The Muscular System

1
The Muscular System

• Chapter 6

2
Muscular System

• Responsible for all types of body movement
• Three basic muscle types in the body
• Skeletal
• Cardiac
• Smooth
• Know these characteristics! ?

3
Characteristics of Skeletal Muscles

• Cells are elongated (cigar-shaped)
• Muscle cell Muscle fiber
• Most are attached to bones by tendons
• Cells are multinucleate
• Have striations (bands, stripes)
• Voluntary (we consciously control them)
• They are also activated by reflexes
• Cells are surrounded and bundled by connective
  tissue
• Key words skeletal, striated, voluntary

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Connective Tissue Wrappings of Skeletal Muscle

• Endomysium
• Covering around single fiber/cell
• Perimysium
• Around a bundle of fibers
• Bundle of fibers known as Fascicle
• Course, fibrous membrane around a bundle of
  muscle cells

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Connective Tissue Wrappings of Skeletal Muscle

• Epimysium
• Covers the entire skeletal muscle
• Outer covering
• Fascia
• On the outside of the epimysium
• Like skin on muscle
• Think skin like membrane on skinless raw chicken

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Skeletal Muscle Attachments

• Epimysium blends into connective tissue
  attachment
• Tendon
• Cord-like structure
• Tough Small pass over body projections
• Aponeuroses
• Sheet-like structure
• Both attaches muscle to bones, cartilage,
  connective tissue, or each other

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Smooth Muscle Characteristics

• No striations
• Spindle-shaped cells
• Single nucleus
• Involuntary-we cant control them
• Found mainly in walls of hollow organs
• Stomach
• Intestines
• Bladder
• Respiratory passages
• Key words Visceral, Involuntary, Non-striated

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Cardiac Muscle Characteristics

• Has striations
• Usually has one nucleus
• Joined to other muscle cells at intercalated disc
• Junction between cardiac muscle cells
• Involuntary
• Found in heart
• Key words cardiac, striated, involuntary

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Muscle Function

• Produce movement
• Maintain posture
• Stabilize joints
• Tendons help reinforce joints
• Generate heat
• Heat is a byproduct of muscle activity

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Microscopic Anatomy of Skeletal Muscle

• Cells are multinucleate
• Nuclei are beneath sarcolemma
• Fancy word for cell membrane of muscle
• Literally muscle husk

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Microscopic Anatomy of Skeletal Muscle

• Myofibril
• Bundles of myofilaments
• Prefix myo means muscle
• Aligned to give distinct bands (striations)
• I band light band
• A band dark band
• Sarcomere
• Contractile unit of muscle fiber

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Microscopic Anatomy of Skeletal Muscle

• Organization of the sarcomere
• Thick filaments Myosin filaments (RED)
• LARGE
• Made of the protein Myosin
• Length of A band
• Contains enzymes that help with muscle
  contraction

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Microscopic Anatomy of Skeletal Muscle

• Organization of the sarcomere
• Thin filaments Actin filaments (BLUE)
• Made of protein Actin
• Anchored to Z-disc
• Light I band is made only of thin filaments

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Microscopic Anatomy of Skeletal Muscle

• Myosin filaments have heads (extentions, or cross
  bridges)
• Look like balloons in the picture
• Myosin and actin overlap
• At rest, there is a bare zone that lacks actin
• Known as H zone

15
Skeletal Muscle Activity

• Muscles need to have
• Irratibility
• Ability to receive and respond to a stimulus
• Contractility
• Ability to shorten when an adequate stimulus is
  received

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Nerve Stimulus to Muscles

• Muscles must be stimulated by a nerve to contract
• Parts of a Motor Unit ?
• One Neuron (nerve cell)
• All the muscle cells stimulated by that neuron

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Nerve Stimulus to Muscles

• Neuromuscular Junction
• Association site of nerve and muscle
• Where nerve/muscle come together
• Synaptic cleft
• Gap between nerve and muscle
• Nerve and muscle dont make contact
• This area is filled with fluid

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Transmission of Nerve Impulse to Muscle

• Neurotransmitter (NT)
• Chemical released by nerve when impulse arrives
• Acetylcholine is the NT for skeletal muscle
• (uh-seet-ill-co-lean is pronunciation)
• NT attaches to receptors on sarcolemma (muscle
  cell membrane)
• The sarcolemma is then temporarily permeable to
  sodium (Na)
• Sodium rushes into the cell
• This changes electrical conditions in the cell
  (because of extra ions)
• Generates an action potential (electrical
  current)
• Causes contractionImpulse goes from one end of
  cell to the other
• Once contraction starts, you cant stop it.

