic model of nerve signaling to muscle through the acetylcholine receptor. The mammalian neuromuscular junction is the prototypical and most extensively studied synapse. Research has provided more detailed information on the processes that, within the classic scheme, can modify neurotransmission and response to drugs. One example of this is the role of qualitative or quantitative changes in acetylcholine receptors modifying neurotransmission and response to drugs. In myasthenia gravis, for example, the decrease in acetylcholine receptors results in decreased efficiency of neurotransmission (and therefore muscle weakness) and altered sensitivity to neuromuscular relaxants. Another example is the importance of nerve-related (prejunctional) changes that alter neurotransmission and response to drugs. At still another level is the evidence that muscle relaxants act in ways that are not encompassed by the classic scheme of unitary site of action. The observation that muscle relaxants can have prejunctional effects or that some nondepolarizers can also have agonist-like stimulatory actions on the receptor while others have effects not explainable by purely postsynaptic events has provided new insights into some previously unexplained observations. Although this multifaceted action-response scheme makes the physiology and pharmacology of neurotransmission more complex, these added insights also bring experimentally derived knowledge much closer to clinical observations.