Chapter 10 - Muscular Tissue Flashcards
Explain the structural differences among the three types of muscular tissue.
Skeletal muscle tissue is so named because most skeletal muscles move the bones of the skeleton. Skeletal muscle tissue is striated: Alternating light and dark protein bands (striations) are seen when the tissue is examined with a microscope. Skeletal muscle tissue works mainly in a voluntary manner.
Cardiac muscle tissue, is found only in the heart, where it forms most of the heart wall. Cardiac muscle is also striated, but its action is involuntary. The heart beats because it has a natural pacemaker that initiates each contraction. This built-in rhythm is termed autorhythmicity.
Smooth muscle tissue is located in the walls of hollow internal structures, such as blood vessels, airways, and most organs in the abdominopelvic cavity. It is also found in the skin, attached to hair follicles. Smooth muscle tissue is nonstriated, which is why it is referred to as smooth. The action of smooth muscle is usually involuntary, and some smooth muscle tissue, such as the muscles that propel food through your gastrointestinal tract, has autorhythmicity.
What are the 4 key functions of muscular tissue?
Producing body movements
Stabilizing body positions
Storing and moving substances within the body
Generating heat
What are the 4 special properties of muscular tissue?
Electrical excitability (the ability to respond to electrical or chemical stimuli by producing electronic signals called action potentials)
Contractility (the ability to contract forcefully when stimulated by an action potential)
Extensibility (the ability to stretch without being damaged)
Elasticity (the ability to return to their original length and shape after being stretched or contracted)
Explain the importance of connective tissue components, blood vessels, and nerves to skeletal muscles.
Connective tissue surrounds and protects muscular tissue. The subcutaneous layer or hypodermis, which is composed of areolar and adipose tissue and separates muscle from skin, provides a pathway for nerves, blood vessels, and lymphatic vessels to enter and exit muscles. The adipose tissue of the subcutaneous layer stores most of the body’s triglycerides, serves as an insulating layer that reduces heat loss, and protects muscles from physical trauma. Fascia is a dense sheet or broad band of irregular connective tissue that lines the body wall and limbs and supports and surrounds muscles and other organs of the body.
Skeletal muscles are well supplied with nerves and blood vessels. Generally, an artery and one or two veins accompany each nerve that penetrates a skeletal muscle. The neurons that stimulate skeletal muscle to contract are somatic motor neurons. Especially during contraction, a muscle fiber synthesizes and uses considerable ATP (adenosine triphosphate). These reactions require oxygen, glucose, fatty acids, and other substances that are delivered to the muscle fiber via blood vessels.
Describe the microscopic anatomy of a skeletal muscle fiber.
The multiple nuclei of a skeletal muscle fiber are located just beneath the sarcolemma, the plasma membrane of a muscle cell. Thousands of tiny invaginations of the sarcolemma, called transverse (T) tubules, tunnel in from the surface toward the center of each muscle fiber. Because T tubules are open to the outside of the fiber, they are filled with interstitial fluid. Muscle action potentials travel along the sarcolemma and through the T tubules, quickly spreading throughout the muscle fiber. This arrangement ensures that an action potential excites all parts of the muscle fiber at essentially the same instant. Within the sarcolemma is the sarcoplasm, the cytoplasm of a muscle fiber. Sarcoplasm includes a substantial amount of glycogen, which is a large molecule composed of many glucose molecules. Glycogen can be used for synthesis of ATP. In addition, the sarcoplasm contains a red-colored protein called myoglobin. This protein, found only in muscle, binds oxygen molecules that diffuse into muscle fibers from interstitial fluid. Myoglobin releases oxygen when it is needed by the mitochondria for ATP production. The mitochondria lie in rows throughout the muscle fiber, strategically close to the contractile muscle proteins that use ATP during contraction so that ATP can be produced quickly as needed.
