PowerPoint ® Lecture Slides prepared by Janice Meeking, Mount Royal College C H A P T E R Copyright © 2010 Pearson Education, Inc. 9 Muscles and Muscle Tissue: Part A

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 (esp. skeletal muscle)

Copyright © 2010 Pearson Education, Inc. Overview of Muscle Tissue Three Types of Muscle Tissue 1.Skeletal muscle tissue: Attached to bones and skin Striated Voluntary Powerful

Copyright © 2010 Pearson Education, Inc. Three Types of Muscle Tissue 2.Cardiac muscle tissue: Walls of heart Striated and Involuntary Contains intercalated discs

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

Copyright © 2010 Pearson Education, Inc. Table 9.3

Copyright © 2010 Pearson Education, Inc. Skeletal Muscle Structure Each muscle is served by one artery, one nerve, and one or more veins Three Layers of connective tissue sheaths surround skeletal muscle: Epimysium: dense regular CT surrounding entire muscle Perimysium: fibrous CT surrounding fascicles (groups of muscle fibers) Endomysium: fine areolar CT surrounding each muscle fiber Tendon – muscles CT wrappings extend beyond the muscle as a ropelike tendon which connects it to bone

Copyright © 2010 Pearson Education, Inc. Figure 9.1 Perimysium Endomysium (between individual muscle fibers) Muscle fiber Fascicle (wrapped by perimysium) Epimysium Tendon Epimysium Muscle fiber in middle of a fascicle Perimysium Endomysium Fascicle (a) (b)

Copyright © 2010 Pearson Education, Inc. Muscle Action Action – muscles main function Attachments Origin- fixed/immovable point of muscle attachment Insertion – muslces attachment on the movable bone

Copyright © 2010 Pearson Education, Inc. Functional Classification Prime mover – has the major responsibility for producing a specific movement Synergist - helps the prime mover by adding a little extra force to the same movement or reducing undesirable movements that might occur as the prime mover contracts Antagonist – muscles that oppose a movement Muscle Action

Copyright © 2010 Pearson Education, Inc. Skeletal Muscle Fiber Structure Myofibrils A muscle fiber is composed of thousands of stacked, cylindrical, contractile structures called: myofibrils Each myofibril consists of smaller contractile units called: sarcomeres Each sarcomere contains an orderly arrangement of rod-like protein filaments called: myofilaments Thick filaments – contain myosin Thin filaments – contain actin

Copyright © 2010 Pearson Education, Inc. Microscopic Anatomy Sarcoplasm – cytoplasm of a muscle cell Myoglobin – red, oxygen storing pigment Sarcolemma- plasma membrane of muscle cell T (transverse) tubules – areas of the sarcolemma that penetrate the cells interior at regular intervals

Copyright © 2010 Pearson Education, Inc. NucleusLight I bandDark A band Sarcolemma Mitochondrion Myofibril

Copyright © 2010 Pearson Education, Inc. Microscopic Anatomy Sarcoplasmic Reticulum – Smooth ER of the muscle cell Surround each myofibril Stores and regulates intracellular calcium Most tubules of SR run longitudinally along myofibril, others form perpendicular cross channels Terminal cisternae- paired cross channels with a T-tubule running between them form a triad

Copyright © 2010 Pearson Education, Inc. Figure 9.5 Myofibril Myofibrils Triad: Tubules of the SR Sarcolemma Mitochondria I band A band H zoneZ disc T tubule Terminal cisternae of the SR (2) M line

Copyright © 2010 Pearson Education, Inc. Sarcomere Structure Sarcomere – smallest contractile unit of a muscle fiber; the region of a myofibril between two successive Z discs Myofilments- filaments that constitute myofi brils

Copyright © 2010 Pearson Education, Inc. Sarcomere Structure A sarcomere extends from Z disc to Z disc Z line (band): proteins that anchor thin filaments; connects myofibrils to one another Sarcomere regions H zone (band)- lighter mid region within the A band where filaments don’t overlap M line- middle dark line of sarcomere; holds adjacent thick filaments together A band – dark band; contains thick and thin filaments I band – Light band; contains thin filaments only

Copyright © 2010 Pearson Education, Inc. Figure 9.2c, d I band A band Sarcomere H zone Thin (actin) filament Thick (myosin) filament Z disc M line (c) Part of one myofibril Z disc M line Sarcomere Thin (actin) filament Thick (myosin) filament Elastic (titin) filaments (d)

Copyright © 2010 Pearson Education, Inc. The Neuromuscular Junction Motor Neurons – Located in the brain or spinal cord Skeletal Muscle is innervated by: somatic motor neurons Each muscle is served by: one artery, one nerve and one/more veins Neuron axons: travel from origin all the way to the skeletal muscle it innervates Motor unit: one motor neuron and all of the skeletal muscle fibers it innervates

