The body contains over 600 different skeletal muscles and each consists of thousands of muscle fibres ranging in length from a few millimetres to several centimetres. The motor nerve fibres innervating them, which arise in the spinal cord, can be more than 1 m in length. However, the area of apposition between the terminal tip of each nerve fibre and the ‘endplate’ of each muscle fibre is usually less than 50 μm (1/20 mm) in diameter. This particular type of synapse is called the neuromuscular junction (see figure). In order to generate the complex and finely controlled movements that we all take for granted, there has to be a very efficient, fail-safe, and one-way transmission between the nerve and the muscle that ensures that muscle contractions faithfully follow commands from the central nervous system. Such neuromuscular transmission depends on the release from the motor nerve terminal of the chemical acetylcholine (ACh), and its binding to a protein receiver on the surface of the muscle, called the acetylcholine receptor (AChR).
When a nerve impulse reaches the motor nerve terminal, specialized proteins forming ion-channels in its cell membrane open transiently, allowing a short-lived entry of calcium into the terminal. Stored inside the nerve terminal, and attached to special sites on the inside of the cell membrane, are small round vesicles filled with ACh. The sudden inrush of calcium causes some of the vesicle membranes to fuse with the nerve terminal membrane, and to release their contents into the synaptic cleft between the nerve and the surface of the muscle fibre (see figure (d) ).
ACh diffuses rapidly across the ultramicroscopic 50 nm gap and binds to the AChRs that are very densely packed on the tops of the synaptic folds on the muscle fibre (see figure (c), (d) ). When two ACh molecules bind to each AChR, its central pore (channel) opens, allowing small positively charged ions, mainly sodium, to enter the muscle, resulting in a local reduction in the potential across the membrane (depolarization). The release of many ACh-containing vehicles by a nerve impulse leads to a large depolarization called the endplate potential, which in turn opens the voltage-sensitive sodium channels situated at the base of each synaptic fold (see figure (d) ). These are responsible for starting an ‘all or nothing’ action potential that is propagated along the muscle fibre in each direction and initiates muscle contraction.
After about a millisecond, the AChR pore closes and ACh unbinds and is broken down by an enzyme, ACh esterase (AChE), that sits in the synaptic cleft (see figure (d) ). Choline is then taken back into the nerve terminal by special transporters, and used to make more ACh; this is stored in newly-formed synaptic vesicles, themselves made up of recycled nerve terminal membrane. The whole sequence of events, from the inrush of calcium to the initiation of the action potential, takes place in less than two milliseconds.
Many of the earliest studies on chemical synaptic transmission began with the autonomic nervous system, but they were soon extended to skeletal muscles when Dale and his colleagues (1936) showed that stimulation of motor nerves released ACh, and that ACh can induce muscle contraction. The action of ACh could be increased by using a drug, eserine, that inhibits the ACh esterase, and the action of ACh on the muscle could be blocked by the arrow poison, curare. Katz and his co-workers subsequently used intracellular micro-electrodes to measure the endplate potentials and showed that these followed the release of many vesicles of ACh, and that a similar depolarization of the muscle occurred when ACh was applied directly onto the neuromuscular junction with a micropipette.
(a) A silver-stained nerve trunk and three myelinated nerves synapsing on the surface of the muscle. Each myelinated nerve ends as a series of fine, unmyelinated terminals that spread out over the neuromuscular junction. (b) An electron micrograph of a single nerve terminal and the adjacent muscle fibre. There are many mitochondria in both the nerve and the muscle. The surface of the muscle fibre apposed to the nerve terminal is thrown into a series of folds. (c) A higher magnification of part of (b). The nerve terminal contains many synaptic vesicles. At the top of the muscle folds the membrane is strongly stained because of the high density of AChRs. Just beneath the folds the muscle fibrils, which are responsible for muscle contraction, can be seen in cross-section. (d) A diagram depicting the main components of neuromuscular transmission and their roles (see text). (a), (b), and (c) courtesy of Dr Clarke Slater, University of Newcastle
(Click to enlarge)
The neuromuscular junction, unlike most of the nervous system, is accessible to factors circulating in the blood. This can be both an advantage and a disadvantage. For many surgical operations, one of the important roles of the anesthetist is to relax the patient's muscles using an intravenous injection of the otherwise poisonous curare-like drugs — whilst taking over artificially the muscular function of breathing. Similarly, many species of venomous animals, such as snakes and scorpions, make toxins that paralyse their prey, and in some parts of the world this can also be a serious hazard for humans. Such toxins are rapidly absorbed and carried to the neuromuscular junction where they bind with extraordinary efficiency to the AChRs and other ion channel proteins, leading to muscle paralysis which can prevent breathing. Another important toxin is botulinum, which is produced by bacteria contaminating certain foods. Botulinum toxin blocks the release of ACh from the motor nerve terminals, and can cause fatal paralysis in babies; on the other hand it has recently found use as a treatment by local injection into muscles that are subject to uncontrollable severe spasm.
These neurotoxins have also provided marvellous tools for investigating function. For instance, a particular snake toxin, alpha-bungarotoxin, binds very strongly to AChRs and has been of immense use in the study of diseases that affect neuromuscular transmission. In myasthenia gravis (mys: muscle: aesthenia: weakness), the patient suffers from serious weakness and fatigue that can be life-threatening if it involves swallowing and breathing muscles. Myasthenia was first described in 1672 by the very distinguished London physician and anatomist, Thomas Willis. Over three hundred years later, Jim Patrick and Jon Lindstrom at the Salk Institute in California induced a myasthenia gravis-like disease in rabbits by injecting them with AChR protein purified from the electric organs of certain fish. The rabbits responded to the ‘foreign’ protein by making antibodies to it, but these antibodies gained access to the rabbits' neuromuscular junctions, recognized the muscle AChRs, and reduced their function, producing muscle weakness. Following these experimental observations, radioactively-labelled alpha-bungarotoxin was used to show that patients with myasthenia have reduced numbers of AChRs at their neuromuscular junctions, and subsequently that this is caused by serum antibodies that bind to AChRs — just as in the rabbits. Drugs that inhibit the ACh esterase enzyme cause clinical improvement because they prolong the action of ACh, as first demonstrated in 1934 by Mary Walker, a young doctor in London, but nowadays the most important treatment is to reduce the circulating antibodies that bind AChR.
— Angela Vincent
Bibliography
- Hughes, J. T. (1991). Thomas Willis 1621-1675. His life and work. Royal Society of Medicine Services Ltd., London.
- Katz, B. (1966). Nerve muscle and synapse. McGraw Hill, Inc., New York.
- Vincent, A. and Wray, D. (1992). Neuromuscular transmission: basic and applied aspects. Pergammon Press, Oxford
See also skeletal muscle; synapse.