At high magnification, the sarcoplasm appears stuffed with little threads. These small structures are the myofibrils, the contractile organelles of skeletal muscle. Their prominent striations make the entire skeletal muscle fiber appear striped (striated). A fluid-filled system of membranous sacs called the sarcoplasmic reticulum (SR) encircles each myofibril. This elaborate system is similar to smooth endoplasmic reticulum in non-muscular cells. Dilated end sacs of the sarcoplasmic reticulum called terminal cisterns butt against the T tubule from both sides. A transverse tubule
and the two terminal cisterns on either side of it form a triad. In a relaxed muscle fiber, the sarcoplasmic reticulum stores calcium ions (Ca 2+). Release of Ca2+ from the terminal cisterns of the sarcoplasmic reticulum triggers muscle contraction.
Distinguish thick filaments from thin filaments and describe the parts of a sarcomere.
Within myofibrils are smaller protein structures called filaments or myofilaments. Thin filaments are 8 nm in diameter and 1–2 μm long and composed of the protein actin, while thick filaments are 16 nm in diameter and 1–2 μm long and composed of the protein myosin. The filaments inside a myofibril do not extend the entire length of a muscle fiber. Instead, they are arranged in compartments called sarcomeres, which are the basic functional units of a myofibril. Narrow, plate-shaped regions of dense protein material called Z discs separate one sarcomere from the next. The components of a sarcomere are organized into a variety of bands and zones. The darker middle part of the sarcomere is the A band, which extends the entire length of the thick filaments. Toward each end of the A band is a zone of overlap, where the thick and thin filaments lie side by side. The I band is a lighter, less dense area that contains the rest of the thin filaments but no thick filaments. A narrow H zone in the center of each A band contains thick but not thin filaments. Supporting proteins that hold the thick filaments together at the center of the H zone form the M line, so named because it is at the middle of the sarcomere.
Describe the functions of skeletal muscle proteins.
Myofibrils are built from three kinds of proteins: (1) contractile proteins, which generate force during contraction; (2) regulatory proteins, which help switch the contraction process on and off; and (3) structural proteins, which keep the thick and thin filaments in the proper alignment, give the myofibril elasticity and extensibility, and link the myofibrils to the sarcolemma and extracellular matrix.
The two contractile proteins in muscle are myosin and actin, components of thick and thin filaments, respectively.
Smaller amounts of two regulatory proteins – tropomyosin and troponin – are also part of the thin filament, covering the myosin-binding cites on actin until Ca2+ (calcium ions) cause the troponin to change shape and move the tropomyosin away from the cites.
Besides contractile and regulatory proteins, muscle contains about a dozen structural proteins, which contribute to the alignment, stability, elasticity, and extensibility of myofibrils. Several key structural proteins are titin, α-actinin, myomesin, nebulin, and dystrophin. Titin, named for its large size, is the third most plentiful protein in skeletal muscle (after actin and myosin).
Outline the steps involved in the sliding filament mechanism of muscle contraction.
Skeletal muscle shortens during contraction because the thick and thin filaments slide past one another. The model describing this process is known as the sliding filament mechanism. Myosin heads attach to and “walk” along the thin filaments at both ends of a sarcomere, progressively pulling the thin filaments toward the M line. As a result, the thin filaments slide inward and meet at the center of a sarcomere. They may even move so far inward that their ends overlap. As the thin filaments slide inward, the I band and H zone narrow and eventually disappear altogether when the muscle is maximally contracted. At the onset of contraction, the sarcoplasmic reticulum releases calcium ions (Ca 2+) into the sarcoplasm. There, they bind to troponin. Troponin then moves tropomyosin away from the myosin-binding sites on actin. Once the binding sites are “free,” the contraction cycle - the repeating sequence of events that causes the filaments to slide - begins. The contraction cycle consists of four steps:
(1). ATP hydrolysis. A myosin head includes an ATP-binding site that functions as an ATPase - an enzyme that hydrolyzes ATP into ADP (adenosine diphosphate) and a phosphate group. The energy generated from this hydrolysis reaction is stored in the myosin head for later use during the contraction cycle. The myosin head is said to be energized when it contains stored energy.
(2). Attachment of myosin to actin. The energized myosin head attaches to the myosin-binding site on actin and releases the previously hydrolyzed phosphate group. When a myosin head attaches to actin during the contraction cycle, the myosin head is referred to as a cross-bridge.
