Chapter 7: Excitation of Skeletal Muscle (PDF)

CHAPTER 7 Excitation of Skeletal Muscle: UNIT II Neuromuscular Transmission and...

CHAPTER 7 Excitation of Skeletal Muscle: UNIT II Neuromuscular Transmission and Excitation-Contraction Coupling membrane. Acetylcholine is synthesized in the cytoplasm Transmission of of the terminal, but it is absorbed rapidly into many small Impulses from Nerve synaptic vesicles, about 300,000 of which are normally in Endings to Skeletal the terminals of a single end plate. In the synaptic space Muscle Fibers: The are large quantities of the enzyme acetylcholinesterase, Neuromuscular which destroys acetylcholine a few milliseconds after it Junction has been released from the synaptic vesicles. The skeletal muscle fibers are innervated by large, myeli- Secretion of Acetylcholine by the Nerve Terminals nated nerve fibers that originate from large motoneurons in the anterior horns of the spinal cord. As pointed out When a nerve impulse reaches the neuromuscular junc- in Chapter 6, each nerve fiber, after entering the muscle tion, about 125 vesicles of acetylcholine are released belly, normally branches and stimulates from three to from the terminals into the synaptic space. Some of the several hundred skeletal muscle fibers. Each nerve ending details of this mechanism can be seen in Figure 7-2, which makes a junction, called the neuromuscular junction, with shows an expanded view of a synaptic space with the neu- the muscle fiber near its midpoint. The action potential ral membrane above and the muscle membrane and its initiated in the muscle fiber by the nerve signal travels in subneural clefts below. both directions toward the muscle fiber ends. With the On the inside surface of the neural membrane are lin- exception of about 2 percent of the muscle fibers, there is ear dense bars, shown in cross section in Figure 7-2. To only one such junction per muscle fiber. each side of each dense bar are protein particles that pen- etrate the neural membrane; these are voltage-gated cal- Physiologic Anatomy of the Neuromuscular cium channels. When an action potential spreads over the Junction—The Motor End Plate. Figure 7-1A and B terminal, these channels open and allow calcium ions to shows the neuromuscular junction from a large, myeli- diffuse from the synaptic space to the interior of the nerve nated nerve fiber to a skeletal muscle fiber. The nerve fiber terminal. The calcium ions, in turn, are believed to exert forms a complex of branching nerve terminals that invagi- an attractive influence on the acetylcholine vesicles, draw- nate into the surface of the muscle fiber but lie outside the ing them to the neural membrane adjacent to the dense muscle fiber plasma membrane. The entire structure is bars. The vesicles then fuse with the neural membrane called the motor end plate. It is covered by one or more and empty their acetylcholine into the synaptic space by Schwann cells that insulate it from the surrounding fluids. the process of exocytosis. Figure 7-1C shows an electron micrographic sketch Although some of the aforementioned details are spec- of the junction between a single axon terminal and the ulative, it is known that the effective stimulus for causing muscle fiber membrane. The invaginated membrane is acetylcholine release from the vesicles is entry of calcium called the synaptic gutter or synaptic trough, and the space ions and that acetylcholine from the vesicles is then emptied between the terminal and the fiber membrane is called through the neural membrane adjacent to the dense bars. the synaptic space or synaptic cleft. This space is 20 to 30 nanometers wide. At the bottom of the gutter are numer- Effect of Acetylcholine on the Postsynaptic Muscle ous smaller folds of the muscle membrane called subneu- Fiber Membrane to Open Ion Channels. Figure 7-2 ral clefts, which greatly increase the surface area at which also shows many small acetylcholine receptors in the mus- the synaptic transmitter can act. cle fiber membrane; these are acetylcholine-gated ion In the axon terminal are many mitochondria that sup- channels, and they are located almost entirely near the ply adenosine triphosphate (ATP), the energy source that mouths of the subneural clefts lying immediately below is used for synthesis of an excitatory transmitter, acetyl- the dense bar areas, where the acetylcholine is emptied choline. The acetylcholine in turn excites the muscle fiber into the synaptic space. 