Download Muscular System Muscle Fibers and Contraction Muscle Fibers

Muscular System
Muscle Fibers and Contraction
Muscle Fibers
 Functional characteristics of muscle tissue
o Excitability: ability to receive and respond to stimuli
o Contractility: the ability to shorten forcibly
o Extensibility: the ability to be stretched or extended
o Elasticity: the ability to recoil and resume original resting length
 Muscle function
o Skeletal muscle responsible for all locomotion
o Cardiac muscle responsible for moving blood through body
o Smooth muscle helps maintain blood pressure and squeezes/propels substances (food) through organs
o Muscles maintain postures, stabilize joints and generate heat
 Types of contractile cells
Skeletal muscle fibers
Cardiac muscle fibers
Smooth muscle fibers
 Elongated cells
 Branching cells
 Spindle-shaped cells
 Presence of visible striations
 Presence of visible striations
 Absence of striations
 Multiple peripheral nuclei
 Single, centrally-located
 Single, centrally-located
nuclei
nuclei
 Move bone
 Move blood
 Move everything else
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Structure of skeletal muscle
o Tendons: attach skeletal muscles to bones
o Fascicles: bundles of skeletal muscle fibers
o Connective tissues surround muscle structures
 Epimysium : connective tissue surround a muscle
 Perimysium: connective tissue surrounding fascicles
 Endomysium: connective tissue surround individual muscle fibers
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Structure of muscle fiber
o Muscle fibers: alternative name for skeletal muscle cell
o Mitochondria: sites of ATP synthesis
o Sarcolemma: plasma membrane of muscle cell
o Sarcoplasmic reticulum: interconnecting tubules of endoplasmic reticulum that surround each myofibril
o Terminal cisternae: sac-like regions of sarcoplasmic reticulum that contain calcium ions
o T-tubules: invaginations of the sarcolemma that project deep into the cell
o Triad: group of one T-tubule lying between two adjacent terminal cisternae
o Myofibril : contractile filaments within the skeletal muscle cell
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Myofibril
o Organelles composed of bundles of myofilaments
o Contain myosin and actin
o Arrangement of myosin and actin creates skeletal muscle’s striated appearance
o Sarcomere
 Contractile unit of a muscle cell
 Between two successive Z discs
 I bands: contain only thin (actin) filaments
 H zone: contains only thick (myosin) filaments
 A band: contains both thin and thick filaments
 Z lines connect actin filaments; are perpendicular to myofilaments
 M lines connect myosin filaments; are perpendicular to myofilaments
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Muscle contraction
o Skeletal muscles must be stimulate by nerve impulses
o Motor neuron may stimulate a few muscle cells or hundreds
o Motor unit
 One neuron and all of the fibers it stimulates
 All muscle fibers in a motor unite are stimulated at the same time
o Muscles are made up of motor units and can be stimulated (recruited) singly or in groups
 Strength of stimulation determines number of motor units recruited
Neuromuscular Junction
 Anatomy of neuromuscular junction
o Motor neuron: nerve that stimulates muscle
o Synaptic vesicles: structures within axon terminal that contain acetylcholine
o Synaptic cleft: space between axon terminal and motor end plate
o Motor end plate: folded region of sarcolemma at neuromuscular junction
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Process of neuromuscular junction
o Action potential arrives at axon terminal
 Action potential arrives at axon terminal
 Voltage-regulated calcium channels open
 Calcium ions enter the axon terminal
o Fusion of synaptic vesicles
 Calcium ions cause synaptic vesicles to fuse with membrane
o Release of acetylcholine
 Acetylcholine is released from synaptic vesicles into synaptic cleft
 Calcium ions pumped out of axon terminal
o Acetylcholine binds to receptor sites
 Acetylcholine binds to motor end plate
 Ion channels open
 Sodium ions move into the muscle cell
 Potassium ion move out of muscle cell
 Depolarization of motor end plate occurs
o Breakdown of acetylcholine
 Acetylcholine diffuses from receptor site
 Ion channels close
 Acetylcholine broken down by acetylcholinesterase into acetic acid and choline
o Action potential propagation
 Action potential propagates along sarcolemma and down the T-tubules
o Calcium release from terminal cisternae
 Action potential causes release of calcium ions from terminal cisternae into cytosol
o Contraction of the muscle cell occurs
Sliding Filament Theory
 When a muscle cell contracts, the thing filaments slide past the thick filaments and the sarcomere shortens
 Chemicals involved
o Myosin, actin, tropomyosin, troponin, ATP, calcium ions
 Anatomy of myosin
o Contains tail
 Has hinge for vertical movement
o Contains two heads (cross bridge)
 Has binding site for ATP and actin
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Thin filaments of sarcomere
o Actin
 Double strand of actin subunits
 Contains myosin binding sites
o Tropomyosin
 Twists around actin
 When sarcomere is relaxed, tropomyosin covers binding sites on the actin and prevents myosin
from binding
o Troponin
 Moves tropomyosin aside to expose myosin binding sites on actin
Calcium ions
o Bind to troponin
o Troponin moves tropomyosin away from myosin binding sites
o Myosin binds to actin
Process of sliding filament theory
o Exposure of binding sites on actin
 Calcium ions released from terminal cisternae
 Calcium ions bind to troponin
 Troponin moves tropomyosin aside
 Binding sites on actin are exposed
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Binding of myosin to actin
 Myosin tail hinge bends and myosin attaches to actin
Power stroke of cross bridge
 ADP and Pi released from myosin
 Myosin head tilts forward, pulling the thin filament inward toward the center of the sarcomere
Disconnecting the cross bridge
 ATP binds to the cross bridge
 Cross bridge disconnects from actin
Re-energizing the cross bridge
 ATP is hydrolyzed into ADP and Pi
 Myosin head extend forward slightly into position, ready for the next attachment to occur
Removal of calcium ions
 Calcium ions fall off the troponin
 Calcium is taken back up into sarcoplasmic reticulum
 Tropomyosin covers binding sites on actin
Calcium pumps
 Calcium ions are pumped back into the sarcoplasmic reticulum through active transport
Botox
 Purified and crystallized botulinum toxin type A
 Toxin produced by bacterium
 Prevents acetylcholine release, preventing muscle from contracting
Rigor Mortis
 After death, calcium levels rise and body’s level of ATP drops
 Muscles continue to contract without relaxing
 Before contracted and stiff: condition called rigor mortis