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Overview and Description

The neuromuscular junction acts as an intermediary between the peripheral nervous system and muscle tissue. The small current transmitted by motor axons is transferred into a chemical signal that then triggers a propagating action potential in the much larger muscle tissue. This complicated system is mediated by three specific structures or areas: the presynaptic region, the synaptic space, and the postsynaptic region.1 Abnormalities in these areas, due to disease or exposure to drugs or toxins, can produce the clinical picture of weakness or tetany, and potentially, disability. Impaired neuromuscular transmission can also affect multiple body systems and functions including eye movements, swallowing function, and autonomic function. Medications such as Botulinum toxin can be utilized to purposely impair neuromuscular transmission at the presynaptic terminal while other medications block transmission at the postsynaptic terminal and are utilized in anesthesia to produce paralysis.

Relevance to Clinical Practice

Normal neuromuscular transmission can be conceptualized as a number of steps in a sequence. In actuality, many of the following steps occur simultaneously as continued transmission occurs. The neurotransmitter acetylcholine (ACh) is packaged into synaptic vesicles in the presynaptic nerve terminal, the distal aspect of the motor axon, which lies approximately 25nm from the muscle membrane.2 These vesicles cluster in a region of the presynaptic terminal known as the active zone. A large number of ACh-filled vesicles are available for rapid release, and many more vesicles remain in a secondary store. After a single nerve action potential, approximately 200 vesicles, or quanta, are released, each containing approximately 5000 molecules of ACh.2 Docking of these vesicles, and subsequent release of ACh is largely dependent on the influx of calcium into the presynaptic terminal. This influx of calcium through voltage-gated calcium channels is triggered by sodium influx from the nerve action potential. The released ACh molecules traverse the short synaptic space and bind to ACh receptors in the postsynaptic region. These receptors at the neuromuscular junction are classified as nicotinic receptors, in contrast to muscarinic receptors which typically mediate innervation of the viscera. They are located on a large number of clefts formed by post-junctional folds of postsynaptic muscle cell membrane.1 Binding to the ACh receptor allows sodium to move into the muscle cell, changing the resting membrane potential enough to achieve a muscle action potential and subsequent muscle contraction. Acetylcholinesterase (AChE) is located in the synaptic cleft and functions to rapidly hydrolyze ACh into acetate and choline. Secondary methods of ACh clearance include simple diffusion and reuptake into the presynaptic cleft. Given the relative number of receptors and the amount of AChE available in the synaptic space, at least 50% of ACh molecules interact with nicotinic ACh post-synaptic receptors. In a resting state, a small amount of ACh is continuously released from the presynaptic terminal. An overabundance of ACh molecules provides a safety factor that ensures that a nerve impulse will ultimately result in a muscle action potential. However, clearance of ACh molecules is also vital to neuromuscular junction function. For example, organophosphate poisoning can cause flooding of post-synaptic receptors, leading to weakness, fasciculations, and myoclonic jerks. This can eventually lead to flaccid paralysis due to prevention of repolarization of the post-synaptic membrane. This is most often seen in dermal contact or inhalation with pesticides in agricultural workers.3

Normal neuromuscular transmission can be impaired due to abnormalities in several of the steps described above. In the most common disorder of neuromuscular transmission, Myasthenia Gravis, antibodies directed against the ACh receptor are present.4 These antibodies render the receptors unavailable for ACh and therefore limit the influx of sodium ions into the muscle cell. In significant disease, this sodium influx may not reach the threshold for muscle action potential. The presence of ACh in the synaptic cleft is extremely short, given the effectiveness of AChE. In these situations, drugs such as Edrophonium and Pyridostigmine can be used to inhibit AChE and provide diagnostic information or improve function.

