Muscular System

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Chapter 12

• Muscular System

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Points to Ponder

• What are the three types of muscle tissue?
• What are the functions of the muscular system?
• How are muscles named and what are the muscles of
  the human body?
• How are skeletal muscles and muscle fibers
  structured?
• How do skeletal muscles contract?
• How do skeletal muscle cells acquire ATP for
  contraction?
• What is rigor mortis?
• What are some common muscular disorders?
• What are some serious muscle diseases?
• How do the skeletal and muscular system help
  maintain homeostasis?
• How are these 2 systems related to other systems
  in maintaining homeostasis?

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Muscle Tissue

• Skeletal muscle
• Voluntary striated muscle
• controlled by nerves of the central nervous
  system
• Multinucleated and tubular
• Attacked to skeleton
• Cardiac muscle
• Involuntary striated muscle
• Uninucleated, tubular, and branched
• Intercalated disks contain gap junctions
• spread contractions quickly throughout the heart
  wall
• Cardiac fibers relax completely between
  contraction
• Prevent fatigue
• Smooth muscle
• Involuntary nonstriated muscle
• Uninucleated
• Cells arranged in parallel lines, forming sheets
• Located in the walls of hollow internal organs
• Slower to contract, sustain prolonged
  contractions
• does not fatigue easily

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Review 3 types of muscle tissue
12.1 Overview of the muscular system
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Functions of skeletal muscles

• Support the body by allowing us to stay upright
• Allow for movement by attaching to the skeleton
• Help maintain a constant body temperature
• - Contraction causes ATP to break down, releasing
  heat
• Assist in movement in the cardiovascular and
  lymphatic vessels via muscular contractions
• Protect internal organs and stabilize joints
• Muscles pad bones
• Tendons hold bones together at joints

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How are skeletal muscles arranged?
12.1 Overview of the muscular system

• Attachments
• Tendon connective tissue that connects muscle
  to bone
• Muscles covered with fascis that extends beyond
  the muscle and becomes the tendon
• Origin attachment of a muscle on a stationary
  bone
• Insertion attachment of a muscle on a bone that
  moves
• Muscle contraction pulls on the tendon at its
  insertion causing the bone to move
• Action
• Nervous system does not stimulate a single
  muscle, it stimulates an appropriate group of
  muscles
• 1. Prime mover muscle that does most of the
  work
• 2. Antagonistic muscles that work in opposite
  pairs
• - biceps brachii and triceps brachii are
  antagonist
• 3. Synergistic muscles working in groups for a
  common action
• - assist the prime mover, enhances action

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An example of muscle arrangement
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How to Name Skeletal Muscles

• Size
• Maximus largerst Minimus smallest
• Vastus huge Longus long Brevis short
• Shape
• Deltoid triangular (Greek letter delta is ?)
• Trapezius trapezoid Latissimus wide Terres
  round
• Location
• Frontalis overlies the frontal bone
• External and internal obliques pectoralis
  gluteus brachii

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How to Name Skeletal Muscles

• Direction of muscle fiber
• Rectus abdominus (rectus means straight)
• Transverse across oblique diagonal
• Attachment
• Brachioradialis attached to the brachium and
  radium
• Sternocleidomastoid attached to sternum,
  clavicle, and mastoid
• Number of attachments
• the biceps brachii has two attachments
• Action
• extensor digitorum extends the digits
• adductor (to midline), flexor (flexes), levator
  (lift)

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Muscle fibers/cells

• Terminology for cell structure
• The plasma membrane is called the sarcolemma
• Transverse tubules penetrate cell to contact with
  sarcoplasmic reticulum
• The cytoplasm is called the sarcoplasm
• Contains glycogen (store energy) and myoglobin
  (binds oxygen) needed for muscle contraction
• The SER of a muscle cell is called the
  sarcoplasmic reticulum and stores calcium

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Skeletal Muscle Fibers

• Sarcoplasm contains networks of SER called
  sarcoplasmic reticulum (SR)
• Sarcoplasmic Reticulum
• Function
• store calcium and help transmit action potential
  to myofibril
• SR forms chambers attached to T-tubules
• Concentrate Ca2 (via ion pumps)
• Release Ca2 into sarcomeres to begin muscle
  contraction
• All calcium is actively pumped from sarcoplasm to
  SR (SR has 1000X more Ca2 than sarcoplasm)

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Skeletal Muscle Fibers

• Terminology for structure within a whole muscle
• Muscle fibers are arranged in bundles called
  fascicles
• Myofibrils are a bundle of myofilaments that run
  the length of a fiber
• Myofilaments are proteins (actin and myosin) that
  are arranged in repeating units
• Sarcomeres are the repeating units of actin and
  myosin found along a myofibril

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12.2 Skeletal muscle fiber contraction
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The sarcomere
12.2 Skeletal muscle fiber contraction

