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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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The muscular system an introduction l.jpg
The muscular system: An introduction


What is it l.jpg
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 l.jpg
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 l.jpg
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 l.jpg
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 l.jpg
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 l.jpg
How do muscles attach to bone?

  • Direct (fleshy) attachment

    • epimysium fused to periosteum or perichondrium


How do muscles attach to bone8 l.jpg
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 l.jpg
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


The muscular system cellular anatomy l.jpg
The muscular system: cellular anatomy


What special features are found in muscle fibers l.jpg
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 l.jpg
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 l.jpg
How do muscles contract?

  • First, look at anatomy of myofilaments

  • Appears as a striation pattern

  • Sliding filament animation


What creates the striations l.jpg
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 l.jpg
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 l.jpg
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 l.jpg
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 l.jpg
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 l.jpg
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 l.jpg
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 l.jpg
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


The muscular system the neuromuscular junction l.jpg
The muscular system: The neuromuscular junction


How do nerves tell muscles to contract l.jpg
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 l.jpg
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 l.jpg
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 l.jpg
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 l.jpg
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 l.jpg
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 l.jpg
What are action potentials?

2. Propagation

  • action potential spreads across sarcolemma

  • Opens voltage-sensitive Ca2+ gates


What are action potentials30 l.jpg
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 l.jpg
    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 l.jpg
    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 l.jpg
    TO REVIEW:What happens during excitation-contraction coupling?

    Excitation

    • 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 l.jpg
    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


    The muscular system dynamics and energy requirements l.jpg
    The muscular system: dynamics and energy requirements


    What makes a contraction strong l.jpg
    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 l.jpg
    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 l.jpg
    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 l.jpg
    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 l.jpg
    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 l.jpg
    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 l.jpg
    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 l.jpg
    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 l.jpg
    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 l.jpg
    What happens after 40 secs?

    • Long-term energy

      • Oxygen delivery catches up with demands

      • Aerobic respiration supported


    Why do muscles fatigue l.jpg
    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 l.jpg
    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 l.jpg
    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 l.jpg
    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 l.jpg
    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 l.jpg
    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 l.jpg
    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 l.jpg
    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 l.jpg
    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 l.jpg
    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


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