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The muscular system: An introduction . What is it?. Collection of ~600 skeletal muscles Also many smooth muscles and heart tissue Myology: study of muscle Myo-, mys-, sarco- all refer to muscle. What do muscles do for me?. Move you and your internal organs Provide stability

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what is it
What is it?
  • Collection of ~600 skeletal muscles
    • Also many smooth muscles and heart tissue
  • Myology: study of muscle
  • Myo-, mys-, sarco- all refer to muscle
what do muscles do for me
What do muscles do for me?
  • Move you and your internal organs
  • Provide stability
  • Maintain posture
  • Control body openings/passages
  • Produce body heat (~85% of it)
what are the parts of a muscle
What are the parts of a muscle?
  • Watch animation on your CD
  • Muscle cell = muscle fiber
    • long (up to 30 cm)
  • Epimysium:
  • Perimysium and fascicles
    • Fascicle = group of muscle fibers
      • Deep vs. superficial
  • Endomysium
  • CT sheaths = for blood vessels and nerves
what are the characteristics of muscle tissue
What are the characteristics of muscle tissue?
  • Excitability: ability to receive/respond to stimuli
    • Receive neurotransmitter (e.g. acetylcholine)
    • Response: contract
  • Contractility: ability to shorten
  • Extensibility: ability to be stretched/extended
  • Elasticity: ability to recoil
where do muscles attach to bone
Where do muscles attach to bone?
  • Attached in a minimum of two places
    • Insertion: point which moves toward immovable bone
    • Origin: point of insertion at immovable bone
      • Limbs: usually origin is proximal to insertion
    • Also it (or its tendon) MUST cross a joint between the origin and insertion!
      • If a muscle (or its tendon) didn’t, what would happen?
how do muscles attach to bone
How do muscles attach to bone?
  • Direct (fleshy) attachment
    • epimysium fused to periosteum or perichondrium
how do muscles attach to bone8
How do muscles attach to bone?
  • Indirect attachment
    • Collagen fibers of epimysium become tendon
    • Tendon merges with periosteum
    • Much more common
    • Aponeurosis: sheetlike tendon connection
      • Scalp, abdomen, lumbar, hand, foot
how do muscles move together
How do muscles move together?
  • Agonist(prime mover)
    • Synergist helps
  • Antagonist: Opposes prime mover
  • Antagonistic pair:act on opposite side of joint
  • Fixator: e.g. to scapula so biceps move radius and not scapula
  • muscle movement movie
what special features are found in muscle fibers
What special features are found in muscle fibers?
  • Multiple nuclei: fusion of many myoblasts
    • Satellite cells nearby
  • Sarcolemma
  • Sarcoplasm
    • Many glycosomes (with glycogen)
    • Lots of myoglobin
      • Stores oxygen like hemoglobin
what special features are found in muscle fibers12
What special features are found in muscle fibers?
  • Sarcoplasmic reticulum (SR):releases calcium
  • Myofibrils: many in each myocyte
    • long, contractile elements, parallel to myocyte length
      • Contains many myofilaments
how do muscles contract
How do muscles contract?
  • First, look at anatomy of myofilaments
  • Appears as a striation pattern
  • Sliding filament animation
what creates the striations
What creates the striations?
  • Alternating A (dark) and I (light) bands
  • A band
    • H zone: light zone in middle
    • M line
    • Only visible when muscle relaxed
  • I band
    • Z disc: dark line in middle
  • Sarcomere = distance between two Z disks
what three myofilaments are found in myofibrils
What three myofilaments are found in myofibrils?
  • Thick filaments: myosin
    • Extends the length of A band
    • Looks like golf club with two heads (cross bridges)
what three myofilaments are found in myofibrils16
What three myofilaments are found in myofibrils?
  • Thin filaments: actin
      • Extend across I band, partially into A band
        • Anchored to Z disk
      • G actin subunit is binding site for myosin head
    • Tropomysin lines actin grooves
      • Troponin bound to tropomysin; binds calcium
what three myofilaments are found in myofibrils17
What three myofilaments are found in myofibrils?
  • Elastic filaments: titin (AKA connectin)
    • Strong recoil, found in center of thick filament
      • Anchor thick filament to Z disc
what do all these proteins do
What do all these proteins do?
  • Contractile proteins
    • Myosin and actin
    • Workhorses
  • Regulatory proteins
    • Tropomysin and troponin
    • Control when contraction happens
    • Based on calcium availability
what determines calcium availability
What determines calcium availability?
  • Sarcoplasmic reticulum
    • Smooth ER, forming parallel tubules
      • Tubules surround each myofibril
      • Stores calcium, releases it when NERVE demands it
  • Terminial cisternae: perpendicular ER tubules
what determines calcium availability20
What determines calcium availability?
  • T tubules: at A band-I band junction
    • Extension of sarcolemma
    • Extends as tubule through paired terminal cisternae
      • Forms triad
    • Transmits nerve signal deep into myocyte
      • Stimulates calcium release from sarcoplasmic ret.
so how do all these parts make muscles contract
So how do all these parts make muscles contract?
  • Sliding filament theory
    • Time for that CD!
  • Result: thin filaments slide along thick filaments causing contraction
how do nerves tell muscles to contract
How do nerves “tell” muscles to contract?
  • Via action potentials
    • Changes in electrical charge across sarcoplasm
    • Stimulates calcium release
    • action potential and contraction animation
  • Called Excitation-Contraction Coupling
    • First, we need to know more about neuromuscular junctions and action potentials
where do nerve and muscle touch
Where do nerve and muscle touch?
  • Neuromuscular junction
    • Motor neuron stimulates skeletal muscle
    • Axon divides to form numerous neuromuscular junctions
      • One neuromuscular junction per muscle fiber
      • Motor unit = all myocytes innervated by one nerve (20-1,000)
        • More motor units = finer control
      • Muscles have > 1 motor unit to prevent fatigue
where do nerve and muscle touch25
Where do nerve and muscle touch?
  • Synaptic cleft separates muscle fiber from nerve
    • 60-100 nm space
    • Synaptic knob (end of axon)
    • Motor end plate (depression in muscle fiber)
if nerve and muscle don t touch how do they communicate
If nerve and muscle don’t touch, how do they communicate?
  • Nerve impulse reaches axon
    • Voltage-regulated calcium gates open, cause
    • Synaptic vesicles to be exocytosized
      • Release acetylcholine (ACh)
      • ACh traverses synaptic cleft
  • ACh receptors on myocyte bind Ach
    • Stimulates opening of calcium gates
    • Action potential propagates down T tubules
      • Calcium released to stimulate contraction
what is a polarized cell
What is a polarized cell?
  • All cells are polarized (resting potential)
    • Differential charge across PM
    • High K+ inside, high Na+ outside
    • Also DNA, RNA high negative charge
  • Overall, inside of cell is neg., outside pos.
  • resting potential animation
what are action potentials
What are action potentials?