Copyright © 2010 Pearson Education, Inc. Neuromuscular Junction includes axonal endings, synaptic cleft and junctional folds of sarcolemma Each axon terminal forms: a neuromuscular junction with a single muscle fiber Synaptic Cleft –the small space between the axon terminal and the sarcolemma The Neuromuscular Junction

Copyright © 2010 Pearson Education, Inc. Axon Terminal Synaptic vesicles – small membranous sacs that store the neurotransmitter acetylcholine Acetylcholine (Ach) – When a nerve impulse reaches the axon terminal Ca2+ floods into the axon terminal This causes synaptic vesicles fuse to axon terminal leading to Exocytosis of: acetylcholine into synaptic cleft The Neuromuscular Junction

Copyright © 2010 Pearson Education, Inc. Figure 9.8 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.

Copyright © 2010 Pearson Education, Inc. The Motor End Plate – Contains Ach receptors Highly-excitable region of the sarcolemma Responsible for initiation of action potentials End plate potential:a local electrical event that ignites the action potential The Neuromuscular Junction

Copyright © 2010 Pearson Education, Inc. Figure 9.11, step 1 Axon terminal of motor neuron Muscle fiber Triad One sarcomere Synaptic cleft Setting the stage Sarcolemma Action potential is generated Terminal cisterna of SR ACh Ca 2+

Copyright © 2010 Pearson Education, Inc. Resting Membrane Potential Sarcolemma – is polarized prior to a nerve impulse = cells interior is negative relative to the outside The difference between the charge inside and outside the membrane is called: resting membrane potential

Copyright © 2010 Pearson Education, Inc. Action Potential Generation Steps: Ach Is released from axon terminal Diffuses across synaptic cleft Binds to Ach receptors on junctional folds of sarcolemma Binding of Ach to receptors causes: chemically gated cation channels to open This allows Na+ and K+ to pass

Copyright © 2010 Pearson Education, Inc. Depolarization Chemically gated cation channels open: allowing Na+ and K+ to pass More Na+ diffuses in than K+ diffuses out This produces a local change in the membrane potential or depolarization = end plate potential This causes the interior of the muscle fiber (just below sarcolemma) to become less negative Action Potential Generation

Copyright © 2010 Pearson Education, Inc. Figure 9.9, step 1 Na + Open Na + Channel Closed K + Channel K+K+ Na + K+K+ Action potential Axon terminal Synaptic cleft ACh Sarcoplasm of muscle fiber K+K+ 1 Local depolarization: generation of the end plate potential on the sarcolemma 1 W a v e o f d e p o l a r i z a t i o n

Copyright © 2010 Pearson Education, Inc. Propagation The depolarization wave spreads to: areas adjacent to the neuromuscular junction Depolarization travels down the sarcolemma: opening voltage-gated sodium channels, so Na+ enters Once a threshold voltage is reached the AP is initiated The AP propagates (spreads) along the length of the sarcolemma, opening more voltage-gated Na+ channels Action Potential Generation

Copyright © 2010 Pearson Education, Inc. Figure 9.9, step 2 Na + Open Na + Channel Closed K + Channel K+K+ Na + K+K+ Action potential Axon terminal Synaptic cleft ACh Sarcoplasm of muscle fiber K+K+ Generation and propagation of the action potential (AP) 1 Local depolarization: generation of the end plate potential on the sarcolemma 2 1 W a v e o f d e p o l a r i z a t i o n

Copyright © 2010 Pearson Education, Inc. Repolarization – Restores the sarcolemma to its initial polarized state Repolarization follows the depolarization wave Due to the closing of voltage gated Na+ channels and opening of voltage-gated K+ channels K+ diffuses out (K+efflux) of muscle fiber Absolute refractory period- is the interval from the beginning of the AP until the fiber is able to conduct another AP Action Potential Generation

Copyright © 2010 Pearson Education, Inc. Figure 9.9, step 3 Na + Closed Na + Channel Open K + Channel K+K+ Repolarization 3

Copyright © 2010 Pearson Education, Inc. Figure 9.9 Na + Open Na + Channel Closed Na + Channel Closed K + Channel Open K + Channel Action potential Axon terminal Synaptic cleft ACh Sarcoplasm of muscle fiber K+K+ 2 Generation and propagation of the action potential (AP) 3 Repolarization 1 Local depolarization: generation of the end plate potential on the sarcolemma K+K+ K+K+ Na + K+K+ W a v e o f d e p o l a r i z a t i o n

