(a) Skeletal muscle Description: Long, cylindrical, multinucleate cells; obvious striations. Function: Voluntary movement; locomotion; manipulation of the environment; facial expression; voluntary control. Location: In skeletal muscles attached to bones or occasionally to skin. Photomicrograph: Skeletal muscle (approx. 460x). Notice the obvious banding pattern and the fact that these large cells are multinucleate. Nuclei Striations Part of muscle fiber (cell)

(b) Cardiac muscle Description: Branching, striated, generally uninucleate cells that interdigitate at specialized junctions (intercalated discs). Function: As it contracts, it propels blood into the circulation; involuntary control. Location: The walls of the heart. Photomicrograph: Cardiac muscle (500X); notice the striations, branching of cells, and the intercalated discs. Intercalated discs Striations Nucleus

(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 passage- ways; involuntary control. Location: Mostly in the walls of hollow organs. Photomicrograph: Sheet of smooth muscle (200x). Smooth muscle cell Nuclei

Figure 9.1a Connective tissue sheaths of skeletal muscle: epimysium, perimysium, and endomysium. Bone Perimysium Endomysium (between individual muscle fibers) Muscle fiber Fascicle (wrapped by perimysium) Epimysium Tendon Blood vessel Fascicle

Figure 9.1b Connective tissue sheaths of skeletal muscle: epimysium, perimysium, and endomysium. Epimysium Muscle fiber in middle of a fascicle Perimysium Endomysium

Figure 9.2a Microscopic anatomy of a skeletal muscle fiber. Nuclei Fiber Dark A band Light I band

Figure 9.8 Events at the Neuromuscular Junction (1 of 4) Nucleus Action potential (AP) Myelinated axon of motor neuron Axon terminal of neuromuscular junction Sarcolemma of the muscle fiber

Figure 9.8 Events at the Neuromuscular Junction Nucleus Action potential (AP) Myelinated axon of motor neuron Axon terminal of neuromuscular junction Sarcolemma of the muscle fiber Ca 2+ Axon terminal of motor neuron Synaptic vesicle containing ACh Mitochondrion Synaptic cleft Junctional folds of sarcolemma Fusing synaptic vesicles ACh Sarcoplasm of muscle fiber Postsynaptic membrane ion channel opens; ions pass. Na + K+K+ Ach – Na + K+K+ Degraded ACh Acetyl- cholinesterase Postsynaptic membrane ion channel closed; ions cannot pass. 1 Action potential arrives at axon terminal of motor neuron. 2 Voltage-gated Ca 2+ channels open and Ca 2+ enters the axon terminal. 3 Ca 2+ 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. 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 by its enzymatic breakdown in the synaptic cleft by acetylcholinesterase.

Action potential arrives at axon terminal of motor neuron. Voltage-gated Ca 2+ channels open and Ca 2+ enters the axon terminal. Ca 2+ entry causes some synaptic vesicles to release their contents (acetylcholine) by exocytosis. Acetylcholine, a neurotransmitter, diffuses across the synaptic cleft and binds to receptors in the sarcolemma. Ca 2+ Axon terminal of motor neuron Synaptic vesicle containing ACh Mitochondrion Synaptic cleft Junctional folds of sarcolemma Fusing synaptic vesicles ACh Sarcoplasm of muscle fiber Ca

NucleusLight I bandDark A band Sarcolemma Mitochondrion Myofibril

Figure 9.2c Microscopic anatomy of a skeletal muscle fiber. I band A band Sarcomere H zone Thin (actin) filament Thick (myosin) filament Z disc M line

Figure 9.2d Microscopic anatomy of a skeletal muscle fiber. Z disc M line Sarcomere Thin (actin) filament Thick (myosin) filament Elastic (titin) filaments

I band thin filaments only Actin filament Myosin filament H zone thick filaments only M line thick filaments linked by accessory proteins Outer edge of A band thick and thin filaments overlap