83 Unit II Membrane Physiology, Nerve, and Muscle Figure 7-1 Different views of the Myelin Axon motor end plate. A, Longitudinal sheath section through the end plate. Terminal nerve B, Surface view of the end branches plate. C, Electron micrographic Teloglial cell appearance of the contact point between a single axon terminal Myofibrils Muscle and the muscle fiber membrane. nuclei (Redrawn from Fawcett DW, as modified from Couteaux R, in Bloom W, Fawcett DW: A Textbook A B of Histology. Philadelphia: WB Saunders, 1986.) Axon terminal in Synaptic vesicles synaptic trough C Subneural clefts Release Neural Vesicles sites membrane trated in Figure 7-3. The channel remains constricted, as shown in section A of the figure, until two acetylcholine molecules attach respectively to the two alpha subunit Dense bar proteins. This causes a conformational change that opens Calcium the channel, as shown in section B of the figure. channels The acetylcholine-gated channel has a diameter of Basal lamina about 0.65 nanometer, which is large enough to allow the and acetylcholinesterase important positive ions—sodium (Na+), potassium (K+), and calcium (Ca++)—to move easily through the opening. Acetylcholine Conversely, negative ions, such as chloride ions, do not receptors pass through because of strong negative charges in the mouth of the channel that repel these negative ions. Subneural In practice, far more sodium ions flow through the cleft Voltage activated acetylcholine-gated channels than any other ions, for two Na+ channels reasons. First, there are only two positive ions in large concentration: sodium ions in the extracellular fluid and Muscle potassium ions in the intracellular fluid. Second, the neg- membrane ative potential on the inside of the muscle membrane, Figure 7-2 Release of acetylcholine from synaptic vesicles at the −80 to −90 millivolts, pulls the positively charged sodium neural membrane of the neuromuscular junction. Note the prox- ions to the inside of the fiber, while simultaneously pre- imity of the release sites in the neural membrane to the acetyl- venting efflux of the positively charged potassium ions choline receptors in the muscle membrane, at the mouths of the subneural clefts. when they attempt to pass outward. As shown in Figure 7-3B, the principal effect of opening the acetylcholine-gated channels is to allow large numbers Each receptor is a protein complex that has a total of sodium ions to pour to the inside of the fiber, carrying molecular weight of 275,000. The complex is composed with them large numbers of positive charges. This creates a of five subunit proteins, two alpha proteins and one each local positive potential change inside the muscle fiber mem- of beta, delta, and gamma proteins. These protein mol- brane, called the end plate potential. In turn, this end plate ecules penetrate all the way through the membrane, lying potential initiates an action potential that spreads along the side by side in a circle to form a tubular channel, illus- muscle membrane and thus causes muscle contraction. 84  Chapter 7 Excitation of Skeletal Muscle: Neuromuscular Transmission and Excitation-Contraction Coupling End Plate Potential and Excitation of the Skeletal Muscle Fiber. The sudden insurgence of sodium ions – – – – into the muscle fiber when the acetylcholine-gated chan- – – nels open causes the electrical potential inside the fiber at the local area of the end plate to increase in the posi- UNIT II tive direction as much as 50 to 75 millivolts, creating a local potential called the end plate potential. Recall from Chapter 5 that a sudden increase in nerve membrane potential of more than 20 to 30 millivolts is normally suf- ficient to initiate more and more sodium channel open- ing, thus initiating an action potential at the muscle fiber membrane. Figure 7-4 shows the principle of an end plate poten- tial initiating the action potential. This figure shows three A separate end plate potentials. End plate potentials A and Na+ Ach C are too weak to elicit an action potential, but they do produce weak local end plate voltage changes, as recorded in the figure. By contrast, end plate potential B is much – – stronger and causes enough sodium channels to open so – – that the self-regenerative effect of more and more sodium – – ions flowing to the interior of the fiber initiates an action potential. The weakness of the end plate potential at point A was caused by poisoning of the muscle fiber with curare, a drug that blocks the gating action of acetylcholine on the acetylcholine channels by competing for the acetylcholine receptor sites. The weakness of the end plate potential at point C resulted from the effect of botulinum toxin, a bac- terial poison that decreases the quantity of acetylcholine release by the nerve terminals. Safety Factor for Transmission at the Neuro- B muscular Junction; Fatigue of the Junction. Ordinarily, Figure 7-3 Acetylcholine-gated channel. A, Closed state. B, After each impulse that arrives