Calcium influx into the presynaptic terminal occurs via P/Q type calcium channels that are located near the areas of vesicle aggregation, the active zones.1 When an action potential reaches the presynaptic area, sodium enters the presynaptic terminal and, subsequently, calcium channels are activated allowing calcium influx. Vesicles in the active zone bind to the docking proteins, described below, and exocytosis of ACh can take place. Impairment of this calcium influx due to antibodies directed against the P/Q type calcium channel forms the physiologic basis of Lambert-Eaton myasthenic syndrome.5 In normal neuromuscular transmission, calcium influx is limited by voltage-gated potassium channels that allow potassium to be released from the cell as calcium influx occurs. The efflux of potassium ultimately reverses the membrane potential and closes the calcium channels, stopping calcium influx. Potassium channel blockers can be utilized to slow potassium release and, therefore, prolong the calcium influx, increasing the amount of vesicle exocytosis.

The key step of vesicle docking, and ultimate exocytosis, in the presynaptic terminal, is mediated by membrane proteins termed SNAREs, soluble N-ethylmaleimide-sensitive attachment protein receptors. Synaptobrevin, located on the vesicle itself, when exposed to calcium, attaches to syntaxin and SNAP-25 which are located on the internal aspect of the presynaptic plasma membrane. Formation of this complex allows vesicle exocytosis.1This mechanism is relevant clinically as Botulinum toxin A cleaves SNAP-25 and botulinum toxin B cleaves synaptobrevin, preventing ACh release into the synaptic space.

Assessment of patients with potential disorders of neuromuscular transmission is made easier through an understanding of the physiology of the neuromuscular junction. Patients will commonly present with weakness in both Myasthenia Gravis and Lambert-Eaton syndrome; however, given the different pathophysiology described above, patients may exhibit different patterns of weakness. With repeated attempted contraction, more calcium will enter the presynaptic terminal. In Lambert-Eaton syndrome, such influx will improve the patient’s weakness and their strength. In contrast, strength in Myasthenia Gravis may be relatively normal at the outset but is described as, and can be shown to be, fatigable. Continued attempt at contraction, of the eyelid for example, will utilize the available ACh receptors and subsequent attempts at neuromuscular transmission will be unsuccessful.

The neuromuscular junction is commonly assessed with electrophysiologic methods through the use of specific tests, such as repetitive stimulation and single-fiber electromyography (EMG). During repetitive stimulation at slow rates (2-3 Hz), the number of ACh receptors available for attachment by ACh is decreased from one stimulation to the next. Normally the safety factor is large enough to overcome this and all stimulations will lead to a muscle action potential.6 In the case of Myasthenia Gravis, some stimulations will not create an action potential and subsequently a detrimental response of the recorded compound muscle action potential will be seen as availability of ACh receptors continues to decrease. Repetitive stimulation at high rates, on the other hand, will increase the amount of calcium in the nerve terminal. Low rates of stimulation do not cause this increased level, as the calcium quickly disperses. In disorders of impaired vesicle release, botulism and Lambert-Eaton syndrome, fast repetitive stimulation will lead to more ACh availability for binding to ACh receptors and subsequently improvement in neuromuscular transmission, seen as an increment in amplitude with successive stimulation.7 Edrophonium, a short-acting anticholinesterase, can also be used to assess for objective improvements in Myasthenia Gravis symptoms. This test should be done with monitoring equipment and where resuscitation is available.1

Knowledge of neuromuscular transmission is essential to provide care for those with neuromuscular disorders, as many will require inpatient rehabilitation after their acute illness. It is also crucial to understanding several treatments for spasticity. Botulinum toxin ultimately impairs neuromuscular transmission by binding to and entering the presynaptic terminal through endocytosis. Once intracellular, Botulinum toxin cleaves the membrane docking proteins and prevents ACh exocytosis. Botulinum toxin A cleaves SNAP 25 while Botulinum toxin B cleaves synaptobrevin to limit exocytosis.8

Cutting Edge/Unique Concepts/Emerging Issues

Critical illness myopathy is an increasingly common source of debility in the hospitalized patient. In critical illness, an up-regulation of immature ACh receptors can be seen.9 These immature receptors have a longer open time than the mature version. These immature receptors with a prolonged open time could potentially explain weakness in critical illness myopathy as long mean open time is associated with weakness and atrophy in other conditions such as congenital myasthenic syndromes; however, this idea is disputed.10 The connection between increased relative numbers of immature ACh receptors in critical illness is an area of research directed at a potential therapeutic solution.

