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(a) Skeletal muscle Description: Long, cylindrical, multinucleate cells; obvious striations. Striations Function: Voluntary movement; locomotion; manipulation of the environment; facial expression; voluntary control. Location: In skeletal muscles attached to bones or occasionally to skin. Nuclei Part of muscle fiber (cell) Photomicrograph: Skeletal muscle (approx. 460x). Notice the obvious banding pattern and the fact that these large cells are multinucleate. (b) Cardiac muscle Description: Branching, striated, generally uninucleate cells that interdigitate at specialized junctions (intercalated discs). Striations Intercalated discs Function: As it contracts, it propels blood into the circulation; involuntary control. Location: The walls of the heart. Nucleus Photomicrograph: Cardiac muscle (500X); notice the striations, branching of cells, and the intercalated discs. (c) Smooth muscle Description: Spindle-shaped cells with central nuclei; no striations; cells arranged closely to form sheets. Function: Propels substances or objects (foodstuffs, urine, a baby) along internal passageways; involuntary control. Location: Mostly in the walls of hollow organs. Smooth muscle cell Nuclei Photomicrograph: Sheet of smooth muscle (200x). Bone Epimysium Tendon Blood vessel Figure 9.1a Connective tissue Fascicle (wrapped by perimysium) sheaths of skeletal muscle: Endomysium (between individual epimysium, perimysium, and muscle fibers) endomysium. Muscle fiber Perimysium Fascicle Epimysium Perimysium Figure 9.1b Connective tissue sheaths of skeletal muscle: Endomysium epimysium, perimysium, and fiber endomysium. Muscle in middle of a fascicle Nuclei Dark A band Figure 9.2a MicroscopicLight I band anatomy of a skeletal muscle fiber. Fiber Axon terminal of neuromuscular junction Action potential Sarcolemma (AP) of the muscle fiber Nucleus Myelinated axon of motor neuron Figure 9.8 Events at the Neuromuscular Junction (1 of 4) Myelinated axon of motor neuron Axon terminal of neuromuscular junction Action potential (AP) Nucleus Sarcolemma of the muscle fiber Figure 9.8 Events at the Neuromuscular Junction 1 Action potential arrives at axon terminal of motor neuron. 2 Voltage-gated Ca2+ channels Ca2+ Ca2+ open and Ca2+ enters the axon terminal. Axon terminal of motor neuron 3 Ca2+ entry causes some Fusing synaptic vesicles synaptic vesicles to release their contents (acetylcholine) by exocytosis. ACh 4 Acetylcholine, a neurotransmitter, diffuses across the synaptic cleft and binds to receptors in the sarcolemma. by its enzymatic breakdown in the synaptic cleft by acetylcholinesterase. Junctional folds of sarcolemma Sarcoplasm of muscle fiber 5 ACh binding opens ion channels that allow simultaneous passage of Na+ into the muscle fiber and K+ out of the muscle fiber. 6 ACh effects are terminated Synaptic vesicle containing ACh Mitochondrion Synaptic cleft Na+ K+ Ach– Degraded ACh Na+ Acetylcholinesterase K+ Postsynaptic membrane ion channel opens; ions pass. Postsynaptic membrane ion channel closed; ions cannot pass. 1 Action potential arrives at axon terminal of motor neuron. 2 Voltage-gated Ca2+ channels open and Ca2+ enters the axon terminal. 3 Ca2+ entry causes some synaptic vesicles to release their contents (acetylcholine) by exocytosis. 4 Acetylcholine, a neurotransmitter, diffuses across the synaptic cleft and binds to receptors in the sarcolemma. Ca2+ Ca2+ Axon terminal of motor neuron Synaptic vesicle containing ACh Mitochondrion Synaptic cleft Fusing synaptic vesicles ACh Junctional folds of sarcolemma Sarcoplasm of muscle fiber Sarcolemma Mitochondrion Myofibril Dark A band Light I band Nucleus Thin (actin) filament Z disc H zone Z disc Figure 9.2c Microscopic anatomy of a skeletal muscle Thick (myosin) I band A band I band M line fiber. filament Sarcomere Sarcomere Z disc M line Z disc (actin) Figure 9.2d Microscopic Thin filament Elastic (titin) filaments anatomy of a skeletal muscle Thick (myosin) fiber. filament Myosin filament Actin filament I band thin filaments only H zone thick filaments only M line Outer edge of A band thick filaments linked by thick and thin accessory filaments overlap proteins Longitudinal section of filaments within one sarcomere of a myofibril Thick filament Figure 9.3 Composition of thick and thin filaments. Thin