Chapter 10: Muscle Tissue

Transcription

1 Chapter 10: Muscle Tissue Muscle is one of the 4 primary types of tissue. It is subdivided into skeletal, cardiac and smooth muscle. I. Skeletal Muscle Tissue and the Muscular System, p. 284 Objective 1. Specify the functions of skeletal muscle tissue. Skeletal muscles are the muscles attached to the skeletal system, which allow us to move. The muscular system includes only skeletal muscles. Skeletal muscles are made up of muscle tissue (composed of muscle cells or fibers), connective tissues, nerves and blood vessels. The 5 functions of skeletal muscles are: 1. To produce skeletal movement. 2. To maintain posture and body position. 3. To support soft tissues. 4. To guard the entrances and exits of the body. 5. To maintain body temperature. II. Functional Anatomy of Skeletal Muscle, p. 284 Objectives 1. Describe the organization of muscle at the tissue level 2. Explain the unique characteristics of skeletal muscle fibers. 3. Identify the structural components of a sarcomere. Organization of Connective Tissues, p. 284 Figure 10-1 Muscles have 3 layers of connective tissues: 1. the epimysium: an exterior collagen layer connected to the deep fascia which separates the muscle from surrounding tissues. 2. the perimysium: surrounds bundles of muscles fibers called fascicles. Perimysium holds the blood vessels and nerves that supply the fascicles. 3. the endomysium: surrounds individual muscle cells (the muscle fibers), and contains the capillaries and nerve fibers that directly contact the muscle cells. Endomysium also contains satellite cells (stem cells) that repair damaged muscles. At each end of the muscle, the endomysium, perimysium and epimysium come together to form a connective tissue attachment to the bone matrix, either a tendon (a bundle) or an aponeurosis (a sheet).

2 Blood Vessels and Nerves, p. 285 Skeletal muscles are voluntary muscles, controlled by nerves from the central nervous system. An extensive vascular system supplies large amounts of oxygen to muscles, and carries away wastes. Skeletal Muscle Fibers, p. 286 Figure 10-2 Skeletal muscle cells (fibers) are very different from typical cells. The long fibers develop through the fusion of mesodermal cells (myoblasts) until they become very large and contain hundreds of nuclei. Figure 10-3 The cell membrane of a muscle cell is called the sarcolemma, which surrounds the sarcoplasm or cytoplasm of the muscle fiber. Muscle contractions begin with a change in the transmembrane potential. Because the whole muscle fiber must contract at the same time, the signal (action potential) is conducted through the cell by transverse tubules (T tubules) which have the same properties as the sarcolemma. Within each muscle fiber are hundreds of lengthwise subdivisions called myofibrils. Myofibrils are made up of bundles of the protein filaments (myofilaments) that are responsible for muscle contraction. The 2 types of myofilaments are: 1. thin filaments: made of the protein actin, and 2. thick filaments: made of the protein myosin. Sarcoplasmic Reticulum: Surrounding each myofibril is a membranous structure called the sarcoplasmic reticulum, which is involved in transmitting the action potential to the myofibril. The sarcoplasmic reticulum is similar in structure to the smooth endoplasmic reticulum, forming chambers called terminal cisternae which attach to T tubules. One T tubule and a pair of terminal cisternae are called a triad. Ion pumps concentrate calcium ions (Ca++) in the cisternae. The calcium ions are released into the contractile units of the muscle (sarcomeres) at the beginning of a muscle contraction. Figure 10-4 Sarcomeres (the contractile units of muscle) are structural units of myofibrils resulting from the organization or pattern of thick and thin filaments within the

3 myofibril. Skeletal muscles appear striped or striated because of the arrangement of alternating dark, thick filaments (A bands) and light, thin filaments (I bands) within their myofibrils. The center of the A band is the midline or M line of the sarcomere. The centers of the I bands are Z lines. One sarcomere is measured from one Z line to another. Thick filaments and thin filaments overlap in the zone of overlap, which is the densest, darkest area on a light micrograph. The area around the M line, which has thick filaments but no thin filaments, is called the H zone. Strands of protein (titin) reach from the tips of the thick filaments to the Z line and stabilize the filaments. Figure 10-5 Two transverse tubules encircle each sarcomere near the 2 zones of overlap. When calcium ions are released by the sarcoplasmic reticulum, thin and thick filaments interact. Figure 10-6 (Review the functional organization of a skeletal muscle fiber.) Figure 10-7 The complex interactions of thick and thin filaments which cause muscle contraction are determined by the structures of their protein molecules. Thin filaments contain 4 proteins: 1. F actin (2 twisted rows of globular G actin. Active sites on G actin strands bind to myosin.) 2. nebulin (holds F actin strands together) 3. tropomyosin (a double strand, prevents actin-myosin interaction) 4. troponin (a globular protein, binds tropomyosin to G actin, controlled by Ca++) When a Ca++ ion binds to the receptor on a troponin molecule, the troponintropomyosin complex changes, exposing the active site of the F actin and initiating contraction. Thick Filaments contain twisted myosin subunits. The tail binds to other myosin molecules. The free head, made of 2 globular protein subunits, reaches out to the nearest thin filament.

