Muscular System The Muscular System Three basic muscle

Muscular System

The Muscular System · Three basic muscle types are found in the body · Skeletal muscle · Cardiac muscle · Smooth muscle

Characteristics of Muscles · (muscle cell = muscle fiber) · Excitability (responsiveness or irritability): · Contractility: · Extensibility: · Elasticity:

Skeletal Muscle Characteristics · Most are attached by tendons to bones · Cells are multinucleate · Striated · Voluntary · surrounded and bundled by connective tissue

Smooth Muscle Characteristics · Has no striations · Single nucleus · Involuntary · Found mainly in the walls of hollow organs

Cardiac Muscle Characteristics · Has striations · Usually has a single nucleus · Joined to another muscle cell at an intercalated disc · Involuntary · Found only in the heart

Table 9. 3

Connective Tissue Wrappings of Skeletal Muscle · Endomysium · Dense regular · Perimysium · fibrous · Epimysium · loose · Fascia

Epimysium Bone Epimysium Perimysium Endomysium Tendon (b) Perimysium Fascicle (a) Muscle fiber in middle of a fascicle Blood vessel Fascicle (wrapped by perimysium) Endomysium (between individual muscle fibers) Muscle fiber Figure 9. 1

Table 9. 1

Skeletal Muscle Attachments · Sites of muscle attachment · Bones · Cartilages · Connective tissue coverings

Function of Muscles · Produce movement · Maintain posture · Stabilize joints · Generate heat

Microscopic Anatomy of a Skeletal Muscle Fiber • Cylindrical cell 10 to 100 m in diameter, up to 30 cm long • Multiple peripheral nuclei • Many mitochondria • Glycosomes for glycogen storage, myoglobin for O 2 storage • Also contain myofibrils, sarcoplasmic reticulum, and T tubules

Microscopic Anatomy of Skeletal Muscle

Microscopic Anatomy of Skeletal Muscle · Myofibril · Bundles of myofilaments

Features of a Sarcomere • Contractile unit • Thick filaments: run the entire length of an A band • Thin filaments: run the length of the I band partway into the A band • Z disc: coin-shaped sheet of proteins that anchors the thin filaments and connects myofibrils to one another • H zone: lighter midregion where filaments do not overlap • M line: line of protein myomesin that holds adjacent thick filaments together

Thin (actin) filament Thick (myosin) filament Z disc I band H zone A band Sarcomere Z disc 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. 2 c, d

Sarcomere

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 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-binding sites ATPbinding site Heads Tail Flexible hinge region Myosin molecule Active sites for myosin attachment Actin subunits Figure 9. 3

Microscopic Anatomy of Skeletal Muscle

Microscopic Anatomy of Skeletal Muscle · Sarcolemma – (carry electrical current and connect to tendons · Sarcoplasmic reticulum- store Ca.

Sarcoplasmic Reticulum (SR) • Network of smooth endoplasmic reticulum surrounding each myofibril • Pairs of terminal cisternae form perpendicular cross channels • Functions in the regulation of intracellular Ca 2+ levels

T Tubules • Continuous with the sarcolemma • Penetrate the cell’s interior at each A band –I band junction • Associate with the paired terminal cisternae to form triads that encircle each sarcomere

Part of a skeletal muscle fiber (cell) Myofibril I band A band I band Z disc H zone Z disc M line Sarcolemma Triad: • T tubule • Terminal cisternae of the SR (2) Tubules of the SR Myofibrils Mitochondria Figure 9. 5

Nerve Stimulus to Muscles · Skeletal muscles must be stimulated by a nerve to contract

Nerve Stimulus to Muscles · Neuromuscular junctions · Synaptic cleft

Transmission of Nerve Impulse to Muscle · Neurotransmitter · for skeletal muscle is acetylcholine · Neurotransmitter attaches to receptors on the sarcolemma · Sarcolemma becomes permeable to sodium (Na+) · Sodium rushing into the cell generates an action potential

Myelinated axon of motor neuron Axon terminal of neuromuscular junction Sarcolemma of the muscle fiber Action potential (AP) Nucleus 1 Action potential arrives at axon terminal of motor neuron. 2 Voltage-gated Ca 2+ channels open and Ca 2+ enters the axon terminal. Ca 2+ Axon terminal of motor neuron 3 Ca 2+ entry causes some Fusing synaptic vesicles to release their contents (acetylcholine) by exocytosis. ACh 4 Acetylcholine, a 5 ACh binding opens ion Na+ K+ channels that allow simultaneous passage of Na+ into the muscle fiber and K+ out of the muscle fiber. by its enzymatic breakdown in the synaptic cleft by acetylcholinesterase. Junctional folds of sarcolemma Sarcoplasm of muscle fiber neurotransmitter, diffuses across the synaptic cleft and binds to receptors in the sarcolemma. 6 ACh effects are terminated Synaptic vesicle containing ACh Mitochondrion Synaptic cleft Ach– Degraded ACh Na+ Acetylcholinesterase Postsynaptic membrane ion channel opens; ions pass. Postsynaptic membrane ion channel closed; ions cannot pass. K+ Figure 9. 8

