Table 9.3

Special Characteristics of Muscle Tissue Excitability (responsiveness or irritability): ability to receive and respond to stimuli Contractility: ability to shorten when stimulated Extensibility: ability to be stretched Elasticity: ability to recoil to resting length

Muscle Functions Movement of bones or fluids (e.g., blood) Maintaining posture and body position Stabilizing joints Heat generation (especially skeletal muscle)

Skeletal Muscle Each muscle is served by one artery, one nerve, and one or more veins

(wrapped by perimysium) Epimysium Epimysium Bone Perimysium Tendon Endomysium Muscle fiber in middle of a fascicle (b) Blood vessel Fascicle (wrapped by perimysium) Endomysium (between individual muscle fibers) Perimysium Fascicle Muscle fiber (a) Figure 9.1

Microscopic Anatomy of a Skeletal Muscle Fiber Cylindrical cell 10 to 100 m in diameter, up to 30 cm long Multiple nuclei Many mitochondria Glycogen storage, myoglobin for O2 storage

Sarcolemma Mitochondrion Myofibril Dark A band Light I band Nucleus (b) Diagram of part of a muscle fiber showing the myofibrils. One myofibril is extended afrom the cut end of the fiber.

Thin (actin) filament Z disc H zone Z disc Thick (myosin) filament I band A band Sarcomere I band M line (c) Small part of one myofibril enlarged to show the myofilaments responsible for the banding pattern. Each sarcomere extends from one Z disc to the next. Sarcomere Z disc M line Z disc Thin (actin) filament Elastic (titin) filaments Thick (myosin) filament (d) Enlargement of one sarcomere (sectioned lengthwise). Notice the myosin heads on the thick filaments. Figure 9.2c, d

Longitudinal section of filaments within one sarcomere of a myofibril Thick filament 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 myosin molecules whose heads protrude at opposite ends of the 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 thick filament Portion of a thin filament Myosin head Tropomyosin Troponin Actin Actin-binding sites Heads Tail Active sites for myosin attachment ATP- binding site Actin subunits Flexible hinge region Myosin molecule Actin subunits Figure 9.3

Part of a skeletal muscle fiber (cell) I band A band I band Z disc H zone Z disc Myofibril M line Sarcolemma Triad: • T tubule • Terminal cisternae of the SR (2) Sarcolemma Tubules of the SR Myofibrils Mitochondria Figure 9.5

1 2 Figure 9.6 Z H Z I A I Fully relaxed sarcomere of a muscle fiber Z Fully contracted sarcomere of a muscle fiber Figure 9.6

Myelinated axon of motor neuron Action potential (AP) Axon terminal of neuromuscular junction Nucleus Sarcolemma of the muscle fiber Action potential arrives at axon terminal of motor neuron. 1 Ca2+ Synaptic vesicle containing ACh Ca2+ Voltage-gated Ca2+ channels open and Ca2+ enters the axon terminal. 2 Mitochondrion Synaptic cleft Axon terminal of motor neuron Fusing synaptic vesicles Figure 9.8 Figure 9.8

Figure 9.8 1 2 3 4 5 6 Myelinated axon of motor neuron Action potential (AP) Axon terminal of neuromuscular junction Nucleus Sarcolemma of the muscle fiber Action potential arrives at axon terminal of motor neuron. 1 Ca2+ Synaptic vesicle containing ACh Ca2+ Voltage-gated Ca2+ channels open and Ca2+ enters the axon terminal. 2 Mitochondrion Synaptic cleft Axon terminal of motor neuron Ca2+ entry causes some synaptic vesicles to release their contents (acetylcholine) by exocytosis. 3 Fusing synaptic vesicles Junctional folds of sarcolemma ACh Acetylcholine, a neurotransmitter, diffuses across the synaptic cleft and binds to receptors in the sarcolemma. 4 Sarcoplasm of muscle fiber ACh binding opens ion channels that allow simultaneous passage of Na+ into the muscle fiber and K+ out of the muscle fiber. 5 Na+ K+ Postsynaptic membrane ion channel opens; ions pass. ACh effects are terminated by its enzymatic breakdown in the synaptic cleft by acetylcholinesterase. 6 Ach– Degraded ACh Postsynaptic membrane ion channel closed; ions cannot pass. Na+ Acetyl- cholinesterase K+ Figure 9.8

Terminal cisterna of SR Setting the stage Axon terminal of motor neuron Synaptic cleft Action potential is generated ACh Sarcolemma Terminal cisterna of SR Muscle fiber Ca2+ Triad One sarcomere Figure 9.11, step 1