(3). Power stroke. After a cross-bridge forms, the myosin head pivots, changing its position from a 90° angle to a 45° angle relative to the thick and thin filaments. As the myosin head changes to its new position, it pulls the thin filament past the thick filament toward the center of the sarcomere, generating tension (force) in the process. This event is known as the power stroke.
(4). Detachment of myosin from actin. At the end of the power stroke, the cross-bridge remains firmly attached to actin until it binds another molecule of ATP. As ATP binds to the ATP-binding site on the myosin head, the myosin head detaches from actin.
Describe how muscle action potentials arise at the neuromuscular junction.
Muscle action potentials arise at the neuromuscular junction (NMJ), the synapse between a somatic motor neuron and a skeletal muscle fiber. At the NMJ, the end of the motor neuron, called the axon terminal, divides into a cluster of synaptic end bulbs, the neural part of the NMJ. Suspended in the cytosol within each synaptic end bulb are hundreds of membrane-enclosed sacs called synaptic vesicles. Inside each synaptic vesicle are thousands of molecules of acetylcholine (ACh), the neurotransmitter released at the NMJ. The region of the sarcolemma opposite the synaptic end bulbs, called the motor end plate is the muscular part of the NMJ. Within each motor end plate are 30 million to 40 million acetylcholine receptors. A nerve impulse (nerve action potential) elicits a muscle action potential in the following way:
(1). Release of acetylcholine. Arrival of the nerve impulse at the synaptic end bulbs stimulates voltage-gated channels to open, liberating ACh into the synaptic cleft.
(2). Activation of ACh receptors. Binding of two molecules of ACh to the receptor on the motor end plate opens an ion channel in the Ach receptor. Once the channel is open, small cations, most importantly Na+, can flow across the membrane.
(3). Production of muscle action potential. The inflow of Na+ (down its electrochemical gradient) makes the inside of the muscle fiber more positively charged. This change in the membrane potential triggers a muscle action potential. The muscle action potential then propagates along the sarcolemma into the system of T tubules. This causes the sarcoplasmic reticulum to release its stored Ca2+ into the sarcoplasm, and the muscle fiber subsequently contracts.
(3.) Termination of ACh activity. The effect of ACh binding lasts only briefly because ACh is rapidly broken down by an enzyme called acetylcholinesterase (AChE).
The sequence of events that links excitation (a muscle action potential) to contraction (sliding of the filaments) is referred to as excitation–contraction coupling.
Describe the 3 reactions by which muscle fibers produce ATP.
(1). Creatine Phosphate. While muscle fibers are relaxed, they produce more ATP than they need for resting metabolism. Most of the excess ATP is used to synthesize creatine phosphate, an energy-rich molecule that is found in muscle fibers. The enzyme creatine kinase (CK) catalyzes the transfer of one of the high-energy phosphate groups from ATP to creatine, forming creatine phosphate and ADP. When contraction begins and the ADP level starts to rise, CK catalyzes the transfer of a high-energy phosphate group from creatine phosphate back to ADP. This direct phosphorylation reaction quickly generates new ATP molecules.
(2). Anaerobic Glycolysis. When muscle activity continues and the supply of creatine phosphate within the muscle fiber is depleted, glucose is catabolized to generate ATP. A series of reactions known as glycolysis quickly breaks down each glucose molecule into two molecules of pyruvic acid. Glycolysis occurs in the cytosol and produces a net gain of two molecules of ATP. Glycolysis can occur whether oxygen is present (aerobic conditions) or absent (anaerobic conditions). Ordinarily, the pyruvic acid formed by glycolysis in the cytosol enters mitochondria, where it undergoes a series of oxygen-requiring reactions called aerobic respiration that produce a large amount of ATP. During heavy exercise, however, not enough oxygen is available to skeletal muscle fibers. Under these anaerobic conditions, the pyruvic acid generated from glycolysis is converted to lactic acid. The entire process by which the breakdown of glucose gives rise to lactic acid when oxygen is absent or at a low concentration is referred to as anaerobic glycolysis. Each molecule of glucose catabolized via anaerobic glycolysis yields 2 molecules of lactic acid and 2 molecules of ATP. Anaerobic glycolysis provides enough energy for about 2 minutes of maximal muscle activity.
