Muscle Tissue

1
CHAPTER 10

• Muscle Tissue

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OVERVIEW OF MUSCLE TISSUE

• Skeletal muscle tissue
• attached to bones
• striated and voluntary
• Cardiac muscle tissue
• forms wall of heart
• striated and involuntary
• Smooth (visceral) muscle tissue
• walls of hollow organs, blood vessels
• nonstriated in appearance
• involuntary

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Functions of Muscle Tissue

• Produce body movements
• Stabilize body positions
• Storage/movement of substances within the body
• bands of smooth muscle called sphincters
• blood, lymph, urine, air, food and fluids, sperm
• Heat production
• involuntary contractions of skeletal muscle
  (shivering)

4
Properties of Muscle Tissue

• Excitability ability to generate electrical
  signals (a.p.s)
• in response to autorhythmic electrical signal in
  heart
• in response to chemicals released from nerve
  cells (NTs)
• Contractility
• ability to contract forcefully when stimulated by
  act. pot.
• Extensibility
• ability to be stretched
• can forcefully contract even when stretched
• Elasticity ability to return to original shape
  after being stretched

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Skeletal Muscle CT components

• Superficial fascia loose C.T. underlying the
  sub-Q layer of skin
• Deep fascia dense irregular C.T. around muscle
• C.T. components of the muscle include
• epimysium surrounds whole muscle
• perimysium surrounds bundles (fascicles) of
  10-100 muscle cells
• endomysium separates individual muscle cells
• All these connective tissue layers extend beyond
  the muscle belly to form the tendon

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Nerve and Blood Supply

• Well-vascularized highly innervated
• Somatic motor neurons
• axon extends from brain to group of fibers
• axon branches _at_ muscle, each branch supplies
  diff. fiber
• Each muscle cell supplied by at least one
  capillary
• bring in oxygen/nutrients
• remove wastes
• Nerve fibers capillaries are found in the
  endomysium between individual cells

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Microscopic Anatomy of Skeletal Muscle

• Muscle fibers muscle cells
• Sarcolemma plasma membrane of muscle cell
• T tubules tiny in-folds of the sarcolemma
• tunnel from surface of cell to center
• open to outside of fiber ? filled with
  interstitial fluid
• allows muscle action potential to propagate to
  all parts of muscle fiber simultaneously
• Sarcoplasm muscle cell cytoplasm
• contains a large amount of glycogen for energy
  production
• myoglobin binds oxygen for storage

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Microscopic Anatomy of Skeletal Muscle

• Sarcoplasmic reticulum
• fluid-filled system of membranous sacs
• similar to endoplasmic reticulum
• encircles each myofibril
• terminal cisternae store Ca2 ions in relaxed
  muscle
• release of Ca2 ions triggers muscle contraction
• intersection w/ T-tubule forms triad

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Microscopic Anatomy of Skeletal Muscle

• Myofibrils contractile organelles of muscle
  fibers
• extend entire length of muscle fiber
• dark light bands give striated appearance
• sarcomere functional unit of myofibril
• contain myofilaments which do not extend entire
  length of cell
• defined as the region btwn two Z discs
• thin filaments
• make up I band of sarcomere
• actin protein
• anchored together by Z disc in middle of I band
• slide relative to myosin during muscle contraction

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Microscopic Anatomy of Skeletal Muscle

• Myofibrils, ctd.
• thick filaments
• myosin protein
• A band of sarcomere ? dark-staining
• zone of overlap ends of A bands where
  thick/thin filaments overlap
• H zone of sarcomere region of only thick
  filaments
• middle of A band where thin filaments do not
  reach
• M line in center anchors myosin vertically

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Filaments and the Sarcomere

• Myofilaments do not extend entire length of
  muscle fiber ? arranged into sarcomeres
• Sarcomere functional unit of myofibril
• Thick and thin filaments overlap each other in a
  pattern that creates striations (light I bands
  and dark A bands)
• I band region contains only thin filaments
• Separated by Z discs