4 During a contraction, myosin heads interact with actin filaments to form crossbridges. The myosin head pivots, producing motion. Thick filaments contain titin strands that recoil after stretching. Sliding Filaments and Muscle Contraction, p. 291 Figure 10-8 In skeletal muscle contraction, the thin filaments of the sarcomere slide toward the M line, in between the thick filaments. This is called the sliding filament theory. The width of the A zone stays the same, but the Z lines move closer together. III. The Contraction of Skeletal Muscle, p. 292 Objectives 1. Identify the components of the neuromuscular junction, and summarize the events involved in the neural control of skeletal muscles. 2. Explain the key steps involved in the contraction of a skeletal muscle fiber. Figure 10-9 Muscle fiber contraction is initiated by neural stimulation of a sarcolemma, causing excitation-contraction coupling. The cisternae of the sarcoplasmic reticulum release calcium ions, which trigger the interaction of thick and thin filaments, consuming ATP and producing a pulling force called tension. We will now look at each stage of skeletal muscle contraction in detail. The Control of Skeletal Muscle Activity, p. 293 Figure Neural stimulation occurs at the neuromuscular junction (NMJ). The electrical signal or action potential travels along the nerve axon and ends at a synaptic terminal which releases a chemical neurotransmitter called acetylcholine (ACh). ACh travels across a short gap called the synaptic cleft and binds to membrane receptors on the sarcolemma called the motor end plate, causing sodium ions to rush into the sarcoplasm. An enzyme in the sarcolemma (acetylcholinesterase or AChE) then breaks down the ACh. The increase in sodium ions generates an action potential in the sarcolemma which travels along the T tubules, leading to the excitation-contraction coupling. Excitation - Contraction Coupling, p. 295

5 Figure When the action potential reaches a triad, calcium ions are released, triggering contraction. This step requires the myosin heads to have previously broken down ATP and stored the potential energy in the cocked position. Figure The Contraction Cycle has 5 steps: 1. Exposure of active sites 2. Formation of cross-bridges 3. Pivoting of myosin heads 4. Detachment of cross-bridges 5. Reactivation of myosin Figure As the sarcomeres shorten, the muscle pulls together, producing tension that moves whatever it is attached to. Relaxation, p. 298 Since AChE quickly breaks down ACh, the duration of a contraction depends on: 1. the duration of the neural stimulus 2. the number of free calcium ions in the sarcoplasm 3. the availability of ATP As calcium ion concentrations in the sarcoplasm fall, calcium ions detach from troponin, and the active sites are recovered by tropomyosin. The sarcomeres will remain in the contracted state unless an outside force returns them to their stretched position. Upon death, ion pumps cease to function and calcium builds up in the sarcoplasm, causing a fixed muscular contraction called rigor mortis. Table 10-1: A review of muscle contraction from ACh release to the end of contraction. Key Skeletal muscle fibers shorten as thin filaments interact with thick filaments and sliding occurs. The trigger for contraction is the appearance of free calcium ions in the sarcoplasm; the calcium ions are released by the sarcoplasmic reticulum when the muscle fiber is stimulated by the associated motor neuron. Contraction is an active process; relaxation and return to resting length is entirely passive. IV. Tension Production, p. 300

