Muscle tissue is specialized for contraction and is responsible for body movements and changes in size and shape of internal organs. Muscle cells are usually elongated and arranged in parallel arrays. Muscle is classified based on the appearance of its cells. The two principle types of muscle include striated and smooth muscle. Striated muscle appears to have cross striations when viewed under the light microscope, whereas smooth muscle does not. Striated muscle includes two subtypes (1) skeletal muscle that is attached to bone and responsible for the movements of the axial and appendicular skeleton, and cardiac muscle that makes up most of the heart. (2) Skeletal muscle and cardiac muscle are sometimes referred to as voluntary and involuntary striated muscle, respectively. The prefixes myo and sarco refer to muscle. Consequently, terms like myofibril or myofilament refer to structures within a muscle. For example, the plasma membrane of a muscle cell is called the sarcolemma, the cytoplasm the sarcoplasm, and the endoplasmic reticulum the sarcoplasmic reticulum (SR). In addition, a single skeletal muscle cell is also called a muscle fiber. The four properties of muscles enable them to perform their functions. These properties include Muscles serve four major functions including production of movement, maintenance of posture, stabilization of joints, and generation of heat: Skeletal muscle accounts for approximately 40% of body weight. Each skeletal muscle is made up of muscle fibers, connective tissue, blood vessels, and nerves. As with neurons, each muscle has three connective tissue layers (Fig. 7.1). Fig. 7.1 Connective tissue sheaths in skeletal muscle. Each skeletal muscle represents skeletal muscle fibers grouped into a muscle surrounded by a thin connective tissue sheath called the epimysium. Within the muscle are groupings of muscle fibers called fascicles, which are surrounded by the perimysium. Within each fascicle are individual muscle fibers surrounded by the endomysium. Near the ends of the muscle, the epimysium, perimysium, and endomysium blend together forming either a bundle called a tendon, or a broad sheet called an aponeurosis. Tendons and aponeuroses attach muscle to bones merging with the periosteum of the bone. These attachments allow contraction of the muscles to move the bones. The two innermost layers of connective tissue within the muscle each contains blood vessels and nerves. Skeletal muscle is generally under voluntary nervous control, and therefore requires stimulation from nerve fibers to initiate contraction. Therefore, individual nerve fibers must innervate each muscle fiber to control contraction. While the diaphragm consists of skeletal muscle, it usually is under involuntary control but can be under voluntary control as well. Skeletal muscle tissue consists of large, multinucleated cells called muscle fibers (Fig. 7.2). Muscle fibers can be 100 μm in diameter and run the entire length of a muscle and can contain hundreds of nuclei. These cells form from the fusion of small, individual muscle cells called myoblasts during development (Fig. 7.2). However, some of these cells remain unfused and become satellite cells. While skeletal muscle fibers are incapable of dividing, new muscle fibers are produced from satellite cells located in the adult muscle. Not all myoblasts fuse to form muscle fibers. The satellite cells can later enlarge, divide, and then fuse with damaged muscle cells, thus regenerating the muscle. The muscle fiber nuclei are located immediately under the plasma membrane, which, in skeletal muscle is called the sarcolemma. There is a resting membrane potential present due to the unequal distribution of ions across the sarcolemma like that found in neurons. The cytoplasm of skeletal muscle is called sarcoplasm. Within the sarcoplasm there are storage granules of glycogen and myoglobin, a red pigmented protein that stores oxygen. Although skeletal muscle fibers are large, an electrical signal must propagate throughout the cell quickly to induce contraction. Transverse, or T, tubules are small diameter tubes running perpendicular to the sarcolemma traversing into the sarcoplasm. These tubes are continuous with the extracellular space, and thus they contain extracellular fluid. They can be thought of as extensions of the sarcolemma. As we will see later, the action potential (AP) can travel along the sarcolemma and down the T tubules. Muscle fibers are composed of functional subunits called myofibrils. Each muscle fiber contains hundreds to thousands of myofibrils that run longitudinally the length of the fiber. The myofibrils consist of bundles of myofilaments that are protein filaments composed primarily of actin and myosin, the two contractile proteins in muscle. Actin forms the bulk of the thin filaments and myosin forms the bulk of the thick filaments. The myofibrils are packed tightly into the muscle fiber forcing the mitochondria, nuclei, and other organelles to be squeezed toward the periphery of the