Download Muscle Tissue

Chapter 9
Types of Muscle Tissue
 Derived from mesoderm
 Skeletal
 Skeletal, striated, voluntary, multinucleated
 Fast, fatigued contractions
 Adaptable (paperclip vs textbook)
 Smooth
 Visceral, nonstriated, involuntary, uninucleated
 Slow, sustained contractions
 Cardiac
 Cardiac, striated, involuntary, uninucleated
 Contraction rate stabilized by pacemaker cells

Neural control can alter
 Intercalated discs
Functional Characteristics
 Excitability
 Receive and respond to a stimulus (pH or NT)

Electrical impulse along the sarcolemma
 Contractibility
 Shorten and thicken w/ appropriate stimulation
 Extensibility
 Stretch or extend without damage
 Elasticity
 Return to normal shape after a stretch
Muscle Function
 Produce movement
 Sk: locomotion, manipulation, and response
 Sm: squeeze substances through
 Car: keep blood moving
 Maintain posture and position
 Adjustments to stay erect or seated despite gravity
 Protection
 Encloses viscera and forms valves (control)
 Generate heat
 Contractions keep body temp at 98.6
 Stabilize joints
 Pull on bones for movement, but strengthen joints
Gross Anatomy of Skeletal Muscle
 Discrete organs of all 4 tissue types
 Nerves and blood
 1 nerve, 1 artery, & 1+ vein per muscle


Enter centrally; 1 nerve ending per muscle fiber (cell)
Constant O2 and nutrients b/c contractions are high E demand
 Connective tissue
 Support and reinforce
 3 layers (internal to external)
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Endomysium: cover muscle fiber
Perimysium: cover fasicle
Epimysium: cover muscle
 Attachments
 Direct: epimysium fused to periosteum
 Indirect: epimysium beyond muscle = tendon
Microscopic Anatomy
 Sarcolemma
 Sacroplasm
 Glycogen, myoglobin, and mitochondria
 Myofibrils
 Actin (thin) and myosin (thick) proteins arranged into
repeating sarcomeres
 Sarcoplasmic reticulum (SR)

Smooth ER surrounding myofibrils
 Triads
 Terminal cisternae

SR is enlarged and joins with T tubules; occur in pairs
 Transverse (T) tubules
 Deep indentions of sarcolemma into sarcoplasm; conduct Ca2+ into
cell
Sarcomere
Organization
 Smallest functional unit of skeletal muscle fiber
 A bands dark b/c contain thick and thin filaments
 H zone is lighter middle because it lacks thin filaments
 M line created by a protein that link thick filaments
 I bands light b/c contain thin filaments only
 Z line connect thin filaments together in a zig zag
pattern

Marks end of sarcomere
 Zone of overlap
 6 thin surround 1 thick; 3 thick surround 1 thin
Myofilament
Structure
 Thick filaments
 Bundles of myosin proteins
 Composed of a rod-like tail and globular head

Head forms cross bridges; attach to site on actin; contain ATPases
 Thin filaments
 Twisted strands of F actin, composed of G actin

G actin contains ‘active sites’ where myosin can attach
 Tropomyosin forms stiffening chains that cover ‘active sites’
 Troponin holds the tropomyosin in place
 Changes shape to expose active sites
 G-actin = pearl, F-actin = strand, tropomysin = strands together
Sliding Filament Theory
 During contraction,
sarcomeres (not filaments)
shorten
 Z lines closer, shortening
sarcomere
 H band and I band narrow
 Zone of overlap increases
 A band doesn’t change
Sliding Mechanism
 Cross bridges detached
 Tropomyosin blocks ‘active sites’
 Active site exposed  cross
bridge attach
 Ca2+ binds troponin  shape
change
 Myosin head pivots toward M
line  thin filaments to center
 Cross bridges detach and
mysoin reactivated
 ATP binds  ATPase resets
Neuromuscular Junction (NMJ)
 Innervation of muscle fiber by an
axon terminal
 1 NMJ per muscle fiber
 Motor unit: motor neuron and all
muscle fibers innervated


Fewer fibers = more precise
Number determines strength of muscle
 Separated by a synaptic cleft
 Axon terminal houses synaptic
vesicles filled with acetylcholine
(Ach)
 Impulse opens Ca2+ channels to
http://www.colorado.edu/intphys/Class/IPHY3430-200/image/figure7m.jpg
release
 Motor end plate is depression in the
sarcolemma for the axon
 Contains Ach receptors
 Propogates an action potential (AP)
Introduction to Action Potentials
 Resting membrane is polarized (charge separation)
 NT binds = opens gated ion channels (Na + and K +)
 Depolarize cell (less ‘–’ or more ‘+’) locally
 Spreads throughout plasma membrane in waves
 Initiates AP
 Adjacent Na + open  more depolarization to threshold
 Na + close, K + open = repolarization
 Refractory period because no stimuli can initiate
 Resets electrical condition to resting state
 Na + /K + pump restore ionic condition
 All or none response, b/c unstoppable once started
Excitation – Contraction Coupling
 Stimuli releases ACh, depolarizes
end plate
 AP propagated down T tubules
 Termianl cisternae of SR release Ca 2+
 Electrical signal raise Ca2+ levels by
opening Ca2+ channels
 Ca2+ binds troponin, removing
tropomyosin block
 Contraction occurs (see earlier)
 Ca2+ levels decrease, tropomyosin
replaced = relaxation
 ATP dependent Ca2+ pump into SR
 Repeat with stimulation
Skeletal Muscle Contractions
 Muscle Tone
 Alternating active motor units while
muscle at rest
 No active movements produced
 Stabilize joints and maintain posture
 Ensure response ready
 Isotonic
 Tension constant as muscle length changes
 Isometric
 Tension increases to peak, but muscle
length unchanged

Moving a load greater than developable
tension
 Concentric : force w/shortening
 Eccentric : force w/lengthening
(gastrocnemius & hills)
Muscle Twitch
 Response to a single stimulation
 Quick contract, relax cycle in 3 phases
 Latent period
 Excitation – coupling is occurring
 Muscle tension increases, but no contraction
 Contraction period
 Cross bridges active
 Peak tension, muscle shortens
 Relaxation period
 Reentry Ca2+ into SR
 Muscle tension to zero, resting
 Varies between muscle types
 Strength depends on # of motor units,
recruitment
Graded Muscle Responses
 Contraction varies depending on circumstance
 Wave summation (time)
 2 stimuli in rapid succession = larger contraction 2nd time
 Refractory period unaltered
 Tetanus (speed)
 Sustained contraction w/ or w/o partial relaxation
 Unfused
 Fused
 Treppe
 Increase tension with repeated contractions
 Warming up  stronger later to same stimulus
Muscle Metabolism
Muscle Disorders
 Myasthenia gravis: autoimmune disease, loss of Ach

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receptors
Rigor mortis: ATP depletion prevents cross bridge
detachment
Atrophy: degeneration of muscle from disuse
Duchenne muscular dystrophy: sex-linked disease that
destroys muscle
Hernia: organ protrudes through abdominal wall
Myalgia: muscle pain
Fibromyositis: inflammation of a muscle and CT coverings
Strain: excessive stretching and tearing of muscle or tendon