Axon terminal Open Na+ Channel Na+ Synaptic cleft ACh K+ iza tio n K+ ++ ++ + + Action potential + + +++ + 2 Generation and propagation of the action potential (AP) of d ep o l ar ACh Na+ K+ Na+ Closed K+ Channel ve Wa 1 Local depolarization: generation of the end plate potential on the sarcolemma Sarcoplasm of muscle fiber Figure 9. 9, step 2

1 Action potential is Steps in E-C Coupling: propagated along the sarcolemma and down the T tubules. Voltage-sensitive tubule protein Sarcolemma T tubule Ca 2+ release channel Terminal cisterna of SR 2 Calcium ions are released. Ca 2+ Figure 9. 11, step 4

Actin Ca 2+ Troponin Tropomyosin blocking active sites Myosin 3 Calcium binds to troponin and removes the blocking action of tropomyosin. Active sites exposed and ready for myosin binding Myosin cross bridge 4 Contraction begins The aftermath Figure 9. 11, step 7

Actin Ca 2+ Myosin cross bridge Thin filament ADP Pi Thick filament Myosin 1 Cross bridge formation. Figure 9. 12, step 1

ADP Pi 2 The power (working) stroke. Figure 9. 12, step 3

ATP 3 Cross bridge detachment. Figure 9. 12, step 4

Sliding Filament Model of Contraction • In the relaxed state, thin and thick filaments overlap only slightly • During contraction, myosin heads bind to actin, detach, and bind again, to propel the thin filaments toward the M line • As H zones shorten and disappear, sarcomeres shorten, muscle cells shorten, and the whole muscle shortens

The Sliding Filament Theory of Muscle Contraction

The Sliding Filament Theory

Contraction of a Skeletal Muscle · Muscle fiber contraction is “all or none” · not all fibers may be stimulated during the same interval · Graded responses

Types of Graded Responses · Twitch · Not a normal muscle function

Types of Graded Responses · Tetanus (summing of contractions) · One contraction is immediately followed by another · The muscle does not completely return to a resting state · The effects are added

Types of Graded Responses · Unfused (incomplete) tetanus · Fused (complete) tetanus Figure 6. 9 a, b

Muscle Response to Strong Stimuli · force depends upon the number of fibers stimulated · can continue to contract unless they run out of energy (ATP)

Energy for Muscle Contraction · Initially, muscles used stored ATP for energy · Only 4 -6 seconds worth of ATP is stored by muscles · After this initial time, other pathways must be utilized to produce ATP

Energy for Muscle Contraction · Direct phosphorylation · Muscle cells contain creatine phosphate (CP) · After ATP is depleted, ADP is left · CP transfers energy to ADP, to regenerate ATP · CP supplies are exhausted in about 20 seconds

Energy for Muscle Contraction · Aerobic Respiration · This is a slower reaction that requires continuous oxygen

Energy for Muscle Contraction · Anaerobic glycolysis · Pyruvic acid is converted to lactic acid · This reaction is not as efficient, but is fast · Huge amounts of glucose are needed · Lactic acid produces muscle fatigue Figure 6. 10 b Slide 6. 26 a

Muscle Fatigue and Oxygen Debt · When a muscle is fatigued, it is unable to contract · The common reason for muscle fatigue is oxygen debt · Oxygen is required to get rid of accumulated lactic acid · Increasing acidity (from lactic acid) and lack of ATP causes the muscle to contract less

Types of Muscle Contractions · Isotonic contractions · The muscle shortens · Isometric contractions · The muscle is unable to shorten

Muscle Tone · Some fibers are contracted even in a relaxed muscle · under involuntary control · The more muscle is used the more tone it becomes.

Muscles and Body Movements

Effects of Exercise on Muscle · Results of increased muscle use · Increase in muscle size · Increase in muscle strength · Increase in muscle efficiency · Muscle becomes more fatigue resistant

Large number of muscle fibers activated Large muscle fibers High frequency of stimulation Muscle and sarcomere stretched to slightly over 100% of resting length Contractile force Figure 9. 21

Muscle Fiber Type Classified according to two characteristics: 1. Speed of contraction: slow or fast, according to: – Speed at which myosin ATPases split ATP – Pattern of electrical activity of the motor neurons

Muscle Fiber Type 2. Metabolic pathways for ATP synthesis: – Oxidative fibers—use aerobic pathways – Glycolytic fibers—use anaerobic glycolysis

Muscle Fiber Type Three types: – Slow oxidative fibers – Fast glycolytic fibers

Table 9. 2

Longitudinal layer of smooth muscle (shows smooth muscle fibers in cross section) Small intestine (a) Mucosa (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

Microscopic Structure • Spindle-shaped fibers: thin and short compared with skeletal muscle fibers • Connective tissue: endomysium only • SR: less developed than in skeletal muscle • Pouchlike infoldings (caveolae) of sarcolemma sequester Ca 2+ • No sarcomeres, myofibrils, or T tubules

Myofilaments in Smooth Muscle • Ratio of thick to thin filaments (1: 13) is much lower than in skeletal muscle (1: 2) • Thick filaments have heads along their entire length • No troponin complex; protein calmodulin binds Ca 2+

Myofilaments in Smooth Muscle • Myofilaments are spirally arranged, causing smooth muscle to contract in a corkscrew manner • Dense bodies: proteins that anchor noncontractile intermediate filaments to sarcolemma at regular intervals

Figure 9. 28 a

Figure 9. 28 b