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Muscle Proteins

• Contractile proteins generate force during muscle
  contraction
• myosin motor protein
• push/pull cell structures to produce movement
• actin main component of thin filaments
• myosin binding site on each molecule
• Regulatory proteins
• tropomyosin
• covers myosin binding sites in relaxed muscle
• prevents myosin binding in absence of Ca2 ions
• troponin holds tropomyosin in place
• binds calcium
• moves tropomyosin exposes myosin binding sites
• Structural proteins contribute to proper
  alignment, elasticity and extensibility

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Sliding Filament Mechanism Of Contraction

• Myosin cross bridges pull on thin filaments
• Thin filaments slide toward M-line
• Z Discs come toward each other
• H I bands narrow
• Sarcomeres shorten at same time? muscle fiber
  shortens? muscle shortens
• Notice Thick/thin filaments do not change in
  length!!

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How Does Contraction Begin?

1. Nerve impulse reaches axon terminal synaptic
   vesicles release acetylcholine (ACh)
2. ACh diffuses across synaptic cleft binds to
   receptors on the sarcolemma Na channels open
   Na rushes into cell
3. A muscle action potential spreads over sarcolemma
   and down into the transverse tubules
4. SR releases Ca2 into the sarcoplasm
5. Ca2 binds to troponin causes
   troponin-tropomyosin complex to move reveal
   myosin binding sites on actin? contraction cycle
   begins

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Contraction Cycle

• Once Ca2 is released from terminal cisternae
  has bound troponin, myosin binding sites on actin
  are available for crossbridge formation
• 4 steps to contraction cycle
• ATP hydrolysis ? puts myosin in hi-energy
  position
• attachment of myosin to actin to form
  crossbridges
• Release of ADP Pi causes conformational change
  in myosin which pulls thin filaments toward
  center of sarcomere ? this is the power stroke
• detachment of myosin from actin when new ATP
  binds ? mysoin returns to hi-energy conformation
  (this is position of myosin in resting muscle)
• Cycle repeats as long as ATP is available
  Ca2 is high near the filaments

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Steps in the Contraction Cycle

• Refer to figure 10.7
• Notice how myosin head attaches pulls on thin
  filament with energy released from ATP

Excitation - Contraction Coupling

• All the steps that occur from the muscle action
  potential reaching the T tubule to contraction of
  the muscle fiber
• Refer to figure 10.8

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Relaxation

• Acetylcholinesterase (AChE) breaks down ACh _at_
  synaptic cleft
• Muscle action potentials cease
• Ca2-release channels close
• Ca2 actively resequestered by SR/ Ca2-ATPase
  pump
• ? in sarcoplasmic Ca2 returns troponin-tropomyosi
  n to original conformation
• myosin binding sites unavailable
• Contraction stops but myosin heads remain in
  high-energy conformation, ready for contraction
  cycle to begin again
• Remember ATP is required to detach
  crossbridges, so resting muscle is in energized
  state

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Length-Tension Relationship

• Fiber length _at_ onset of contration influenes
  contraction strength
• Maximal tension develops when fiber contracts _at_
  30 of its optimal length
• fibers shorter/longer than optimal length result
  in decreased force of contraction
• too long no overlap between myosin/actin ? no
  Xbridge formation
• too short too much overlap, thick filaments
  compressed by Z lines ? fewer interactions btwn
  myson/actin
• Figure 10.9 illustrates this relationship

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Structures of NMJ Region

• NMJ synapse btwn somatic motor neuron (SMN)
  skeletal muscle fiber
• synapse area of communication btwn neuron
  another cell (in this case, a muscle fiber)
• synaptic cleft gap separating neuron muscle
  cell
• neurotransmitter (NT) chem released by neuron
  into cleft which activates the muscle fiber
• axon terminal end of motor neuron
• synaptic end bulbs are swellings axon terminal
• contain synaptic vesicles filled w/ ACh
• motor end plate region of sarcolemma opposite
  axon terminal
• contains ACh receptors/cation channels

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Generation of Muscle Action Potential

1. Arrival of nerve impulse at nerve terminal causes
   release of ACh from synaptic vesicles
2. ACh binds to receptors on motor end plate opens
   gated ion channels ? Na rushes into muscle cell
3. Inside of muscle cell becomes more positive,
   triggering a muscle action potential that travels
   over the cell and down the T tubules
4. The release of Ca2 from the SR is triggered and
   the muscle cell will shorten generate force
5. Acetylcholinesterase breaks down the ACh attached
   to the receptors on the motor end plate so the
   muscle action potential will cease and the muscle
   cell will relax.