(3). Aerobic Respiration. If sufficient oxygen is present, the pyruvic acid formed by glycolysis enters the mitochondria, where it undergoes aerobic respiration, a series of oxygen-requiring reactions (the Krebs cycle and the electron transport chain) that produce ATP, carbon dioxide, water, and heat. Muscular tissue has two sources of oxygen: (1) oxygen that diffuses into muscle fibers from the blood and (2) oxygen released by myoglobin within muscle fibers. They bind oxygen when it is plentiful and release oxygen when it is scarce. Aerobic respiration supplies enough ATP for muscles during periods of rest or light to moderate exercise of several minutes to an hour or more, provided sufficient oxygen and nutrients are available. Although aerobic respiration is slower than anaerobic glycolysis, it yields much more ATP. Each molecule of glucose catabolized under aerobic conditions yields about 30 or 32 molecules of ATP.
Distinguish between anaerobic glycolysis and aerobic respiration.
Aerobic respiration requires oxygen and anaerobic glycolysis does not. Aerobic respiration produces 30 or 32 molecules of ATP, as well as carbon dioxide, water and heat. Anaerobic glycolysis produces only 2 molecules of ATP as well as lactic acid.
Describe the factors that contribute to muscle fatigue.
The inability of a muscle to maintain force of contraction after prolonged activity is called muscle fatigue. Although the precise mechanisms that cause muscle fatigue are still not clear, several factors are thought to contribute. One is inadequate release of calcium ions from the SR, resulting in a decline of Ca2+ concentration in the sarcoplasm. Depletion of creatine phosphate also is associated with fatigue, but surprisingly, the ATP levels in fatigued muscle often are not much lower than those in resting muscle. Other factors that contribute to muscle fatigue include insufficient oxygen, depletion of glycogen and other nutrients, buildup of lactic acid and ADP, and failure of action potentials in the motor neuron to release enough acetylcholine.
The term oxygen debt has been used to refer to the added oxygen, over and above the resting oxygen consumption, that is taken into the body after exercise. This extra oxygen is used to “pay back” or restore metabolic conditions to the resting level in three ways: (1) to convert lactic acid back into glycogen stores in the liver, (2) to resynthesize creatine phosphate and ATP in muscle fibers, and (3) to replace the oxygen removed from myoglobin. Oxygen use after exercise also is boosted by ongoing changes. First, the elevated body temperature after strenuous exercise increases the rate of chemical reactions throughout the body. Faster reactions use ATP more rapidly, and more oxygen is needed to produce the ATP. Second, the heart and the muscles used in breathing are still working harder than they were at rest, and thus they consume more ATP. Third, tissue repair processes are occurring at an increased pace. For these reasons, recovery oxygen uptake is a better term than oxygen debt for the elevated use of oxygen after exercise.
Describe the structure and function of a motor unit and define motor unit recruitment.
Even though each skeletal muscle fiber has only a single neuromuscular junction, the axon of a somatic motor neuron branches out and forms neuromuscular junctions with many different muscle fibers. A motor unit consists of a somatic motor neuron plus all of the skeletal muscle fibers it stimulates.
The total force or tension that a single muscle fiber can produce depends mainly on the rate at which nerve impulses arrive at the neuromuscular junction. The number of impulses per second is the frequency of stimulation. Maximum tension is also affected by the amount of stretch before contraction and by nutrient and oxygen availability. The total tension a whole muscle can produce depends on the number of muscle fibers that are contracting in unison.
