Histology of Muscle PDF

Overview and Classification =========================== - A muscle is a tissue characterized by the aggregation of specialized elongated cells arranged in a parallel fashion - The functions of the muscular system are movement, stability, communication, control of body openings and pass...

Overview and Classification =========================== - A muscle is a tissue characterized by the aggregation of specialized elongated cells arranged in a parallel fashion - The functions of the muscular system are movement, stability, communication, control of body openings and passages, heat production, and glycaemic control. To carry out those functions, all muscle cells have the following characteristics: - **Excitability (responsiveness)** -- Excitability is a property of all living cells, but muscle and nerve cells have developed this property to the highest degree. When stimulated by chemical signals, stretch, and other stimuli, muscle cells respond with electrical changes across the plasma membrane. - **Conductivity** -- Stimulation of a muscle cell produces more than a local effect. Local electrical excitation sets off a wave of excitation that travels rapidly along the cell and initiates processes leading to contraction - **Contractility** -- Muscle cells are unique in their ability to shorten substantially when stimulated. This enables them to pull on bones and other organs to create movement. - **Extensibility** -- In order to contract, a muscle cell must also be extensible---able to stretch again between contractions. Most cells rupture if they are stretched even a little, but skeletal muscle cells can stretch to as much as three times their contracted length. - **Elasticity** -- When a muscle cell is stretched and then released, it recoils to a shorter length. If it were not for this elastic recoil, resting muscles would be too slack. - Contraction is mediated by the interaction between myofilaments - Two principal myofilaments - **Thin** filaments (6 -- 8 nm diameter): composed of **actin**. A polymer of fibrous actin formed from globular actin (**G actin**) - **Thick** filaments (15 nm diameter): composed of **myosin II protein** - The thin and thick filaments occupy the cytoplasm (sarcoplasm) - Importantly, actin and myosin also function in other cell types to mediate cytokinesis, exocytosis and cell migration - Classification: - Striated muscle -- cells exhibit cross-striations at LM - Smooth muscle -- cells lack cross-striations at LM - The cross-striations are due to the architectural organization of the actin and myosin myofilaments - Striated muscle is further sub classified: - Skeletal muscle -- attached to bone, produces skeletal movement and maintains posture - Visceral Striated muscle -- morphologically indistinct from skeletal muscle, localized to tongue, pharynx, diaphragm, and upper oesophagus - Cardiac muscle Skeletal Muscle ==================================== **Skeletal muscle** may be defined as voluntary striated muscle that is usually attached to one or more bones. A skeletal muscle exhibits alternating light and dark transverse bands, or **striations**, that reflect an overlapping arrangement of their internal contractile proteins. Skeletal muscle is called **voluntary** because it is usually subject to conscious control. The other types of muscle are **involuntary** (not usually under conscious control), and they are never attached to bones. Skeletal muscle is composed of multinucleated **syncytia** formed by the fusion of multiple individual myoblast cells that vary in length. Because of their extraordinary length, skeletal muscle cells are usually called **muscle fibres** or **myofibers.** A skeletal muscle is composed not only of muscular tissue, but also of fibrous connective tissue: - the ***endomysium*** -- reticular fibres that surrounds each muscle fibre - the ***perimysium*** -- bundles muscle fibres together into fascicles/bundles - the ***epimysium*** -- dense connective tissue that surrounds collections of fascicles to form muscle (see fig. 10.1). These connective tissues are continuous with the collagen fibres of tendons and those, in turn, with the collagen of the bone matrix. Thus, when a muscle fibre contracts, it pulls on these collagen fibres and typically moves a bone. The plasma membrane of a muscle fibre is called the **sarcolemma,** and its cytoplasm is called the **sarcoplasm.** The sarcoplasm is occupied mainly by long protein cords called **myofibrils** (fig. 11.2). It also contains an abundance of **glycogen,** a starchlike carbohydrate that provides energy for the cell during heightened levels of exercise, and the red oxygen-binding pigment **myoglobin,** which provides some of the oxygen needed for muscular activity. The multiple flattened or sausage-shaped nuclei in the syncytia are pressed against the inside of the sarcolemma. Most other organelles of the cell, such as mitochondria, are packed into the spaces between the myofibrils. The smooth endoplasmic reticulum, here called the **sarcoplasmic reticulum (SR),** forms a network around each myofibril (the blue web in fig. 11.2). It periodically exhibits dilated end-sacs called **terminal cisterns,** which cross the muscle fibre from one side to the other. The sarcolemma has tubular infoldings called **transverse (T) tubules,** which penetrate through the cell and emerge on the other side. Each T tubule is closely associated with two terminal cisterns running alongside it, one on each side. The T tubule and the two cisterns associated with it constitute a *triad.