• Made of two protein myofilaments
• Myosin are the thick filaments shaped likea
  golf club
• Globular head allows for cross-bridges
• Actin are the thin filaments
• Two other proteins are present
• Tropomyosin and troponin
• These filaments slide over one another during
  muscle contraction

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Actin
Myosin
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Regions of the Sarcomere

• A-band
• - whole width of thick filaments, looks dark
• microscopically
• M line at midline of sarcomere
• - Center of each thick filament, middle of
  A-band
• - Attaches neighboring thick filaments
• H-zone
• - Light region on either side of the M line
• - Contains thick filaments only
• Zone of overlap
• - ends of A-bands
• - place where thin filaments intercalate between
  thick
• filaments (triads encircle zones of
  overlap)

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Regions of the Sarcomere

• I-band
• - Contains thin filaments outside zone of
  overlap
• - Not whole width of thin filaments
• Z lines/disc
• - the centers of the I bands

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Sliding Filaments
Figure 108
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Sliding Filament Theory

• Contraction of skeletal muscle is due to thick
  filaments and thin
• filament sliding past each other
• not compression of the filaments
• H-zones and I-bands decrease width during
  contraction
• Zones of overlap increase width
• Z-lines move closer together
• A-band remains constant
• Sliding causes shortening of every sarcomere in
  every myofibril in every fiber
• Overall result shortening of whole skeletal
  muscle

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The beginning of muscle contraction The sliding
filament model
12.2 Skeletal muscle fiber contraction

• Nerve impulses travel down motor neurons to a
  neuromuscular junction
• Acetylcholine (ACh) is released from the neurons
  and bind to the muscle fibers
• This binding stimulates fibers causing calcium to
  be released from the sarcoplasmic reticulum

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Skeletal Muscle Neuromuscular Junction
Figure 1010a, b (Navigator)
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Muscle contraction continued
12.2 Skeletal muscle fiber contraction

• Released calcium combines with troponin, a
  molecule associated with actin
• This causes the tropomyosin threads around actin
  to shift and expose myosin binding sites
• Myosin heads bind to these sites forming
  cross-bridges
• ATP bind to the myosin heads and is used as
  energy to pull the actin filaments towards the
  center of the sarcomere contraction now occurs
• ATP is bind to myosin
• ATP is split to ADP and 2 Phosphates
• ADP and P remain on the myosin heads
• - Heads attach to an actin filament forming cross
  bridges
• ADP and P is released, cross-bridges bend sharply
  
• - Power stroke pulls action filament toward the
  center of the sarcomere
• ATP molecule binds again to myosin to break cross
  bridge

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12.2 Skeletal muscle fiber contraction
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What role does ATP play in muscle contraction and
rigor mortis?
12.2 Skeletal muscle fiber contraction

• ATP is needed to attach and detach the myosin
  heads from actin
• After death muscle cells continue to produce ATP
  through fermentation and muscle cells can
  continue to contract
• When ATP runs out some myosin heads are still
  attached and cannot unattach rigor mortis
• Body temperature and rigor mortis helps to
  estimate the time of death

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Tension Production

• For a single muscle fiber contraction is
  allornone
• as a whole, a muscle fiber is either contracted
  or relaxed

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Resting Length

• Greatest tension produced at optimal resting
  length
• Optimal resting length Optimum overlap
• Overlap determines the number of pivoting
  cross-bridges

Figure 1014
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Frequency of Stimulation

• Twitch single contraction due to a single
  neural stimulation, 3 phases
• Latent period post stimulation but not tension
• Action potential moves across the sarcolemma
• Ca2 is released
• Contraction phase peak tension production
• - Ca2 bind
• - Active cross bridge formation
• Relaxation phase decline in tension
• Ca2 is reabsorbed
• Cross bridges decline

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Terms in whole muscle contraction
12.3 Whole muscle contraction

• Motor unit a nerve fiber and all of the muscle
  fibers it stimulates
• Muscle twitch a single contraction lasting a
  fraction of a second
• Summation an increase in muscle contraction
  until the maximal sustained contraction is
  reached
• Tetanus maximal sustained contraction
• Tone a continuous, partial contraction of
  alternate muscle fibers causing the muscle to
  look firm

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Physiology of skeletal muscle contraction
12.3 Whole muscle contraction
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Total Number of Muscle Fibers Stimulated

• Each skeletal muscle has thousands of fibers
  organized into motor units
• Motor units all fibers controlled by a single
  motor neuron
• Axon branches to contact each fiber
• Number of fibers in a motor unit depends on the
  function
• Fine control 4/unit (e.g. eye muscles)
• Gross control 2000/unit (e.g. leg muscles)
• Fibers from different units are intermingled in
  the muscle so that the activation of one unit
  will produce equal tension across the whole muscle