6 Objectives 1. Describe the mechanism responsible for tension production in a muscle fiber, and discuss the factors that determine the peak tension developed during a contraction. 2. Discuss the factors that affect peak tension production during the contraction of an entire skeletal muscle, and explain the significance of the motor unit in this process. 3. Compare the different types of muscle contractions. Tension Production by Muscle Fibers, p. 300 As a whole, a muscle fiber is either contracted or relaxed (the all-or-none principal). The tension produced by the contraction of an individual muscle fiber can vary, depending on the number of pivoting cross-bridges; the fiber s resting length at the time of stimulation, and the frequency of stimulation. Figure Length-Tension Relationships: The number of pivoting cross bridges depends on the amount of overlap between thick and thin fibers. There is an optimum amount of overlap to produce the greatest amount of tension; too much or too little overlap reduces efficiency. The normal range of resting sarcomere length is 75 to The Frequency of Stimulation: A single neural stimulation produces a single contraction or twitch which lasts about milliseconds. Sustained muscular contractions require many repeated stimuli. Figure The length of a twitch depends on the type of muscle. A graph of twitch tension development is called a myogram. Twitches are divided into 3 phases: 1. The latent period before contraction. The action potential moves through the sarcolemma, causing calcium ions to be released. 2. The contraction phase: Calcium ions bind to troponin, tension builds to a peak. 3. The relaxation phase: Calcium levels fall, active sites are covered, and tension falls to resting levels. Figure 10-16a Repeated stimulations immediately after the relaxation phase (stimulus frequency < 50 per second) causes a series of contractions with increasing tension. This stair-step type increase in twitch tension is called treppe. Figure 10-16b

7 Repeated stimulations before the end of the relaxation phase (stimulus frequency > 50 per second) causes increasing tension called a summation of twitches (or wave summation). Figure 10-16c If rapid stimulation continues and the muscle is not allowed to relax, the twitches will reach a maximum level of tension called incomplete tetanus. Figure 10-16d If stimulation frequency is so high that the muscle never begins a relaxation phase, the muscle reaches complete tetanus, or continuous contraction. Tension Production by Skeletal Muscles, p.304 Skeletal muscle motion results from the coordinated action of many fibers in a muscle. Figure The amount of tension a whole muscle can produce depends on: 1. The internal tension produced by the muscle fibers 2. The external tension the muscle fibers exert on their elastic extracellular fibers (series elastic elements such as tendons) 3. The total number of muscle fibers stimulated Figure A single motor neuron can control hundreds of muscle fibers (a motor unit) that contract at the same time. In a whole muscle or group of muscles, smooth motion and increasing tension are produced by slowly increasing the size or number of motor units stimulated. This is called recruitment or multiple motor unit summation. Maximum tension is achieved when all motor units reach tetanus, but this can only be sustained for a very short time. Sustained tension is less than maximum tension, allowing some motor units to rest in rotation. Key All voluntary muscle contractions and intentional movements involve the sustained, tetanic contractions of skeletal muscle fibers. The force exerted can be increased by increasing the number of stimulated motor units (recruitment). The normal tension and firmness of a muscle at rest is called muscle tone. Though not producing motion, some muscle units are always actively maintaining body position. Increasing muscle tone leads to more active muscle fibers, which increases the metabolic energy used, even at rest.

8 Figure There are 2 basic patterns of muscle tension: isotonic contraction and isometric contraction. In isotonic contraction, the muscle changes length, resulting in motion. If muscle tension exceeds the resistance, the skeletal muscle shortens (concentric contraction). If muscle tension is less than the resistance, the muscle lengthens (eccentric contraction). In isometric contraction, the muscle is prevented from changing length, even though tension is developed. Figure Resistance and speed of contraction are inversely related. The heavier the resistance on a muscle, the longer it will take for the muscle to begin to shorten, and the less the muscle will shorten. Muscle Relaxation and Return to Resting Length: After a contraction, a muscle fiber returns to its original length by a combination of elastic forces, opposing muscle contractions and gravity. 1. Elastic forces are the pull of the elastic elements returning to normal length. 2. Opposing muscle contractions reverse the direction of the original motion, the work of opposing muscle pairs. 3. Gravity can take the place of opposing muscle contraction to return a muscle to its resting state. V. Energy Use and Muscular Activity, p. 309 Objectives 1. Describe the mechanisms by which muscle fibers obtain the energy to power contractions. 2. Describe the factors that contribute to muscle fatigue, and discuss the stages and mechanisms involved in the muscle s subsequent recovery. It takes a lot of energy, in the form of ATP, to sustain muscle contraction. Muscles store enough energy to get the contraction started; the rest of the ATP must be manufactured by the muscle fiber as it is needed. ATP and CP Reserves, p. 309 ATP is the active energy molecule. If a resting muscle has more ATP than it needs, it transfers the excess energy to a storage molecule called creatine phosphate (CP).

