Muscle Overview

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Muscle Overview

• 3 different types of muscle tissue provide
  movement

• Skeletal
• attached to the bones of the skeleton
• controlled consciously (voluntary)

• Cardiac
• Heart
• controlled sub consciously (involuntary)

• Smooth
• airways of the lungs
• blood vessels
• the digestive, urinary, and reproductive tracts
• controlled sub consciously (involuntary)

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Characteristics of Muscle Tissue

• Excitability, or irritability
• the ability to receive and respond to stimuli

• Conductivity
• the ability to create and conduct an action
  potential along the cell membrane

• Contractility
• the ability to shorten forcibly

• Extensibility
• the ability to be stretched or extended

• Elasticity
• the ability to recoil after being stretched

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Muscle Terminology

• Prefixes
• sarco- flesh
• Sarcolemma muscle plasma membrane
• Sarcoplasm cytoplasm of a muscle fiber (cell)

• my- muscle
• Myocyte muscle fiber
• Epimysium the sheath of connective tissue that
  surrounds a skeletal muscle

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Skeletal Muscle Gross Anatomy
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Skeletal Muscle Gross Anatomy

• Three connective tissue sheaths surround a muscle
  and holds the cells together

• Epimysium
• connective tissue on the outside of the muscle

• Perimysium
• connective tissue that surrounds a group of
  muscle fibers
• organizes muscle fibers into fascicles

• Endomysium
• connective tissue inside the muscle that wraps
  around an individual muscle fiber

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Motor Unit The Nerve-Muscle Functional Unit

• A skeletal fiber will contract only after it is
  excited
• stimulated to generate an action potential

-A skeletal fiber stimulated by the exocytosis of
neurotransmitters from a motor neuron at a
synapse called the neuromuscular junction (NMJ)
-generates an action potential in the
skeletal fiber which triggers contraction
-A single motor neuron is capable of stimulating
multiple skeletal muscle fibers
-one axon branches creating multiple axon
termini
-the anatomical relationship between a
motor neuron and all skeletal fibers that it
causes to contract is called a motor unit
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Motor Unit The Nerve-Muscle Functional Unit
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• The number of muscle fibers per motor unit can
  range
• few (small motor unit)

-control fine movements (fingers,
eyes)

• several hundred (large motor unit)
• large weight-bearing muscles (back)
• control gross movements (arms, legs)

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Neuromuscular Junction

• The axon termini have synaptic vesicles that
  contain the neurotransmitter acetylcholine (ACh)

-ACh receptors (ligand-gated Na channels) are
localized to a portion of the sarcolemma called
the motor end plate
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Neuromuscular Junction
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Neuromuscular Junction
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Muscle Twitch
-The contraction followed by the relaxation of a
muscle fiber to a single, brief threshold
stimulus by a motor neuron is called a twitch
-There are three phases of a muscle twitch

• Latent (lag) period
• time between the stimulation by a motor neuron
  and the beginning of contraction (few
  milliseconds)

• Contractile period
• contractile proteins within the fiber hydrolyze
  ATP causing the fiber to shorten resulting in an
  increase in tension (force)

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• Relaxation period
• fiber lengthens causing tension to decrease

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Muscle Twitch
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Contraction of Skeletal Muscle
-The two types of muscle contractions are

• Isometric contraction same length
• muscle contracts and produces tension, but does
  not shorten
• trying to lift a car

• Isotonic contraction same tension
• muscle contracts and produces tension
• shortens as it contracts
• lifting a pencil

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Isometric Contractions

• Tension increases to the muscles capacity, but
  the muscle neither shortens nor lengthens
• Occurs if the load is greater than the tension
  the muscle is able to develop

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Isotonic Contractions

• In isotonic contractions, the muscle changes in
  length and moves the load

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Variety of Muscle Responses
-Variations in the force of muscle contraction is
required for proper control of skeletal movement
-moving a pencil vs. a textbook with your
hand uses the same muscles, but requires a
different amount of force

• Skeletal muscle contractions are varied by
• altering the frequency of muscle stimulation

-determined by the frequency of
action potentials traveling down a motor neuron
arriving at a fiber

• altering the number of muscle fibers that will
  contract

• determined by the number of motor units that are
  propagating action potentials to a muscle

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Muscle Response Stimulation Frequency

• A single stimulus results in a single muscle
  twitch producing a constant amount of tension

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Muscle Response Stimulation Frequency

• More rapidly delivered stimuli result in the
  summation of muscle twitches resulting in an
  incomplete tetanus
• muscle tension does not return to baseline
• If stimuli are given quickly enough, complete
  tetanus is observed where the contractile force
  reaches a maximum, but individual twitches
  blended together