* Muscle contraction requires a lot of calcium ions (Ca^2+^), as you will see, but this presents a problem. A high concentration of Ca^2+^ in the cytosol is lethal -- it can react with phosphate ions to precipitate as calcium phosphate crystals, and can trigger cell death by apoptosis. Therefore, at rest, a muscle cell stores its Ca^2+^ in the sarcoplasmic reticulum, safely bound to a protein called **calsequestrin.** In a resting muscle fibre, Ca^2+^ is about 10,000 times as concentrated in the SR as it is in the cytosol. When the cell is stimulated, ion gates in the SR membrane open and Ca^2+^ floods into the cytosol to activate contraction. The T tubule signals the SR when to release these calcium bursts. Slow- and Fast-Twitch Fibers ---------------------------- Skeletal muscle fibres can be divided, based on their contraction speed (time required to reach maximum tension), into **slow-twitch**, or **type I, fibres**, and **fast-twitch**, or **type II, fibres.** These differences are associated with different myosin ATPase isoenzymes, which can also be designated as "slow" and "fast." The extraocular muscles that position the eyes, for example, have a high proportion of fast-twitch fibres. The soleus muscle in the leg, by contrast, has a high proportion of slow-twitch fibres. - **Type I / slow oxidative** - Muscles like the soleus are *postural muscles*; they are able to sustain a contraction for a long period of time without fatigue; Slow-twitch fatigue-resistant motor units. - The resistance to fatigue demonstrated by these muscles is aided by other characteristics of slow-twitch (type I) fibers that endow them with a high oxidative capacity for aerobic respiration. Hence, the type I fibers are often referred to as **slow oxidative fibers**. - These fibers have a rich capillary supply, numerous mitochondria and aerobic respiratory enzymes, and a high concentration of myoglobin and cytochromes. - Myoglobin is a red pigment, similar to the hemoglobin in red blood cells, that improves the delivery of oxygen to the slow-twitch fibers. Because of their high myoglobin content, slow-twitch fibers are also called **red fibers.** - These fibres have low myosinATP-aseactivity and are found mostly in high endurance athletes. - **Type IIa / fast oxidative** - Intermediate color in fresh tissue - High numbers of mitochondria and myoglobin - They contain large amounts of glycogen and perform anaerobic glycolysis - fast-twitch but also have a high oxidative capacity; therefore, they are relatively resistant to fatigue - Fast twitch fatigue resistant - **Type IIb / fast glycolytic** - These fibres have less myoglobin, and are therefore paler, and contain fewer capillaries and mitochondria than Type I or IIa fibers. - They are adapted to metabolize anaerobically by a large store of glycogen and a high concentration of glycolytic enzymes. - Low levels of oxidative enzymes - Fast twitch fatigue prone units that generate high tension - Adapted for rapid contraction and precise fine movements Myofilaments ------------ Muscle fibres are composed of longitudinally arrayed structural units called **myofibrils**. Myofibrils are composed of bundles of **myofilaments** that extend through the entire length of myocytes. Myofilaments are the individual filamentous polymers of **myosin II (thick)** and **actin (thin)** and their associated proteins. They are the actual contractile units in skeletal muscle. 1. **Thick filaments** (fig. 11.3a, b, d) - Each is made of several hundred molecules of a protein called **myosin.** A myosin molecule is shaped like a golfclub, with two chains intertwined to form a shaft like *tail* and a double globular *head* projecting from it at an angle. A thick filament may be likened to a bundle of golf clubs, with their heads directed outward in a helical array around the bundle. The heads on one half of the thick filament angle to the left, and the heads on the other half angle to the right; in the middle is a *bare zone* with no heads. Convert ATP to energy of motion. 2. **Thin filaments** (fig. 11.3c, d), are composed primarily of two intertwined strands of a protein called **fibrous (F) actin.** Each F actin is like a bead necklace---a string of subunits called **globular (G) actin.** Each G actin has an **active site** that can bind to the head of a myosin molecule. A thin filament also has 40 to 60 molecules of yet another protein, **tropomyosin.