Chapter 10 Muscle Tissue Lecture Presentation by Lee

An Introduction to Muscle Tissue • A primary tissue type, divided into: • Skeletal

10 -1 Functions of Skeletal Muscle Tissue • Skeletal Muscles • Are attached to the skeletal system • Allow us to move • The muscular system • Includes only skeletal muscles © 2015 Pearson Education, Inc.

10 -1 Functions of Skeletal Muscle Tissue • Six Functions of Skeletal Muscle Tissue 1. 2. 3. 4. 5. 6. Produce skeletal movement Maintain posture and body position Support soft tissues Guard entrances and exits Maintain body temperature Store nutrient reserves © 2015 Pearson Education, Inc.

10 -2 Organization of Muscle • Skeletal Muscle • • Muscle tissue (muscle cells

10 -2 Organization of Muscle • Organization of Connective Tissues • Muscles have three

10 -2 Organization of Muscle • Epimysium • Exterior collagen layer • Connected to

10 -2 Organization of Muscle • Perimysium • Surrounds muscle fiber bundles (fascicles) •

10 -2 Organization of Muscle • Endomysium • Surrounds individual muscle cells (muscle fibers) • Contains capillaries and nerve fibers contacting muscle cells • Contains myosatellite cells (stem cells) that repair damage © 2015 Pearson Education, Inc.

Figure 10 -1 The Organization of Skeletal Muscles (Part 1 of 3). Skeletal Muscle (organ) Epimysium Perimysium Endomysium Muscle fascicle Blood vessels Muscle fibers Epimysium Blood vessels and nerves Tendon Endomysium Perimysium © 2015 Pearson Education, Inc. Nerve

Figure 10 -1 The Organization of Skeletal Muscles (Part 2 of 3). Muscle Fascicle (bundle of fibers) Perimysium Muscle fiber Epimysium Blood vessels and nerves Endomysium Tendon Endomysium Perimysium © 2015 Pearson Education, Inc.

Figure 10 -1 The Organization of Skeletal Muscles (Part 3 of 3). Muscle Fiber (cell) Capillary Myofibril Sarcoplasm Epimysium Blood vessels and nerves Mitochondrion Myosatellite cell Sarcolemma Nucleus Tendon Axon of neuron Endomysium Perimysium © 2015 Pearson Education, Inc. Endomysium

10 -2 Organization of Muscle • Organization of Connective Tissues • Muscle Attachments • Endomysium, perimysium, and epimysium come together: • At ends of muscles • To form connective tissue attachment to bone matrix • i. e. , tendon (bundle) or aponeurosis (sheet) © 2015 Pearson Education, Inc.

10 -2 Organization of Muscle • Blood Vessels and Nerves • Muscles have extensive vascular systems that: • Supply large amounts of oxygen • Supply nutrients • Carry away wastes • Skeletal muscles are voluntary muscles, controlled by nerves of the central nervous system (brain and spinal cord) © 2015 Pearson Education, Inc.

10 -3 Characteristics of Skeletal Muscle Fibers • Skeletal Muscle Cells • Are very long • Develop through fusion of mesodermal cells (myoblasts) • Become very large • Contain hundreds of nuclei © 2015 Pearson Education, Inc.

Figure 10 -2 The Formation of a Multinucleate Skeletal Muscle Fiber. Muscle fibers develop through the fusion of embryonic cells called myoblasts. Myoblasts a A muscle fiber forms by the fusion of myoblasts. Muscle fiber LM × 612 Sarcolemma Striations Nuclei Myofibrils Myosatellite cell Nuclei Immature muscle fiber Mitochondria b A diagrammatic view and a Myosatellite cell micrograph of one muscle fiber. Up to 30 cm in length Mature muscle fiber © 2015 Pearson Education, Inc.

Figure 10 -2 a The Formation of a Multinucleate Skeletal Muscle Fiber. Muscle fibers develop through the fusion of embryonic cells called myoblasts. Myoblasts a A muscle fiber forms by the fusion of myoblasts. Myosatellite cell Nuclei Immature muscle fiber Myosatellite cell Up to 30 cm in length Mature muscle fiber © 2015 Pearson Education, Inc.

Figure 10 -2 b The Formation of a Multinucleate Skeletal Muscle Fiber. Muscle fiber LM × 612 Sarcolemma Striations Nuclei Myofibrils Mitochondria b A diagrammatic view and a © 2015 Pearson Education, Inc. micrograph of one muscle fiber.

10 -3 Characteristics of Skeletal Muscle Fibers • The Sarcolemma and Transverse Tubules • The sarcolemma • The cell membrane of a muscle fiber (cell) • Surrounds the sarcoplasm (cytoplasm of muscle fiber) • A change in transmembrane potential begins contractions © 2015 Pearson Education, Inc.

10 -3 Characteristics of Skeletal Muscle Fibers • The Sarcolemma and Transverse Tubules • Transverse tubules (T tubules) • Transmit action potential through cell • Allow entire muscle fiber to contract simultaneously • Have same properties as sarcolemma © 2015 Pearson Education, Inc.

