Muscular System

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Muscular System

• Ch. 7

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Muscle Types

• All muscle cells are elongated and are called
  muscle fibers.
• Muscle cells contract because of the presence of
  two types of muscle filaments - myosin and actin.
• The prefixes myo and sarco refer to muscles.

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Smooth Muscle

• Found in the walls of the hollow organs and blood
  vessels
• The fibers have no striations
• The fibers are under involuntary control
• Fibers are large, spindle shaped, and have a
  single nucleus
• Fibers are arranged in sheets or layers often
  there are two sheets - one running circularly and
  one running longitudinally
• As the layers contract and relax they change the
  size of the organs
• Contractions are slow and sustained
• 3 key words to remember - visceral, nonstriated,
  involuntary

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Cardiac Muscle

• Cardiac muscle is a type of muscle found in the
  heart.
• Muscle fibers are striated
• Fibers are under involuntary control
• Fibers are arranged in spiral shaped bundles
• Fibers are branching cells that are joined by
  intercalated disks
• 3 key words - cardiac, striated, involuntary

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Skeletal Muscle

• Skeletal muscle is the type of muscle found
  attached to the bodys skeleton.
• The muscle fibers are cigar shaped and contain
  multiple nuclei.
• Is also called striated muscle because its fibers
  appear striped
• Attaches to the skeleton to produce movement
• Is under voluntary control

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Skeletal Muscle

• Muscle fibers can produce strong contractions
  because of several layers of connective tissue
  sheathes that provide strength.
• Contractions are powerful and rapid the muscle
  tires quickly and must rest
• 3 key words - skeletal, striated, voluntary

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Muscle Functions
 

• A. Producing movement
• 1. skeletal muscles are responsible for all
  voluntary movement and communication
• 2. smooth muscles are responsible for the
  movement of internal organs
• 3. cardiac muscle produces our heart beat
•  
• B. Maintaining posture
• 1. counteracts that force of gravity
• 2. allows us to maintain a sitting or standing
  position
• 3. muscles contract almost continuously making
  tiny adjustments

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Muscle Functions

• C. Skeletal muscles and tendons are important
  for stabilizing some joints such as the shoulder
  or the knee.
•  
• D. As a by-product of muscle contraction heat is
  produced.
• 1. skeletal muscle is the most important
  source of body heat
• 2. vital for maintaining our constant body
  temperature

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Functional characteristics of muscle
 

• A. Excitability, or irritability - the ability
  to receive and respond to a stimulus
•  
• B. Contractility - ability to shorten forcibly
•  
• C. Extensibility - ability to be stretched or
  extended beyond their resting length
•  
• D. Elasticity - ability to recoil and resume its
  resting length after being stretched

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Gross Anatomy of Skeletal Muscle

• A. Skeletal muscle consists mostly of skeletal
  muscle tissue, but also contains nerves, blood
  vessels, and connective tissue.
• B. Several connective tissue wrappings surround,
  support, and contain skeletal muscles

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1. epimysium - dense connective tissue that
surrounds an entire muscle (many muscle
fascicles) many epimysia blend into a tendon
that attaches the muscle to a bone or cartilage
2. perimysium - within a muscle, muscle fibers
are grouped into bundles called fascicles. The
perimysium surrounds an individual fascicle. 3.
endomysium - a fine sheath that surrounds an
individual muscle cell (called a fiber).
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• C. Most muscles span across joints attach to
  bones in at least two places.
• 1. The bone that moves when a muscle contracts
  is called the muscle's insertion.
• 2. The bone that does not move, or is less
  moveable, is called the muscle's origin.
• 3. The motion produced by the contraction is
  called the muscle's action.