** When a muscle fibre is relaxed, each tropomyosin blocks the active sites of six or seven G actins and prevents myosin from binding to them. Each tropomyosin molecule, in turn, has a smaller calcium-binding protein called **troponin** bound to it. 3. **Elastic filaments** (see fig. 11.5b), are made of a huge springy protein called **titin.** They run through the core of each thick filament and anchor it to structures called the *Z disc* at one end and *M line* at the other. Titin stabilizes the thick filament, centres it between the thin filaments, prevents overstretching, and recoils like a spring after a muscle is stretched. It accounts for much of the elasticity and extensibility of myofibrils. Myosin and actin are called **contractile proteins** because they do the work of shortening the muscle fibre. Titin and dystrophin are called **structural proteins**. Tropomyosin and troponin are called **regulatory proteins** because they act like a switch to determine when the fibre can contract and when it cannot. Several clues as to how they do this may be apparent from what has already been said -- calcium ions are released into the sarcoplasm to activate contraction; calcium binds to troponin; troponin is also bound to tropomyosin; and tropomyosin blocks the active sites of actin, so that myosin cannot bind to it when the muscle is not stimulated. At least seven other accessory proteins occur in the thick and thin filaments or are associated with them. Among other functions, they anchor the myofilaments, regulate their length, and keep them aligned with each other for optimal contractile effectiveness. The most clinically important of these is **dystrophin,** an enormous protein located between the sarcolemma and the outermost myofilaments. It links actin filaments to the inner face of the sarcolemma. Through a series of links (fig. 11.4), this leads ultimately to the fibrous endomysium surrounding the muscle fibre. Therefore, when the thin filaments move, they pull on the dystrophin, and this ultimately pulls on the extracellular connective tissues leading to the tendon. Genetic defects in dystrophin are responsible for the disabling disease, *muscular dystrophy*. Striations ---------- Myosin and actin are not unique to muscle; they occur in nearly all cells, where they function in cellular motility, mitosis, and transport of intracellular materials. In skeletal and cardiac muscle they are especially abundant, however, are organized in a precise array that accounts for the striations of these muscle types (fig. 11.5). Striated muscle has dark **A bands** alternating with lighter **I bands**. (*A* stands for *anisotropic* and *I* for *isotropic,* which refer to the way these bands affect polarized light. To help remember which band is which, think "d**A**rk" and "l**I**ght"). Each A band consists of thick filaments lying side by side. Part of the A band, where thick and thin filaments overlap, is especially dark. In this region, each thick filament is surrounded by a hexagonal array of thin filaments. In the middle of the A band, there is a lighter region called the **H band,** into which the thin filaments do not reach. In the middle of the H band, the thick filaments are linked to each other through a dark, transverse protein complex called the **M line.** Each light I band is bisected by a dark narrow **Z disc (Z line),** which provides anchorage for the thin and elastic filaments. Each segment of a myofibril from one Z disc to the next is called a **sarcomere**, the functional contractile unit of the myofibril. A muscle shortens because its individual sarcomeres shorten and pull the Z discs closer to each other, and dystrophin and the linking proteins pull on the extracellular proteins of the muscle. As the Z discs are pulled closer together, they pull on the sarcolemma to achieve overall shortening of the cell. The Sliding Filament Mechanism ------------------------------ The process of muscle contraction and relaxation has four major phases: (1) excitation, (2) excitation--contraction coupling, (3) contraction, and (4) relaxation. Each phase occurs in several smaller steps, which we will now examine in detail. ### Excitation **Excitation** is the process in which action potentials in the nerve fibre lead to action potentials in the muscle fibre. 1. A nerve signal arrives at the axon terminal and opens voltage-gated calcium channels. Calcium ions enter the terminal. 2. Calcium stimulates the synaptic vesicles to release acetylcholine (ACh) into the synaptic cleft. One action potential causes exocytosis of about 60 vesicles, and each vesicle releases about 10,000 molecules of ACh. 3. ACh diffuses across the synaptic cleft and binds to receptors on the sarcolemma. 4. These receptors are ligand-gated ion channels. Two Ach molecules must bind to each receptor to open the channel. When it opens, Na+ flows quickly into the cell and K+ flows out. The voltage on the sarcolemma quickly rises to a less negative value as Na+ enters the cell, then falls back to the RMP as K+ exits. This rapid up-and-down fluctuation in voltage at the motor end plate is called the **end-plate potential (EPP).