10 -3 Characteristics of Skeletal Muscle Fibers • Myofibrils • Lengthwise subdivisions within muscle fiber • Made up of bundles of protein filaments (myofilaments) • Myofilaments are responsible for muscle contraction • Types of myofilaments: • Thin filaments • Made of the protein actin • Thick filaments • Made of the protein myosin © 2015 Pearson Education, Inc.

10 -3 Characteristics of Skeletal Muscle Fibers • The Sarcoplasmic Reticulum (SR) • A membranous structure surrounding each myofibril • Helps transmit action potential to myofibril • Similar in structure to smooth endoplasmic reticulum • Forms chambers (terminal cisternae) attached to T tubules © 2015 Pearson Education, Inc.

10 -3 Characteristics of Skeletal Muscle Fibers • The Sarcoplasmic Reticulum (SR) • Triad • Is formed by one T tubule and two terminal cisternae • Concentrate Ca 2+ (via ion pumps) • Release Ca 2+ into sarcomeres to begin muscle contraction © 2015 Pearson Education, Inc.

Figure 10 -3 The Structure and Internal Organization of a Skeletal Muscle Fiber. Myofibril Muscle fiber Sarcolemma Nuclei Sarcoplasm Mitochondria Terminal cisterna Sarcolemma Sarcoplasm Myofibrils Thin filament Thick filament Triad Sarcoplasmic T tubules reticulum © 2015 Pearson Education, Inc.

Figure 10 -3 The Structure and Internal Organization of a Skeletal Muscle Fiber (Part 2 of 4). Mitochondria Terminal cisterna Sarcolemma Sarcoplasm Myofibrils Thin filament Thick filament Triad Sarcoplasmic reticulum © 2015 Pearson Education, Inc. T tubules

Figure 10 -3 The Structure and Internal Organization of a Skeletal Muscle Fiber (Part 4 of 4). Terminal cisterna Sarcolemma Sarcoplasm Myofibrils Triad Sarcoplasmic reticulum © 2015 Pearson Education, Inc. T tubules

10 -3 Structural Components of a Sarcomere • Sarcomeres • • The contractile units of muscle Structural units of myofibrils Form visible patterns within myofibrils A striped or striated pattern within myofibrils • Alternating dark, thick filaments (A bands) and light, thin filaments (I bands) © 2015 Pearson Education, Inc.

10 -3 Structural Components of a Sarcomere • Sarcomeres • The A Band • M line • The center of the A band • At midline of sarcomere • The H Band • The area around the M line • Has thick filaments but no thin filaments • Zone of overlap • The densest, darkest area on a light micrograph • Where thick and thin filaments overlap © 2015 Pearson Education, Inc.

10 -3 Structural Components of a Sarcomere • Sarcomeres • The I Band •

Figure 10 -4 a Sarcomere Structure, Longitudinal Views. I band A band H band a Z line Titin A longitudinal view of a sarcomere, showing bands of thick and thin filaments Zone of overlap M line Sarcomere © 2015 Pearson Education, Inc. Thin Thick filament

Figure 10 -5 Sarcomere Structure, Superficial and Cross-Sectional Views. Sarcomere Myofibril a A superficial view of a sarcomere Thin filament Actinin filaments Thick filament Titin filament Thin filaments Thick filaments Attachment of titin Z line b Cross-sectional views of different regions of a sarcomere © 2015 Pearson Education, Inc. I band M line H band Zone of overlap

Figure 10 -6 Levels of Functional Organization in a Skeletal Muscle Myofibril Surrounded by: Sarcoplasmic reticulum Surrounded by: Epimysium Contains: Muscle fascicles Consists of: Sarcomeres (Z line to Z line) Sarcomere I band A band Muscle Fascicle Perimysium Contains: Thick filaments Surrounded by: Perimysium Contains: Muscle fibers Thin filaments Z line M line H band Muscle Fiber Endomysium Surrounded by: Endomysium Contains: Myofibrils © 2015 Pearson Education, Inc. Titin Z line

Figure 10 -6 Levels of Functional Organization in a Skeletal Muscle (Part 1 of

Figure 10 -6 Levels of Functional Organization in a Skeletal Muscle (Part 2 of

Figure 10 -6 Levels of Functional Organization in a Skeletal Muscle (Part 3 of

Figure 10 -6 Levels of Functional Organization in a Skeletal Muscle (Part 4 of 5). Myofibril Surrounded by: Sarcoplasmic reticulum Consists of: Sarcomeres (Z line to Z line) © 2015 Pearson Education, Inc.

Figure 10 -6 Levels of Functional Organization in a Skeletal Muscle (Part 5 of 5). Sarcomere I band A band Contains: Thick filaments Thin filaments Z line M line H band © 2015 Pearson Education, Inc. Titin Z line

10 -3 Structural Components of a Sarcomere • Thin Filaments • Tropomyosin • Is a double strand • Prevents actin–myosin interaction • Troponin • A globular protein • Binds tropomyosin to G-actin • Controlled by Ca 2+ © 2015 Pearson Education, Inc.