cell. Fig. 7.2 Formation of skeletal muscle cell. During embryonic development, myoblasts begin to fuse forming a large, multinucleated skeletal muscle cell called a muscle fiber. Unfused myoblasts remain as satellite cells that function as muscle stem cells. The myofibrils contain three types of proteins that will be discussed in more detail when describing the structure of thin and thick filaments below: The myofibrils are attached to the inner surface of the sarcolemma. The outer surface of the muscle fibers is attached to collagen fibers that help connect the cells to the tendon or aponeuroses. Therefore, as the muscle fibers contract, they exert force on the bones causing them to move. Like the endoplasmic reticulum in other cells, the SR forms a tubular network surrounding each myofibril (Fig. 7.3). The terminal cisternae (end sacs) of the SR are always found in pairs, with an intervening T tubule. The combination of a terminal cisterna, a T tubule, and the adjacent terminal cisterna form a triad. Note that the T tubule communicates with the extracellular space while the SR is intracellular. The terminal cisternae have an active calcium pump that moves calcium from the sarcoplasm into the SR. This maintains a low concentration of free calcium within the sarcoplasm, whereas the free calcium concentration inside the SR may be a 1000‐fold greater. Also found within the terminal cisternae is the protein calsequestrin that reversibly binds Ca2+. The free and calsequestrin‐bound calcium concentrations can be 40,000 times that in the sarcoplasm. As we will discuss later, the terminal cisternae of the SR are the source of calcium for skeletal muscle contraction. The functional unit of skeletal muscle is the sarcomere. A myofibril consists of thousands of sarcomeres (Fig. 7.4). In stained cross sections of skeletal muscle, alternating light and dark bands are evident, which are called the I band and A band, respectively. These bands give skeletal muscle its striated appearance. The dark bands alter the plane of the polarized light and are therefore anisotropic (i.e., not having the same properties in all directions), whereas the light bands do not alter the plane of polarized light and are therefore isotropic (i.e., appear the same in all directions), thus the names A band and I band, respectively. Fig. 7.3 T tubule and sarcoplasmic reticulum. The transverse, or T tubules are inwardly directed invaginations of the sarcolemma found near the junction of the A and I bands. The sarcoplasmic reticulum is a network of tubules found inside the cell and which have terminal cisternae near the T tubules. Two terminal cisterna and the intervening T tubule make a triad. Fig. 7.4 Sarcomere. A sarcomere, the functional unit of skeletal muscle, runs from Z‐line to Z‐line. The various segments of the sarcomere are identified in the top portion of the figure, and a photomicrograph of a sarcomere is shown in the bottom portion of the figure. The sarcomere is composed of thick and thin filaments, proteins that stabilize those filaments, and proteins that regulate the interactions between thick and thin filaments. As shown in Figure 7.4, a sarcomere is the region between two adjacent Z discs (or Z‐lines). It consists of one‐half of an I band, an A band, and one‐half of an I band. The A band is the length of the thick filament and can contain both thick and thin filaments. In a muscle at rest, a lighter region can be found in the center of the A band called the H zone (from helle, meaning bright), which contains only myosin. This region disappears as skeletal muscle contracts and the actin filaments overlap, thus entering this area. The M‐line, named for being in the middle of the sarcomere, transects the H zone and is composed of proteins that stabilize the position of the thick filaments. Near the ends of the A band are zones of overlap where thin and thick filaments are found side by side. The I band, located between each intervening A band, contains thin filaments. The I band is bisected by the Z‐line that consists of proteins called actinins, which interconnect adjacent thin filaments. There are several structural proteins associated with the myofibrils making the sarcomere. Titin (from titan, meaning gigantic) is a large protein, and the third most abundant protein in the sarcomere behind myosin and actin. Each titin molecule extends from the Z‐line to the M‐line and helps anchor a thick filament to both the Z‐line and M‐line. This provides stabilization for the position of the thick filaments. As shown in Figure 7.4, the portion of the titin molecule located between the Z‐line and the end of the thick filament is very elastic and can stretch up to four times its resting length. Therefore, titin probably assists in returning the muscle to its resting length following stretching. The Z‐line is composed of the protein nebulin. Nebulin anchors thin filaments and connects myofibrils to each other throughout the muscle cell. The M‐line is composed of the protein myomesin. The