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Muscle Contraction, continued

• Like lighting a match under a dry twig
• Twig is charred, becomes hot enough to burst into
  flame
• Flame then consumes twig
• Flame Action Potential that sweeps over the cell

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Sliding Filament Theory of Contraction

• Activation by nerve causes myosin heads to attach
  to actin
• They then bind to the next site of the actin
  (thin filament)
• This continues, causing the myosin to slide
  along the actin
• Result shortened muscle (contracted muscle)

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Sliding Filament Theory of Contraction
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Contraction of a Skeletal Muscle

• Muscle fiber contraction is all or none
• Muscles cant partially contract
• Not all muscle fibers may be stimulated at the
  same time
• Different combos of fiber contractions produce
  different responses
• Graded responses
• Different degrees of skeletal muscle shortening

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Types of Graded Responses

• Twitch
• Single, brief contraction
• NOT a normal function
• Tetanus (summing of contractions)
• Once contraction is immediately followed by
  another
• Muscle does not completely return to resting
  state
• Effects get added
• Unfused (incomplete) tetanus
• Some relaxation occurs between contractions
• Results are summed
• Fused (complete) tetanus
• No relaxation between contractions
• Sustained muscle contraction

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Muscle Response to Strong Stimuli

• Muscle force depends on how many fibers are
  stimulated
• More fibers contracting means greater muscle
  tension
• Muscles can continue to contract unless they run
  out of energy

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Energy for Muscle Contraction

• Muscles use stored ATP for energy
• Bonds of ATP are broken, energy is released
• There is only 4-6 seconds worth of ATP stored in
  muscles
• What happens after this is used?
• See next slide

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Energy For Muscle Contraction

• Direct Phosphorylation
• Muscles contain creatine phosphate (CP)
• High energy molecule
• CP transfers energy to ADP (whats left over
  after ATP is used) to make ATP again
• Creatine Phosphate supplies are depleted after
  about 20 seconds

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Energy For Muscle Contraction

• Aerobic Respiration
• Series of pathways that occur in mitochondria
• Glucose (sugar) is broken down to release energy
• Produces carbon dioxide and water
• Slow reaction that requires continuous oxygen

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Energy For Muscle Contraction

• Anaerobic Glycolysis
• Breaks down glucose without oxygen
• Glucose is converted to pyruvic acid to make ATP
• Pyruvic acid is converted to lactic acid
• NOT efficient, but is FAST
• Need huge amounts of glucose
• Lactic acid produces muscle fatigue

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How it Works

• Running a race
• We tend to breathe shallowly when running
• Not getting enough O2 to muscles, so body cant
  use Aerobic respiration to make ATP to power
  muscles to keep going
• Uses Anaerobic respiration to make ATP instead
• This produces lactic acid, which builds up in
  muscles and causes cramping
• Proper breathing technique can reduce or
  eliminate cramps related to this

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Muscle Fatigue and Oxygen Debt

• Muscle fatigue unable to contract muscle
• Common cause Oxygen debt
• Oxygen debt must be repaid to tissue to remove
  it
• Oxygen is required to get rid of accumulated
  lactic acid in muscle cells
• This is why athletes with cramps commonly have
  the cramping muscle massaged-it increases blood
  flow to the area
• Blood carries oxygen, when then helps remove
  lactic acid from muscle
• Increasing acidity (from lactic acid) and lack of
  ATP cause muscle to contract less
• This is why you get tired after running for long
  periods of time (or doing some other physical
  activity)

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Types of Muscle Contractions

• Isotonic Contractions
• Myofilaments slide past each other
• Muscle shortens
• Ex Lifting weights
• Isometric Contractions
• Increases tension in the muscle
• Muscle does not shorten
• Ex Pushing against a brick wall

32
Muscle Tone

• In relaxed muscle, some fibers are still
  contracted
• Different fibers contract at different times to
  provide muscle tone
• This is involuntary

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Muscles and Movements

• Movement is due to a muscle moving an attached
  bone
• Muscles are attached to at least two points
• Origin
• Attachment to IMMOVABLE bone
• Insertion
• Attachment to MOVEABLE bone

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What About Exercise?

• Results of Increased Muscle Use
• Increase in muscle size
• Increase in muscle strength
• Increase in muscle efficiency
• Muscle becomes more fatigue resistant
• Decreased muscle use has the opposite affect
• Muscle atrophy (wasting away of muscle)

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Types of Muscles

• Prime Mover
• Has major responsibility for a certain movement
• Antagonist
• Muscle that opposes or reverses a prime mover
• Returns limb to normal position)
• Synergist
• Helps a prime mover in a movement, prevents
  rotation
• Smaller muscles often help large muscle neighbors
• Fixator
• Stabilizes the origin of a prime mover
• Stabilize joints so motion is smooth

• Prime Mover/ Antagonist Pairs
• Biceps/Triceps
• Forearm Flexors/ Extensors
• Pectorals/Latissimus Dorsi
• Trapezius/Deltoid
• Quadriceps/Hamstrings

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Naming Skeletal Muscles

• 7 Criteria
• Direction of muscle fibers
• Ex rectus (straight)
• Relative size of muscle
• Ex gluteus maximus (largest)
• Location of muscle
• Many are named for bones
• Ex Temporalis
• Number of origins
• Ex Triceps (3 heads) Biceps (2 heads)
• Location of muscles origin and insertion
• Ex Sternocleidomastoid (sterno for sternum)
• Shape of the muscle
• Ex Deltoid (triangular, like Greek letter delta)
• Action of the muscle
• Flexor or extensor (flexes or extends a bone)