Figure 10 -7 ab Thin and Thick Filaments. Sarcomere H band Actinin Z line Myofibril Titin a The gross structure of a thin filament, showing the attachment at the Z line M line Troponin Active site Nebulin Tropomyosin G-actin molecules F-actin strand b The organization of G-actin subunits in an F-actin strand, and the position of the troponin–tropomyosin complex © 2015 Pearson Education, Inc.

10 -3 Structural Components of a Sarcomere • Initiating Contraction • Ca 2+ binds to receptor on troponin molecule • Troponin–tropomyosin complex changes • Exposes active site of F-actin © 2015 Pearson Education, Inc.

10 -3 Structural Components of a Sarcomere • Thick Filaments • Contain about 300 twisted myosin subunits • The mysosin molecule • Tail • Binds to other myosin molecules • Head • Made of two globular protein subunits • Reaches the nearest thin filament © 2015 Pearson Education, Inc.

Figure 10 -7 cd Thin and Thick Filaments. Titin c The structure of thick filaments, showing the orientation of the myosin molecules M line d The structure of a myosin molecule © 2015 Pearson Education, Inc. Myosin head Myosin tail Hinge

10 -3 Structural Components of a Sarcomere • Myosin Action • During contraction, myosin heads: • Interact with actin filaments, forming cross-bridges • Pivot, producing motion © 2015 Pearson Education, Inc.

10 -3 Structural Components of a Sarcomere • Sliding Filaments and Muscle Contraction • Sliding filament theory • Thin filaments of sarcomere slide toward M line, alongside thick filaments • The width of A zone stays the same • Z lines move closer together © 2015 Pearson Education, Inc.

Figure 10 -8 a Changes in the Appearance of a Sarcomere during the Contraction of a Skeletal Muscle Fiber. Myofibril at rest I band Z line A band H band Z line a A relaxed sarcomere showing location of the A band, Z lines, and I band. © 2015 Pearson Education, Inc.

Figure 10 -8 b Changes in the Appearance of a Sarcomere during the Contraction of a Skeletal Muscle Fiber. Contracted myofibril I band A band Z line H band Z line b During a contraction, the A band stays the same width, but the Z lines move closer together and the I band gets smaller. When the ends of a myofibril are free to move, the sarcomeres shorten simultaneously and the ends of the myofibril are pulled toward its center. © 2015 Pearson Education, Inc.

10 -3 Structural Components of a Sarcomere • Skeletal Muscle Contraction • The process of contraction • Neural stimulation of sarcolemma • Causes excitation–contraction coupling • Muscle fiber contraction • Interaction of thick and thin filaments • Tension production © 2015 Pearson Education, Inc.

10 -4 Components of the Neuromuscular Junction • The Control of Skeletal Muscle Activity • The neuromuscular junction (NMJ) • Special intercellular connection between the nervous system and skeletal muscle fiber • Controls calcium ion release into the sarcoplasm © 2015 Pearson Education, Inc.

Figure 10 -9 Events at the Neuromuscular Junction (Part 3 of 9). A single axon may branch to control more than one skeletal muscle fiber, but each muscle fiber has only one neuromuscular junction (NMJ). At the NMJ, the axon terminal of the neuron lies near the motor end plate of the muscle fiber. Motor neuron Path of electrical impulse (action potential) Axon Neuromuscular junction Axon terminal SEE BELOW Sarcoplasmic reticulum Motor end plate Myofibril © 2015 Pearson Education, Inc. Motor end plate

Figure 10 -9 Events at the Neuromuscular Junction (Part 5 of 9). 1 The cytoplasm of the axon terminal contains vesicles filled with molecules of acetylcholine, or ACh. Acetylcholine is a neurotransmitter, a chemical released by a neuron to change the permeability or other properties of another cell’s plasma membrane. The synaptic cleft and the motor end plate contain molecules of the enzyme acetylcholinesterase (ACh. E), which breaks down ACh Vesicles The synaptic cleft is a narrow space that separates the axon terminal of the neuron from the opposing motor end plate. © 2015 Pearson Education, Inc. Junctional ACh. E fold of motor end plate

Figure 10 -9 Events at the Neuromuscular Junction (Part 6 of 9). 2 The stimulus for ACh release is the arrival of an electrical impulse, or action potential, at the axon terminal. An action potential is a sudden change in the membrane potential that travels along the length of the axon. Arriving action potential © 2015 Pearson Education, Inc.

Figure 10 -9 Events at the Neuromuscular Junction (Part 7 of 9). 3 When the action potential reaches the neuron’s axon terminal, permeability changes in its membrane trigger the exocytosis of ACh into the synaptic cleft. Exocytosis occurs as vesicles fuse with the neuron’s plasma membrane. Motor end plate © 2015 Pearson Education, Inc.