** 5. Areas of sarcolemma next to the end plate have voltage-gated ion channels that open in response to the EPP. Some of these are specific for Na+ and admit it to the cell, while others are specific for K+ and allow it to leave. These ion movements create an *action potential.* The muscle fibre is now excited. ### Excitation--Contraction Coupling **Excitation--contraction coupling** refers to events that link action potentials on the sarcolemma to activation of the myofilaments, thereby preparing them to contract. 6. A wave of action potentials spreads from the motor end plate in all directions, like ripples on a pond. When this wave of excitation reaches the T tubules, it continues down them into the cell interior. 7. Action potentials open voltage-gated ion channels in the T tubules. These are linked to calcium channels in the terminal cisterns of the sarcoplasmic reticulum (SR). Thus, channels in the SR open as well and calcium diffuses out of the SR, down its concentration gradient into the cytosol. 8. Calcium binds to the troponin of the thin filaments. 9. The troponin-tropomyosin complex changes shape and exposes the active sites on the actin filaments. This makes them available for binding to myosin heads. ### Contraction **Contraction** is the step in which the muscle fibre develops tension and may shorten. The mechanism of contraction is called the **sliding filament theory.** It holds that the myofilaments do not become any shorter during contraction; rather, the thin filaments slide over the thick ones and pull the Z discs behind them, causing each sarcomere as a whole to shorten. 10. The myosin head must have an ATP molecule bound to it to initiate contraction. **Myosin ATPase,** an enzyme in the head, hydrolyses this ATP into ADP and phosphate (P~i~). The energy released by this process activates the head, which "cocks" into an extended, high-energy position. The head temporarily keeps the ADP and P~i~ bound to it. 11. The cocked myosin binds to an exposed active site on the thin filament, forming a **cross-bridge** between the myosin and actin. 12. Myosin releases the ADP and P~i~ and flexes into a bent, low-energy position, tugging the thin filament along with it. This is called the **power stroke.** The head remains bound to actin until it binds a new ATP. 13. The binding of a new ATP to myosin destabilizes the myosin--actin bond, breaking the cross-bridge. The myosin head now undergoes a **recovery stroke.** It hydrolyses the new ATP, recocks (returns to step 10), and attaches to a new active site farther down the thin filament, ready for another power stroke. It may seem as if releasing the thin filament at step 13 would simply allow it to slide back to its previous position, so nothing would have been accomplished. However, when one myosin head releases actin in preparation for the recovery stroke, there are many other heads on the same thick filament holding onto the thin filament so it doesn't slide back. At any given moment during contraction, about half of the heads are bound to the thin filament and the other half are extending forward to grasp it farther down. That is, the myosin heads don't all stroke at once but contract sequentially. Each head acts in a jerky manner, but hundreds of them working together produce a smooth, steady pull on the thin filament. Note that even though the muscle fibre contracts, the *myofilaments do not become shorter* any more than a rope becomes shorter as you pull in an anchor. The thin filaments slide over the thick ones, as the name of the sliding filament theory implies. A single cycle of power and recovery strokes by all myosin heads in a muscle fibre would shorten the fibre about 1%. A fibre, however, may shorten by as much as 40% of its resting length, so obviously the cycle of power and recovery must be repeated many times by each myosin head. Each head carries out about five strokes per second, and each stroke consumes one ATP. ### Relaxation When the nerve fibre stops stimulating it, a muscle fibre relaxes and returns to its resting length. 14. Nerve signals stop arriving at the neuromuscular junction, so the axon terminal stops releasing ACh. 15. As ACh dissociates (separates) from its receptor, AChE breaks it down into fragments that cannot stimulate the muscle. The axon terminal reabsorbs these fragments for recycling. All of this happens continually while the muscle is stimulated, too, but when nerve signals stop, no more Ach is released to replace that which breaks down. Therefore, stimulation of the muscle fibre by ACh ceases. 16. From excitation through contraction, the SR simultaneously releases and reabsorbs Ca2+; but when the nerve fibre stops firing and excitation ceases, Ca2+ release also ceases and only its reabsorption continues. 17. Owing to reabsorption by the SR, the level of free calcium in the cytosol falls dramatically. Now, when calcium dissociates from troponin, it is not replaced. 