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Muscle Response Stimulation Strength

• The number of muscle fibers actively contracting
  determines the force that muscle produces
• directly correlated to the number of active motor
  units

• The first observable muscle contraction occurs
  following a threshold stimulus
• activates one motor unit

• As stimulus strength is increased more motor
  units are activated
• recruitment

• The maximum force that a muscle is capable of
  generating is reached when all motor units are
  activated
• an increase in stimulus intensity results in no
  further increase in force generated

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Types of Skeletal Muscle Fibers

• There are 3 different types skeletal muscle
  fibers based on the duration of a twitch and the
  method of ATP production

• slow oxidative fibers
• fast oxidative fibers
• fast glycolytic fibers

• Skeletal muscles of your body contain a
  combination of all three fiber types, but their
  ratio determines the overall function of that
  muscle

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Oxidative vs. Glycolytic fibers
-Oxidative fibers contain greater amounts of
mitochondria compared to glycolytic fibers

• Oxidative fibers contain an oxygen-binding
  protein called myoglobin to maintain a high
  concentration of oxygen within the fiber for
  aerobic respiration

• similar in structure to the blood protein
  hemoglobin
• provides a red color to oxidative fibers
• a lack of myoglobin in glycolytic fibers results
  in a white color

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Characteristics of Skeletal Muscle Fiber Types

• Slow oxidative fibers
• have a slow twitch (use ATP slowly)
• fatigue resistant
• muscle fibers used to maintain posture

• Fast oxidative fibers
• have a fast twitch (use ATP quickly)
• moderate resistance to fatigue
• muscle fibers used for non-exertive movement
  (walking)

• Fast glycolytic fibers
• have a fast twitch (use ATP quickly)
• easily fatigued
• muscle fibers used for powerful movements
  (jumping and sprinting)

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Microscopic Anatomy of a Skeletal Muscle Fiber
-Each fiber is long (up to 30 cm) and cylindrical
with multiple nuclei just beneath the sarcolemma

• the sarcolemma contains both voltage-gated Na
  and K capable of generating an action potential

• portions of the sarcolemma called transverse (t)
  -tubules fold inward toward the center of the
  fiber
• propagate APs to the center of the muscle cell

• Muscle fibers contain an elaborate, smooth
  sarcoplasmic (endoplasmic) reticulum (SR)

• physically associated with the t-tubules
• storage site of intracellular calcium (Ca2)

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• An action potential in the t-tubules causes the
  release of Ca2 from the SR into the
  sarcoplasm which increases the cytoplasmic level
  of Ca2
• triggers the contraction of a muscle fiber

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Skeletal Muscle

• Has repeating pattern of contractile proteins
• (striations)
• Easily fatigable (tired)

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Microscopic Anatomy of a Skeletal Muscle Fiber
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Contractile Proteins
-The contractile proteins (myofilaments) are
arranged within the sarcoplasm in long bundles
called myofibrils

• composed of 2 types of myofilaments that overlap
  and slide past one another during contraction and
  relaxation
• thin
• thick

• The arrangement of contractile proteins within
  myofibrils creates a repeating pattern of
  striations called sarcomeres

• the basic/repeating contractile unit of a muscle
• thousands of sarcomeres per myofibril

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Myofilaments Banding Pattern
-The overlapping arrangement of thin and thick
filaments in a sarcomere creates an ordered
banding pattern within a single sarcomere

• Z disc
• constitutes one end of a sarcomere
• anchors the thin filaments

• A band
• the length of the thick filaments

• I band
• the length of thin filaments within a sarcomere
  that is not overlapping with the thick filaments

• H (bare) zone
• the length of thick filaments within in a
  sarcomere that is not overlapping with the thin
  filaments

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Microscopic Anatomy of a Skeletal Muscle Fiber
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Sarcomeres
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Arrangement of the Filaments in a Sarcomere
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Sliding Filament Model of Contraction
-In the relaxed state, thin and thick filaments
overlap only slightly

• Upon stimulation, the thick filaments pull the
  thin filaments toward the center of the sarcomere

• filaments overlap to a greater degree
• shortening the sarcomere

• As all of the sarcomeres in a muscle shortens,
  the entire muscle shortens

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Sliding Filament Model of Contraction
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Structure of Thin Filaments

• Thin filaments are composed of 3 proteins
• Actin is a helical polymer of protein subunits
• each subunit contains a binding site for the
  myosin head

• Tropomyosin blocks the interaction between actin
  and myosin
• prevents an unstimulated muscle from contracting

• Troponin C is attached to tropomyosin
• binds to Ca2 in the sarcoplasm during
  contraction