Figure 10 -9 Events at the Neuromuscular Junction (Part 8 of 9). 4 ACh molecules diffuse across the synaptic cleft and blind to ACh receptors on the surface of the motor end plate. ACh binding alters the membrane’s permeability to sodium ions. Because the extracellular fluid contains a high concentration of sodium ions, and sodium ion concentration inside the cell is very low, sodium ions rush into the cytosol. Na+ ACh receptor site © 2015 Pearson Education, Inc. Na+

Figure 10 -9 Events at the Neuromuscular Junction (Part 9 of 9). 5 The sudden inrush of sodium ions results in the generation of an action potential in the sarcolemma. ACh is removed from the synaptic cleft in two ways. ACh either diffuses away from the synapse, or it is broken down by ACh. E into acetic acid and choline. This removal inactivates the ACh receptor sites. The muscle fiber pictured above indicates the propagation of the action potential along the sarcolemma. Action potential Break down ACh. E of ACh © 2015 Pearson Education, Inc.

10 -4 Components of the Neuromuscular Junction • Excitation–Contraction Coupling • Action potential reaches a triad • Releasing Ca 2+ • Triggering contraction • Requires myosin heads to be in “cocked” position • Loaded by ATP energy © 2015 Pearson Education, Inc.

Figure 10 -10 Excitation-Contraction Coupling. © 2015 Pearson Education, Inc.

10 -4 Skeletal Muscle Contraction • The Contraction Cycle 1. 2. 3. 4. 5. 6. Contraction Cycle Begins Active-Site Exposure Cross-Bridge Formation Myosin Head Pivoting Cross-Bridge Detachment Myosin Reactivation © 2015 Pearson Education, Inc.

Figure 10 -11 The Contraction Cycle and Cross-Bridge Formation (Part 3 of 10). 1 Contraction Cycle Begins The contraction cycle involves a series of interrelated steps. It begins with the arrival of calcium ions (Ca 2+) within the zone of overlap in a sarcomere. ADP + P Tropomyosin © 2015 Pearson Education, Inc. Myosin head Ca 2+ Troponin Actin ADP P+ Ca 2+

Figure 10 -11 The Contraction Cycle and Cross-Bridge Formation (Part 4 of 10). 2 Active-Site Exposure Calcium ions bind to troponin, weakening the bond between actin and the troponin–tropomyosin complex. The troponin molecule then changes position, rolling the tropomyosin molecule away from the active sites on actin and allowing interaction with the energized myosin heads. ADP + P Cytosol Ca 2+ Active site © 2015 Pearson Education, Inc. ADP P+

Figure 10 -11 The Contraction Cycle and Cross-Bridge Formation (Part 5 of 10). 3 Cross-Bridge Formation Once the active sites are exposed, the energized myosin heads bind to them, forming cross-bridges. ADP + P Ca 2+ ADP P+ © 2015 Pearson Education, Inc.

Figure 10 -11 The Contraction Cycle and Cross-Bridge Formation (Part 6 of 10). 4 Myosin Head Pivoting After cross-bridge formation, the energy that was stored in the resting state is released as the myosin head pivots toward the M line. This action is called the power stroke; when it occurs, the bound ADP and phosphate group are released. ADP + P Ca 2+ ADP + P © 2015 Pearson Education, Inc.

Figure 10 -11 The Contraction Cycle and Cross-Bridge Formation (Part 7 of 10). 5 Cross-Bridge Detachment When another ATP binds to the myosin head, the link between the myosin head and the active site on the actin molecule is broken. The active site is now exposed and able to form another cross-bridge. ATP Ca 2+ ATP © 2015 Pearson Education, Inc.

Figure 10 -11 The Contraction Cycle and Cross-Bridge Formation (Part 8 of 10). 6 Myosin Reactivation Myosin reactivation occurs when the free myosin head splits ATP into ADP and P. The energy released is used to recock the myosin head. ADP + P Ca 2+ ADP P+ © 2015 Pearson Education, Inc.

Figure 10 -11 The Contraction Cycle and Cross-Bridge Formation (Part 9 of 10). RESTING SARCOMERE In the resting sarcomere, each myosin head is already “energized”—charged with the energy that will be used to power a contraction. Each myosin head points away from the M line. In this position, the myosin head is “cocked” like the spring in a mousetrap. Cocking the myosin head requires energy, which is obtained by breaking down ATP; in doing so, the myosin head functions as ATPase, an enzyme that breaks down ATP. At the start of the contraction cycle, the breakdown products, ADP and phosphate (represented as P), remain bound to the myosin head. © 2015 Pearson Education, Inc. M line Zone of Overlap (shown in sequence above)

Figure 10 -11 The Contraction Cycle and Cross-Bridge Formation (Part 10 of 10). CONTRACTED SARCOMERE The entire cycle is repeated several times each second, as long as Ca 2+ concentrations remain elevated and ATP reserves are sufficient. Calcium ion levels will remain elevated only as long as action potentials continue to pass along the T tubules and stimulate the terminal cisternae. Once that stimulus is removed, the calcium channels in the SR close and calcium ion pumps pull Ca 2+ from the cytosol and store it within the terminal cisternae. Troponin molecules then shift position, swinging the tropomyosin strands over the active sites and preventing further cross-bridge formation. © 2015 Pearson Education, Inc.

10 -4 Skeletal Muscle Contraction • Fiber Shortening • As sarcomeres shorten, muscle pulls together, producing tension • Muscle shortening can occur at both ends of the muscle, or at only one end of the muscle • This depends on the way the muscle is attached at the ends © 2015 Pearson Education, Inc.

