The Muscular System Chapter 10

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					                                The Muscular System Chapter 10

3 Types of Muscle Tissue:
1.) Skeletal: voluntary, striated

2.) Cardiac: involuntary, striated

3.) Smooth: involuntary, non-striated

Functions of Skeletal Muscle:
1.) Movement

2.) Maintenance of posture

3.) Support soft tissues: abdominal wall & pelvic floor muscles

4.) Guard entrances/exits – sphincters
      Openings to digestive & urinary systems

5.) Maintenance of body temperature

6.) Store nutrient reserves

Characteristics of Skeletal Muscle:

    1.) Excitability or irritability: ability to respond to stimulus

    2.) Extensibility:

    3.) Elasticity:

    4.) Contractility: ability to shorten and contract

Anatomy of Skeletal Muscle
   A. Connective Tissue Coverings: (pg. 281)

      •Epimysium: surrounds entire muscle

      •Perimysium: surrounds each fasciculus
          note: epimysium & perimysium contain blood vessels & nerves that
          supply muscle fibers


Skeletal Muscle Fibers
    - muscle fiber is muscle cell

    -are very large and long

    -multinucleate (hundreds per cell)
                                                                      Sarcolemma: cell membrane of a
                                                                      muscle cell (fiber)

                                                                      Sarcoplasm: cytoplasm in muscle
                                                                       contains mitochondria,

                                                                      Transverse Tubules (T tubules):

                                                                      Myofibrils: cylindrical structures –
                                                                      long as the cell

                                                                      made up of myofilaments or
                                                                      muscle proteins
Sarcoplasmic Reticulum (SR):
    membrane channel network stores

    terminal cisternae:

    triad: 3 structures: T tubule & 2 terminal cisternae

      Myofibrils are formed from repeating units termed Sarcomeres
         Sarcomere: basic contractile units of muscle

          Alternating dark (thick) and light (thin) filaments cause striped or
          striated pattern.

                Thick filament: myosin        Thin filament: actin
                                                           Anatomy of a Sarcomere

                                                           -Z line (disc):

                                                           -A band: dark band: all myosin
                                                           ( zone of overlap)

                                                           -I band: light band: only actin

                                                           -H zone:

                                                           -M line:                                   2
Myofilaments: protein
1. Actin:
    -G actin: has active site for myosin

    -F actin:
       held together with nebulin

      -Troponin and Tropomysosin: T-T complex


2. Myosin:
    thick protein consists of tail and head
       head pivots

    forms cross bridges when grabbing actin’s active site

Other important muscle proteins:
   Titin: anchors myosin & assists with muscle fiber elasticity

    Dystrophin: achors actin to sarcomere

muscle fibers contract (sarcomere shortens) results in fascicles shortening, then muscle shortens
and pulls on tendon which pulls bone (creates tension)

skeletal muscle is under neural control – must receive impulse from axon of neuron from CNS

•Motor neuron:

    Axon terminal (synaptic knob)

•Action Potential (nerve impulse): an event which occurs when a stimulus of sufficient intensity
(threshold) is applied to a neuron.

      -cell has a negative charge inside (RMP) & the             concentration is greatest within
      the cell (cytosol) and     concentration greatest outside of cell (IF)

      threshold stimulus applied which open Na+ channels and Na+ ions rush into the cell -
      inside becomes             this is termed               .

      -then Na+ channels close and K+ channels open (K+ leaves cell) which then causes it to

   -Na-K pump operates at rest


•Sliding Filament Theory:
       During contraction, the actin slide towards the H-zone. The sarcomere shortens but the
       length of the actin and myosin remain the same.

   Myosin cross-bridges connect w/ portions of actin
   move like a oars of a boat and slide, which causes the H-zone to disappear, Z       line
   becomes closer

   **Must have stimulus from motor neuron

•Neuromuscular Junction
   synaptic (axon) terminal (synaptic knobs) of motor neuron’s axon
      contains vesicles which stores/releases acetylcholine (ACh)

   synaptic cleft

   Motor End Plate: contains receptors for ACh
         (also contains acetylcholinesterase – AChE)

      Sequence of Events at Neuromuscular Junction (see pages 292-293)

      1.) Action potential arrives at neuromuscular junction

      2.) Ca++ ions pumped into axon terminal

      3.) Synaptic vesicles fuse to membrane – exocytosis of Ach

      4.) Ach diffuses across synaptic cleft – binds to receptors on motor end plate

      5.) Opens Na+ channels – depolarizes sarcolemma

      6.) AChE (AChase) breaks down ACh

THEORY IN DETAIL: (pages 294-295 & Table 10-1 page 297)
1. When muscle is relaxed:

