Co 7

Table 7.2 TABLE 7.2 Comparison of Muscle Types Smooth Muscle Skeletal Muscle Cardiac Muscle Location Appearance Cell Shape Nucleus Special Features Striations Autorhythmic Control Function Attached to boneHeart Walls or hollow organs, blood vessels, and glands Move the whole body Voluntary No Yes Multiple, peripheral Long, cylindricalBranched Usually single, central Intercalated disks Yes Involuntary Compress organs, ducts, tubes, and so on Involuntary Yes No Cell-to-cell attachments Single, central Spindle-shaped Copyright © The McGraw-Hill Companies, Inc. Permission required for reproduction or display. Contract heart to propel blood through the body

Fig Copyright © The McGraw-Hill Companies, Inc. Permission required for reproduction or display. A I I Perimysium Muscle fasciculi Skeletal muscle Tendon Bone Myofibril Actin myofilament Myosin myofilament Muscle fiber Endomysium (surrounding muscle fibers) Nuclei Capillary Sarcoplasmic reticulum Sarcolemma (cell membrane) Transverse (T) tubule Z disk Mitochondrion Myofibrils Sarcomere (c) (a)(b) Epimysium (muscular fascia)

Fig. 7.3 Copyright © The McGraw-Hill Companies, Inc. Permission required for reproduction or display. (b) Sarcomere Z disk A band I band Z disk A band H zone Z disk M line Actin myofilament Myosin myofilament (a) Sarcomere Mitochondria Sarcomere Myofibrils M line Z disk H zone A band I band (a): © Steven Gschmeissner/SPL/Getty Images RF

Fig Actin and myosin myofilaments in a muscle. Myofilaments do not change length during muscle contraction. Myosin myofilamentActin myofilament Sarcomere Z disk Relaxed muscle A band I band A band H zone Z disk H zone Z disk Copyright © The McGraw-Hill Companies, Inc. Permission required for reproduction or display.

Fig Actin and myosin myofilaments in a relaxed muscle (right) and a contracted muscle (#4 below) are the same length. Myofilaments do not change length during muscle contraction. During contraction, actin myofilaments at each end of the sarcomere slide past the myosin myofilaments toward each other. As a result, the Z disks are brought closer together, and the sarcomere shortens. As the actin myofilaments slide over the myosin myofilaments, the H zones (yellow) and the I bands (blue) narrow. The A bands, which are equal to the length of the myosin myofilaments, do not narrow, because the length of the myosin myofilaments does not change. In a fully contracted muscle, the ends of the actin myofilaments overlap, and the H zone disappears. Fully contracted muscle H zone disappears.I band narrows further. A band remains unchanged. A band I band A band H zone narrows.I band narrows. A band does not narrow. Contracting muscle Actin myofilaments move toward each other. Sarcomere shortens as Z disks move toward each other. Z disk Contracting muscle Myosin myofilamentActin myofilament Sarcomere Z disk Relaxed muscle A band I band A band H zone Z disk H zone Z disk Copyright © The McGraw-Hill Companies, Inc. Permission required for reproduction or display.

Fig Copyright © The McGraw-Hill Companies, Inc. Permission required for reproduction or display. Capillary Muscle fiber (a) Axon branch Synaptic vesicles Synaptic cleft Sarcolemma Neuromuscular junction Presynaptic terminal Postsynaptic membrane (sarcolemma) Mitochondrion Presynaptic terminal (axon terminal) Myofibrils

Fig An action potential arrives at the presynaptic terminal, causing Ca 2+ channels to open. Calcium ions (Ca 2+ ) enter the presynaptic terminal and initiate the release of a neurotransmitter, acetylcholine (ACh), from synaptic vesicles into the presynaptic cleft. Diffusion of ACh across the synaptic cleft and binding of ACh to ACh receptors on the postsynaptic muscle fiber membrane opens Na + channels. Sodium ions (Na + ) diffuse down their concentration gradient, which results in depolarization of the muscle fiber membrane; once threshold has been reached, a postsynaptic action potential results. Postsynaptic membrane (sarcolemma) Action potential Na + Action potential Receptor molecule Action potential Presynaptic terminal Ca 2+ channel Ca 2+ Synaptic cleft ACh Na + Copyright © The McGraw-Hill Companies, Inc. Permission required for reproduction or display.

The heads of the myosin myofilaments bend, causing the actin to slide past the myosin. As long as Ca 2+ is present, the cycle repeats. Fig An action potential travels along an axon membrane to a neuromuscular junction. Ca 2+ channels open and Ca 2+ enters the presynaptic terminal. Acetylcholine is released from presynaptic vesicles. Acetylcholine stimulates Na + channels on the postsynaptic membrane to open. Na + diffuses into the muscle fiber, initiating an action potential that travels along the sarcolemma and T-tubule membranes. Action potentials in the T-tubules cause the sarcoplasmic reticulum to release Ca 2+. On the actin, Ca 2+ binds to troponin, which moves tropomyosin and exposes myosin attachment sites. ATP molecules are broken down to ADP and P, which releases energy needed to move the myosin heads P Sarcoplasmic reticulum ADP 1 2 AP Ca 2+ 4 Ach Sarcolemma Na + Ca 2+ 5 T-tubule Copyright © The McGraw-Hill Companies, Inc. Permission required for reproduction or display. 3 6

Fig PPP PPP PP P Exposure of attachment sites. During contraction of a muscle, Ca 2+ binds to troponin molecules, causing tropomyosin molecules to move, which exposes myosin attachment sites on actin myofilaments. Cross-bridge formation. The myosin heads bind to the exposed attachment sites on the actin myofilaments to form cross-bridges, and phosphates are released from the myosin heads. Power stroke. Energy stored in the myosin heads is used to move the myosin heads (green arrows), causing the actin myofilament to slide past the myosin myofilament (purple arrow), and ADP molecules are released from the myosin heads (black arrows). ATP binds to myosin heads. ATP molecules bind to the myosin heads. Cross-bridge release. As ATP is broken down to ADP and phosphates, the myosin heads release from the actin attachment sites. Cross-bridge Z disk Actin myofilament Sarcomere Myosin myofilament Z disk TropomyosinTroponin Ca 2+ ADP ATP ADP Copyright © The McGraw-Hill Companies, Inc. Permission required for reproduction or display.

Fig Copyright © The McGraw-Hill Companies, Inc. Permission required for reproduction or display. Tension Stimulus applied Lag phase Time Contraction phase Relaxation phase

Fig Copyright © The McGraw-Hill Companies, Inc. Permission required for reproduction or display Tension Twitch Incomplete tetanus Complete tetanus Time (ms)

Fig Copyright © The McGraw-Hill Companies, Inc. Permission required for reproduction or display. P+ P + P + P++ P At rest During exercise Aerobic respiration Aerobic respiration Anaerobic respiration Maintains muscle tone and posture Energy ADP ATPADP Energy Creatine phosphate Energy ATP ADPEnergy Active muscle contraction At rest, ATP is produced by aerobic respiration. Excess ATP is used to produce creatine phosphate, an energy-storage molecule. As exercise begins, ATP already in the cell is used first. During moderate exercise, aerobic respiration provides most of the ATP necessary for muscle contraction. Energy stored in creatine phosphate can also be used to produce ATP. Small amounts of ATP are used in muscle contractions that maintain muscle tone and posture. During times of extreme exercise, anaerobic respiration provides small amounts of ATP that can sustain muscle contraction for brief periods. Throughout the time of exercise, ATP from all of these sources (4–7) provides energy for active muscle contraction. ADP