Figure 10 -12 Shortening during a Contraction. a When both ends are free to move, the ends of a contracting muscle fiber move toward the center of the muscle fiber. b When only one end of a myofibril is fixed in position, the free end is pulled toward the fixed end. © 2015 Pearson Education, Inc.

10 -4 Skeletal Muscle Relaxation • Contraction duration • Depends on: • Duration of

10 -4 Skeletal Muscle Relaxation • Ca 2+ concentrations fall • Ca 2+ detaches from troponin • Active sites are re-covered by tropomyosin • Rigor Mortis • A fixed muscular contraction after death • Caused when: • Ion pumps cease to function; ran out of ATP • Calcium builds up in the sarcoplasm © 2015 Pearson Education, Inc.

10 -4 Skeletal Muscle Contraction and Relaxation • Summary • Skeletal muscle fibers shorten as thin filaments slide between thick filaments • Free Ca 2+ in the sarcoplasm triggers contraction • SR releases Ca 2+ when a motor neuron stimulates the muscle fiber • Contraction is an active process • Relaxation and return to resting length are passive © 2015 Pearson Education, Inc.

Figure 10 -13 Steps Involved in Skeletal Muscle Contraction and Relaxation (Part 1 of 2). Steps That Initiate a Muscle Contraction 1 Axon terminal ACh released ACh is released at the neuromuscular junction and binds to ACh receptors on the sarcolemma. Sarcolemma Cytosol 2 An action potential is generated and spreads across the membrane surface of the muscle fiber and along the T tubules. 3 Sarcoplasmic reticulum releases Ca 2+ The sarcoplasmic reticulum releases stored calcium ions. 4 Active site exposure and cross-bridge formation Calcium ions bind to troponin, exposing the active sites on the thin filaments. Cross-bridges form when myosin heads bind to those active sites. 5 T tubule Action potential reaches T tubule Contraction cycle begins The contraction cycle begins as repeated cycles of cross-bridge binding, pivoting, and detachment occur—all powered by ATP. © 2015 Pearson Education, Inc. Sarcoplasmic reticulum Ca 2+ Actin Myosin

Figure 10 -13 Steps Involved in Skeletal Muscle Contraction and Relaxation (Part 2 of 2). Steps That End a Muscle Contraction 6 ACh is broken down Axon terminal ACh is broken down by acetylcholinesterase (ACh. E), ending action potential generation 7 Cytosol T tubule Sarcoplasmic reticulum reabsorbs Ca 2+ As the calcium ions are reabsorbed, their concentration in the cytosol decreases. 8 Sarcolemma Active sites covered, and cross-bridge formation ends Without calcium ions, the tropomyosin returns to its normal position and the active sites are covered again. Sarcoplasmic reticulum Ca 2+ Actin Myosin 9 Contraction ends Without cross-bridge formation, contraction ends. 10 Muscle relaxation occurs The muscle returns passively to its resting length. © 2015 Pearson Education, Inc.

10 -5 Tension Production and Contraction Types • Tension Production by Muscles Fibers • As a whole, a muscle fiber is either contracted or relaxed • Depends on: • The number of pivoting cross-bridges • The fiber’s resting length at the time of stimulation • The frequency of stimulation © 2015 Pearson Education, Inc.

10 -5 Tension Production and Contraction Types • Tension Production by Muscle Fibers • The Frequency of Stimulation • A single neural stimulation produces: • A single contraction or twitch • Which lasts about 7– 100 msec. • Sustained muscular contractions • Require many repeated stimuli © 2015 Pearson Education, Inc.

10 -5 Tension Production and Contraction Types • Tension Production by Muscle Fibers • Twitches 1. Latent period • The action potential moves through sarcolemma • Causing Ca 2+ release 2. Contraction phase • Calcium ions bind • Tension builds to peak 3. Relaxation phase • Ca 2+ levels fall • Active sites are covered and tension falls to resting levels © 2015 Pearson Education, Inc.

Figure 10 -15 a The Development of Tension in a Twitch. Eye muscle Gastrocnemius Tension Soleus 0 Stimulus 10 20 30 40 50 60 Time (msec) 70 80 90 100 a A myogram showing differences in tension over time for a twitch in different skeletal muscles. © 2015 Pearson Education, Inc.

Figure 10 -15 b The Development of Tension in a Twitch. Tension Maximum tension development Stimulus Time (msec) 0 5 10 Resting Latent Contraction phase period phase 20 30 40 Relaxation phase b The details of tension over time for a single twitch in the gastrocnemius muscle. Notice the presence of a latent period, which corresponds to the time needed for the conduction of an action potential and the subsequent release of calcium ions by the sarcoplasmic reticulum. © 2015 Pearson Education, Inc.

10 -5 Tension Production and Contraction Types • Tension Production by Muscle Fibers • Treppe • A stair-step increase in twitch tension • Repeated stimulations immediately after relaxation phase • Stimulus frequency 50/second • Causes a series of contractions with increasing tension © 2015 Pearson Education, Inc.