   *ATP concentration is high

    *Myosin cross-bridges are prevented from

2. When the stimulus is released, travels to axon terminal (synaptic knob)
      • calcium enters
      • causes the synaptic vesicle to fuse with cell membrane of neuron
      • triggers exocytosis of ACh

3. ACh diffuses across synaptic cleft and binds to receptors at motor end plate
      • depolarizes sarcolemma which opens Na+ channels and depolarizes sarcolemma
      results in the nerve impulse being spread over surface of muscle fiber, down the T tubules
      to the SR

4. This causes Calcium to be released from the           Ca++ combines with
    causing it to weaken the troponin-tropomyosin complex.

    exposes active sites on G actin

    termed excitation-contraction coupling (action potential reaches triad and stimulates release
    of calcium which then causes muscle to contract)

5. Contraction Cycle
     requires ATP which is located on myosin head
     ATP  ADP + P + E

      Myosin heads bind to actin (form crossbridge)
      power stroke


      reactivation (“recock” myosin head)

      sarcomere shortens

6. AChase (acetylcholinesterase) inactivates ACh which stops action potential &
    depolarization of sarcolemma & SR

    Ca++ ions leave troponin and the T-T complex is restored

    Ca++ ions are actively transported back into the                    for storage.

    *ADP resynthesized to ATP which attaches to the ATP binding site on myosin cross-bridge

Motor unit: one motor neuron and

    *the more muscle fibers to neuron, more gross the movement:
      More precise movements have smaller ratios

    All fibers in motor unit contract & relax as unit

  The all–or–none principle
     As a whole, a muscle fiber is either contracted or relaxed

 Tension of a Single Muscle Fiber Depends on
      The fiber’s resting length at the time of stimulation
      The number of pivoting cross-bridges
      The frequency of stimulation

Increases in strength/tension:
    1. increased stretch

      sarcomere has optimal length which ensures greatest number of cross
      bridges can form

    2. increase # motor units


    3. increase frequency of stimulation

note: tension must overcome resistance to move bone/limb
    lighter objects are lifted quicker than heavier objects

Muscle Contractions resulting from increased frequency of stimulus (pg. 301)
1. Twitch:

    •3 phases:
      1. latent phase: time period between when the stimulus is applied and the muscle starts
      to contract (shorten)

      2. contraction phase:

      3. relaxation phase:

2. Treppe or staircase phenomenon:
    •muslce stimulated with the same stimulus intensity with complete relaxation between
    stimuli but muscle exhibits a stronger contraction

    •due to increased availability of Ca++ and

    •heat generated by muscle activity, increases efficiency of enzymes

3. Wave Summation
   •muscle stimulated

    •the response to the subsequent stimuli will be greater (stronger force of contraction)

    •occurs because muscle is

4. Tetany (tetanus)
    •stimulation of a muscle at higher frequencies produces a fusion (tetanization) of the
    summated twitches.

    •a single sustained contraction

    Maximum tension achieved when all motor units reach tetanus

    •leads to muscle fatigue caused by:
       1. inability to generate

      2. increased

      3. damage to sarcolemma

Muscle Tone
The normal tension and firmness of a muscle at rest

Muscle units actively maintain body position, without motion
   some motor units always active – muscle is contracting but not enough for movement –
   important balance/posture

Increasing muscle tone increases metabolic energy used, even at rest
    exercise increases muscle tone of muscles – requires more energy

1. Isotonic
     a. concentric

    b. eccentric

2. Isometric
     tension produced doesn’t exceed resistance

Muscles Work in Groups:
   1. Prime Movers:

    2. Antagonists:

    3. Synergist:

    4. Fixator

Energy Sources for Contraction
      ATP is the immediate direct source of energy.

      Sustained muscle contraction uses a lot of ATP energy
      Muscles store enough energy to start contraction
      Muscle fibers must manufacture more ATP as needed

Muscle fibers contain ATP and CP (creatine phosphate) to start contraction
      ADP + creatine phosphate → ATP + creatine (e=creatine kinase or ck)

Then need to make ATP by the metabolic breakdown of glucose or glycogen in muscle cells
   aerobic or anaerobic

      anaerobic: glycolysis

                 each glucose forms             ATP molecules

                 breaks down stored glycogen into glucose

                 pyruvic acid cannot enter aerobic pathways so it is converted into lactic acid

                 occurs in cytoplasm of cell

                 anaerobic threshold: point when muscle switches over to anaerobic rather than

      aerobic: glycolysis, TCA (krebs cycle), ETC (electron transport chain)

                each glucose forms             ATP molecules

                uses glucose, glycerol, fatty acids & amino acids for fuel

                mainly occurs in mitochondria of cell

                uses pyruvic acid produced from glycolysis

The Recovery Period
    muscle conditions return to normal

    The time required after exertion for muscles to return to normal
           Oxygen becomes available
           Mitochondrial activity resumes