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NMJ Antagonists

• Botulinum toxin (C. botulinim)
• blocks exocytosis of ACh ? muscle cannot contract
• causes flaccid paralysis
• death occurs from paralysis of the diaphragm
• Curare (plant poison from poison arrows)
• binds ACh receptors but does not activate them
• causes flaccid paralysis
• used to relax skeletal muscle during surgery
  because does not affect heart muscle
• Black widow spider venom
• causes massive ACh release
• spastic paralysis results ? exaggerated reflexes,
  ? tone

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Muscle MetabolismProduction of ATP in Muscle
Fibers

• Active muscle has high demand for ATP
• required for contraction cycle
• required for calcium uptake by SR
• required for other metabolic reactions
• Sarcoplasmic ATP only lasts for few seconds
• 3 sources of ATP production within muscle
• creatine phosphate
• anaerobic cellular respiration
• aerobic cellular respiration

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Creatine Phosphate

• Hi-energy compound found only in muscle
• Creatine phosphate 3-6 times greater than ATP
  in muscle
• Excess ATP within resting muscle used to form
  creatine phosphate
• ATP Cr ? ADP CrP (at rest)
• As ATP is used during exercise, ADP ? ? energy
  transferred from CrP to ADP
• ADP CrP ? ATP Cr (in active muscle)
• Sustains short, powerful bursts of maximal
  contraction
• lasts about 15 sec
• as in 100 meter dash

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Anaerobic Cellular Respiration

• Generates ATP w/o requiring O2
• When CrP used up, glucose broken down to form
  ATP
• glycolysis yields 2 net ATP
• pyruvate converted to lactic acid instead of
  entering aerobic metabolic cycles
• Can provide ATP for 30 to 40 sec of maximal
  activity (200 meter race)

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Aerobic Cellular Respiration

• Series of reactions requiring O2 to generate ATP
• If sufficient O2 is available, glc completely
  oxidized to yield 36 or 38 ATP
• pyruvate enters mitochondria rather than being
  converted to lactate
• 2 sources of oxygen
• myoglobin
• diffusion from blood
• Provides ATP for prolonged activity if O2
  nutrients are available

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Muscle Fatigue

• Inability to contract after prolonged activity
• Factors that contribute to fatigue
• central (psychological) fatigue feeling of
  tiredness desire to stop physical activity
• neuromuscular fatigue insufficient release of
  ACh from motor neurons
• muscular fatigue
• depletion of creatine phosphate, glycogen
• inadequate release of Ca2 from SR
• insufficient oxygen
• lactate accumulation
• Oxygen debt amount of O2 required to replenish
  ATP/CrP used during exercise

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The Motor Unit

• Motor unit one somatic motor neuron all
  skeletal muscle cells (fibers) it stimulates
• Muscle fibers of a unit normally scattered
  throughout belly of muscle (not all in one
  location)
• All fibers within one unit contract in unison
• Total strength of a contraction depends on
  motor units activated size of motor units
• precise movements require large of small motor
  units (few fibers/unit)
• large/powerful movements involve large motor
  units (many fibers/unit)

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Twitch Contraction

• Brief contraction of all fibers in a motor unit
  in response to single action potential in its
  motor neuron
• Latent period delay btwn stimulus onset of
  contraction
• Muscle a.p. moving along sarcolemma Ca2 is
  released from SR
• Contraction period period where crossbridges
  form and peak muscle tension develops
• Relaxation period time during which Ca2 is
  actively taken up by SR pump
• Refractory period period when muscle cannot
  contract even if it receives an additional
  stimulus

