Muscles • From Latin mus • “Little mouse” • ~ 700 skeletal muscles – 60 in face • >40 for frowning • 20 for smiling
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Function • 1) Movement of skeleton • 2) Maintain posture and body position • 3) Support soft tissues • 4) Guard entrances and exits • 5) Maintain body temp
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Types of Muscle Tissue • Three types: – Skeletal – Smooth – Cardiac
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Types of Muscle Tissue
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Skeletal Muscle Tissue Review • • • • • • •
Attach to skeleton Voluntary Long cylindrical shape Multinucleated Nuclei near edge Striated Can contract rapidly – Need rest
• Largest of three types – Up to 24 inches long! 6
Skeletal Muscle Cells • • • •
Called muscle fibers Mys = muscle Sarkos = flesh All contract
3 Connective Tissue Layers • Bundled together by connective tissue – Provides strength and support
• 1) Endomysium – wraps around individual muscle fiber
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3 Connective Tissue Layers • 2) Perimysium – coarser membrane – encloses a bundle of muscle fibers (called a fascicle) – Collagen and elastic fibers, nerves and blood vessels
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3 Connective Tissue Layers • 3) Epimysium – Surrounds entire muscle – Several fascicles wrapped together – Separates muscle from surrounding tissues and organs – Collagen fibers
• Fascia - Encloses entire muscle and epimysium – Separates individual skeletal muscles and holds muscles in place
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Tendon • Bundle of collagen fibers where muscle meets bone • All 3 connective tissue layers taper and blend
• Interwoven with periosteum of bone • Gives “pulling power” 12
Aponeuroisis • Broad sheet of collagen fibers • Attach skeletal muscles to other skeletal muscles, not to bone coverings
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Muscle structure
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Skeletal Fibers (cells) • Sarcolemma – cell membrane • Sarcoplasm – cytoplasm • Transverse tubules (T tubules) – form passageways and help with contractions – Encircle myofibril
• Sarcoplasmic reticulum (SR) – specialized SER
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Sarcoplasmic Reticulum • Surround each myofibril – Like a crocheted sweater around your arm
• Major role – Store and release calcium • Essential for muscle contraction
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Myofibrils • Responsible for muscle fiber contraction • Make up muscle fibers (cells) • Made of bundles of myofilaments • Each contains ~10,000 sarcomeres
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Sarcomeres • Smallest functional unit of muscle cell • Each can contract • Arranged end-to-end • Segment from z line to z line
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Myofilaments • Made of protein • Two types: – Actin – Myosin
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Actin Filaments • • • •
Thin filaments At ends of sarcomere Made up of protein actin Also contain regulatory proteins troponin and tropomyosin
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Actin Filaments • Form light band – Parts of two adjacent sarcomeres – Contain only thin filaments – Actin filaments slide toward each other in contraction
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Myosin Filaments • Larger, thicker • In middle of sarcomere • Made up of protein myosin • Ends have projections: – Myosin heads = Cross bridges • Link thick and thin filaments during contraction 22
Striations • I bands (light) – Thin actin attached to Z line (midline boundary)
• A bands (dark) – Thick myosin overlapping actin – Contains H zone (central region) – Contains M line (thickening)
Sliding Filament Model • Explanation of sarcomere contraction • Sarcomeres shorten when crossbridges pull on thin filaments at their binding sites • Must have calcium to start contraction!
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Sliding Filament Theory – 1) Myosin crossbridge attaches to actin binding site – 2) Actin filament is pulled closer – 3) Filament is let go – 4) Myosin Crossbridge attaches to another, pulls and lets it go – *Cycle repeats as long as there is ATP – *A-band does not change size 29
Review of Muscle Structure • Large to Small: • Muscle is covered by epimysium (which is covered by fascia) Composed Of:
• Fascicles surrounded by perimysium Composed Of:
• Muscle Fibers (Cells) surrounded by endomysium Composed Of:
• Myofibrils Composed Of:
• Filaments of actin (thin) and myosin (thick)
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Myofibril song
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Muscle Impulse • Similar to nerve impulse • Muscle stimulated to contract • Needs neurotransmitter from neuron to occur – Skeletal muscles must have the neurotransmitter acetylcholine – synthesized in neuron’s cytoplasm
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Muscle Contraction
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Skeletal Muscle Activity • Functional properties: – Irritability • Ability to receive and respond to stimuli
– Contractility • Ability to shorten
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Muscles and Nerves • Muscle must be stimulated Neuron (nerve cell) • Motor neuron = nerve cell that interacts with muscle cells – Can stimulate one muscle fiber or 1000s (usually NOT just one) – Axon branches within perimysium 35
Motor Unit • One neuron and the skeletal muscle cells it stimulates (controls) • Neuron structure: – – – –
Dendrites Cell body Axon Axon terminals
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Motor Unit
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Neuromuscular Junction (NMJ) • Site where muscle and nerve cell meet • Each muscle cell connects w/ a neuron’s axon • Synaptic cleft = • Gap between neuron and muscle • Neurotransmitter released near skeletal muscle = Acetylcholine (Ach)
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Neuromuscular Junction • Motor end plate – area where muscle fiber meets neuron •
Nuclei and mitochondria abundant
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Sarcolemma is folded extensively
• Neurotransmitters – contained in vesicles in distal ends of motor neuron axons (synaptic terminals) •
Stimulate muscle fiber to contract by changing permeability of sarcolemma
• neuromuscular juntion 40 sec 39
Neuromuscular junction
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Muscle Contraction • Muscle cells – “All or none” law of muscle contraction – Muscle cell contracts totally not partially
• Muscles are organs made of thousands of muscle fibers (cells)
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Muscle Twitch • Twitch – single contraction • very short • Consists of : • Stimulus • Contraction • Relaxation
• Latent period – delay b/w the time the stimulus was applied and muscle responded • No muscle tension 42
Muscle Twitch • Contraction – muscle tension rises to a peak •
Cross-bridges interacting w/ active sites
• Relaxation – muscle tension falls to resting levels
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Muscle Twitch • 1 stimulation = 1 twitch • Does not accomplish anything useful
• Need repeated stimulations for sustained muscle contractions
Summation • When individual twitches combine • One twitch is added to another
• Second stimulus arrives before relaxation phase has ended
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Recruitment • Increase in the number of motor units being activated • Muscle will contract w/ max tension if all are used 46
Tetanus • Occurs when the frequency of stimulation is increased so that the muscle contracts without relaxation •
2 types: •
1) Incomplete – muscle produces almost peak tension •
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All normal muscular contractions
2) Complete – rate of stimulation so high muscle never relaxes