10 -5 Tension Production and Contraction Types • Tension Production by Muscle Fibers • Wave summation • Increasing tension or summation of twitches • Repeated stimulations before the end of relaxation phase • Stimulus frequency 50/second • Causes increasing tension or summation of twitches © 2015 Pearson Education, Inc.

Figure 10 -16 ab Effects of Repeated Stimulations. = Stimulus Tension Maximum tension (in tetanus) Maximum tension (in treppe) Time a Treppe is an increase in peak tension with each successive stimulus delivered shortly after the completion of the relaxation phase of the preceding twitch. © 2015 Pearson Education, Inc. Time b Wave summation occurs when successive stimuli arrive before the relaxation phase has been completed.

10 -5 Tension Production and Contraction Types • Tension Production by Muscle Fibers • Incomplete tetanus • Twitches reach maximum tension • If rapid stimulation continues and muscle is not allowed to relax, twitches reach maximum level of tension • Complete tetanus • If stimulation frequency is high enough, muscle never begins to relax, and is in continuous contraction © 2015 Pearson Education, Inc.

Figure 10 -16 cd Effects of Repeated Stimulations. Tension Maximum tension (in tetanus) Time c Incomplete tetanus occurs if the stimulus frequency increases further. Tension production rises to a peak, and the periods of relaxation are very brief. © 2015 Pearson Education, Inc. Time d Complete tetanus. During complete tetanus, the stimulus frequency is so high that the relaxation phase is eliminated. Tension plateaus at maximum levels.

10 -5 Tension Production and Contraction Types • Motor Units and Tension Production • Motor units in a skeletal muscle: • Contain hundreds of muscle fibers • That contract at the same time • Controlled by a single motor neuron © 2015 Pearson Education, Inc.

Figure 10 -17 a The Arrangement and Activity of Motor Units in a Skeletal Muscle. Axons of motor neurons Motor nerve KEY SPINAL CORD Muscle fibers Motor unit 1 Motor unit 2 Motor unit 3 a Muscle fibers of different motor units are © 2015 Pearson Education, Inc. intermingled, so the forces applied to the tendon remain roughly balanced regardless of which motor units are stimulated.

10 -5 Tension Production and Contraction Types • Motor Units and Tension Production • Sustained tension • Less than maximum tension • Allows motor units to rest in rotation • Muscle tone • The normal tension and firmness of a muscle at rest • Muscle units actively maintain body position, without motion • Increasing muscle tone increases metabolic energy used, even at rest © 2015 Pearson Education, Inc.

10 -5 Tension Production and Contraction Types • Motor Units and Tension Production • Contractions are classified based on pattern of tension production • Isotonic contraction • Isometric contraction © 2015 Pearson Education, Inc.

10 -5 Tension Production and Contraction Types • Isotonic Contraction • Skeletal muscle changes length • Resulting in motion • If muscle tension load (resistance): • Muscle shortens (concentric contraction) • If muscle tension load (resistance): • Muscle lengthens (eccentric contraction) © 2015 Pearson Education, Inc.

Figure 10 -18 a Concentric, Eccentric, and Isometric Contractions. Tendon Muscle 4 tension (kg) 2 Muscle contracts (concentric contraction) Amount of load Muscle relaxes Peak tension production 0 Contraction begins Resting length 2 kg a In this experiment, a muscle is attached to a weight one-half its peak tension potential. On stimulation, it develops enough tension to lift the weight. Tension remains constant for the duration of the contraction, although the length of the muscle changes. This is an example of isotonic contraction. © 2015 Pearson Education, Inc. Time 100 Muscle 90 length (percent 80 of resting length) 70

Figure 10 -18 b Concentric, Eccentric, and Isometric Contractions. When the eccentric contraction ends, the unopposed load stretches the muscle until either the muscle tears, a tendon breaks, or the elastic recoil of the skeletal muscle is sufficient to oppose the load. Support removed when contraction begins (eccentric contraction) 140 4 Muscle tension 2 (kg) Peak tension production 0 Support removed, contraction begins 6 kg Resting length 130 120 Muscle length 110 (percent of resting 100 length) 90 80 6 kg b In this eccentric contraction, the muscle elongates as it generates tension. © 2015 Pearson Education, Inc. Time 70

10 -5 Tension Production and Contraction Types • Isometric Contraction • Skeletal muscle develops tension, but is prevented from changing length • iso- same, metric measure © 2015 Pearson Education, Inc.

Figure 10 -18 c Concentric, Eccentric, and Isometric Contractions. 6 Muscle 4 tension (kg) 2 Muscle contracts (isometric contraction) 6 kg c Muscle relaxes Peak tension production 0 Contraction begins Length unchanged 6 kg The same muscle is attached to a weight that exceeds its peak tension capabilities. On stimulation, tension will rise to a peak, but the muscle as a whole cannot shorten. This is an isometric contraction. © 2015 Pearson Education, Inc. Amount of load Time 100 Muscle 90 length (percent 80 of resting length) 70

10 -5 Tension Production and Contraction Types • Load and Speed of Contraction • Are inversely related • The heavier the load (resistance) on a muscle: • The longer it takes for shortening to begin • And the less the muscle will shorten © 2015 Pearson Education, Inc.