1.) Lactic Acid Removal and Recycling (Cori Cycle)

    oxygen is available again

    lactic acid converted back to pyruvic acid which is used to make ATP or converted back to
    glucose 6-PO4 and glycogen (reverse glycolysis)

    some lactic acid enters blood and taken to liver and converted back to glucose

    some lactic acid used by heart for fuel

2.) Oxygen Debt:
    extra amount of oxygen necessary to replenish the muscle (Cori cycle: convert lactic acid,
      make ATP)

    so must increase RR

Energy Use:
   At rest skeletal muscle metabolizes fatty acids

    During light activity

    At peak activity: anaerobic metabolism

Muscle Performance
   power: maximum amount of tension produced by a muscle or muscle group
   endurance: amount of time during which an individual can perform a particular activity

Hormones and Muscle Metabolism

Hormone             Released from                               Action


Thyroid Hormone


Types of Muscle Fibers: (pg. 309 table 10-3)
   1. Slow Fibers - Type I (ST slow twitch or slow oxidative):
      aerobic or red fibers

      most myoglobin
      smallest diameter

      most mitochondria

      uses carbs, fats and amino acids for energy

    2. Intermediate Fibers - Type IIA (FT or fast glycolytic)
          Mainly anaerobic but contain some myoglobin

           Some mitochondria, intermediate diameter

    3. Fast Fibers - Type IIB (FT or fast oxidative)
          anaerobic or white fibers
          largest diameter

           fewest mitochondria

           low myoglobin

           uses carbs only

    •all muscle fibers within any given motor unit

    •training can increase the O2 capacity of type I and change intermediate to fast and fast to
    intermediate BUT CANNOT change slow to intermediate/fast or vice versa



Physical Conditioning
Anaerobic endurance: length of time contraction can continue by glycolysis

          limited by:
          1.) ATP & CP
          2.) Glycogen
          3.) lactic acid tolerance

Aerobic endurance

      1.) Convert IIB (fast fiber) into a IIA (intermediate)

      2.) Cardiovascular increases

Specific Adaptations to Exercise:
   1. increase capillaries

    2. increase # mitochondria

    3. more myoglobin produced

    4. muscle hypertrophy (mainly resistance exercise)

    5. Heart hypertrophy and increase stroke volume

    6. Arteries cleaned

    7. Increase efficiency of metabolic pathways


Comparison of Skeletal, Cardiac & Smooth Muscle Tissues (pg. 316 table 10-4)

Characteristic      Skeletal             Cardiac               Smooth (Visceral)


Appearance                               intercalated discs


Fiber Size


Ca++ Source


Energy Source

Clinical Information:

Muscular Dystrophies
   Duchenne’s muscular dystrophy (DMD)
     Genetic disease (usually X-linked)

      Malfunctioning protein: dystrophin (provides strength to sarcomere and anchors actin)

      Symptoms start in childhood – b/w 3-7 yrs.

   a. botulism: Clostridium botulinum

           anaerobic bacteria: prevents release of Ach = paralysis

           botulism toxin most deadly toxin

    b. curare
           prevents ACh from binding to receptors on motor end plate

    c. tetanus: (page 291) Clostridium tetani
       Anaerobic bacteria – mortality rate – 40-80%

       Blocks inhibitory pathway to stop motor neuron = continuous contraction

       Headache, muscle spasms – lockjaw

       Tetanus toxin is top 3 deadliest toxins

       Tetanus shot every 10 yrs.

Myasthenia gravis
   Autoimmune disease

    decrease in ACh receptors on motor end plate

Rigor mortis: (pg. 296)
    Lack of ATP traps Ca++ in cell muscles contract but without ATP myosin cannot detach

     Viral disease

    Attacks motor neurons

    Post Polio Syndrome

Delayed Onset Muscle Soreness (DOMS) (pg. 312)

    caused by microtears in myofilaments/sarcomere, connective tissue, and/or muscle spasms


    characterized by musculoskeletal pain caused by chronic inflammation of muscle,
     connective tissue, joint capsules

    diagnosed by presence of “trigger points”






Torn Muscle:

Remember: skeletal and smooth muscle has some ability to regenerate: cardiac does not


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