18. Tropomyosin moves back into the position where it blocks the active sites of the actin filament. Myosin can no longer bind to actin, and the muscle fibre ceases to produce or maintain tension. Relaxation alone does not return muscle to its resting length. That must be achieved by some force pulling the muscle and stretching it. For example, if the bicep flexes the elbow and then relaxes, it stretches back to its resting length only if the elbow is extended by contraction of the triceps or by the pull of gravity on the forearm. The Length--Tension Relationship and Muscle Tone ------------------------------------------------ The tension generated by a muscle, and therefore the force of its contraction, depends on how stretched or contracted it was at the outset; depends on the length of the sarcomere. This principle is called the **length--tension relationship.** The reasons for it can be seen in figure 11.12. If a fibre was already extremely contracted, its thick filaments would be rather close to the Z discs, as on the left side of the figure. The fibre could not contract very much farther before the thick filaments would butt against the Z discs and stop. The contraction would be brief and weak. On the other hand, if a muscle fibre was extremely stretched, as on the right, there would be little overlap between the thick and thin filaments. The myosin heads would be unable to "get a grip" on the thin filaments, and again the contraction would be weak. Between these extremes, there is an optimum resting length at which a muscle responds with the greatest force. In this range (the flat top of the curve), the sarcomeres are 2.0 to 2.25 μm long. If the sarcomeres are less than 60% or more than 175% of their optimal length, they develop no tension at all in response to a stimulus. The complete length-tension curve is derived from muscles isolated from an animal (often the frog gastrocnemius muscle) for laboratory stimulation. In reality, a muscle *in situ* (in its natural position in the living body) is never as extremely stretched or contracted as the far right and left sides of the figure depict. For one thing, the attachments of muscles to the bones and limitations on bone movement restrict muscle contraction to the midrange of the curve. For another, the central nervous system continually monitors and adjusts the length of the resting muscles, maintaining a state of partial contraction called **muscle tone.** This maintains optimum sarcomere length and makes the muscles ideally ready for action. The elastic filaments of the sarcomere also help to maintain enough myofilament overlap to ensure effective contraction when the muscle is called into action. Muscle Metabolism ----------------- ### Production of ATP in Muscle Fibres - A large amount of ATP is needed to: - Power the contraction cycle - Pump Ca^2+^ into the SR - The ATP inside muscle fibres will power contraction for only a few seconds - ATP must be produced by the muscle fibre after reserves are used up - Muscle fibres have three ways to produce ATP 1. From creatine phosphate 2. By anaerobic cellular respiration 3. By aerobic cellular respiration #### Creatine Phosphate In a short, intense exercise such as a 100 m dash, the myoglobin in a muscle fibre supplies oxygen for a limited amount of aerobic respiration at the outset, but this oxygen supply is quickly depleted. Until the respiratory and cardiovascular systems catch up with the heightened oxygen demand, the muscle meets most of its ATP needs by borrowing phosphate groups (P~i~) from other molecules and transferring them to ADP. **Creatine kinase** obtains P~i~ from a phosphate-storage molecule, **creatine phosphate (CP),** and donates it to ADP to make ATP. This is a fast-acting system that helps to maintain the ATP level while other ATP-generating mechanisms are being activated. Excess ATP is used to synthesize creatine phosphate. ATP and CP, collectively called the phosphagen system, provide nearly all the energy used for short bursts of intense activity; provide enough energy for contraction for about 15 seconds. #### Anaerobic respiration As the phosphagen system is exhausted, the muscles transition to anaerobic respiration, a series of ATP producing reactions that do not require oxygen, to generate ATP by glycolysis. The point at which this occurs is called the **anaerobic threshold,** or sometimes the **lactate threshold** because one can begin to detect a rise in blood lactate levels at this time. During the anaerobic phase, the muscles obtain glucose from the blood and their own stored glycogen. Glycolysis breaks down glucose into molecules of pyruvic acid and produces two molecules of ATP. If sufficient oxygen is present, pyruvic acid formed by glycolysis enters aerobic respiration pathways producing a large amount of ATP. If oxygen levels are low, anaerobic reactions convert pyruvic acid to lactic acid which is carried away by the blood. It can produce enough ATP for 30 to 40 