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Microscopic anatomy of skeletal muscle
 

• A. The muscle cell is called a muscle fiber
• 1. the cell membrane is called the sarcolemma
• 2. the cytoplasm of a muscle fiber is called
  the sarcoplasm
• 3. the fiber contains many nuclei that are
  pushed against the sarcolemma
• 4. the fiber is filled with myofibrils
• 5. the endoplasmic reticulum stores and
  releases calcium in muscle, it is the
  sarcoplasmic reticulum

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Microanatomy of a Muscle Fiber (Cell)
transverse (T) tubules
__________________________
________________
terminal cisternae
____________
__________________
______________
myofibril
triad
________
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• B. The myofibrils are composed of contractile
  units called sarcomeres
• 1. each sarcomere is composed of two types
  of protein filaments known as myofilaments
  - thick and thin filaments
• 2. the protein myosin composes the thick
  filaments
• 3. the protein actin composes the thin
  filaments

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• 4. Striations represent alternating dark and
  light bands
• ?dark bands (called A bands) myosin and actin
• ?light bands (called I bands) actin
• 5. The I bands are interrupted by the Z disks.
• ?connects sarcomeres
• ?a sarcomere extends from Z disk to the next Z
  disk ?anchors the actin filaments of a sarcomere
• 6. The A bands interrupted by the M line
• ?M line represents a protein that anchors the
  myosin filaments

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Sarcomere
__________
_________
______
H zone
_________
Zone of overlap
Zone of overlap
_____
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Myofilaments of sarcomere
Z line
M line
Z line
_____________________________
_________________________________________
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• C. Structure of myofilaments
• 1. Thick filament myosin molecules
• ?each myosin molecule has a rod-like tail that
  ends in two heads
• ?the heads act as cross-bridges that allow
  temporary connections to form between myosin
  and actin during contraction

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• 2. Thin filaments actin molecules,
  tropomyosin, and troponin
• ?actin molecules contain active sites where the
  myosin cross-bridges attach
• ?tropomyosin - stiffens and thins filament
• ?troponin - binds to and blocks the active
  sites on actin

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Figure 6.5 Slide 6.4B
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Sliding Filament Theory

• When muscles contraction occurs the actin
  filaments slide over the myosin filaments and
  cause the sarcomere to shorten
• 1. Actin molecules are pulled toward the
  center of the sarcomere
• 2. The distance between Z lines becomes less
  as each sarcomere shortens
• 3. The combined effect of shortened sarcomeres
  is the shortened muscle fiber
• 4. Contraction results from the attachments of
  the cross-bridges on the myosin molecules to
  the active sites on the actin molecules. Each
  cross-bridge attaches and detaches several
  times during one contraction.

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• 5. Contraction requires the presence of calcium
  ions
• ?Troponin blocks the active sites on actin
• ?Calcium binds with and alters the shape of
  troponin
• ?The altered shape of troponin opens up the
  active sites on actin

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• 6. When the active sites on actin are exposed,
  the following four events occur
•   a. a cross- bridge attaches to an actin active
  site
• b. the myosin head pivots into a bent shape
  that drags the actin toward the center of the
  sarcomere
• c. the cross-bridge detaches
• d. the cross-bridge re-forms its straighter,
  "cocked" position

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Physiology of Skeletal Muscle Contraction
 

• A. Skeletal muscles must be stimulated by nerve
  impulses to contract.
• 1. The nerve input is carried by a motor
  neuron.
• 2. One motor neuron may stimulate several or
  hundreds of muscle fibers.
• 3. A motor unit is defined as a motor neuron
  and the muscle fibers it stimulates.
• 4. The neuron branches into a number of axonal
  terminals as it nears the fiber. Each
  terminal forms a junction with a different
  muscle fiber. These junctions are called
  neuromuscular junctions.

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• 5. The neuron and the fiber never actually
  touch. There is a space between the two known as
  the synaptic cleft.
• 6. When a nerve impulse (action potential)
  reaches the neuromuscular junction, a chemical,
  called a neurotransmitter is released. The
  neurotransmitter for all skeletal muscles is
  acetylcholine.
• 7. The nerve impulse causes the muscle fiber to
  contract.

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• Once an action potential (AP) is generated at the
  motor end plate it will spread like an electrical
  current along the sarcolemma of the muscle fiber

• The AP will also spread into the T-tubules,
  exciting the terminal cisternae of the
  sarcoplasmic reticula

• This will cause Calcium (Ca2 ) gates in the SR
  to open, allowing Ca2 to diffuse into the
  sarcoplasm

• Calcium will bind to troponin (on the thin
  myofilament), causing it to change its shape.
  This then pulls tropomyosin away from the active
  sites of actin molecules.