at the neuromuscular junction acetylcholine (Ach) has become attached and a conformational causes about three times as much end plate potential as change has opened the channel, allowing sodium ions to enter the muscle fiber and excite contraction. Note the negative charges at that required to stimulate the muscle fiber. Therefore, the channel mouth that prevent passage of negative ions such as the normal neuromuscular junction is said to have a high chloride ions. safety factor. However, stimulation of the nerve fiber at rates greater than 100 times per second for several min- utes often diminishes the number of acetylcholine ves- Destruction of the Released Acetylcholine by icles so much that impulses fail to pass into the muscle Acetylcholinesterase. The acetylcholine, once released into the synaptic space, continues to activate the acetyl- +60 choline receptors as long as the acetylcholine persists in +40 the space. However, it is removed rapidly by two means: +20 (1) Most of the acetylcholine is destroyed by the enzyme 0 Millivolts acetylcholinesterase, which is attached mainly to the –20 spongy layer of fine connective tissue that fills the syn- –40 Threshold aptic space between the presynaptic nerve terminal and –60 the postsynaptic muscle membrane. (2) A small amount –80 of acetylcholine diffuses out of the synaptic space and –100 A B C is then no longer available to act on the muscle fiber 0 15 30 45 60 75 membrane. Milliseconds The short time that the acetylcholine remains in the synaptic space—a few milliseconds at most—normally Figure 7-4 End plate potentials (in millivolts). A, Weakened end plate potential recorded in a curarized muscle, too weak to elicit is sufficient to excite the muscle fiber. Then the rapid an action potential. B, Normal end plate potential eliciting a mus- removal of the acetylcholine prevents continued muscle cle action potential. C, Weakened end plate potential caused by re-excitation after the muscle fiber has recovered from its botulinum toxin that decreases end plate release of acetylcholine, initial action potential. again too weak to elicit a muscle action potential. 85 Unit II Membrane Physiology, Nerve, and Muscle fiber. This is called fatigue of the neuromuscular junc- Drugs That Enhance or Block Transmission at the tion, and it is the same effect that causes fatigue of syn- Neuromuscular Junction apses in the central nervous system when the synapses are overexcited. Under normal functioning conditions, Drugs That Stimulate the Muscle Fiber by Acetylcholine- measurable fatigue of the neuromuscular junction occurs Like Action. Many compounds, including methacholine, rarely, and even then only at the most exhausting levels carbachol, and nicotine, have the same effect on the muscle of muscle activity. fiber as does acetylcholine. The difference between these drugs and acetylcholine is that the drugs are not destroyed by cholinesterase or are destroyed so slowly that their action Molecular Biology of Acetylcholine often persists for many minutes to several hours. The drugs Formation and Release work by causing localized areas of depolarization of the mus- cle fiber membrane at the motor end plate where the acetyl- choline receptors are located. Then, every time the muscle Because the neuromuscular junction is large enough to be fiber recovers from a previous contraction, these depolarized studied easily, it is one of the few synapses of the nervous sys- areas, by virtue of leaking ions, initiate a new action poten- tem for which most of the details of chemical transmission tial, thereby causing a state of muscle spasm. have been worked out. The formation and release of acetyl- Drugs That Stimulate the Neuromuscular Junction choline at this junction occur in the following stages: by Inactivating Acetylcholinesterase. Three particularly 1. Small vesicles, about 40 nanometers in size, are formed well-known drugs, neostigmine, physostigmine, and diiso- by the Golgi apparatus in the cell body of the motoneuron propyl fluorophosphate, inactivate the acetylcholinesterase in the spinal cord. These vesicles are then transported by in the synapses so that it no longer hydrolyzes acetylcho- axoplasm that “streams” through the core of the axon from line. Therefore, with each successive nerve impulse, addi- the central cell body in the spinal cord all the way to the tional acetylcholine accumulates and stimulates the muscle neuromuscular junction at the tips of the peripheral nerve fiber repetitively. This causes muscle spasm when even a fibers. About 300,000 of these small vesicles collect in the few nerve impulses reach the muscle. Unfortunately, it can nerve terminals of a single skeletal muscle end plate. also cause death due to laryngeal