seconds of maximum activity. #### Aerobic respiration After 40 seconds or so, the respiratory and cardiovascular systems "catch up" and deliver oxygen to the muscles fast enough for aerobic respiration to once again meet most of the ATP demand. Aerobic respiration supplies ATP for prolonged activity. Aerobic respiration produces much more ATP than glycolysis does - typically 36 ATP per glucose. Thus, it is a very efficient means of meeting the ATP demands of prolonged exercise. Pyruvic acid entering the mitochondria is completely oxidized generating ATP, carbon dioxide, water and heat. Muscle tissue has two sources of oxygen: 1) Oxygen from haemoglobin in the blood, 2) Oxygen released by myoglobin in the muscle cell. Myoglobin and haemoglobin are oxygen binding proteins. ### Fatigue and Endurance Muscle **fatigue** is the progressive inability of muscle to maintain force of contraction after prolonged use of the muscles. Factors that contribute to muscle fatigue: - Inadequate release of calcium ions from the SR - Depletion of creatine phosphate - Insufficient oxygen - Depletion of glycogen and other nutrients - Buildup of lactic acid and ADP - Failure of the motor neuron to release enough acetylcholine ### Oxygen Consumption after exercise You have probably noticed that you breathe heavily not only during strenuous exercise but also for several minutes afterward. This is to meet a metabolic demand called **excess postexercise oxygen consumption (EPOC),** also known by an older popularized term, **oxygen debt.** EPOC is the difference between the elevated rate of oxygen consumption at the end of an exercise and the normal rate at rest. This added oxygen is used to restore muscle cells to the resting level in several ways. It occurs in part because oxygen is needed to regenerate ATP aerobically, and that ATP goes in part to regenerate creatine phosphate. A small amount of oxygen serves to reoxygenate the muscle myoglobin, and the liver consumes oxygen in disposing of the lactate generated by exercise. In addition, exercise raises the body temperature and overall metabolic rate, which in itself consumes more oxygen. Control of Muscle Tension ------------------------- ### Threshold, Latent Period, and Twitch The timing and strength of a muscle's contraction can be shown in a chart called a **myogram** (fig. 11.13). A sufficiently weak (subthreshold) electrical stimulus to a muscle produces no reaction. By gradually increasing the voltage and stimulating the muscle again, one can determine the **threshold,** or minimum voltage necessary to generate an action potential in the muscle fibre. At threshold or higher, a single stimulus causes a quick cycle of contraction and relaxation called a **twitch.** Therefore, a twitch contraction is the brief contraction of the muscle fibres in a motor unit in response to an action potential. There is a brief delay, or **latent period,** of about 2 milliseconds between the stimulus and twitch. This is the time required for excitation, excitation--contraction coupling, and tensing of the elastic components of the muscle. The force generated during this time is called *internal tension.* It is not visible on the myogram because it causes no shortening of the muscle. On the left side, the myogram is therefore flat. Once the elastic components are taut, the muscle begins to produce *external tension* and move a resisting object, or load, such as a bone or body limb. This is called the **contraction phase** of the twitch. By analogy, imagine lifting a weight suspended from a rubber band. At first, internal tension would only stretch the rubber band. Then, as the rubber band became taut, external tension would lift the weight. The contraction phase is short-lived, because the SR quickly reabsorbs Ca^2+^ before the muscle develops maximal force. As the Ca^2+^ level in the cytoplasm falls, myosin releases the thin filaments and muscle tension declines. This is seen in the myogram as the **relaxation phase.** As shown by the asymmetry of the myogram, the muscle contracts more quickly than it relaxes. The entire twitch lasts from about 10 to 100 ms. When a muscle fibre contracts, it temporarily cannot respond to another action potential. This occurs during the **refractory period**. Skeletal muscle has a refractory period of 5 milliseconds and cardiac muscle has a refractory period of 300 milliseconds. ### Contraction Strength of Twitches We have seen that a subthreshold stimulus induces no muscle contraction at all, but at threshold intensity, a twitch is produced. Increasing the stimulus voltage still more, however, produces twitches no stronger than those at threshold. Superficially, the muscle fibre seems to be giving its maximum response once the stimulus intensity is at threshold or higher. However, even for a constant voltage, twitches vary in strength. This is so for a variety of reasons: - Twitch strength depends on how stretched the muscle was just before it was stimulated, as we have just seen in the length--tension