Distance shortened Figure 10 -19 Load and Speed of Contraction. Small load Intermediate load

10 -5 Tension Production and Contraction Types • Muscle Relaxation and the Return to Resting Length • Elastic forces • The pull of elastic elements (tendons and ligaments) • Expands the sarcomeres to resting length • Opposing muscle contractions • Reverse the direction of the original motion • Are the work of opposing skeletal muscle pairs © 2015 Pearson Education, Inc.

10 -6 Energy to Power Contractions • ATP Provides Energy for Muscle Contraction • Sustained muscle contraction uses a lot of ATP energy • Muscles store enough energy to start contraction • Muscle fibers must manufacture more ATP as needed © 2015 Pearson Education, Inc.

10 -6 Energy to Power Contractions • ATP and CP Reserves • Adenosine triphosphate (ATP) • The active energy molecule • Creatine phosphate (CP) • The storage molecule for excess ATP energy in resting muscle • Energy recharges ADP to ATP • Using the enzyme creatine kinase (CK) • When CP is used up, other mechanisms generate ATP © 2015 Pearson Education, Inc.

10 -6 Energy to Power Contractions • ATP Generation • Cells produce ATP in two ways 1. Aerobic metabolism of fatty acids in the mitochondria 2. Anaerobic glycolysis in the cytoplasm © 2015 Pearson Education, Inc.

10 -6 Energy to Power Contractions • Aerobic Metabolism • Is the primary energy source of resting muscles • Breaks down fatty acids • Produces 34 ATP molecules per glucose molecule • Glycolysis • Is the primary energy source for peak muscular activity • Produces two ATP molecules per molecule of glucose • Breaks down glucose from glycogen stored in skeletal muscles © 2015 Pearson Education, Inc.

10 -6 Energy to Power Contractions • Energy Use and the Level of Muscular Activity • Skeletal muscles at rest metabolize fatty acids and store glycogen • During light activity, muscles generate ATP through anaerobic breakdown of carbohydrates, lipids, or amino acids • At peak activity, energy is provided by anaerobic reactions that generate lactic acid as a by-product © 2015 Pearson Education, Inc.

10 -6 Energy to Power Contractions • Muscle Fatigue • When muscles can no longer perform a required activity, they are fatigued • Results of Muscle Fatigue • Depletion of metabolic reserves • Damage to sarcolemma and sarcoplasmic reticulum • Low p. H (lactic acid) • Muscle exhaustion and pain © 2015 Pearson Education, Inc.

10 -6 Energy to Power Contractions • The Recovery Period • The time required after exertion for muscles to return to normal • Oxygen becomes available • Mitochondrial activity resumes © 2015 Pearson Education, Inc.

10 -6 Energy to Power Contractions • Lactic Acid Removal and Recycling • The Cori Cycle • The removal and recycling of lactic acid by the liver • Liver converts lactate to pyruvate • Glucose is released to recharge muscle glycogen reserves © 2015 Pearson Education, Inc.

10 -6 Energy to Power Contractions • The Oxygen Debt • After exercise or other exertion: • The body needs more oxygen than usual to normalize metabolic activities • Resulting in heavy breathing • Also called excess postexercise oxygen consumption (EPOC) © 2015 Pearson Education, Inc.

10 -6 Energy to Power Contractions • Heat Production and Loss • Active muscles produce heat • Up to 70 percent of muscle energy can be lost as heat, raising body temperature © 2015 Pearson Education, Inc.

10 -7 Types of Muscle Fibers and Endurance • Three Major Types of Skeletal

10 -7 Types of Muscle Fibers and Endurance • Fast Fibers • Contract very quickly • Have large diameter, large glycogen reserves, few mitochondria • Have strong contractions, fatigue quickly © 2015 Pearson Education, Inc.

10 -7 Types of Muscle Fibers and Endurance • Slow Fibers • • Are slow to contract, slow to fatigue Have small diameter, more mitochondria Have high oxygen supply Contain myoglobin (red pigment, binds oxygen) © 2015 Pearson Education, Inc.

10 -7 Types of Muscle Fibers and Endurance • Intermediate Fibers • Are mid-sized

Figure 10 -21 Fast versus Slow Fibers. Capillary Slow fibers Smaller diameter, darker color due to myoglobin; fatigue resistant LM × 170 Fast fibers Larger diameter, paler color; easily fatigued LM × 170 © 2015 Pearson Education, Inc. LM × 783

Table 10 -2 Properties of Skeletal Muscle Fiber Types. © 2015 Pearson Education, Inc.

10 -7 Types of Muscle Fibers and Endurance • Muscle Performance and the Distribution of Muscle Fibers • White muscles • Mostly fast fibers • Pale (e. g. , chicken breast) • Red muscles • Mostly slow fibers • Dark (e. g. , chicken legs) • Most human muscles • Mixed fibers • Pink © 2015 Pearson Education, Inc.

10 -7 Types of Muscle Fibers and Endurance • Muscle Hypertrophy • Muscle growth from heavy training • Increases diameter of muscle fibers • Increases number of myofibrils • Increases mitochondria, glycogen reserves • Muscle Atrophy • Lack of muscle activity • Reduces muscle size, tone, and power © 2015 Pearson Education, Inc.

