The neuromuscular junction

The neuromuscular junction is a synapse between the tightly apposed presynaptic motor neuron terminal and the postsynaptic muscle fiber; this is where a chemical process (release of acetylcholine [ACh] from the nerve ending) leads to an electrical event (muscle membrane depolarization) resulting in a mechanical effect (muscle contraction) (Figure 44-1). Large motor nerve axons branch as they course distally within skeletal muscle. Ultimately, the axons divide into 10 to 100 smaller terminal nerve fibers, lose their myelin sheath, and innervate a single muscle fiber. The combination of the terminal neural fibers that originate from 1 axon and the muscle fibers they innervate form a motor unit. The average number of muscle fibers innervated by a single motor neuron defines the innervation ratio, which, in humans, varies between 1:5 and 1:2000. For smaller muscles that are specialized in fine and precise movement (such as the hand and ocular muscles), the innervation ratio is low (1:5, or 5 muscle fibers per neuron), whereas large antigravity muscles have very high innervation ratios (1:2000). Transmission from nerve to muscle is mediated by ACh, which is synthesized in the nerve terminal and stored in specialized vesicles. Each nerve terminal contains approximately 500,000 vesicles (also called quanta, and each containing 6-10,000 ACh molecules) arranged in a specialized region of the membrane where the synaptic vesicles gather, the active zone. ACh is released by exocytosis into the junctional cleft after an appropriate nerve impulse reaches the nerve terminal and diffuses across the 50- to 70-nm cleft to bind the nicotinic cholinoceptors on the postjunctional muscle membrane and initiate muscle contraction.

ACh is synthesized from acetyl coenzyme A and choline under the catalytic influence of choline O-acetyltransferase (CAT) enzyme in the axoplasm (Figure 44-2). The ACh is transported into vesicles by a specific carrier-mediated system. Approximately 80% of the ACh present in the nerve terminal is located in the vesicles, with the remainder dissolved in the axoplasm.

Transmitter mobilization

The rate at which available ACh stores are replaced is termed transmitter mobilization. Evidence suggests that a positive feedback loop exists for ACh.

Postjunctional events

Released ACh diffuses across the synaptic cleft and binds to a receptor on the postjunctional membrane (see later). This receptor forms a membrane ion channel. Two molecules of ACh must bind the receptor (one molecule to each of the two recognition sites—see later) before the receptor undergoes the conformational change necessary to open the receptor channel to ion flow. These channels are chemically sensitive but cannot discriminate between sodium and potassium ions. Once the channels are opened, ion flow makes the immediate area more positive. Each elementary current pulse is additive and summates to produce an end plate current. The end plate current depolarizes the end plate membrane to produce the end plate potential. Once the end plate potential reaches the critical threshold, a propagating action potential is triggered that is directed away from the end plate and results in the activation of muscle fiber contraction.

Junctional cholinesterase

The neuromuscular junction contains two forms of acetylcholinesterase (AChE): a dissolved form in the nerve terminal axoplasm and a membrane-bound form anchored to the basement membrane of the junctional cleft. The enzyme acts to rapidly hydrolyze released ACh to choline and acetate. The kinetics of this enzyme in the neuromuscular junction is such that a single ACh molecule reacts with a single cholinoceptor before inactivation by the AChE.

Postsynaptic receptors

The postsynaptic muscle membrane at the cleft contains multiple invaginations that markedly increase the membrane surface area. At the top of these folds are high concentrations (up to 10,000-20,000 receptors/μm²) of nicotinic acetylcholine receptors (nAChRs). Outside the synaptic cleft area, the concentration of receptors is at least 1000 times lower. The nAChRs are pentameric proteins consisting of two α subunits (protomers), and one β, δ, and ε subunit each; the receptors are anchored to the postsynaptic muscle membrane by proteins such as agrin and rapsyn. In the adult mammal, the receptors are designated as α₂ β, δ and ε (Figure 44-3). Stereochemically, they are arranged in a counterclockwise order as α, ε, α, δ, β. The fetal nAChR is similar to that of the adult, except that the fetal nAChR has a γ protomer that is replaced in the adult by the ε protomer. The five subunits of the nAChR form a rosette surrounding a central transmembrane pore with a diameter of approximately 0.7 nm. Each α subunit possesses a recognition site for ACh at the αε and αδ subunit interfaces. When ACh binds to the two α recognition sites, the receptor undergoes a conformational change and the central pore opens, allowing sodium flux that produces a brief (6.5-ms) current.

Presynaptic receptors

A second type of nicotinic receptor is believed to exist on the nerve terminal. Similar to the postsynaptic receptors, these presynaptic receptors are also blocked by neuromuscular blocking agents (NMBAs) but are relatively selective for calcium fluxes. Their function is thought to help mobilize ACh during periods of high ACh demand, such as high-frequency (tetanic) stimulation. Blockade of these receptors is thought to account for the tetanic fade produced by partial nondepolarizing block.

Upregulation and downregulation

Clinical hypersensitivity and hyposensitivity (resistance) to NMBAs are observed in a number of pathologic states. The concepts of upregulation and downregulation of receptor sites have been introduced to provide a cohesive theory of receptor-drug interaction that can explain a mechanism for abnormal effects of NMBAs in the clinical setting.

Acknowledgment

The author and editors would like to thank Jerald O. Van Beck, MD, for his work on this chapter in previous editions.