Whole muscles that control precise movements consist of many small motor units. For instance, muscles of the larynx (voice box) that control voice production have as few as two or three muscle fibers per motor unit, and muscles controlling eye movements may have 10 to 20 muscle fibers per motor unit. In contrast, skeletal muscles responsible for large-scale and powerful movements, such as the biceps brachii muscle in the arm and the gastrocnemius muscle in the calf of the leg, have as many as 2000 to 3000 muscle fibers in some motor units. Because all of the muscle fibers of a motor unit contract and relax together, the total strength of a contraction depends, in part, on the size of the motor units and the number that are activated at a given time.
The process in which the number of active motor units increases is called motor unit recruitment. Typically, the different motor units of an entire muscle are not stimulated to contract in unison. While some motor units are contracting, others are relaxed. This pattern of motor unit activity delays muscle fatigue and allows contraction of a whole muscle to be sustained for long periods.
Explain the 3 phases of a twitch contraction.
A twitch contraction is the brief contraction of all muscle fibers in a motor unit in response to a single action potential in its motor neuron. A brief delay occurs between application of the stimulus and the beginning of contraction. The delay, which lasts about 2 msec, is termed the latent period. During the latent period, the muscle action potential sweeps over the sarcolemma and calcium ions are released from the sarcoplasmic reticulum. The second phase, the contraction period, lasts 10–100 msec. During this time, Ca 2+ binds to troponin, myosin-binding sites on actin are exposed, and cross-bridges form. Peak tension develops in the muscle fiber. During the third phase, the relaxation period, also lasting 10–100 msec, Ca2+ is actively transported back into the sarcoplasmic reticulum, myosin-binding sites are covered by tropomyosin, myosin heads detach from actin, and tension in the muscle fiber decreases. The actual duration of these periods depends on the type of skeletal muscle fiber. Some fibers, such as the fast-twitch fibers that move the eyes (described shortly), have contraction periods as brief as 10 msec and equally brief relaxation periods. Others, such as the slow-twitch fibers that move the legs, have contraction and relaxation periods of about 100 msec each.
If two stimuli are applied, one immediately after the other, the muscle will respond to the first stimulus but not to the second. When a muscle fiber receives enough stimulation to contract, it temporarily loses its excitability and cannot respond for a time. The period of lost excitability, called the refractory period, is a characteristic of all muscle and nerve cells. The duration of the refractory period varies with the muscle involved. Skeletal muscle has a short refractory period of about 1 msec; cardiac muscle has a longer refractory period of about 250 msec.
Describe how frequency of stimulation affects muscle tension, and how muscle tone is produced.
When a second stimulus occurs after the refractory period of the first stimulus is over, but before the skeletal muscle fiber has relaxed, the second contraction will actually be stronger than the first. This phenomenon, in which stimuli arriving at different times cause larger contractions, is called wave summation. When a skeletal muscle fiber is stimulated at a rate of 20 to 30 times per second, it can only partially relax between stimuli. The result is a sustained but wavering contraction called unfused (incomplete) tetanus. When a skeletal muscle fiber is stimulated at a higher rate of 80 to 100 times per second, it does not relax at all. The result is fused (complete) tetanus, a sustained contraction in which individual twitches cannot be detected.
Wave summation and both kinds of tetanus occur when additional Ca 2+ is released from the sarcoplasmic reticulum by subsequent stimuli while the levels of Ca 2+ in the sarcoplasm are still elevated from the first stimulus. Because of the buildup in the Ca2+ level, the peak tension generated during fused tetanus is 5 to 10 times larger than the peak tension produced during a single twitch.
Even at rest, a skeletal muscle exhibits muscle tone, a small amount of tautness or tension in the muscle due to weak, involuntary contractions of its motor units. This is why your head stays upright, and why the walls of the gastrointestinal tract maintain constant pressure on their contents.
Distinguish between isotonic and isometric contractions.
Muscle contractions may be either isotonic or isometric. In an isotonic contraction, the tension (force of contraction) developed in the muscle remains almost constant
while the muscle changes its length. Isotonic contractions are used for body movements and for moving objects. The two types of isotonic contractions are concentric and eccentric. In a concentric contraction the muscle shortens and in an eccentric contraction the muscle lengthens. In an isometric contraction, the tension generated is not enough to exceed the resistance of the object to be moved, and the muscle does not change its length. Think of holding a book at arm’s length as opposed to picking up the book (concentric contraction) and putting down the book (eccentric contraction).