Figure 9.11, step 2 Steps in E-C Coupling: The aftermath Sarcolemma Voltage-sensitive tubule protein T tubule 1 Action potential is propagated along the sarcolemma and down the T tubules. Ca2+ release channel Calcium ions are released. 2 Terminal cisterna of SR Ca2+ Actin Troponin Tropomyosin blocking active sites Ca2+ Myosin Calcium binds to troponin and removes the blocking action of tropomyosin. 3 Active sites exposed and ready for myosin binding Contraction begins 4 Myosin cross bridge The aftermath Figure 9.11, step 2

Figure 9.12 Thin filament Actin Ca2+ Myosin cross bridge Thick ADP Pi Thick filament Myosin 1 Cross bridge formation. ADP ADP Pi ATP hydrolysis Pi 4 Cocking of myosin head. 2 The power (working) stroke. ATP ATP 3 Cross bridge detachment. Figure 9.12

Motor Unit: The Nerve-Muscle Functional Unit Motor unit = a motor neuron and all (four to several hundred) muscle fibers it supplies

neuromuscular junctions Spinal cord Motor neuron cell body Muscle Nerve Motor unit 1 unit 2 fibers neuron axon Axon terminals at neuromuscular junctions Axons of motor neurons extend from the spinal cord to the muscle. There each axon divides into a number of axon terminals that form neuromuscular junctions with muscle fibers scattered throughout the muscle. Figure 9.13a

Motor Unit Small motor units in muscles that control fine movements (fingers, eyes) Large motor units in large weight-bearing muscles (thighs, hips)

Motor Unit Muscle fibers from a motor unit are spread throughout the muscle so that a single motor unit causes weak contraction of entire muscle Motor units in a muscle usually contract asynchronously; helps prevent fatigue

Motor unit 1 Recruited (small fibers) unit 2 recruited (medium unit 3 (large Figure 9.17

Muscle Tone Constant, slightly contracted state of all muscles Due to spinal reflexes that activate groups of motor units alternately in response to input from stretch receptors in muscles Keeps muscles firm, healthy, and ready to respond

Muscle Metabolism: Energy for Contraction ATP is the only source used directly for contractile activities Available stores of ATP are depleted in 4–6 seconds

Muscle Metabolism: Energy for Contraction ATP is regenerated by: Direct phosphorylation of ADP by creatine phosphate Anaerobic pathway Aerobic respiration

At 70% of max contractile activity: Anaerobic Pathway At 70% of max contractile activity: Bulging muscles compress blood vessels Oxygen delivery is impaired Build up lactic acid

Glucose (from glycogen breakdown or delivered from blood) 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 O2 ATP Figure 9.19b

Effects of Exercise Aerobic (endurance) exercise: Leads to increased: Muscle capillaries Number of mitochondria Myoglobin synthesis Results in greater endurance, strength, and resistance to fatigue May convert fast glycolytic fibers into fast oxidative fibers

Effects of Resistance Exercise Resistance exercise (typically anaerobic) results in: Muscle hypertrophy (due to increase in fiber size) Increased mitochondria, myofilaments, glycogen stores, and connective tissue

The Overload Principle Forcing a muscle to work hard promotes increased muscle strength and endurance Muscles adapt to increased demands Muscles must be overloaded to produce further gains

Smooth Muscle Found in walls of most hollow organs (except heart) Usually in two layers (longitudinal and circular)

Longitudinal layer of smooth muscle (shows smooth muscle fibers in cross section) Small intestine Mucosa (a) (b) Cross section of the intestine showing the smooth muscle layers (one circular and the other longitudinal) running at right angles to each other. Circular layer of smooth muscle (shows longitudinal views of smooth muscle fibers) Figure 9.26

Peristalsis Alternating contractions and relaxations of smooth muscle layers that mix and squeeze substances through the lumen of hollow organs Longitudinal layer contracts; organ dilates and shortens Circular layer contracts; organ constricts and elongates

Contraction of Smooth Muscle Very energy efficient (slow ATPases) Myofilaments may maintain for prolonged contractions

Special Features of Smooth Muscle Contraction Hyperplasia: Smooth muscle cells can divide and increase their numbers Example: estrogen effects on uterus at puberty and during pregnancy

Developmental Aspects With age, connective tissue increases and muscle fibers decrease By age 30, loss of muscle mass (sarcopenia) begins Regular exercise reverses sarcopenia Atherosclerosis may block distal arteries, leading to intermittent claudication and severe pain in leg muscles