Figure 9.3 Composition of thick and thin filaments. Flexible hinge region Tail Tropomyosin TroponinActin Myosin head ATP- binding site Heads Active sites for myosin attachment Actin subunits Actin-binding sites Thick filament Each thick filament consists of many myosin molecules whose heads protrude at opposite ends of the filament. 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). Thin filament Thick filament In the center of the sarcomere, the thick filaments lack myosin heads. Myosin heads are present only in areas of myosin-actin overlap. Longitudinal section of filaments within one sarcomere of a myofibril Portion of a thick filament Portion of a thin filament Myosin molecule Actin subunits

Figure 9.3 Composition of thick and thin filaments (1 of 3). Thin filament Thick filament Longitudinal section of filaments within one sarcomere of a myofibril

Figure 9.3 Composition of thick and thin filaments (2 of 3). Flexible hinge region Tail Myosin head ATP- binding site Heads Actin-binding sites 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 molecule

Figure 9.3 Composition of thick and thin filaments (3 of 3). TropomyosinTroponinActin Active sites for myosin attachment Actin subunits 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 Actin subunits

Thin filament (actin)Thick filament (myosin)Myosin heads

Figure 9.5 Relationship of the sarcoplasmic reticulum and T tubules to myofibrils of skeletal muscle. Myofibril Myofibrils Triad: Tubules of the SR Sarcolemma Mitochondria I band A band H zoneZ disc Part of a skeletal muscle fiber (cell) T tubule Terminal cisternae of the SR (2) M line

I Fully relaxed sarcomere of a muscle fiber Fully contracted sarcomere of a muscle fiber I A ZZ H IIA ZZ 1 2

Figure 9.6 Sliding filament model of contraction (1 of 2). II A ZZ H 1 Fully relaxed sarcomere of a muscle fiber

Figure 9.6 Sliding filament model of contraction (2 of 2). IIA ZZ 2 Fully contracted sarcomere of a muscle fiber

Figure 9.14a The muscle twitch. Latent period Single stimulus Period of contraction Period of relaxation

Figure 9.14b The muscle twitch. Latent period Extraocular muscle (lateral rectus) Gastrocnemius Soleus Single stimulus

Figure 9.15a Muscle response to changes in stimulation frequency. Contraction Relaxation Stimulus Single stimulussingle twitch

Figure 9.15b Muscle response to changes in stimulation frequency. Stimuli Partial relaxation Low stimulation frequency unfused (incomplete) tetanus

Figure 9.15c Muscle response to changes in stimulation frequency. Stimuli High stimulation frequency fused (complete) tetanus

Stimulus strength Proportion of motor units excited Strength of muscle contraction Maximal contraction Maximal stimulus Threshold stimulus

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)

Figure 9.19a Pathways for regenerating ATP during muscle activity. Coupled reaction of creatine phosphate (CP) and ADP Energy source: CP (a) Direct phosphorylation Oxygen use: None Products: 1 ATP per CP, creatine Duration of energy provision: 15 seconds Creatine kinase ADPCP Creatine ATP

Figure 9.19b Pathways for regenerating ATP during muscle activity. Energy source: glucose Glycolysis and lactic acid formation (b) Anaerobic pathway Oxygen use: None Products: 2 ATP per glucose, lactic acid Duration of energy provision: 60 seconds, or slightly more Glucose (from glycogen breakdown or delivered from blood) Glycolysis in cytosol Pyruvic acid Released to blood net gain 2 Lactic acid O2O2 O2O2 ATP

Figure 9.19c Pathways for regenerating ATP during muscle activity. Energy source: glucose; pyruvic acid; free fatty acids from adipose tissue; amino acids from protein catabolism (c) Aerobic pathway Aerobic cellular respiration Oxygen use: Required Products: 32 ATP per glucose, CO 2, H 2 O Duration of energy provision: Hours Glucose (from glycogen breakdown or delivered from blood) 32 O2O2 O2O2 H2OH2O CO 2 Pyruvic acid Fatty acids Amino acids Aerobic respiration in mitochondria Aerobic respiration in mitochondria ATP net gain per glucose