Three Types of Muscle Tissue 1 Skeletal muscle

Three Types of Muscle Tissue 1. Skeletal muscle tissue: • Attached to bones and skin • Striated • Voluntary (i. e. , conscious control) • Powerful • Primary topic of this chapter Copyright © 2010 Pearson Education, Inc.

Three Types of Muscle Tissue 2. Cardiac muscle tissue: • Only in the heart • Striated • Involuntary • More details in Chapter 18 Copyright © 2010 Pearson Education, Inc.

Three Types of Muscle Tissue 3. Smooth muscle tissue: • In the walls of hollow organs, e. g. , stomach, urinary bladder, and airways • Not striated • Involuntary Copyright © 2010 Pearson Education, Inc.

Copyright © 2010 Pearson Education, Inc. 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 Copyright © 2010 Pearson Education, Inc.

Muscle Functions 1. Movement of bones or fluids (e. g. , blood) 2. Maintaining posture and body position 3. Stabilizing joints 4. Heat generation (especially skeletal muscle) Copyright © 2010 Pearson Education, Inc.

Skeletal Muscle • Each muscle is served by one artery, one nerve, and one or more veins Copyright © 2010 Pearson Education, Inc.

Skeletal Muscle • Connective tissue sheaths of skeletal muscle: • Epimysium: dense regular connective tissue surrounding entire muscle • Perimysium: fibrous connective tissue surrounding fascicles (groups of muscle fibers) • Endomysium: fine areolar connective tissue surrounding each muscle fiber Copyright © 2010 Pearson Education, Inc.

Epimysium Bone Epimysium Perimysium Endomysium Tendon (b) Perimysium Fascicle (a) Copyright © 2010 Pearson Education, Inc. Muscle fiber in middle of a fascicle Blood vessel Fascicle (wrapped by perimysium) Endomysium (between individual muscle fibers) Muscle fiber Figure 9. 1

Skeletal Muscle: Attachments • Muscles attach: • Directly—epimysium of muscle is fused to the periosteum of bone or perichondrium of cartilage • Indirectly (more common) —connective tissue wrappings extend beyond the muscle as a ropelike tendon or sheetlike aponeurosis Copyright © 2010 Pearson Education, Inc.

Copyright © 2010 Pearson Education, Inc. Table 9. 1

Microscopic Anatomy of a Skeletal Muscle Fiber • Surrounded by sarcolemma (plasma membrane) • Long (huge) cylindrical cells (up to 30 cm!!!) • Multiple nuclei • Many mitochondria (Why So Many? ) • Glycosomes (for glycogen storage) & myoglobin (for O 2 storage) • Also contain myofibrils, sarcoplasmic reticulum, and T tubules Copyright © 2010 Pearson Education, Inc.

Myofibrils • Densely packed, rodlike elements (100’s to 1000’s per muscle fiber) • Makes up to 80% of muscle cell volume • Exhibit striations: perfectly aligned repeating series of dark A bands and light I bands Copyright © 2010 Pearson Education, Inc.

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. Copyright © 2010 Pearson Education, Inc.

Sarcomere • Smallest contractile unit (functional unit) of a muscle fiber • The region of a myofibril between two successive Z discs • Composed of thick and thin myofilaments made of contractile proteins responsible for muscle contraction Copyright © 2010 Pearson Education, Inc.

Features of a Sarcomere • 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 mid-region on either side of the M line. Only seen in resting muscle fibers • M line: Found in the center of the H zone, it is a line of protein myomesin (M for middle) Copyright © 2010 Pearson Education, Inc.

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. Copyright © 2010 Pearson Education, Inc. Figure 9. 2 c, d

Thick Filament (Myosin) • Composed of the protein myosin • Myosin tails contain: • 2 interwoven, heavy polypeptide chains • Myosin heads contain: • The “Business End” that act as cross bridges during contraction • Binding sites for actin of thin filaments • Binding sites for ATP • ATPase enzymes (split ATP to generate energy) Copyright © 2010 Pearson Education, Inc.

Thin Filament (Actin) • Composed mostly of protein actin • Bears active sites for the cross-bridges (heads of myosin) during contraction • Contains tropomyosin and troponin: regulatory proteins bound to actin • Elastic filament (made of titan protein) allows muscle to spring back into place Copyright © 2010 Pearson Education, Inc.

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 Copyright © 2010 Pearson Education, Inc. Myosin molecule Active sites for myosin attachment Actin subunits Figure 9. 3

Sarcoplasmic Reticulum (SR) • Network of smooth endoplasmic reticulum surrounding each myofibril • Pairs of terminal cisternae (reservoirs) form perpendicular cross channels • Regulates calcium - stores and releases Ca+ for contraction (we’ll talk about Ca+ later) Copyright © 2010 Pearson Education, Inc.