spasm, which smothers 2. Acetylcholine is synthesized in the cytosol of the nerve the person. fiber terminal but is immediately transported through Neostigmine and physostigmine combine with acetyl- the membranes of the vesicles to their interior, where it cholinesterase to inactivate the acetylcholinesterase for is stored in highly concentrated form, about 10,000 mol- up to several hours, after which these drugs are displaced ecules of acetylcholine in each vesicle. from the acetylcholinesterase so that the esterase once again 3. When an action potential arrives at the nerve terminal, becomes active. Conversely, diisopropyl fluorophosphate, it opens many calcium channels in the membrane of the which is a powerful “nerve” gas poison, inactivates acetyl- nerve terminal because this terminal has an abundance of cholinesterase for weeks, which makes this a particularly voltage-gated calcium channels. As a result, the calcium lethal poison. ion concentration inside the terminal membrane increases Drugs That Block Transmission at the Neuromuscular about 100-fold, which in turn increases the rate of fusion Junction. A group of drugs known as curariform drugs can of the acetylcholine vesicles with the terminal membrane prevent passage of impulses from the nerve ending into the about 10,000-fold. This fusion makes many of the vesicles muscle. For instance, D-tubocurarine blocks the action of rupture, allowing exocytosis of acetylcholine into the syn- acetylcholine on the muscle fiber acetylcholine receptors, aptic space. About 125 vesicles usually rupture with each thus preventing sufficient increase in permeability of the action potential. Then, after a few milliseconds, the ace- muscle membrane channels to initiate an action potential. tylcholine is split by acetylcholinesterase into acetate ion and choline and the choline is reabsorbed actively into the neural terminal to be reused to form new acetylcholine. Myasthenia Gravis Causes Muscle Paralysis This sequence of events occurs within a period of 5 to 10 milliseconds. Myasthenia gravis, which occurs in about 1 in every 20,000 4. The number of vesicles available in the nerve ending is persons, causes muscle paralysis because of inability of the sufficient to allow transmission of only a few thousand neuromuscular junctions to transmit enough signals from nerve-to-muscle impulses. Therefore, for continued func- the nerve fibers to the muscle fibers. Pathologically, antibod- tion of the neuromuscular junction, new vesicles need ies that attack the acetylcholine receptors have been demon- to be re-formed rapidly. Within a few seconds after each strated in the blood of most patients with myasthenia gravis. action potential is over, “coated pits” appear in the termi- Therefore, it is believed that myasthenia gravis is an autoim- nal nerve membrane, caused by contractile proteins in mune disease in which the patients have developed antibod- the nerve ending, especially the protein clathrin, which is ies that block or destroy their own acetylcholine receptors at attached to the membrane in the areas of the original ves- the postsynaptic neuromuscular junction. icles. Within about 20 seconds, the proteins contract and Regardless of the cause, the end plate potentials that cause the pits to break away to the interior of the mem- occur in the muscle fibers are mostly too weak to initiate brane, thus forming new vesicles. Within another few sec- opening of the voltage-gated sodium channels so that muscle onds, acetylcholine is transported to the interior of these fiber depolarization does not occur. If the disease is intense vesicles, and they are then ready for a new cycle of acetyl- enough, the patient dies of paralysis—in particular, paraly- choline release. sis of the respiratory muscles. The disease can usually be 86  Chapter 7 Excitation of Skeletal Muscle: Neuromuscular Transmission and Excitation-Contraction Coupling ameliorated for several hours by administering neostigmine 3. Velocity of conduction: 3 to 5 m/sec—about 1/13 the or some other anticholinesterase drug, which allows larger velocity of conduction in the large myelinated nerve than normal amounts of acetylcholine to accumulate in the fibers that excite skeletal muscle. synaptic space. Within minutes, some of these paralyzed people can begin to function almost normally, until a new dose of neostigmine is required a few hours later. Spread of the Action Potential to the Interior of UNIT II the Muscle Fiber by Way of “Transverse Tubules” Muscle Action Potential The skeletal muscle fiber is so large that action poten- tials spreading along its surface membrane cause almost Almost everything discussed in Chapter 5 regarding ini- no current flow deep within the fiber. Yet