10 -7 Types of Muscle Fibers and Endurance • Physical Conditioning • Improves both power and endurance • Anaerobic activities (e. g. , 50 -meter dash, weightlifting) • Use fast fibers • Fatigue quickly with strenuous activity • Improved by: • Frequent, brief, intensive workouts • Causes hypertrophy © 2015 Pearson Education, Inc.

10 -7 Types of Muscle Fibers and Endurance • Physical Conditioning • Improves both power and endurance • Aerobic activities (prolonged activity) • Supported by mitochondria • Require oxygen and nutrients • Improves: • Endurance by training fast fibers to be more like intermediate fibers • Cardiovascular performance © 2015 Pearson Education, Inc.

10 -7 Types of Muscle Fibers and Endurance • Importance of Exercise • What you don’t use, you lose • Muscle tone indicates base activity in motor units of skeletal muscles • Muscles become flaccid when inactive for days or weeks • Muscle fibers break down proteins, become smaller and weaker • With prolonged inactivity, fibrous tissue may replace muscle fibers © 2015 Pearson Education, Inc.

10 -8 Cardiac Muscle Tissue • Cardiac muscle cells are striated and found only in the heart • Striations are similar to that of skeletal muscle because the internal arrangement of myofilaments is similar © 2015 Pearson Education, Inc.

10 -8 Cardiac Muscle Tissue • Structural Characteristics of Cardiac Muscle Tissue • Unlike skeletal muscle, cardiac muscle cells (cardiocytes): • Are small • Have a single nucleus • Have short, wide T tubules • Have no triads • Have SR with no terminal cisternae • Are aerobic (high in myoglobin, mitochondria) • Have intercalated discs © 2015 Pearson Education, Inc.

10 -8 Cardiac Muscle Tissue • Intercalated Discs • Coordination of cardiocytes • Because intercalated discs link heart cells mechanically, chemically, and electrically, the heart functions like a single, fused mass of cells © 2015 Pearson Education, Inc.

Figure 10 -22 a Cardiac Muscle Tissue. Cardiac muscle cell Intercalated discs Nucleus Cardiac

Figure 10 -22 b Cardiac Muscle Tissue. Cardiac muscle cell (intact) Intercalated disc (sectioned) b A diagrammatic view of cardiac muscle. Note the striations and intercalated discs. Mitochondria Nucleus Intercalated discs Myofibrils © 2015 Pearson Education, Inc. Cardiac muscle cell (sectioned)

Figure 10 -22 c Cardiac Muscle Tissue. Entrance to T tubule Sarcolemma Mitochondrion Contact of sarcoplasmic reticulum with T tubule Myofibrils Sarcoplasmic reticulum c Cardiac muscle tissue showing short, broad T tubules and SR that lacks terminal cisternae. © 2015 Pearson Education, Inc.

10 -8 Cardiac Muscle Tissue • Functional Characteristics of Cardiac Muscle Tissue • Automaticity • Contraction without neural stimulation • Controlled by pacemaker cells • Variable contraction tension • Controlled by nervous system • Extended contraction time • Ten times as long as skeletal muscle • Prevention of wave summation and tetanic contractions by cell membranes • Long refractory period © 2015 Pearson Education, Inc.

10 -9 Smooth Muscle Tissue • Smooth Muscle in Body Systems • Forms around other tissues • In integumentary system • Arrector pili muscles cause “goose bumps” • In blood vessels and airways • Regulates blood pressure and airflow • In reproductive and glandular systems • Produces movements • In digestive and urinary systems • Forms sphincters • Produces contractions © 2015 Pearson Education, Inc.

10 -9 Smooth Muscle Tissue • Structural Characteristics of Smooth Muscle Tissue • Nonstriated tissue • Different internal organization of actin and myosin • Different functional characteristics © 2015 Pearson Education, Inc.

Figure 10 -23 b Smooth Muscle Tissue. Relaxed (sectional view) Actin Dense body Myosin Relaxed (superficial view) Intermediate filaments (desmin) Adjacent smooth muscle cells are bound together at dense bodies, transmitting the contractile forces from cell to cell throughout the tissue. Contracted (superficial view) b A single relaxed smooth muscle cell is spindle shaped © 2015 Pearson Education, Inc. and has no striations. Note the changes in cell shape as contraction occurs.

10 -9 Smooth Muscle Tissue • Characteristics of Smooth Muscle Cells • • Long, slender, and spindle shaped Have a single, central nucleus Have no T tubules, myofibrils, or sarcomeres Have no tendons or aponeuroses Have scattered myosin fibers Myosin fibers have more heads per thick filament Have thin filaments attached to dense bodies Dense bodies transmit contractions from cell to cell © 2015 Pearson Education, Inc.

10 -9 Smooth Muscle Tissue • Excitation–Contraction Coupling • Free Ca 2+ in cytoplasm triggers contraction • Ca 2+ binds with calmodulin • In the sarcoplasm • Activates myosin light chain kinase • Enzyme breaks down ATP, initiates contraction © 2015 Pearson Education, Inc.