Compare the structure and function of the three types of skeletal muscle fibers.
Skeletal muscle fibers are not all alike in composition and function. For example, muscle fibers vary in their content of myoglobin, the red-colored protein that binds oxygen in muscle fibers. Skeletal muscle fibers that have a high myoglobin content are termed red muscle fibers and appear darker; those that have a low content of myoglobin are called white muscle fibers and appear lighter. Red muscle fibers also contain more mitochondria and are supplied by more blood capillaries. Skeletal muscle fibers also contract and relax at different speeds, and vary in which metabolic reactions they use to generate ATP and in how quickly they fatigue. For example, a fiber is categorized as either slow or fast depending on how rapidly the ATPase in its myosin heads hydrolyzes ATP. Based on all these structural and functional characteristics, skeletal muscle fibers are classified into three main types: (1) slow oxidative fibers, (2) fast oxidative–glycolytic fibers, and (3) fast glycolytic fibers.
Slow oxidative (SO) fibers appear dark red because they contain large amounts of myoglobin and many blood capillaries. Because they have many large mitochondria, SO fibers generate ATP mainly by aerobic respiration, which is why they are called oxidative fibers. These fibers are said to be “slow” because the ATPase in the myosin heads hydrolyzes ATP relatively slowly, and the contraction cycle proceeds at a slower pace than in “fast” fibers. As a result, SO fibers have a slow speed of contraction.
Fast oxidative–glycolytic (FOG) fibers are typically the largest fibers. Like slow oxidative fibers, they contain large amounts of myoglobin and many blood capillaries. Thus, they also have a dark red appearance. FOG fibers can generate considerable ATP by aerobic respiration, which gives them a moderately high resistance to fatigue. Because their intracellular glycogen level is high, they also generate ATP by anaerobic glycolysis. FOG fibers are “fast” because the ATPase in their myosin heads hydrolyzes ATP three to five times faster than the myosin ATPase in SO fibers.
Fast glycolytic (FG) fibers have low myoglobin content, relatively few blood capillaries, and few mitochondria, and appear white in color. They contain large amounts of glycogen and generate ATP mainly by glycolysis. Due to their ability to hydrolyze ATP rapidly, FG fibers contract strongly and quickly. These fast-twitch fibers are adapted for intense anaerobic movements of short duration, such as weight-lifting or throwing a ball, but they fatigue quickly.
Describe the main structural and functional characteristics of cardiac muscle tissue.
The principal tissue in the heart wall is cardiac muscle tissue. Cardiac muscle fibers have the same arrangement of actin and myosin and the same bands, zones, and Z discs as skeletal muscle fibers. However, intercalated discs are unique to cardiac muscle fibers. These microscopic structures are irregular transverse thickenings of the sarcolemma that connect the ends of cardiac muscle fibers to one another. The discs contain desmosomes, which hold the fibers together, and gap junctions, which allow muscle action potentials to spread from one cardiac muscle fiber to another. Cardiac muscle tissue has an endomysium and perimysium, but lacks an epimysium.
In response to a single action potential, cardiac muscle tissue remains contracted 10 to 15 times longer than skeletal muscle tissue. The long contraction is due to prolonged delivery of Ca 2+ into the sarcoplasm.
Describe the main structural and functional characteristics of smooth muscle tissue and the two kinds of smooth muscle tissue.
Like cardiac muscle tissue, smooth muscle tissue is usually activated involuntarily. Of the two types of smooth muscle tissue, the more common type is visceral (single-unit) smooth muscle tissue. It is found in the skin and in tubular arrangements that form part of the walls of small arteries and veins and of hollow organs such as the stomach, intestines, uterus, and urinary bladder. The second type of smooth muscle tissue, multi-unit smooth muscle tissue, consists of individual fibers, each with its own motor neuron terminals and with few gap junctions between neighboring fibers. Stimulation of one visceral muscle fiber causes contraction of many adjacent fibers, but stimulation of one multi-unit fiber causes contraction of that fiber only. Multi-unit smooth muscle tissue is found in the walls of large arteries, in air-ways to the lungs, in the arrector pili muscles that attach to hair follicles, in the muscles of the iris that adjust pupil diameter, and in the ciliary body that adjusts focus of the lens in the eye.