M‐line binds to titin, thus helping to connect adjacent thick filaments. Dystophin is another structural protein that links thin filaments to integral membrane proteins in the sarcolemma. Other proteins in the sarcolemma then attach to the connective tissue sheath surrounding the muscle. Thus, the contractive forces generated in the sarcomere are transferred throughout the muscle. Thin filaments are 5–6 nm in diameter and 1 μm in length (Fig. 7.5). Each thin filament is composed of three proteins: Thick filaments are 10–12 nm in diameter and 1.6 μm in length (Fig. 7.6). Thick filaments consist of approximately 500 myosin molecules, each composed of two myosin subunits wrapped around each other. The long tails of the myosin molecules line up forming the thick filament, and the heads of the myosin molecules project off the filament toward adjacent thin filaments. The head of the myosin molecule consists of two globular proteins, has ATPase activity, and can bind to the actin filament. A cross bridge is formed when the head of the myosin binds to the actin filament. There is a hinge between the head and the tail of the myosin molecule that allows the head to pivot toward or away from the M‐line. The myosin molecules are arranged so that their tails point toward the M‐line. In the H zone, there are no myosin heads, only tails. Also within each thick filament is a molecule of titin extending from the M‐line to the Z‐line. Fig. 7.5 Thin filament. The thin filaments in skeletal muscle consist of G‐actin, troponin, and tropomyosin. G‐actin polymerizes into F‐actin, or filamentous actin. Troponin is made of three globular proteins binding G‐actin, tropomyosin, and calcium ions, respectively. Two strands of tropomyosin, a rod‐shaped protein, intertwine around the F‐actin covering the myosin‐binding sites while the muscle is at rest. Fig. 7.6 Thick filament. A single myosin molecule is shown at the top. It contains a pair of intertwined subunits each consisting of a tail, a hinge region, and a globular head. The thick filaments contain approximately 500 myosin molecules in which the tails are lined up so that the heads project away from the M‐line. As summarized in Figure 7.7, the control of skeletal muscle contraction involves the voluntary stimulation of motor neurons innervating the muscle. The release of the neurotransmitter from these motor neurons initiates excitation‐coupling‐contraction in which an AP is generated within the skeletal muscle fiber. The AP causes the release of calcium from the SR, which then causes muscle contraction. Fig. 7.7 Summary of skeletal muscle contraction. Stimulation of α‐motor neurons going to skeletal muscle causes the release of acetylcholine at the neuromuscular junction. This causes the production of an action potential in the muscle fiber that spreads along the sarcolemma and down the T tubules where it causes the release of calcium ions from the sarcoplasmic reticulum. Calcium then diffuses to the thin filaments where it binds to troponin to initiate contraction. Fig. 7.8 Neuromuscular junction. The neuromuscular junction is a specialized synapse between an α‐motor neuron and skeletal muscle fibers. The synaptic bouton is imbedded in the sarcolemma. At this site are subneural clefts that increase the surface area surrounding the synapse. Synaptic vesicles containing acetylcholine (ACh) are in the nerve ending. Upon stimulation, the α‐motor neuron releases ACh that can diffuse across the synaptic cleft and bind to nicotinic receptors on the skeletal muscle fiber. Upon binding to the receptor, ACh causes Na+ ions to enter the skeletal muscle fiber, causing a postsynaptic potential. The postsynaptic potential is always large enough to induce an AP in the skeletal muscle fiber. Skeletal muscle is controlled by the somatic nervous system. The cell bodies of the α‐motor neurons, i.e., somatic motor neurons, that innervate skeletal muscle reside in the central nervous system. The axons of these neurons leave the CNS and innervate skeletal muscle fibers at a specialized junction called the neuromuscular junction (NMJ), or myoneural junction (Fig. 7.8). Each muscle fiber is innervated by a neuron, although a single neuron may innervate multiple muscle fibers. The neuron branches as it enters the perimysium, and each branch ends in a synaptic terminal, sometimes called a synaptic bouton. The synapse is the region of contact between a neuron and its target cell, in this case a skeletal muscle fiber. The space between the neuron and the muscle fiber is the synaptic cleft. The sarcolemma in the region of the NMJ is called the motor end plate. Since an electrical signal cannot traverse the synaptic cleft, the signal from the motor neuron is communicated via the release of a neurotransmitter. The neurotransmitter released from α‐motor neurons is ACh. ACh is contained in synaptic vesicles located in the synaptic bouton. When the AP arrives at the synaptic bouton, it causes the release of ACh that then diffuses across the synapse and binds