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 ***NOTE: A skeletal muscle is very long. Ttubules allow the electrical stimulus and ECF to come in contact with deep regions which makes muscle reaction occur quicker Copyright © 2010 Pearson Education, Inc.

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 Copyright © 2010 Pearson Education, Inc. Figure 9. 5

CHECK POINT!!!!! 1. ) Which myofilaments have heads that form cross-bridges that are important during contraction? Thick filaments 2. ) What surrounds the myofibril and regulates Ca+ needed for contraction? Sarcoplasmic Reticulum Copyright © 2010 Pearson Education, Inc.

You have 7 minutes from the time the bell rings…. If you don’t turn it in on time, you don’t get the credit! Briefly explain how actin and myosin work together in the sliding filament model Explain the major role of the sarcoplasmic reticulum (SR), especially the terminal cisternae. What key substance provides the final “go” signal for contraction? Hint: pg 282 What structure works close with the SR and forms a triad relationship? Copyright © 2010 Pearson Education, Inc.

Contraction • The generation of force • Shortening occurs when tension from the cross bridges on the thin filaments pulls the thin filament toward the M line ultimately contracting or shortening the muscle fiber Copyright © 2010 Pearson Education, Inc.

Sliding Filament Model of Contraction • In the relaxed state, thin and thick filaments only overlap 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 and muscle cells shorten, thus, the whole muscle shortens Copyright © 2010 Pearson Education, Inc.

Sarcomere Contraction Animation Z Z H A I I 1 Fully relaxed sarcomere of a muscle fiber Z I Z A I 2 Fully contracted sarcomere of a muscle fiber Copyright © 2010 Pearson Education, Inc. Figure 9. 6

So now we know how a muscle fiber contracts…but what causes it to contract? 1. Activation: neural stimulation at a neuromuscular junction 2. Excitation-contraction coupling: • Creation and increase of an action potential (electrical current) along the sarcolemma • Final trigger: a brief rise in intracellular Ca+ levels Copyright © 2010 Pearson Education, Inc.

Step One- The Activation Step: Takes place at the Neuromuscular Junction • Skeletal muscles are stimulated by somatic motor neurons • Axons of motor neurons travel from the central nervous system (brain or spinal cord) via nerves to skeletal muscles • Each axon forms several branches as it enters a muscle • Each axon ending forms a neuromuscular junction with a single muscle fiber Copyright © 2010 Pearson Education, Inc.

The axon of each motor neuron divides profusely and forms a neuromuscular junction at each muscle fiber Copyright © 2010 Pearson Education, Inc.

Action potential (AP) Myelinated axon of motor neuron Axon terminal of neuromuscular junction Nucleus Sarcolemma of the muscle fiber 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 Synaptic vesicle containing ACh Mitochondrion Synaptic cleft Fusing synaptic vesicles Copyright © 2010 Pearson Education, Inc. Figure 9. 8

Neuromuscular Junction • Situated midway along the length of a muscle fiber • Axon terminal and muscle fiber are separated by a gel-filled space called the synaptic cleft • Synaptic vesicles within the axon terminal contain the neurotransmitter acetylcholine (ACh) • Junctional folds of the sarcolemma contain ACh receptors Copyright © 2010 Pearson Education, Inc.

Events at the Neuromuscular Junction • Nerve impulse arrives at axon terminal • Voltage sensitive Calcium channels open and release Calcium into the axon terminal • Due to increased Calcium levels, ACh is released and binds with receptors on the sarcolemma which triggers electrical events • These electrical events lead to the creation of an action potential (electrical current) which spreads down the sarcolemma Copyright © 2010 Pearson Education, Inc.

Destruction of Acetylcholine • ACh effects are quickly stopped by the enzyme acetylcholinesterase which is located in the synaptic cleft • Prevents continued muscle fiber contraction by breaking down Ach into basic “non-stimulating” components Copyright © 2010 Pearson Education, Inc.

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. Copyright © 2010 Pearson Education, Inc. 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 tio n za of de po l ari ACh Na+ K+ e Wav 1 Local depolarization: generation of the end plate potential on the sarcolemma Sarcoplasm of muscle fiber Copyright © 2010 Pearson Education, Inc. ++ ++ + + Closed K+ Channel K+ Action potential + + +++ + 2 Generation and propagation of the action potential (AP) Closed Na+ Open K+ Channel Na+ K+ 3 Repolarization Figure 9. 9

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 Copyright © 2010 Pearson Education, Inc. Figure 9. 9, step 2

Excitation-Contraction (E-C) Coupling • Sequence of events by which transmission of an AP along the sarcolemma leads to sliding of the myofilaments • Occurs during latent period: • Time between AP initiation and the beginning of contraction Copyright © 2010 Pearson Education, Inc.