Copyright © 2010 Pearson Education, Inc. Destruction of Acetylcholine Effects are quickly terminated by: the enzyme acetylcholinesterase Prevents: continued muscle fiber contraction in the absence of additional stimulation

Copyright © 2010 Pearson Education, Inc. Sliding Filament Mechanism When a muscle fiber contracts: myosin heads bind to actin, detach, and bind again, propelling thin filaments toward M line In the relaxed state: thin and thick filaments slightly overlap During a contraction: thin filaments slide past thick ones so that they overlap to a greater degree until the H zone disappears; sarcomeres & muscle cells shorten, so whole muscle shortens

Copyright © 2010 Pearson Education, Inc. Figure 9.6 I Fully relaxed Fully contracted I A ZZ H IIA ZZ 1 2

Copyright © 2010 Pearson Education, Inc. Ultrastructure of Myofilaments The structure of actin and myosin play a major role in the sliding filament ‖ mechanism Thick Filament (Myosin) Structure - Composed of m yosin tails and heads The head of each myosin molecule: bear actin and ATP- binding sites and contain ATPase enzymes Myosin heads are attracted to: actin of thin filaments Myosin has the ability to: split ATP to generate energy for muscle contraction

Copyright © 2010 Pearson Education, Inc. Ultrastructure of Myofilaments Thin Filament Structure Two actin strands are: twisted around one another Troponin (protein) molecules: regulatory protein of the thin filament that binds calcium; joined to tropomyosin Normally, the tropomyosin strand: covers the myosin binding sites on actin

Copyright © 2010 Pearson Education, Inc. Figure 9.3 Flexible hinge region Tail Tropomyosin TroponinActin Myosin head ATP- binding site Heads Active sites for myosin attachment Actin subunits Actin-binding sites Thick filamentThin filament Thick filament Longitudinal section of filaments within one sarcomere of a myofibril Portion of a thick filament Portion of a thin filament Myosin molecule Actin subunits

Copyright © 2010 Pearson Education, Inc. Steps in the Sliding filament Mechanism In a relaxed state, the actin filaments: are not bound by myosin heads ATP binds to the myosin head and: hydrolyzes ATP The energy released is stored and the myosin head is energized (“cocked “) position

Copyright © 2010 Pearson Education, Inc. Figure 9.12, step 5 Cocking of myosin head. ATP hydrolysis ADP PiPi 4

Copyright © 2010 Pearson Education, Inc. An action potential travels down the sarcolemma and down the T-tubules The T-tubules carry the impulse into the interior of the cell The impulse causes the SR to release calcium Steps in the Sliding filament Mechanism

Copyright © 2010 Pearson Education, Inc. Figure 9.11, step 4 Steps in E-C Coupling: Terminal cisterna of SR Voltage-sensitive tubule protein T tubule Ca 2+ release channel Ca 2+ Sarcolemma Action potential is propagated along the sarcolemma and down the T tubules. Calcium ions are released. 1 2

Copyright © 2010 Pearson Education, Inc. Ca2+ ions: are bound by troponin Troponin and tropomyosin complex: troponin changes shape causing tropomyosin to move away from myosin binding sites on actin The active sites for binding of the myosin head are now uncovered Steps in the Sliding filament Mechanism

Copyright © 2010 Pearson Education, Inc. Figure 9.11, step 5 TroponinTropomyosin blocking active sites Myosin Actin Ca 2+ The aftermath

Copyright © 2010 Pearson Education, Inc. Myosin heads bind to actin Once each myosin head binds to actin: a cross bridge is formed As the myosin head tilts inward: it pulls the actin filament towards the M line This inward pulling of the actin is called the power stroke. Steps in the Sliding filament Mechanism

Copyright © 2010 Pearson Education, Inc. Figure 9.12, step 1 Actin Cross bridge formation. Ca 2+ Myosin cross bridge Thick filament Thin filament ADP Myosin PiPi 1

Copyright © 2010 Pearson Education, Inc. Figure 9.12, step 3 The power (working) stroke. ADP PiPi 2

Copyright © 2010 Pearson Education, Inc. Once the power stroke has occurred: myosin binds ATP Binding of the ATP: causes myosin to detach from the actin The newly attached ATP: is hydrolyzed and the cycle starts again This process continues in a ratchet-like manner until the actin filaments are overlapping Steps in the Sliding filament Mechanism

Copyright © 2010 Pearson Education, Inc. Figure 9.12, step 4 Cross bridge detachment. ATP 3

Copyright © 2010 Pearson Education, Inc. Figure Actin Cross bridge formation. Cocking of myosin head. The power (working) stroke. Cross bridge detachment. Ca 2+ Myosin cross bridge Thick filament Thin filament ADP Myosin PiPi ATP hydrolysis ATP 24 3 ADP PiPi PiPi