filament In the center of the sarcomere, the thick filaments lack myosin heads. Myosin heads are present only in areas of myosin-actin overlap. Thick filament Thin filament Each thick filament consists of many A thin filament consists of two strands myosin molecules whose heads protrude of actin subunits twisted into a helix at opposite ends of the filament. plus two types of regulatory proteins (troponin and tropomyosin). Portion of a thick filament Portion of a thin filament Myosin head Tropomyosin Troponin Actin Actin-binding sites ATPbinding site Heads Tail Flexible hinge region Myosin molecule Active sites for myosin attachment Actin subunits Actin subunits Longitudinal section of filaments within one sarcomere of a myofibril Figure 9.3 Composition of thick and thin filaments (1 of 3). Thick filament Thin filament Thick filament Each thick filament consists of many myosin molecules whose heads protrude at opposite ends of the filament. Portion of a thick filament Myosin head Figure 9.3 Composition of thick and thin filaments (2 of 3). Actin-binding sites Heads ATPbinding site Flexible hinge region Myosin molecule Tail Thin filament A thin filament consists of two strands of actin subunits twisted into a helix plus two types of regulatory proteins (troponin and tropomyosin). Portion of a thin filament Tropomyosin Troponin Actin Figure 9.3 Composition of thick and thin filaments (3 of 3). Active sites for myosin attachment Actin subunits Actin subunits Thin filament (actin) Myosin heads Thick filament (myosin) Part of a skeletal muscle fiber (cell) Myofibril I band A band I band Z disc H zone Z disc M line Figure 9.5 Relationship ofSarcolemma the Triad: sarcoplasmic reticulum and T • T tubule • Terminal cisternae Sarcolemma tubules to myofibrils of skeletal of the SR (2) Tubules of muscle. the SR Myofibrils Mitochondria Z Z H A I 1 Fully relaxed sarcomere of a muscle fiber Z I I Z A I 2 Fully contracted sarcomere of a muscle fiber Figure 9.6 Sliding filament model of contraction (1 of 2). Z I H A 1 Fully relaxed sarcomere of a muscle fiber Z I Figure 9.6 Sliding filament model of contraction (2 of 2). Z Z I A I 2 Fully contracted sarcomere of a muscle fiber Latent Period of period contraction Period of relaxation Figure 9.14a The muscle twitch. Single stimulus Latent period Extraocular muscle (lateral rectus) Gastrocnemius Soleus Figure 9.14b The muscle twitch. Single stimulus Single stimulus single twitch Figure 9.15a Muscle response Contraction to changes in stimulation Relaxation frequency. Stimulus Low stimulation frequency unfused (incomplete) tetanus Partial relaxation Figure 9.15b Muscle response to changes in stimulation Stimuli frequency. High stimulation frequency fused (complete) tetanus Figure 9.15c Muscle response to changes in stimulation frequency. Stimuli Stimulus strength Maximal stimulus Threshold stimulus Proportion of motor units excited Strength of muscle contraction Maximal contraction Figure 9.17 The size principle of recruitment. Motor unit 1 Recruited (small fibers) Motor unit 2 recruited (medium fibers) Motor unit 3 recruited (large fibers) (a) Direct phosphorylation Coupled reaction of creatine phosphate (CP) and ADP Energy source: CP CP ADP Figure 9.19a Pathways for Creatine regenerating ATP kinase during muscle activity. Creatine ATP Oxygen use: None Products: 1 ATP per CP, creatine Duration of energy provision: 15 seconds (b) Anaerobic pathway Glycolysis and lactic acid formation Energy source: glucose Glucose (from glycogen breakdown or delivered from blood) Figure 9.19b Pathways for regenerating ATP during muscle activity. Glycolysis in cytosol 2 O2 ATP net gain Released to blood Pyruvic acid O2 Lactic acid Oxygen use: None Products: 2 ATP per glucose, lactic acid Duration of energy provision: 60 seconds, or slightly more (c) Aerobic pathway Aerobic cellular respiration Energy source: glucose; pyruvic acid; free fatty acids from adipose tissue; amino acids from protein catabolism Glucose (from glycogen breakdown or delivered from blood) Figure 9.19c Pathways for regenerating ATP during muscle activity. O2 Pyruvic acid Fatty acids O2 Aerobic respiration Aerobic respiration in mitochondria mitochondria Amino acids 32 CO2 H2O ATP net gain per glucose Oxygen use: Required Products: 32 ATP per glucose, CO2, H2O Duration of energy provision: Hours