9 The energy in creatine phosphate is used to recharge ADP to ATP (using the enzyme creatine phosphokinase or CPK). When the CP is used up, other mechanisms generate ATP. Table 10-2 compares sources of stored energy in muscle fiber. ATP Generation, p. 310 As we learned in Chapter 3, cells produce ATP in 2 ways: 1. Aerobic metabolism of fatty acids in the mitochondria: - the primary energy source of resting muscles - 34 ATP molecules produced per glucose molecule 2. Anaerobic glycolysis in the cytoplasm: - the breakdown of glucose from glycogen - primary energy source for peak muscular activity - 2 ATP molecules produced per molecule of glucose - skeletal muscles store glycogen Energy Use and the Level of Muscular Activity, p. 311 Figure At peak levels of exertion, muscles can t get enough oxygen to support mitochondrial activity. The muscle then relies on glycolysis for ATP. The pyruvic acid produced by glycolysis, which would normally be used up by the mitochondria, starts to build up and is converted to lactic acid. Muscle Fatigue, p. 312 When muscles can no longer perform a required activity, they are fatigued. Muscle fatigue is associated with: 1. depletion of metabolic reserves 2. damage to the sarcolemma and sarcoplasmic reticulum 3. low ph (lactic acid) 4. muscle exhaustion and pain The Recovery Period, p. 312 After high levels of exertion, it can take hours or days for muscles to return to their normal condition. During the recovery period, oxygen is once again available and mitochondrial activity resumes. Lactic acid is carried by the blood stream to the liver, where it is converted back into pyruvic acid, and glucose is released to recharge the muscles glycogen reserves. This removal and recycling of lactic acid by the liver is called

10 the Cori cycle. To process excess lactic acid and normalize metabolic activities after exercise, the body uses more oxygen than usual. This elevated need for oxygen, called the oxygen debt, is responsible for heavy breathing after exercise. Key Skeletal muscles at rest metabolize fatty acids and store glycogen. During light activity, muscles can generate ATP through the anaerobic breakdown of carbohydrates, lipids or amino acids. At peak levels of activity, most of the energy is provided by anaerobic reactions that generate lactic acid as a byproduct. Heat Production and Loss: The more active muscles are, the more heat they produce. During strenuous exercise, up to 70 percent of the energy produced can be lost as heat, raising body temperature. Hormones and Muscle Metabolism, p. 313 Many hormones of the endocrine system affect muscle metabolism, including growth hormone, testosterone, thyroid hormones, and epinephrine. VI. Muscle Performance, p. 313 Objectives 1. Relate the types of muscle fibers to muscle performance. 2. Distinguish between aerobic and anaerobic endurance, and explain their implications for muscular performance. Muscle performance is measured by the maximum amount of tension produced (power) and the amount of time the activity can be sustained (endurance). Power and endurance depend on the types of muscle fibers and physical conditioning. Types of Skeletal Muscle Fibers, p. 313 There are 3 major types of skeletal muscle fibers: 1. Fast Fibers: - contract very quickly - have large diameter, large glycogen reserves, and few mitochondria - have strong contractions, fatigue quickly 2. Slow Fibers: - are slow to contract, slow to fatigue - have small diameter, more mitochondria - have high oxygen supply - contain myoglobin (a red pigment that binds oxygen)

11 3. Intermediate Fibers: - are mid-sized - have low myoglobin - have more capillaries than fast fiber, are slower to fatigue Table 10-3 compares the properties of the 3 types of skeletal muscle fibers. Muscle Performance and the Distribution of Muscle Fibers, p. 315 Different muscles have different percentages of fast, slow and intermediate fibers. Muscles with mostly fast fibers are pale (white muscle) like chicken breast. Muscles with mostly slow fibers are dark (red muscle) like chicken legs. Most human muscles have mixed fibers and are pink. Muscle Hypertrophy and Atrophy, p. 315 Hypertrophy: Extensive training can cause muscles to grow by increasing the diameter of the muscle fibers, which increases the number of myofibrils, mitochondria and glycogen reserves. Atrophy: Lack of muscle activity causes reduction in muscle size, tone and power. Physical Conditioning, p. 316 Physical conditioning and training improve both power and endurance. Anaerobic endurance: Anaerobic activities (e.g. 50 meter dash or weightlifting) use fast fibers, which fatigue within about 2 minutes of strenuous activity. Frequent, brief, intensive workouts stimulate muscle hypertrophy, which improves anaerobic endurance. Aerobic endurance (prolonged aerobic activity) is supported by mitochondrial activity, requiring oxygen and nutrients provided by circulating blood. Improvements in aerobic endurance result from: 1. repetitive training to alter the neural responses of fast fibers 2. cardiovascular training Key What you don t use, you loose. Muscle tone is an indication of the background level of activity in the motor units in skeletal muscles. When inactive for days or weeks, muscles become flaccid. The muscle fibers break down their contractile proteins and become smaller and weaker. If inactive for long periods of time, muscle fibers may be replaced by fibrous tissue.