to a specialized cholinergic receptor located on the muscle fiber. This receptor is called a nicotinic receptor. This is a transmembrane protein that binds ACh and can also be stimulated by the agonist nicotine (hence the moniker for the receptor). When ACh binds to the nicotinic receptor, it causes the opening of a ligand‐gated ion channel that allows sodium ions to enter the muscle fiber. This causes the production of a postsynaptic potential that produces an AP in the muscle fiber. The amount of neurotransmitter released per nerve impulse is greater than the amount needed to induce a postsynaptic AP, and the number of receptors activated by ACh is more than required to reach threshold, thus providing a “safety factor” assuring that stimulation of a motor neuron results in contraction of skeletal muscle. Since the NMJ is a chemical synapse, it is prone to pharmacological manipulation. Curare, produced by certain frogs, is a compound used by South American Indians to make poisonous arrows and darts. Curare blocks nicotinic receptors and thereby prevents ACh from inducing skeletal muscle contraction. Derivatives of curare are sometimes given prior to surgery to relax the skeletal muscles. Clostridium botulinum is a bacterium often found in contaminated canned foods. The toxin from this organism prevents the release of ACh from somatic motor neurons. Botulinum toxin thus prevents skeletal muscle contraction. A very small amount of this toxin can cause death by paralyzing the diaphragm and other respiratory muscles. Recently, this toxin has been increasingly used in human medicine (Botox) to reduce wrinkles, control strabismus (crossed eyes), blepharospasm (uncontrolled blinking), or cervical dystonia (also known as spasmodic torticollis), which is characterized by involuntary tonic contractions or intermittent spasms of the neck muscles. ACh is normally inactivated by the enzyme acetylcholinesterase (AChE). Agents known as AChE inhibitors can be used to strengthen weak skeletal muscle contractions. An autoimmune disease called myasthenia gravis, in which there is reduced nicotinic receptor function, is treated with the AchE inhibitor neostigmine. This drug can also be used to reverse the effects of curare. The process by which an AP in skeletal muscle fibers induces contraction is called excitation‐contraction coupling (Fig. 7.9). The AP migrates along the sarcolemma and down the T tubules. At the triad, the AP triggers the release of Ca2+ from the terminal cisterns of the SR. The release of Ca2+ from the terminal cisterns involves the direct mechanical connection between the T tubules and the terminal cisterns (also called lateral sacs) of the SR. Located on the T tubule membrane is a T‐tubule voltage sensor which detects a change in membrane potential associated with the AP. A change in voltage causes a conformational change in the T‐tubule voltage sensor that triggers the Ca2+ channels on the terminal cisterns of the SR, causing them to open and release Ca2+ into the cytosol or sarcoplasm. This direct mechanical connection is unique to skeletal muscle. Smooth muscle has a different mechanism and different source of calcium for excitation‐contraction coupling. Cytosolic calcium levels increase at least 10‐fold. As cytosolic calcium levels increase, Ca2+ binds to troponin, causing a conformational change in the shape of this globular protein. This change in shape allows tropomyosin to slide into the grooves of the double helix formed by F‐actin (Fig. 7.10). As tropomyosin slides into the groove, it uncovers the myosin binding sites on G‐actin. Once uncovered, the heads of the myosin filament bind to the myosin‐binding sites, and contraction begins. When stimulation from the motor neurons ends, the AP is no longer propagated down the T tubules. At this point, a calcium active transport pump called calsequestrin actively pumps Ca2+ back into the terminal cisterns of the SR. This process requires ATP and allows for the concentration of Ca2+ in the SR to be 10,000 times higher than in the sarcoplasm. As the Ca2+ levels in the sarcoplasm decrease, troponin returns to its resting configuration and tropomyosin again covers the myosin‐binding sites on the G‐actin. During skeletal muscle contraction, the length of the thin and thick filaments does not change. Instead, the thin filaments slide between the thick filaments as the myosin heads “grab” the actin filaments and pull them toward the M‐line (Fig. 7.11). Hence, as the thin filaments move toward the M‐line, the Z‐lines get closer, thus decreasing the length of the sarcomere and the myofibril. As the sarcomere width decreases, the muscle shortens. Fig. 7.9 Increasing sarcoplasmic calcium concentration. The action potential (AP) migrates along the sarcolemma and down the T tubule. When reaching the triad, the AP activates the enzyme phospholipase C, resulting in the production of 2‐diacylglycerol (2‐DAG) and inositol triphosphate (IP3). The 2‐DAG remains membrane bound while IP3 diffuses through the sarcoplasm