Events of Excitation-Contraction (E-C) Coupling • AP is spread along sarcolemma to the T tubules • Voltage-sensitive proteins stimulate the SR to release Ca+ • Ca+ is necessary for contraction Copyright © 2010 Pearson Education, Inc.

Setting the stage Axon terminal of motor neuron Action potential Synaptic cleft is generated ACh Sarcolemma Terminal cisterna of SR Muscle fiber Ca 2+ Triad One sarcomere Copyright © 2010 Pearson Education, Inc. Figure 9. 11, step 1

Steps in E-C Coupling: Sarcolemma Voltage-sensitive tubule protein T tubule 1 Action potential is propagated along the sarcolemma and down the T tubules. Ca 2+ release channel 2 Calcium ions are released. Terminal cisterna of SR Ca 2+ Actin Troponin Ca 2+ 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 4 Contraction begins Myosin cross bridge The aftermath Copyright © 2010 Pearson Education, Inc. Figure 9. 11, 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 Ca 2+ Copyright © 2010 Pearson Education, Inc. Figure 9. 11, step 3

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+ Copyright © 2010 Pearson Education, Inc. Figure 9. 11, step 4

Actin Ca 2+ Troponin Tropomyosin blocking active sites Myosin The aftermath Copyright © 2010 Pearson Education, Inc. Figure 9. 11, step 5

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 The aftermath Copyright © 2010 Pearson Education, Inc. Figure 9. 11, step 6

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 Copyright © 2010 Pearson Education, Inc. Figure 9. 11, step 7

Steps in E-C Coupling: Sarcolemma Voltage-sensitive tubule protein T tubule 1 Action potential is propagated along the sarcolemma and down the T tubules. Ca 2+ release channel 2 Calcium ions are released. Terminal cisterna of SR Ca 2+ Actin Troponin Ca 2+ 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 4 Contraction begins Myosin cross bridge The aftermath Copyright © 2010 Pearson Education, Inc. Figure 9. 11, step 8

Role of Calcium (Ca 2+) in Contraction • At low intracellular Ca 2+ concentration: • Tropomyosin blocks the active sites on actin • Myosin heads cannot attach to actin • Muscle fiber relaxes Copyright © 2010 Pearson Education, Inc.

Role of Calcium (Ca 2+) in Contraction • At higher intracellular Ca+ concentrations: • Ca+ binds to troponin • Troponin changes shape and moves tropomyosin away from active sites • Events of the cross bridge cycle occur • When nervous stimulation stops, Ca+ is pumped back into the SR and contraction ends Copyright © 2010 Pearson Education, Inc.

Cross Bridge Cycle Continues as long as the Ca+ signal and adequate ATP are present 1. Cross bridge formation—Energized myosin head attaches to actin on the thin filament forming a “cross bridge” 2. Working (power) stroke—ADP and Pi are released and the myosin head pivots and bends (low energy shape), pulling the thin filament toward M line Copyright © 2010 Pearson Education, Inc.

Cross Bridge Cycle 3. Cross bridge detachment—ATP attaches to myosin head weakening the link and the cross bridge detaches 4. “Cocking” of the myosin head—energy from hydrolysis of ATP back to ADP and Pi cocks the myosin head into the high-energy state Copyright © 2010 Pearson Education, Inc.

Thin filament Actin Ca 2+ Myosin cross bridge ADP Pi Thick filament Myosin Cross bridge formation. 1 ADP Pi Pi ATP hydrolysis 2 The power (working) stroke. 4 Cocking of myosin head. ATP 3 Cross bridge detachment. Copyright © 2010 Pearson Education, Inc. Figure 9. 12

Actin Ca 2+ Myosin cross bridge Thin filament ADP Pi Thick filament Myosin 1 Cross bridge formation. Copyright © 2010 Pearson Education, Inc. Figure 9. 12, step 1

ADP Pi 2 The power (working) stroke. Copyright © 2010 Pearson Education, Inc. Figure 9. 12, step 3

ATP 3 Cross bridge detachment. Copyright © 2010 Pearson Education, Inc. Figure 9. 12, step 4

ADP ATP Pi hydrolysis 4 Cocking of myosin head. Copyright © 2010 Pearson Education, Inc. Figure 9. 12, step 5

Thin filament Actin Ca 2+ Myosin cross bridge ADP Pi Thick filament Myosin Cross bridge formation. 1 ADP Pi Pi ATP hydrolysis 2 The power (working) stroke. 4 Cocking of myosin head. ATP 3 Cross bridge detachment. Cross bridge animation Copyright © 2010 Pearson Education, Inc. Figure 9. 12

Copyright © 2010 Pearson Education, Inc.