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Frequency of Stimulation

• Wave summation repeated stimuli arriving _at_
  different times, (but before muscle has
  completely relaxed from previous stimulus)
  produces an additive effect
• results in stronger contraction
• Incomplete (unfused) tetanus sustained muscle
  contraction that permits partial relaxation
  between stimuli
• stimulation rate 20-30x/sec
• Complete (fused) tetanus sustained contraction
  that lacks partial relaxation between stimuli
• stimulation rate 80-100x/sec

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Motor Unit Recruitment

• Process in which of active motor units is
  increased
• Generally, motor units in a whole muscle fire
  asynchronously
• some fibers are active others are relaxed
• Asynchronous contraction delays muscle fatigue ?
  allows for sustained contraction
• Produces smooth muscular contraction
• not series of jerky movements
• Weak motor units are recruited first
• then progressively larger units recruited as
  needed
• size of unit recruited dependent upon amt of
  force needed

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TYPES OF SKELETAL MUSCLE FIBERS

• Slow oxidative fibers
• Least powerful of three fiber types
• Large amounts of myoglobin ? dark in color
• Generate ATP primarily by aerobic cellular
  respiration
• have many large mitochondria for this function
• Highest resistance to fatigue of all fiber types
  ? adapted for endurance activities
• Fast oxidative-glycolytic fibers
• Lots of myoglobin, but less than slow oxidative
  fibers
• Generate ATP via aerobic respiration
• High glycogen content ? can also generate fair
  amt of ATP via anaerobic means
• Fairly fatigue resistant

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TYPES OF SKELETAL MUSCLE FIBERS

• Fast glycolytic fibers
• Generate quickest, most powerful contractions of
  all fiber types
• Low myoglobin content ? appear white
• Lots of glycogen ? primarily generate ATP via
  anaerobic means
• Adapted for short, powerful movements ? weight
  lifting, throwing a ball, sprinting, etc.
• Fatigue quickly
• Relative ratios of fiber types are genetically
  determined
• All muscles contain the three fiber types, but
  distribution of fibers dependent upon particular
  function/need of the muscle

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CARDIAC MUSCLE TISSUE

• Found only in the heart wall
• fibers arranged similarly to skeletal muscle
  fibers but are branched rather than parallel
• fibers connect to adjacent fibers by intercalated
  discs
• desmosomes prevent cell separation during
  contraction
• gap junctions allow a.p. to spread quickly
• contractions last longer than skeletal muscle
  twitch due to prolonged delivery of calcium ions
  from sarcoplasmic reticulum and the extracellular
  fluid.
• Cardiac muscle fibers contract when stimulated by
  their own autorhythmic fibers.
• This continuous, rhythmic activity is a major
  physiological difference between cardiac and
  skeletal muscle tissue.

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SMOOTH MUSCLE

• Nonstriated, involuntary autorhythmic
• Two types
• Visceral (single unit) smooth muscle
• found in walls of hollow viscera small blood
  vessels
• fibers work as single unit, an a.p. stimulates
  all fibers to contract at once
• Multiunit smooth muscle
• found in large blood vessels, large airways,
  arrector pili muscles, eye muscles
• each fiber has own neuron
• fibers operate singly rather than as one unit

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Microscopic Anatomy of Smooth Muscle

• One centrally located nucleus
• Thick and thin filaments are not organized into
  sarcomeres
• Contains intermediate filaments attached to dense
  bodies
• Small SR, but no T-tubules
• Caveolae structures containing XC Ca2 for use
  by sm. musc.
• Lengthwise shortening of fiber results from dense
  bodies pulling on filament
• Twists into helix during contraction
• Reverse motion during relaxation

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Physiology of Smooth Muscle

• Contraction starts slowly lasts longer
• no transverse tubules very little SR
• Ca2 must flow in from outside
• Can contract/stretch to greater extent than
  skeletal muscle
• Regulatory protein calmodulin
• binds Ca in cytosol ? facilitates myosin-actin
  binding
• Smooth muscle tone state of ctd partial
  contraction
• result of prolonged presence of calcium ions
• Stress-relaxation response smooth muscle can
  change in length without generating tension
• retains contractile ability while stretched