1. Depolarization

  • When ACh binds to ACh receptors, Na+ sensitive gates open
    • Inflow of Na+ changes charge (voltage) difference across membrane
what are action potentials29
What are action potentials?

2. Propagation

  • action potential spreads across sarcolemma
  • Opens voltage-sensitive Ca2+ gates
what are action potentials30
What are action potentials?

3. Repolarization

    • K+ gates open to reestablish charge
    • Refractory period: time its takes to reestablish charge
  • Action potential movie
how does a cell return to its resting state
How does a cell return to its resting state?
  • After several rounds of depolarization, too much Na+ on inside
  • Na+/K+ pumps
    • Export 3 Na+ and import 2 K+
meanwhile back at the synaptic cleft
Meanwhile, back at the synaptic cleft…
  • Acetylcholinesterase (AChE) destroys
    • any ACh still in synaptic cleft
    • Prevents continual stimulation
to review what happens during excitation contraction coupling
TO REVIEW:What happens during excitation-contraction coupling?


  • Action potential travels along sarcolemma and down T tubules
  • Action potential reaches triad
    • causes terminal cisternae to release calcium into sarcoplasm
  • Calcium bind to troponin, moves tropomyosin out of the way
what happens during excitation contraction coupling
What happens during excitation-contraction coupling?
  • Contraction
  • Myosin heads attach to actin and pull thin filaments toward H line
    • Attachment, power stroke, reattachment, cocking
  • Relaxation
  • Within 30 ms, calcium removed
    • via ATP-driven calcium pump
  • At same time AChE degrades ACh
  • Tropomyosin blockage reestablished
    • cross bridge activity ceases
what makes a contraction strong
What makes a contraction strong?
  • Length-tension relationship
    • Too little = ___________ contraction
    • Too much overlap = contraction ___________
  • Optimal overlap allows for greatest contraction
    • CNS maintains constant, partial contraction: tonus (muscle tone)
how do whole muscles work
How do whole muscles work?
  • Threshold: minimum voltage for contraction
  • Stimulate a nerve or a myocyte:
    • Latent-period: delay before contraction (for excitation, etc.)
    • Higher voltage does not produce stronger contraction
so how do muscles contract at varying strengths
So how do muscles contract at varying strengths?
  • Recruitment
    • Multiple motor unit summation
  • Temporal summation
    • Produces treppe (staircase phenomenon)
    • If enough stimuli fast enough, produces incomplete tetanus
    • Why?
      • Stimuli too rapid to clear calcium between contractions?
      • Heat released causes enzymes to work more efficiently (e.g. warm-up exercises)?
what happens if muscle stimulated too much
What happens if muscle stimulated too much?
  • Individual contractions fuse to smooth contraction
    • Complete tetanus
    • Different than the disease tetanus
      • This blocks glycine (an inhibitor) release
where do muscles get the atp needed to contract
Where do muscles get the ATP needed to contract?
  • Anaerobic fermentation
    • Pro: don’t need oxygen to make ATP
    • Cons: only makes a little ATP and produces lactic acid
  • Aerobic respiration
    • Pro: much greater ATP yield
    • Con: requires oxygen
how much atp do muscles need
How much ATP do muscles need?
  • Yes, if you only want to shorten your muscle by about 1%
  • Cycle repeated until desired shortening reached
    • Usually this is about 30 to 35%
    • Note: always some myosin heads attached
      • Prevents thin filament from sliding back
  • Cycle stops if SR pumps calcium back out of sarcoplasm
    • Also stops if no more ATP
what if there s no atp
What if there’s no ATP?
  • Rigor mortis
    • Starts at 3 to 4 hrs after death
    • Peaks at 12 hours
    • Dissipates over next 48 to 60 hrs
  • Calcium no longer pumped out
    • Myosin stays stuck to actin