to the terminal cistern of the SR. This opens Ca2+ release channels, causing the release of Ca2+ from the SR into the sarcoplasm. Ca2+ then binds to troponin, which initiates contraction. Fig. 7.10 Excitation‐contraction coupling. (A) At rest, calcium is sequestered in the SR, and the myosin head sits perpendicular to the thin filament. The myosin head is a charged intermediate with ADP and inorganic phosphate (Pi) attached. (B) Calcium released from the SR binds to the TnC component of troponin. (C) The conformational change in troponin results in the tropomyosin filament sliding into the groove of the double helix formed by F‐actin, thus uncovering the myosin‐binding site located on the actin filament. The myosin head binds to actin, releasing ADP and Pi. (D) The power stroke occurs when the myosin head tilts toward the M‐line. Fig. 7.11 Sliding filaments. The top sarcomere is at rest while the bottom sarcomere is in a contracted state. Note that in the contracted state, the Z‐lines move closer together and the I band, and H zone shorten while the A band remains the same width. Contraction of skeletal muscle involves four steps: This cycle then repeats itself as long as the myosin binding sites on the actin remain uncovered and there is sufficient ATP. Each thick filament has about 600 myosin heads. As contraction occurs, these heads are attaching and detaching throughout the cycle such that at any given time, there are many myosin heads attached. Therefore, contraction force is always being generated during this time. The myosin heads are sequentially “walking” the thin filament toward the M‐line throughout the contraction cycle, and therefore pulling the Z‐lines closer together. The elastic components in muscle include titin, tendons, and the connective tissue sheaths (endomysium, perimysium, and epimysium) are also critical for contraction. As the muscle fibers contract, the elastic components are stretched. This stretch is relayed out to the tendons, which then pull on the bones causing them to move. As calcium is sequestered in the SR, tropomyosin again covers the myosin binding sites. Hence, the myosin head, with its hydrolyzed molecule of ATP attached, assumes its resting position poised to attach to actin when a binding site becomes available. As contraction ceases, the elastic components of the muscle help return the muscle to its resting position. After death, the supply of energy within the cells diminishes as metabolism ceases. Consequently, the cells can no longer synthesize ATP. ATP is needed to actively remove sequestered Ca2+ into the terminal cisterns of the SR. In addition, since the cross bridges can be broken only in the presence of ATP, myosin and actin remain attached following death. This occurs in all the skeletal muscles creating a state called rigor mortis (rigidity of death) in which the animal appears rigid. Rigor mortis ceases as proteolytic lysosomal enzymes released by autolysis digest the cross bridges. The following steps summarize the contraction of skeletal muscles: The tension developed by a muscle fiber during contraction is dependent on the length of the sarcomere prior to contraction. At a sarcomere length of approximately 2.0–2.4 μm, or 90–110% of the resting sarcomere length, the overlap between the actin and myosin filaments is optimal, and the muscle can generate the maximum tension (Fig. 7.12). At these lengths, the maximum number of cross bridges can be formed between the myosin head and thin filament. As the muscle is either contracted or stretched, the number of cross bridges that can form decreases, and less tension is generated during contraction. As the muscle fiber is stretched to approximately 170% of its resting length, the thick and thin filaments no longer overlap, and therefore, no tension can be generated. Conversely, as the length of the sarcomere gets too short, the thick filaments are compressed against the Z‐line, decreasing the number of cross bridges that can be produced. A single stimulation of a motor neuron results in a single contraction, or twitch (Fig. 7.13). Although twitches can produce heat during shivering, they are not generally observed during normal muscle contraction. Instead, prolonged stimulation results in more tension being produced than caused by a single twitch. Fig. 7.12 Length‐tension relationship. At the optimal resting sarcomere length, the maximum number of cross bridges can be formed between myosin and the thin filament resulting in the maximum tension. As the length of the sarcomere is stretched or compressed, the number of cross bridges is reduced, resulting in less tension. Fig. 7.13 Muscle twitch. A myogram showing the three stages of an isometric muscle twitch. The stimulus is followed by a latent period during which calcium is released from the sarcoplasmic reticulum (SR) and then binds to troponin. The contraction period is when the myosin head actively binds and pulls on the thin filaments, and the relaxation period occurs when calcium is sequestered in the