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Motor Units in a Skeletal Muscle
Figure 1017
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Recruitment (Multiple Motor Unit Summation)

• In a whole muscle or group of muscles, smooth
  motion and increasing tension is produced by
  slowly increasing size or number of motor units
  stimulated
• Recruitment order of activation of a motor unit
• Slower weaker units are activated first
• Strong units are added to produce steady
  increases in tension

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Contraction Skeletal Muscle

• During sustained contraction of a muscle
• Some units rest while others contract to avoid
  fatigue
• For maximum tension, all units in complete
  tetanus
• Leads to rapid fatigue
• Muscle tone maintaining shape/definition of the
  muscle
• Some units are always contracting
• Exercise Increase of units contraction ?
• Increase in metabolic rate ?
• Increase in speed of recruitment (better tone)

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Where are the fuel sources for muscle contraction?
12.3 Whole muscle contraction

• Stored in the muscle
• Glycogen
• Fat
• In the blood
• Glucose
• Fatty acids

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What are the sources of ATP for muscle
contraction?
12.3 Whole muscle contraction

• Limited amounts of ATP are stored in muscle
  fibers
• Creatine phosphate pathway (CP)
• fastest way to acquire ATP
• sustains a cell for seconds
• builds up when a muscle is resting
• Fermentation
• fast-acting but results in lactate build up
• Cellular respiration (aerobic)
• not an immediate source of ATP
• best long term source

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Acquiring ATP for muscle contraction
12.3 Whole muscle contraction
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Muscle fibers come in two forms

• Fast-twitch fibers
• rely on CP and fermentation (anaerobic)
• Lactate buildup is possible which leads to quick
  fatigue
• Designed for strength
• Provide explosions of energy, develop max.
  tension more rapidly (sprinting, weight lifiting)
• Light in color
• Few mitochondria
• Little or no myoglobin
• Fewer blood vessels than slow-twitch
• Motor units contain many fibers

• Slow-twitch fibers
• Rely on aerobic respiration
• Tire when fuel supply is gone
• Designed for endurance
• Long distance running
• Biking and swimming
• Dark in color
• Many mitochondria
• Myoglobin
• Many blood vessels
• Provide oxygen
• Low maximum tension but high resistance to
  fatigue

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Types of muscle fibers
12.3 Whole muscle contraction
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Health focus Benefits of exercise
12.3 Whole muscle contraction

• Increases muscle strength, endurance and
  flexibility
• Increases cardiorespiratory endurance
• Heart rate and capacity increase
• Air passages dilate so that the heart and lungs
  are able to support prolonged muscular activity
• HDL increases thus improving cardiovascular
  health
• Prevents development of plaque
• Proportion of protein to fat increases favorably
• May prevent certain cancers
• colon, breast, cervical, uterine and ovarian
• Improve density of bones thus decreasing the
  likelihood of osteoporosis
• Enhances mood and may relieve depression

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Common muscle disorders

• Spasms
• sudden, involuntary muscle contractions that are
  usually painful
• Seizure
• multiple spasms of skeletal muscles
• Cramps
• strong, painful spasms often of the leg and foot
• Strain
• stretching or tearing of a muscle

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Common muscle disorders

• Sprain
• twisting of a joint involving muscles, ligaments,
  tendons, blood vessels and nerves
• Tendonitis
• inflammation of a tendon usually due to overuse
  (i.e. tennis elbow)
• Bursitis
• inflammation of a bursa usually from repetitive
  use or frequent pressure

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Muscular diseases

• Fibromyalgia
• chronic achy muscles that is not well understood
• Muscular dystrophy
• group of genetic disorders in which muscles
  progressively degenerate and weaken
• Myasthenia gravis
• autoimmune disorder that attacks ACh receptor and
  weakens muscles of the face, neck and extremities
• Amyotrophic lateral sclerosis (ALS)
• commonly known as Lou Gehrigs disease in which
  motor neurons degenerate and die leading to loss
  of voluntary muscle movement

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Homeostasis the skeletal and muscular systems
12.5 Homeostasis

• Both systems are involved with movement that
  allows us to respond to stimuli, digestion of
  food, return of blood to the heart and moving air
  in and out of the lungs
• Both systems protect body parts
• Bones store and release calcium need for muscle
  contraction and nerve impulse conduction
• Blood cells are produced in the bone
• Muscles help maintain body temperature

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Bioethical focus Anabolic steroids?
12.5 Homeostasis

• Anabolic steroids are a group of steroids that
  usually increase protein production
• Most common side effects are high blood pressure,
  jaundice, acne and great increased risk of cancer
• Abuse of these drugs may also cause impotence and
  shrinking of the testicles
• May lead to increased aggressiveness and violent
  mood swings
• Are they worth the risk?
• Should they be legal to use in athletics?