12 VII. Cardiac Muscle Tissue, p. 317 Objective 1. Identify the structural and functional differences between skeletal muscle fibers and cardiac muscle cells. Structural Characteristics of Cardiac Tissue, p. 317 Figure Cardiac muscle is a striated muscle tissue found only in the heart. Unlike skeletal muscle fibers, cardiac muscle cells (cardiocytes): 1. are small 2. have a single nucleus 3. have short, wide T tubules and no triads 4. have SR with no terminal cisternae 5. are aerobic (high in myoglobin and mitochondria) 6. have specialized contact points called intercalated discs Intercalated discs join the cell membranes of adjacent cardiocytes with gap junctions and desmosomes. They maintain structure and enhance molecular and electrical connections. Action potentials travel easily across intercalated discs. Because heart cells are mechanically, chemically and electrically linked, the heart functions like a single, fused mass of cells. Functional Characteristics of Cardiac Tissue, p. 318 The 4 special functions of cardiac muscle tissue are: 1. automaticity (contraction without neural stimulation, controlled by pacemaker cells) 2. variable contraction tension controlled by the nervous system 3. extended contraction time 4. prevention of wave summation and tetanic contractions by cell membranes VII. Smooth Muscle Tissue, p. 318 Objectives 1. Identify the structural and functional differences between skeletal muscle fibers and smooth muscle cells. 2. Discuss the role that smooth muscle plays in systems throughout the body. Smooth muscle is a nonstriated tissue which forms around other tissues in almost every organ system. - In all systems, smooth muscle in blood vessels regulates blood pressure and

13 flow. - In digestive and urinary systems, smooth muscle forms sphincters and produces contractions. - Smooth muscle also produces movements in the reproductive and glandular systems. - In the integumentary system, goose bumps are caused by arrector pili muscles of hair follicles. Structural Characteristics of Smooth Muscle Tissue, p. 319 Figure 10-24b The internal organization of actin and myosin in smooth muscle is different from that in the striated muscles. Smooth muscle cells: 1. are long and slender 2. are spindle shaped, with a single, central nucleus 3. have no T tubules, myofibrils or sarcomeres 4. have scattered myosin fibers, with more heads per thick filament 5. have thin filaments attached to dense bodies 6. transmit contractile force from cell to cell through dense bodies 7. have no tendons or aponeuroses Functional Characteristics of Smooth Muscle Tissue, p. 320 Smooth muscle functions differently than striated muscles in several ways: 1. Excitation-Contraction Coupling: Free calcium ions in the cytoplasm trigger smooth muscle contraction. In the sarcoplasm, calcium ions bind with the protein calmodulin, which activates the enzyme myosin light chain kinase, which breaks down ATP and initiates the contraction. 2. Length-Tension Relationships: Thick and thin filaments are scattered, so resting length is not related to tension development. The ability of smooth muscle to function over a wide range of lengths is called plasticity. 3. Control of Contractions: Smooth muscle cells are subdivided into multiunit smooth muscle cells, which are connected to motor neurons, and visceral smooth muscle cells, which are not. Visceral smooth muscle networks generally have rhythmic cycles of activity controlled by pacesetter cells. 4. Smooth Muscle Tone: Smooth muscles maintain normal levels of activity, which can be modified by neural, hormonal or chemical factors.

14 Table 10-4 compares the characteristics of skeletal, cardiac and smooth muscle tissues. SUMMARY In Chapter 10 we learned: the 3 types of muscle tissue (skeletal, cardiac and smooth) the functions of skeletal muscles the structure of skeletal muscle cells (endomysium, perimysium, epimysium) the functional anatomy of the skeletal muscle fiber (actin and myosin) the nervous control of skeletal muscle fibers (neuromuscular junctions and action potentials) tension production in skeletal muscle fibers (twitch, treppe and tetanus) tension production by skeletal muscles (motor units and contractions) skeletal muscle activity and energy (ATP, CP, aerobic and anaerobic energy) skeletal muscle fatigue and recovery the 3 types of skeletal muscle fibers (fast, slow, intermediate) skeletal muscle performance (white and red muscles, physical conditioning) the structure and function of cardiac muscle tissue the structure and function of smooth muscle tissue