    • ATP runs out shortly after person stops breathing
    • No ATP means myosin heads can’t detach
    • Rigor mortis disappears as muscle proteins break down
    • Related to liver mortis
when do muscles use each atp generating path
When do muscles use each ATP-generating path?
  • Immediate demand (e.g. quick sprint, ~10 secs)
    • Myoglobin supplies oxygen
    • Phosphate groups donated to ADP from:
      • Another ADP (myokinase orchestrates this)
      • Creatinine phosphate (creatinine kinase directs)
when do muscles use each atp generating path44
When do muscles use each ATP-generating path?
  • Short-term (as phosphate borrowing runs out)
    • Shift to anaerobic while awaiting oxygen
    • Glycogen-lactic acid system
    • 30-40 seconds of energy
what happens after 40 secs
What happens after 40 secs?
  • Long-term energy
    • Oxygen delivery catches up with demands
    • Aerobic respiration supported
why do muscles fatigue
Why do muscles fatigue?
  • Different from psychological fatigue!
    • Run out of glucose and glycogen
    • ATP shortage slows Na/K pumps
      • Resting potential not maintained
    • Release of K+ to interstitial fluid lowers membrane potential and excitability
    • Motor nerves use up acetylcholine
why do i keep breathing heavy after i stop running
Why do I keep breathing heavy after I stop running?
  • Oxygen debt
    • Convert lactic acid to pyruvic acid
      • Most converted back to glucose
    • Replacing oxygen reserves (myoglobin, hemoglobin, etc.)
    • Replenish creatinine phosphate and AMP to ATPs
what are slow and fast twitch muscle fibers
What are slow- and fast-twitch muscle fibers?
  • Slow twitch
    • Small, AKA red fibers, long twitches
    • More mitochondria, myoglobin, capillaries
      • Why? More oxygen consumption and aerobic respiration
    • Postural muscles of back, soleus
what are slow and fast twitch muscle fibers49
What are slow- and fast-twitch muscle fibers?
  • Fast twitch
    • Large, AKA white fibers, short twitches
      • Many creatinine, glycogen-lactic acid pathways
    • Gastrocnemius
adapting to exercise
Adapting to exercise
  • Muscle fibers add filaments, not more myocytes (hypertrophy)
  • Red muscle adds more mitochondria, myoglobin
  • Heart hypertrophies
what s so special about cardiac muscle
What’s so special about cardiac muscle?
  • Recall characteristics
  • Does not require nerve stimulus for contraction
    • Pacemaker
    • Autrorhythmic
  • Aerobic respiration
what makes smooth muscle so smooth
What makes smooth muscle so smooth?
  • Lack striation, Z discs, T tubules
    • Calcium comes from interstitial fluid
  • Fusiform shape
    • Actin filaments anchored to dense body
    • Myosin filaments in between actin filaments
  • Can undergo mitosis
what kinds of smooth muscles are there
What kinds of smooth muscles are there?
  • Multi-unit
    • Muscle fibers structurally independent of one another
    • Arrector pili, larger arteries, pulmonary airways, iris
  • Single-unit (visceral muscle)
    • All cells contract as a unit, joined by gap junctions
    • Contract like a single cell
    • Most smooth muscle in body
how are smooth muscles stimulated
How are smooth muscles stimulated?
  • Can contract without nerve stimulus
    • Hormones, carbon dioxide levels, low pH, lack of oxygen, stretch reflex
  • Also has ANS innervation
    • Varicosities: synaptic vesicles secreted
    • Nerve fiber passes among several myocytes = diffuse junctions
how do smooth muscles contract
How do smooth muscles contract?
  • Calcium gates on PM
    • Voltage-regulated
    • Ligand-regulated (hormones)
    • mechanically-regulated (stretch): peristalsis
  • No troponin; calmodulin instead
    • Calcium binds to calmodulin
    • Long latent period, slow contraction and slow relaxation
      • Latch-bridge mechanism keeps myosin heads bound, prolongs contraction
      • E.g. vasomotor tone