The sarcoplasm of smooth muscle fibers contains both thick filaments and thin filaments, but they are not arranged in orderly sarcomeres as in striated muscle. Smooth muscle fibers also contain intermediate filaments. Because the various filaments have no regular pattern of overlap, smooth muscle fibers do not exhibit striations, causing a smooth appearance. Smooth muscle fibers also lack transverse tubules and have only a small amount of sarcoplasmic reticulum for storage of Ca2+. Although there are no transverse tubules in smooth muscle tissue, there are small pouchlike invaginations of the plasma membrane called caveolae that contain extracellular Ca2+ that can be used for muscular contraction. In smooth muscle fibers, the thin filaments attach to structures called dense bodies, which are functionally similar to Z discs in striated muscle fibers.
Although the principles of contraction are similar, smooth muscle tissue exhibits some important physiological differences from cardiac and skeletal muscle tissue. Contraction in a smooth muscle fiber starts more slowly and lasts much longer than skeletal muscle fiber contraction. Another difference is that smooth muscle can both shorten and stretch to a greater extent than the other muscle types.
Several mechanisms regulate contraction and relaxation of smooth muscle cells. In one such mechanism, a regulatory protein called calmodulin binds to Ca 2+ in the sarcoplasm. (Troponin takes this role in striated muscle fibers.) After binding to Ca2+, calmodulin activates an enzyme called myosin light chain kinase. This enzyme uses ATP to add a phosphate group to a portion of the myosin head. Once the phosphate group is attached, the myosin head can bind to actin, and contraction can occur. Because myosin light chain kinase works rather slowly, it contributes to the slowness of smooth muscle contraction.
Unlike striated muscle fibers, smooth muscle fibers can stretch considerably and still maintain their contractile function. When smooth muscle fibers are stretched, they initially contract, developing increased tension. Within a minute or so, the tension decreases. This phenomenon, which is called the stress–relaxation response, allows smooth muscle to undergo great changes in length while retaining the ability to contract effectively. Thus, even though smooth muscle in the walls of blood vessels and hollow organs such as the stomach, intestines, and urinary bladder can stretch, the pressure on the contents within them changes very little.
Explain how muscle fibers regenerate.
Because mature skeletal muscle fibers have lost the ability to undergo cell division, growth of skeletal muscle after birth is due mainly to hypertrophy, the enlargement of existing cells, rather than to hyperplasia, an increase in the number of fibers. Satellite cells divide slowly and fuse with existing fibers to assist both in muscle growth and in repair of damaged fibers. Thus, skeletal muscle tissue can regenerate only to a limited extent.
New research indicates that, under certain circumstances, cardiac muscle tissue can regenerate. In addition, cardiac muscle fibers can undergo hypertrophy in response to increased workload.
Smooth muscle tissue, like skeletal and cardiac muscle tissue, can undergo hypertrophy. In addition, certain smooth muscle fibers, such as those in the uterus, retain their capacity for division and thus can grow by hyperplasia. Also, new smooth muscle fibers can arise from cells called pericytes, stem cells found in association with blood capillaries and small veins. Smooth muscle fibers can also proliferate in certain pathological conditions, such as occur in the development of atherosclerosis.
What are the three layers of connective tissue that extend from the fascia to protect and strengthen skeletal muscle?
Epimysium is the outer layer, encircling the entire muscle. It consists of dense irregular connective tissue.
Perimysium is also a layer of dense irregular connective tissue, but it surrounds groups of 10 to 100 or more muscle fibers, separating them into bundles called fascicles. Many fascicles are large enough to be seen with the naked eye. They give a cut of meat its characteristic “grain.”
Endomysium penetrates the interior of each fascicle and separates individual muscle fibers from one another. The endomysium is mostly reticular fibers.