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Structure of Thin Filaments
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Structure of Thick Filaments

• Thick filaments are composed of many molecules of
  the protein myosin

-Each myosin protein has a rodlike tail and two
heads

• Myosin heads (also known as cross bridges)
• hydrolyze a molecule of ATP
• uses the chemical energy to contract

• Temporarily bind to actin
• pull on actin causing the shortening sarcomere

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Structure of Thick Filaments
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Skeletal Muscle Contraction
-In order to contract, a skeletal muscle must be
stimulated by a motor neuron

• generates an action potential in the muscle fiber
• causes an increase in the amount of cytoplasmic
  Ca2

• causes the muscle fiber to contract

-Linking the action potential to the contraction
of a muscle fiber is called excitation-contraction
coupling
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Excitation-Contraction Coupling
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Excitation-Contraction Coupling

• Binding of ACh to its receptors opens the channel
  and allows both Na and K to diffuse
• diffusion of more Na than K causes the membrane
  potential to depolarize (endplate potential)

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Excitation-Contraction Coupling

• The endplate potential brings the membrane
  potential to threshold
• opens voltage-gated Na and K channels to
  generate an action potential in the sarcolemma

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Excitation-Contraction Coupling

• Action potentials propagate along the sarcolemma
  into the t-tubules
• action potentials in the t-tubules cause the
  release of Ca2 from the SR into the sarcoplasm

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Excitation-Contraction Coupling

• Ca2 in the sarcoplasm binds to troponin C
• changes the position of troponin C
• moves tropomyosin away from the myosin binding
  site on actin

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Events of Contraction (Cross bridge cycling)

• Muscle fiber shortening occurs as myosin pulls on
  actin in a repetitive ratcheting fashion

• Thin filaments move toward the center of the
  sarcomere

• Activation of the myosin head
• a molecule of ATP is hydrolyzed and the energy is
  used by the myosin head to change the shape of
  myosin into the high-energy state

• Cross bridge formation
• myosin cross bridge attaches to actin filament

• Power stroke
• myosin head pivots and pulls thin filament over
  thick filament

• Cross bridge detachment
• The binding of a molecule of ATP to the myosin
  head causes it to detach from actin

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Events of Contraction (Cross bridge cycling)
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Muscle Fiber Relaxation

• The motor neuron stops the exocytosis of ACh
• The remaining ACh is hydrolyzed into acetate and
  choline by the enzyme Acetylcholine esterase
  located in the synaptic cleft of the NMJ
• ACh receptors close
• membrane potential returns to resting value

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Muscle Fiber Relaxation

• Ca2 is pumped back into the SR by Ca2-ATPase in
  the SR membrane
• decreases Ca2 in the sarcoplasm
• troponin C moves back to resting position
• Tropomyosin covers the binding site for myosin on
  G actin

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Skeletal Muscle Tissue

• Packaged in skeletal muscles that attach to and
  cover the bony skeleton
• Has obvious stripes called striations
• Is controlled voluntarily (i.e., by conscious
  control)
• Contracts rapidly but tires easily
• Is responsible for overall body motility
• Is extremely adaptable and can exert forces
  ranging from a fraction of an ounce to over 70
  pounds

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Skeletal Muscle

• -Striated
• -Multinucleated
• -Cells not defined

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Cardiac Muscle Tissue

• Occurs only in the heart
• Is striated like skeletal muscle but is not
  voluntary (involuntary)
• Contracts at a fairly steady rate set by the
  hearts pacemaker
• Neural controls allow the heart to respond to
  changes in bodily needs

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Cardiac Muscle

• -Striated
• -Defined cells with single nucleus

- only rests between beats
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Smooth Muscle Tissue

• Found in the walls of hollow visceral organs,
  such as the stomach, urinary bladder, and
  respiratory passages
• Forces food and other substances through internal
  body channels
• It is not striated and is involuntary

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Skeletal Muscle Nerve and Blood Supply

• Each muscle is served by one nerve, an artery,
  and one or more veins
• Each skeletal muscle fiber is supplied with a
  nerve ending that controls contraction
• Contracting fibers require continuous delivery of
  oxygen and nutrients via arteries
• Wastes must be removed via veins

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Skeletal Muscle Attachments

• Most skeletal muscles span joints and are
  attached to bone in at least two places
• When muscles contract on the movable bone, the
  muscles insertion moves toward the immovable
  bone, the muscles origin

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Skeletal Muscle Attachments

• Muscles attach
• Directly epimysium of the muscle is fused to
  the periosteum of a bone
• Indirectly connective tissue wrappings extend
  beyond the muscle as a tendon or aponeurosis