SR. A recording of a single muscle twitch is called a myogram. A single twitch can last from 20 to 200 ms, depending on the type of muscle, temperature, stretch of the muscle, etc. A muscle twitch consists of three phases: If skeletal muscle is stimulated a second time, shortly after the relaxation phase of the first twitch, the second twitch will generate greater tension (Fig. 7.14). This increase in tension is known as treppe, German for “stairs.” The increase in tension caused by subsequent stimulations results from the gradual increase in sarcoplasmic calcium since the calcium pumps located in the SR are unable to sequester all the calcium between twitches. While a muscle twitch is an all‐or‐none response, muscle contraction is graded, meaning that it displays varying length and strength of contraction. There are two mechanisms leading to graded responses: (1) changing the frequency of stimulation and (2) changing the strength of the stimulus. While muscle twitches can be observed in the laboratory, muscle contraction generally involves smooth sustained contraction resulting from frequent stimulation. If a second stimulation occurs before the muscle completes its relaxation phase, the second twitch will create greater tension than the original twitch. This process is called wave summation (Fig. 7.15). This generally occurs at stimulation rates of about 50 per second. Stimulation occurs rapidly enough that the SR is no longer able to sequester Ca2+ between twitches. In addition to prolonging contraction, the second contraction causes greater shortening than the first contraction because it is superimposed on an already contracted muscle, thus increasing tension. Fig. 7.14 Treppe. If the muscle is stimulated shortly after the relaxation phase, the subsequent muscle twitches generate greater tension, producing a step‐like increase in magnitude called treppe. The increase in tension in subsequent twitches results from the increase in sarcoplasmic calcium due to the inability of the SR to recapture all the calcium between twitches. Fig. 7.15 Wave summation and tetanus. (1) A single twitch. (2) The muscle is stimulated (↑) before the relaxation phase is complete, causing wave summation and increased contraction force. (3) Frequency of stimulation is more rapid, resulting in unfused tetanus in which the individual twitches can still be discerned. (4) Frequency of stimulation is so rapid that individual twitches cannot be distinguished, resulting in fused tetanus. As the frequency of stimulation increases, the tension developed also increases. Incomplete tetanus (tetan = rigid or tense) occurs when the individual twitches are still distinguishable. When the frequency of stimulation is rapid enough to eliminate the relaxation phase, and the individual twitches are no longer distinguishable, the contraction is termed complete tetanus. Complete tetanus is the normal state observed during muscle contraction. Wave summation results in smooth, continuous muscle contraction. Note that the frequency of nerve stimulation cannot be faster than the absolute refractory period of the neurons. A second type of summation that increases the force of muscle contraction is called multiple motor unit summation, or recruitment. Skeletal muscles have thousands of muscle fibers. All the muscle fibers innervated by a single motor neuron constitute a motor unit (Fig. 7.16). The size of a motor unit can vary with a single motor neuron innervating as few as 4–6 muscle fibers, or as many as several thousand. Where fine, delicate movements are necessary, such as in the lips, motor units are small, whereas in areas where precise movements are less important, such as in the hindquarter of a beef cow, the motor units are much larger. Motor units are intermingled within a muscle so that they always deliver force on the tendon attached to a bone. As an animal begins a task, it generally stimulates the smallest motor units. However, if more force is required, more and larger motor units are recruited to increase the tension produced by the muscle. Skeletal muscle is seldom completely flaccid, but instead maintains a degree of tension called muscle tone. Since skeletal muscle contraction is controlled by motor neurons releasing ACh, muscle tone is established by the central nervous system. If these motor neurons are cut, skeletal muscle becomes flaccid. Muscle tone is due to the alternating stimulation of motor units by the central nervous system. Such tone helps keep an animal upright, keep the head held up, stabilize joints, and maintain posture. Muscle tone is not unique to skeletal muscle, but it also occurs in smooth muscle. For example, blood vessels generally maintain a vascular tone as does the gastrointestinal tract. There are two major categories of muscle contraction: isotonic and isometric (Fig. 7.17). During isotonic (iso = same; tonos = tension), the length of the muscle changes as force is generated, resulting in movement. There are two types of isotonic contractions, concentric