Muscle Tissue

1
Muscle Tissue

• Chapter 9

2
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

3
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

4
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

5
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

6
Microscopic Anatomyof 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

7
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

8
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

9
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 doesnt change length

10
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

11
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
12
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 cant
  initiate
• Resets electrical condition to resting state
• Na /K pump restore ionic condition
• All or none response, b/c unstoppable once
  started

13
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

14
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

15
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

16
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

17
Muscle Metabolism
18
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