relationship. - Twitches become weaker as a muscle fatigues. - Twitches vary with the temperature of the muscle; a warmed-up muscle contracts more strongly because enzymes such as the myosin heads work more quickly. - Twitch strength varies with the muscle's state of hydration, which affects the spacing between thick and thin filaments and therefore the ability to form myosin--actin cross-bridges. - Twitch strength varies with stimulus frequency; stimuli arriving close together produce stronger twitches than stimuli arriving at longer time intervals. It should not be surprising that twitches vary in strength. Indeed, an individual twitch isn't strong enough to do any useful work. Muscles must contract with variable strength for different tasks, such as lifting a glass of champagne compared with lifting barbells at the gym. Let us examine more closely the contrasting effects of stimulus *intensity* versus stimulus *frequency* on contraction strength. Suppose we apply a stimulating electrode to a motor nerve that supplies a muscle, such as a laboratory preparation of a frog sciatic nerve connected to its gastrocnemius muscle. Subthreshold stimulus voltages produce no response (fig. 11.14). At threshold, we see a weak twitch (at *3* in the bottom row of the figure), and if we continue to raise the voltage, we see stronger twitches. The reason for this is that higher voltages excite more and more nerve fibres in the motor nerve (middle row of the figure), and thus stimulate more and more motor units to contract. The process of bringing more motor units into play is called **recruitment,** or **multiple motor unit (MMU) summation.** This is seen not just in artificial stimulation, but is part of the way the nervous system behaves naturally to produce varying muscle contractions. The neuromuscular system behaves according to the **size principle** -- smaller, less powerful motor units with smaller, slower nerve fibres are activated first. This is sufficient for delicate tasks and refined movements, but if more power is needed, then larger motor units with larger, faster nerve fibres are subsequently activated. But even when stimulus intensity (voltage) remains constant, twitch strength can vary with stimulus frequency. High-frequency stimulation produces stronger twitches than low-frequency stimulation. In figure 11.15a, we see that when a muscle is stimulated at low frequency, say 5 to 10 stimuli/s, it produces an identical twitch for each stimulus and fully recovers between twitches. At higher stimulus frequencies, say 20 to 40 stimuli/s, each new stimulus arrives before the previous twitch is over. Each new twitch "rides piggyback" on the previous one and generates higher tension (fig. 11.15b). This phenomenon goes by two names: **temporal summation,** because it results from two stimuli arriving close together in time, or **wave summation,** because it results from one wave of contraction added to another. Wave upon wave, each twitch reaches a higher level of tension than the one before, and the muscle relaxes only partially between stimuli. This effect produces a state of sustained fluttering contraction called **incomplete tetanus.** In the laboratory, an isolated muscle can be stimulated at such high frequency that the twitches fuse into a single, nonfluctuating contraction called **complete (fused) tetanus** (fig. 11.15c). This doesn't happen in the body, however, because motor neurons don't fire that fast. Indeed, there is an inhibitory mechanism in the spinal cord that prevents them from doing so. Complete tetanus is injurious to muscle and associated soft tissues, so spinal inhibition protects the muscles by preventing complete tetanus. Muscle tetanus should not be confused with the disease of the same name caused by the tetanus toxin. Despite the fluttering contraction seen in incomplete tetanus, we know that a muscle taken as a whole can contract very smoothly. This is possible because motor units function asynchronously; when one motor unit relaxes, another contracts and takes over so that the muscle does not lose tension. ### Typed of contractions: Isometric and Isotonic Contraction In muscle physiology, "contraction" doesn't always mean the shortening of a muscle - it may mean only that the muscle produces internal tension while an external resistance causes it to stay the same length or even become longer. Thus, physiologists speak of different kinds of muscle contraction as *isometric* versus *isotonic.