to cause maxi- tiation and conduction of action potentials in nerve fibers mum muscle contraction, current must penetrate deeply applies equally to skeletal muscle fibers, except for quan- into the muscle fiber to the vicinity of the separate myo- titative differences. Some of the quantitative aspects of fibrils. This is achieved by transmission of action poten- muscle potentials are the following: tials along transverse tubules (T tubules) that penetrate all the way through the muscle fiber from one side of 1. Resting membrane potential: about −80 to −90 milli- the fiber to the other, as illustrated in Figure 7-5. The volts in skeletal fibers—the same as in large myelinated T tubule action potentials cause release of calcium ions nerve fibers. inside the muscle fiber in the immediate vicinity of the 2. Duration of action potential: 1 to 5 milliseconds in myofibrils, and these calcium ions then cause contrac- skeletal muscle—about five times as long as in large tion. This overall process is called excitation-contraction myelinated nerves. coupling. Myofibrils Sarcolemma Terminal Z line cisternae Triad of the reticulum Transverse tubule Mitochondrion A band Sarcoplasmic reticulum Transverse tubule I band Sarcotubules Figure 7-5 Transverse (T) tubule–sarcoplasmic reticulum system. Note that the T tubules communicate with the outside of the cell mem- brane, and deep in the muscle fiber, each T tubule lies adjacent to the ends of longitudinal sarcoplasmic reticulum tubules that surround all sides of the actual myofibrils that contract. This illustration was drawn from frog muscle, which has one T tubule per sarcomere, located at the Z line. A similar arrangement is found in mammalian heart muscle, but mammalian skeletal muscle has two T tubules per sarcomere, located at the A-I band junctions. 87 Unit II Membrane Physiology, Nerve, and Muscle Figures 7-6 and 7-7 show that the action potential of the Excitation-Contraction Coupling T tubule causes current flow into the sarcoplasmic reticu- lar cisternae where they abut the T tubule. As the action Transverse Tubule–Sarcoplasmic potential reaches the T tubule, the voltage change is sensed Reticulum System by dihydropyridine receptors that are linked to calcium Figure 7-5 shows myofibrils surrounded by the T tubule– release channels, also called ryanodine receptor channels, sarcoplasmic reticulum system. The T tubules are small in the adjacent sarcoplasmic reticular cisternae (see Figure and run transverse to the myofibrils. They begin at the cell 7-6). Activation of dihydropyridine receptors triggers the membrane and penetrate all the way from one side of the opening of the calcium release channels in the cisternae, as muscle fiber to the opposite side. Not shown in the figure well as in their attached longitudinal tubules. These chan- is the fact that these tubules branch among themselves nels remain open for a few milliseconds, releasing calcium and form entire planes of T tubules interlacing among all ions into the sarcoplasm surrounding the myofibrils and the separate myofibrils. Also, where the T tubules origi- causing contraction, as discussed in Chapter 6. nate from the cell membrane, they are open to the exterior of the muscle fiber. Therefore, they communicate with the Calcium Pump for Removing Calcium Ions from the extracellular fluid surrounding the muscle fiber and they Myofibrillar Fluid After Contraction Occurs. Once the themselves contain extracellular fluid in their lumens. In calcium ions have been released from the sarcoplasmic other words, the T tubules are actually internal extensions tubules and have diffused among the myofibrils, muscle of the cell membrane. Therefore, when an action potential contraction continues as long as the calcium ions remain in spreads over a muscle fiber membrane, a potential change high concentration. However, a continually active calcium also spreads along the T tubules to the deep interior of the pump located in the walls of the sarcoplasmic reticulum muscle fiber. The electrical currents surrounding these pumps calcium ions away from the myofibrils back into T tubules then elicit the muscle contraction. the sarcoplasmic tubules (see Figure 7-6). This pump can Figure 7-5 also shows a sarcoplasmic reticulum, in yel- concentrate the calcium ions about 10,000-fold inside the low. This is composed of two major parts: (1) large cham- tubules. In addition, inside the reticulum is a protein called bers called terminal cisternae that abut the T tubules and calsequestrin that can bind up to 40 times more calcium. (2) long longitudinal tubules that surround all surfaces of the actual contracting myofibrils. Excitatory “Pulse” of Calcium Ions. The normal resting state concentration (