• The exposure of the active sites allow the
  sliding of the filaments

Table 7-1
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• B. Contraction of a muscle as a whole
• 1. Contraction of a muscle fiber is an all or
  none action.
• 2. An entire muscle can contract in varying
  degrees. This is called a graded response.
• 3. A graded response can be produced by 1)
  varying the speed with which the muscle is
  stimulated and by 2) changing the number of
  muscle fibers stimulated.
• 4. In most muscle activity, nerve impulses are
  delivered to the muscle fibers quickly and the
  combined effect of successive fiber contractions
  produces a smooth muscle contraction.
• 5. The force with which a muscle contracts is
  largely determined by the number of fibers that
  are stimulated.

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• C. Energy for muscle contraction
• 1. The energy needed for muscle contraction
  comes from ATP that is produced by aerobic
  respiration.
• 2. When a muscle is over worked there is
  insufficient oxygen for aerobic respiration to
  occur and the fiber switches to anaerobic
  respiration.
• 3. Anaerobic respiration produces lactic acid as
  a by- product. Lactic acid is the cause of
  muscle soreness and stiffness that may occur
  after intense activity.

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ATP

• Principle source of energy ATP
• ATP needed to 1) cock the myosin heads in the
  ready to fire position 2) allows the
  cross-bridge to disengage
• After death, the lack of ATP keeps cross-bridges
  engaged this is the cause of rigor mortis
• ATP replenished by variety of means
• Creatine phosphate (assists in the bonding of P
  to ADP)
• Creatine supplements are not needed or used by
  the body, all you are doing is making expensive
  urine the body makes plenty of creatine, it is
  not stored when a supplement is taken
• Stored glycogen used for initial 3-5 minutes of
  exercise
• Aerobic catabolism of glucose, fatty acids, and
  other high-energy molecules for long term energy
  and exercise

Slide 6.9
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• D. Muscle fatigue
• 1. Muscle fatigue occurs if we exercise our
  muscles continuously for a long time.
• 2. Muscle fatigue occurs when a muscle is
  unable to contract even though it is being
  stimulated.
• 3. This occurs when a muscle is being
  contracted continuously without a period of
  rest. The contractions get shorter and shorter
  until they stop completely.
• 4. Muscle fatigue is believed to be due to the
  bodys inability to supply oxygen to the muscles
  fast enough. Without oxygen the energy produced
  by the muscles decreases until there is no
  energy for contraction. This is called the
  oxygen debt.

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• E. Types of muscle contractions
• 1. Twitch contractions are quick, jerky
  responses to a stimulus. They play a minor role
  in normal muscle activity.
• 2. Tetanic contractions are longer, more
  sustained contractions produced by a series of
  stimuli reaching the muscle fibers in rapid
  succession
• (tetany refers to a smooth, sustained muscle
  contraction and NOT to the disease known as
  tetanus caused by C. tetani)
• 3. Isotonic contractions occur when the muscle
  shortens and movement occurs.

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4. Isometric contractions occur when the muscle
tries to shorten but it is met with resistance.
The muscle does not shorten but the tension in
the muscle increases. 5. Muscle tone is a
state of continuous, partial muscle contractions
that keeps muscles firm and healthy.
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• F. The effect of exercise on muscle contraction
• 1. Aerobic exercises produce stronger, more
  flexible muscles that are more resistant to
  fatigue. This occurs because the blood supply
  to the muscle increases as does the number of
  mitochondria in each fiber. There is little
  increase in the size of the muscles.
• 2. Resistance exercises, or isometric
  exercises, train the muscles against immovable
  objects. Consistent isometric exercises cause
  an increase in the size of muscle cells, not an
  increase in muscle cells. Increase fiber size
  causes an increase in the size of muscles.
  Increase in size, not number of cells is called
  hypertrophy.

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3. Cross training - alternating between aerobic
and resistance exercises provides an effective
program for general health. 4. Prolonged
inactivity may cause muscle fibers to shrink.
This process is called atrophy and the condition
is referred to as disuse atrophy.