Blood-based biomarkers in normal human neuromuscular junction development and maintenance have recently gained ground as a way to study age-related remodeling at the neuromuscular junction and subsequent sarcopenia as well as neuromuscular disorders.11, 12 These include muscle-specific tyrosine kinase (MuSK), low-density lipoprotein receptor-related protein 4 (Lrp4), and agrin. The agrin-MuSK-Lrp4 complex potentiates a signaling cascade that is essential to organizing ACh receptor clustering at the post-synaptic cleft to maintain neuromuscular junction integrity.13, 14

As an established example, disruptions to this pathway are already known to cause issue in myasthenia gravis. In cases with high clinical suspicion and seronegativity for ACh receptor antibodies, blood tests for MuSK antibody can help establish diagnosis.15 Approximately 5-8% of patients with myasthenia gravis have anti-MuSK autoantibodies.16

Other examples of important mediators include neural cell adhesion molecules (NCAMs), brain-derived neurotrophic factors (BDNFs), and glial cell line-derived neurotrophic factors (GDNFs). These have known roles in the innervation and denervation processes associated with exercise and deconditioning that lead to muscle development/maintenance and degradation respectively. Studies have shown that exercise appears to preserve the regulation of these important signaling molecules throughout aging, supporting the idea that exercise helps to maintain the plasticity of a healthy neuromuscular junction.17

Neuromuscular junction physiology plays an important role in anesthesiology as well. Traditionally, AChE inhibitors like neostigmine have been utilized as antagonists to paralytic agents such as rocuronium and vecuronium.  This is commonly seen in reversal of neuromuscular blockade following the intraoperative period. The development of sugammadex, which first received FDA approval in 2015, has offered several advantages over current reversal agents. Sugammadex functions by binding directly binding to free rocuronium and vecuronium to cause inactivation.18 Recent trials have shown sugammadex to be faster in reversing both deep and moderate neuromuscular blockades, avoid anti-cholinergic side effects, and have a better safety profile compared to neostigmine.19, 20

Gaps in Knowledge/Evidence Base

The physiology of the neuromuscular junction is important not only in diseases of this region, but is also implicated in other diseases including critical illness, amyotrophic lateral sclerosis (ALS), muscular dystrophy, spinal muscular atrophy (SMA), sarcopenia, and aging.21 Neuromuscular junction degeneration and loss is known to occur in ALS, but the exact mechanism and sequence is unknown and the focus of further research.22 

Furthermore, there are significant barriers to further study of neuromuscular disease. Direct observations or studies of the neuromuscular junction in humans have been difficult to establish given ethical issues with obtaining suitable neuromuscular tissue.17 However, rat model studies have shown clear evidence of morphology changes related to aging and exercise that provide a template for future directions of human studies and clinical applications.23, 24, 25 Given challenges with studying neuromuscular tissue, the biomarkers discussed above have emerged as high value areas of neuromuscular research.

Although most recent studies have focused on the role of MuSK in aging and Myasthenia Gravis, further insights into MuSK signaling are the current basis for developing novel treatments towards many neuromuscular diseases, including the ones above.26 Additionally, agrin, NCAMs, BDNFs, and GDNFs show great potential as diagnostic and predictive roles in the research, and eventually, clinical settings.17 In a more general sense, further study of rehabilitation and exercise protocols for those with a disability from disorders of neuromuscular transmission will improve care of these patients.


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Original Version of the Topic:

Michael K. Mallow, MD. Neuromuscular Physiology. 11/5/2012

Previous Revision(s) of the Topic:

Michael K. Mallow, MD. Neuromuscular Physiology. 10/3/2016

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Christopher Lee, MD
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Udai Nanda, DO
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