Download Muscle Tissue

Muscle Tissue
Chapter 9
Types of Muscle Tissue
• Derived from mesoderm
• Skeletal
– Striated, voluntary, multinucleated
– Attached to bone
– Adaptable (paperclip vs textbook)
• Smooth
– Nonstriated, involuntary, uninucleated
– Lines visceral organs
• Cardiac
– Striated, involuntary, uninucleated
– Cardiac
• Stabilized by pacemaker cells
• 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 (peristalsis)
– 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
Gross Anatomy of Skeletal Muscle Cells
• Discrete organs of all 4 tissue types
• Nerves and blood
– 1 nerve, 1 artery, & 1+ veins per muscle
• Enter centrally; 1 nerve ending per muscle fiber (cell)
• Constant need for O2 and nutrients
• Connective tissue
– Support and reinforce
– 3 layers (internal to external)
• Endomysium: cover muscle fiber
• Perimysium: cover fasicles
• Epimysium: cover muscle
• Attachments
– Direct: epimysium fused to periosteum
– Indirect: epimysium beyond muscle = tendon
Microscopic Anatomy
of Skeletal Muscle Cells
• 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 (2)
• 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 myofilaments
– 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 myofilaments 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
• Heads form 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
– Myosin heads ratchet thin
filaments into center
• Z lines closer = shortening
sarcomere
• H band and I band narrow
• Zone of overlap increases
• A band doesn’t change
length
Sliding Mechanism
• Cross bridges detached
– Tropomyosin blocks ‘active sites’
• Ca2+ binds troponin  shape change
• Active site exposed  cross bridges
attach
• Myosin head pivots toward M line 
thin filaments to center
• ATP binds  ATPase in head resets
• Cross bridges detach and myosin
reactivated
Neuromuscular Junction (NMJ)
• Innervation of muscle fiber by one
axon terminal
– 1 NMJ per muscle fiber (cell)
– 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
release
• Motor end plate is depression in
the sarcolemma for the axon
– Contains ACh receptors
• Propagates an action potential (AP)
http://www.colorado.edu/intphys/Class/IPHY3430-200/image/figure7m.jpg
Introduction to Action Potentials
• Resting membrane is polarized (charge separation)
• NT binds = opens gated Na + and K + channels
• Cell depolarizes cell (less ‘–’ /more ‘+’) locally
– Spreads throughout plasma membrane in waves
• Initiates AP
– Adjacent Na + open  more depolarization until threshold
• Na + close, K + open = repolarization
– Refractory period because repeated stimuli can’t 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 depolarizing
end plate
• AP propagated down T tubules
• Terminal 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 returns
to 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
• Isometric
– Tension increases to peak, but muscle length
unchanged
• Pressing against a door frame
• Limits range of motion, strengthens specific joint angle
– Maintaing posture/position despite movement
• Isotonic
– Tension constant once peaks and muscle length
changes
– Concentric : force w/shortening
– Eccentric : force w/lengthening
• More forceful
• Delayed soreness onset
Muscle Twitch
• Motor unit 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
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