and eccentric. In a concentric contraction, the muscle gets shorter as it works. In other words, the muscle forms cross bridges and the thin filaments interdigitate within the thick filaments overcoming the resistance of the load on the muscle. During eccentric contraction, the tension developed by the muscle is less than the load on the muscle. As a result, the muscle lengthens. As an animal walks down a steep incline, the animal controls the rate of elongation of muscles as the legs stretch to the next location. Fig. 7.16 Motor units. All the muscle fibers innervated by a single motor neuron constitute a motor unit. Motor unit #1 is larger than motor unit #2 because it innervates more muscle fibers. Motor unit #2 would be involved in more precise motor movement than motor unit #1. Fig. 7.17 Isotonic versus isometric contractions. (A) During isotonic contraction, force is generated as the muscle shortens. (B) In isometric contraction, force is generated, but there is minimal shortening of the muscle. During isometric (metric = measure) contraction, the length of the muscle does not change because the tension produced does not exceed the resistance. Isometric contraction is commonly observed in postural muscles that maintain a constant body position while opposing gravity. When performing various movements, animals use all these types of contractions. Consider the motions as a dog sits and then stands back up. The quadriceps are involved in controlling this motion. As a dog begins to sit, the knees begin to bend, or flex (eccentric). As the dog holds a position part way through the sitting motion, an isometric condition exists. As the dog stands, thus extending the knee, isometric and concentric contractions occur. Muscle Relaxation or Return to Resting Length While muscle contraction is an active process requiring energy, the relaxation of muscle is passive. Elastic forces, opposing muscles, and gravity act to return the muscle to its resting length. Such elastic fibers include connective tissue and many of the muscle proteins such as titin. Contraction of the opposing muscle also helps return a muscle to its resting length. For example, as the triceps brachii muscle in the back of the front leg contracts, it causes the biceps brachii on the anterior portion of the leg to extend. Similarly, gravity can cause muscles to extend. The neck of a horse is extended to look upward, and then when the muscles are relaxed, the head will move toward the ground, thus stretching the neck muscles that originally were involved in extension. The major energy source for muscle metabolism is ATP. It is used for cross bridge formation, to actively pump calcium into the SR, and to pump Na+ out and K+ into the muscle fiber. The endogenous stores of ATP can last only about 4–6 seconds. This means there must be mechanisms to replenish these limited stores. While at rest, skeletal muscle makes sufficient ATP to meet its metabolic needs, and to store surplus energy in the form of creatine phosphate and glycogen. Resting muscle can use fatty acids that are broken down in the mitochondria and the ATP used to make creatine phosphate. The glucose that is delivered through the blood stream can be converted to glycogen. When the demands for ATP become greater, there are three pathways for generating ATP. (1) aerobic respiration, (2) ADP interacting with creatine phosphate, and (3) from stored glycogen through the anaerobic process of glycolysis (Fig. 7.18). Fig. 7.18 ATP production in muscle. There are three mechanisms for generating ATP in muscle fibers. (A) During the direct phosphorylation of creatine phosphate, a phosphate group is moved from ATP to creatine producing creatine phosphate, an energy storage form in muscle. When ADP is plentiful, this reaction is reversed to produce ATP. (B) During glycolysis, glucose is anaerobically broken down to two molecules of pyruvate, which are then converted to lactic acid to regenerate NAD+. (C) In the presence of O2, pyruvate is further metabolized to CO2 and H2O, plus ATP. When O2 is present, pyruvate enters the mitochondria where aerobic respiration occurs. This process produces 36 mol of ATP for every 1 mol of glucose. During aerobic respiration, the following reaction occurs:
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Muscular Tissue
Muscle Tissue Overview
Introduction
Properties of Muscles
Functions of Muscles
Skeletal Muscle
Connective Tissues
Blood Vessels and Nerves
Skeletal Muscle Fibers
Myofibrils
Sarcoplasmic Reticulum
Sarcomeres
Thin Filaments
Thick Filaments
Contraction of Skeletal Muscle
Neuromuscular Junction
Pharmacology of the Neuromuscular Junction
Excitation‐Contraction Coupling
Sliding Filament Theory
Rigor Mortis
Summary of Skeletal Muscle Contraction
Length‐Tension Relationships
A Muscle Twitch
Treppe
Summation
Wave Summation
Multiple Motor Unit Summation
Muscle Tone
Isometric Versus Isotonic Contraction
Metabolism of Skeletal Muscle
Aerobic Mechanism
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