Fig. 7.1 Copyright © McGraw-Hill Education. Permission required for reproduction or display. Gastrocnemius Masseter Skeletal muscle Temporalis Sternocleidomastoid Pectoralis major Biceps brachii Abdominal muscles Sartorius Quadriceps femoris Heart Muscle of the intestines and other internal organs and vessels Cardiac muscle Smooth muscle

Fig Copyright © McGraw-Hill Education. Permission required for reproduction or display. Radial tuberosity (insertion of biceps brachii) Radius Ulna Extension Olecranon process (insertion of triceps brachii) Humerus Origins of triceps brachii Scapula Origins of biceps brachii Biceps brachii (belly) Flexion Tendon

Table 7.2

Fig. 7.2 Copyright © McGraw-Hill Education. Permission required for reproduction or display. Nuclei Z disk A I I Bone Heads Z disk M line (a) (d) (b) (c) Perimysium Epimysium (muscular fascia) Endomysium (surrounding muscle fibers) Muscle fiber Myofibrils Myosin myofilament Sarcomere Actin myofilament Myofibril Sarcomere Cross-bridge Titin Myosin myofilament Actin myofilament Z disk Tropomyosin Actin strands (e) Actin myofilament (thin) (enlarged to show detail) Attachment site (f) Myosin myofilament (thick) Myosin molecule Troponin Rod Mitochondrion Transverse (T) tubule Sarcolemma (cell membrane) Sarcoplasmic reticulum Capillary Muscle fasciculi Skeletal muscle Tendon

Fig. 7.3 Copyright © McGraw-Hill Education. Permission required for reproduction or display. M line Z disk I band A band Z disk Hzone M line Z disk (b) (c) (a) Myosin myofilament Actin myofilament Muscle fasciculi Whole muscle Muscle fiber Myofilaments Myosin myofilament Actin myofilament Actin myofilaments only Myosin myofilaments surrounded by actin myofilaments Myosin myofilaments only Rod portion of myosin myofilaments and M line Sarcomere Myofibril Mitochondria H zone I band A band b: ©SPL/Getty Images RF

Fig. 7.7 Copyright © McGraw-Hill Education. Permission required for reproduction or display. II A ZZ H IIA ZZ In a relaxed muscle, the actin and myosin myofilaments overlap slightly, and the H zone is visible. The sarcomere length is at its normal resting length. As a muscle contraction is initiated, actin myofilaments slide past the myosin myofilaments, the z disks are brought closer together, and the sarcomere begins to shorten. In a contracted muscle, 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, nor does the length of the actin myofilaments. In addition, the ends of the actin myofilaments are pulled to and overlap in the center of the sarcomere, shortening it and the H zone disappears. (b) Fully contracted sarcomere (a) Relaxed sarcomere (all): ©Don W. Fawcett/Science Source

Fig. 7.5 Copyright © McGraw-Hill Education. Permission required for reproduction or display. Capillary (a) (b) Synaptic vesicles Presynaptic terminal (axon terminal) Sarcolemma Presynaptic terminal Neuromuscular junction Axon branch Muscle fiber Myofibrils Axon branch Neuromuscular junction Skeletal muscle fiber Mitochondrion Postsynaptic membrane (sarcolemma) Synaptic cleft LM 250x b: ©Ed Reschke/Photolibrary/Getty Images

Fig. 7.6 Copyright © McGraw-Hill Education. Permission required for reproduction or display. Synaptic cleft Ca 2+ ACh Na Postsynaptic membrane (sarcolemma) 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. Action potential Ca 2+ channel Presynaptic terminal Action potential Action potential Receptor molecule

Fig. 7.8 Copyright © McGraw-Hill Education. Permission required for reproduction or display. SarcolemmaNa + AP ACh P ADP 6 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. Action potentials in the T tubules cause the sarcoplasmic reticulum to release Ca 2+. ATP molecules are broken down to ADP and P, which releases energy needed to move the myosin heads. 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. T tubule Sarcoplasmic reticulum Na + diffuses into the muscle fiber, initiating an action potential that travels along the sarcolemma and T tubule membranes. On the actin, Ca 2+ binds to troponin, which moves tropomyosin and exposes myosin attachment sites. Ca 2+

Fig. 7.9 Copyright © McGraw-Hill Education. Permission required for reproduction or display. ATP Z disk P ADP P P P Power stroke. Energy stored in the myosin heads is used to move the myosin heads (small dark blue arrow), causing the actin myofilaments to slide past the myosin myofilaments (dark blue arrow), and ADP molecules are released from the myosin heads (black arrow). Hydrolysis of ATP. The myosin ATPase portion of the myosin heads split ATP into ADP and phosphate (P), which remain attached to the myosin heads. Recovery stroke. The heads of the myosin molecules return to their resting position (small dark blue arrow), and energy is stored in the heads of the myosin molecules. If Ca 2+ is still attached to troponin, cross-bridge formation and movement are repeated (return to step 2).This cycle occurs many times during a muscle contraction. Not all cross-bridges form and release simultaneously. Exposure of active sites. Before cross-bridges cycle, Ca 2+ binds to the troponins and the tropomyosins move, exposing active sites on actin myofilaments. Active site Tropomyosin Troponin Ca 2+ Cross-bridge formation. The myosin heads bind to the exposed active sites on the actin myofilaments to form cross-bridges, and phosphates are released from the myosin heads. Cross-bridge release. An ATP molecule binds to each of the myosin heads, causing them to detach from the actin. Myosin myofilamentActin myofilament Sarcomere Cross-bridge

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

Fig Copyright © McGraw-Hill Education. Permission required for reproduction or display. Time (ms) Frequency 1Frequency 2Frequency 3Frequency 4 Tension Twitch Complete tetanus Incomplete tetanus

Fig Copyright © McGraw-Hill Education. Permission required for reproduction or display. + + (a)(b) ADP+ P+ P P P Creatine phosphate Energy Creatine Active muscle contraction Maintains muscle tone and posture ATP Aerobic respiration Energy Creatine phosphate Creatine Lactate Exercise Anaerobic respiration ATP At rest, ATP is produced by aerobic respiration. Small amounts of ATP are used in muscle contractions that maintain muscle tone and posture. Excess ATP is used to produce creatine phosphate, an energy-storage molecule. Throughout the time of exercise, ATP from all of these sources (4–6) provides energy for active muscle contraction. Energy stored in creatine phosphate can also be used to produce ATP. During times of extreme exercise, anaerobic respiration provides small amounts of ATP that can sustain muscle contraction for brief periods. As exercise begins, ATP already in the cell is used first, but during moderate exercise, aerobic respiration provides most of the ATP necessary for muscle contraction. Aerobic respiration Rest

Table 7.1