* Suppose you lift a heavy dumbbell. When you first contract the muscles of your arm, you can feel the tension building in them even though the dumbbell isn't yet moving. At this point, your muscles are contracting at a cellular level, but their tension is being absorbed by the elastic components and is resisted by the weight of the load; the muscle as a whole is not producing any external movement. This phase is called **isometric** **contraction** -- The tension generated is not enough for the object to be moved and the muscle does not change its length (fig. 11.16a). Isometric contraction is not merely a prelude to movement. The isometric contraction of antagonistic muscles at a single joint is important in maintaining joint stability at rest, and the isometric contraction of postural muscles is what keeps us from sinking in a heap to the floor. **Isotonic** **contraction** -- contraction with a change in length but no change in tension -- begins when internal tension builds to the point that it overcomes the resistance. The muscle now shortens, moves the load, and maintains essentially the same tension from then on (fig. 11.16b). Isometric and isotonic contraction are both phases of normal muscular action (fig. 11.17). Smooth Muscle ============= - Occurs as bundles of elongated fusiform cells that have tapered ends - Cells vary in size in different tissues 20 200 microns - Interconnected by gap junctions to enable synchronous contraction of bundles of smooth muscle cells - Cells have characteristic nuclei in longitudinal section -- they appear elongate and have tapering ends match the shape of the cell - Contractile apparatus in smooth muscle contains thick and thin filaments - The cells possess a cytoskeleton containing desmin and vimentin intermediate filaments - The sarcoplasm of SM cells contains labile thick myosin filaments that are lost during tissue preparation - SM is specialized for prolonged contraction without fatigue - They can: - Produce peristaltic movements by contracting in a wave like manner - Produce extrusive movements as in the urinary bladder, gallbladder or uterus - Exhibit spontaneous contractile activity in the absence of nerve stimuli - Contraction of SM is regulated by post ganglionic fibers of theautonomic nervous system - Most SM is directly innervated by SNS and PNS - In the GIT the enteric division of the ANS is the primary source of innervation to smooth muscle in the gut - Ca^2+^ enters the sarcoplasm during depolarisation by voltage-gated Ca channels - Some hormones act as ligands on ligand gated Ca channels to initiate SM contraction e.g. oxytocin and ADH from the posterior pituitary during birth - Gap junctions between smooth muscle cells propagate contraction through the muscle layer Cardiac Muscle ============== Cardiac muscle is limited to the heart, where its function is to pump blood. Knowing that, we can predict the properties that it must have: (1) It must contract with a regular rhythm; (2) it must function in sleep and wakefulness, without fail or need of conscious attention; (3) it must be highly resistant to fatigue; (4) the cardiomyocytes of a given heart chamber must contract in unison so that the chamber can effectively expel blood; and (5) each contraction must last long enough to expel blood from the chamber. These functional necessities are the key to understanding how cardiac muscle differs structurally and physiologically from skeletal muscle (table 11.4). Cardiac muscle is striated like skeletal muscle, but cardiomyocytes are shorter and thicker. They have one or two nuclei near the middle of the cell. Each cell is enclosed in an endomysium, but there is no perimysium or epimysium as in skeletal muscle. Cardiomyocytes branch slightly so each is joined end to end with several others. These intercellular connections, called **intercalated discs,** appear as thick dark lines in stained tissue sections. Not a syncytium but composed of end to end alignment of individual cells. An intercalated disc has electrical *gap junctions* that allow each cardiomyocyte to directly stimulate its neighbours, and mechanical junctions that keep the cardiomyocytes from pulling apart when the heart contracts. The sarcoplasmic reticulum is less developed than in skeletal muscle, but the T tubules are larger and admit Ca^2+^ from the extracellular fluid. Damaged cardiac muscle is repaired by fibrosis. Unlike skeletal muscle, cardiac muscle can contract without the need of nervous stimulation. The heart has a built-in **pacemaker** that rhythmically sets off a wave of electrical excitation,which travels through the muscle and triggers the contraction of the heart chambers. The heart is said to be **autorhythmic** because of this ability to contract rhythmically and independently. Stimulation by the autonomic nervous system, however, can increase or decrease the heart rate and contraction strength. Cardiac muscle does not exhibit quick twitches like skeletal muscle. Rather, it maintains tension for about 200 to 250 ms, giving the heart time to expel blood. Cardiac muscle uses aerobic respiration almost exclusively. It is very rich in myoglobin and glycogen, and it has especially large mitochondria that fill about 25% of the cell, compared with smaller mitochondria occupying about 2% of a skeletal muscle fibre. Cardiac muscle is very adaptable with respect to the fuel used, but very vulnerable to interruptions in oxygen supply. Because it makes little use of anaerobic fermentation, cardiac muscle is highly resistant to fatigue.