SKELETAL MUSCLE STRUCTURE

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SKELETAL MUSCLE STRUCTURE
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SKELETAL MUSCLE STRUCTURE
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A MUSCLE FIBER
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ARRANGEMENT OF FILAMENTS
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Events Leading to Muscle Fiber Action
1. A motor neuron releases acetylcholine (ACh).
2. ACh binds to receptors on the sarcolemma.
3. The action potential triggers release of Ca2.
4. The Ca2 binds to troponin on the actin
filament, and the troponin pulls tropomyosin off
the active sites, allowing myosin heads to
attach to the actin filament.
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EVENTS LEADING TO MUSCLE ACTION
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ARRANGEMENT OF FILAMENTS IN A SARCOMERE
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AN ACTIN FILAMENT
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The Sliding Filament Theory
w When myosin cross-bridges are activated, they
bind strongly with actin, resulting in a change
in the cross-bridge.
w The change in the cross-bridge causes the
myosin head to tilt toward the arm of the
cross-bridge and drag the actin and myosin
filaments in opposite directions.
w The tilt of the myosin head is known as a power
stroke.
w The pulling of the actin filament past the
myosin results in muscle shortening and
generation of muscle force.
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CONTRACTING MUSCLE FIBER
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Did You Know?
The difference in force development between FT
and ST motor units is due to the number of muscle
fibers per motor unit, not the force generated by
each fiber.
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What Determines Fiber Type?
w Genetics determine which motor neurons
innervate our individual muscle fibers.
w Muscle fibers become specialized according to
the type of neuron that stimulates them.
w Endurance training and muscular inactivity may
result in small changes in the percentage of FT
and ST fibers.
w Aging may result in changes in the percentage
of FT to ST fibers.
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Slow-Twitch (ST) Muscle Fibers
w High aerobic (oxidative) capacity and fatigue
resistance
w Low anaerobic (glycolytic) capacity and motor
unit strength
w Slow contractile speed (110 ms) and myosin
ATPase
w 10180 fibers per motor neuron
w Low sarcoplasmic reticulum development
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Fast-Twitch (FTa) Muscle Fibers
w Moderate aerobic (oxidative) capacity and
fatigue resistance
w High anaerobic (glycolytic) capacity and motor
unit strength
w Fast contractile speed (50 ms) and myosin ATPase
w 300800 fibers per motor neuron
w High sarcoplasmic reticulum development
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Fast-Twitch (FTb) Muscle Fibers
w Low aerobic (oxidative) capacity and fatigue
resistance
w High anaerobic (glycolytic) capacity and motor
unit strength
w Fast contractile speed (50 ms) and myosin ATPase
w 300800 fibers per motor neuron
w High sarcoplasmic reticulum development
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The All-Or-None-Response
w For a motor unit to be recruited into activity
the motor nerve impulse must meet or exceed the
threshold.
w When this occurs, all muscle fibers in the
motor unit act maximally.
w If the threshold is not met no fibers in that
unit act.
w More force is produced by activating more motor
units.
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RAMPLIKE RECRUITMENT OF FIBERS
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TYPES OF MUSCLE ACTION
How do the control aspects change with joint
angle in two or three joint muscles?
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MUSCLE LENGTH vs FORCE PRODUCTION
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TENSION LENGTH CURVES
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MUSCLE VELOCITY vs FORCE PRODUCTION
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MUSCLE ACTION VELOCITY vs FORCE PRODUCTION
Hamil Knutzen 1995
Kreighbaum Barthels 1996
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MUSCLE ACTION VELOCITY vs. FORCE PRODUCTION
Why would less force be produced bilaterally than
the summed unilateral force for the same muscles?
Dickin Too (2006)
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Mechanical Muscle Model
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Functional Classification of Muscles
Agonistsprime movers responsible for the
movement
Antagonistsoppose the agonists to prevent
overstretching of them
Synergistsassist the agonists and sometimes
fine-tune the direction of movement
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MUSCLE ACTION DURING ELBOW FLEXION
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Key Points
Use of Muscles
w Muscles involved in movement can be classified
as agonists, antagonists, and synergists.
w Three types of muscle action are concentric,
static, and eccentric.
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Key Points
Use of Muscles
w All joints have an optimal angle at which the
muscles crossing the joint produce maximal force.
w The angle of maximal force depends on the
relative position of the muscle's insertion on
the bone and the load placed on the muscle.
w Speed of action affects the amount of force
produced.
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MUSCLE FIBER ARRANGEMENT

• Force production is highly dependent upon CSA
• So why do pinnated muscles produce more force
• What is the downfall of the pinnation?

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Factors Influencing Force Generation
Rate coding plays an important role in the
magnitude of tension produced by each muscle
fiber.
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Factors Influencing Force Generation
w Number of motor units activated
w Type of motor units activated (FT or ST)
w Muscle size
w Initial muscle length
w Joint angle

• Speed of muscle action
• (shortening or lengthening)

w Angle of the muscle fibers (Pinnation)
w Frequency of neural activation
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MEASURING MUSCLE ACTIVITY
EMG Electromyography
Measures electrical changes within the muscle as
a function of the chemical changes going on
within the muscle
Change is a difference between the electrodes
Why is a ground electrode important in EMG?
Why and what is the process of normalization
doing?
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Bi-phasic Control of Movement

• Rapid movement to a target results in a specific
  EMG trace in the agonists and antagonists, and
  velocity profile

• Why do we see both Biceps and Triceps bursts
  when we move rapidly in an aiming task?

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Tri-phasic Control of Movement

• Why would some movement show a tri-phasic
  pattern?

• Auditory tone presented at 0
• Dashed line Extension trials
• Solid line Flexion trials
• Note the velocity curves and also the triphasic
  pattern for each movement.

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SENSORY COMPONENTS OF MUSCLES
w Muscle spindles
w Golgi tendon organs
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Muscle Spindles
w Lie between and are connected to regular
skeletal muscle fibers.
w The middle of the spindle cannot contract but
can stretch.
w When muscles attached to the spindle are
stretched, neurons on the spindle transmit
information to the CNS about the muscle's length.
w Reflexive muscle contraction is triggered to
resist further stretching.
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MUSCLE SPINDLE
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MUSCLE SPINDLE
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MUSCLE SPINDLE
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MUSCLE SPINDLE
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MUSCLE SPINDLE

• Why do muscle spindles contain their own
  contractile elements?

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Golgi Tendon Organs (GTOs)
w Encapsulated sensory organs through which
muscle tendon fibers pass
w Located close to the tendon's attachment to the
muscle
w Sense small changes in tension
w Inhibit contracting (agonist) muscles and
excite antagonist muscles to prevent injury
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GOLGI TENDON ORGAN
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GOLGI TENDON ORGAN
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GOLGI TENDON ORGAN
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BRAIN REGIONS AND THEIR FUNCTIONS
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CEREBELLUM

• Involved with
• regulation of muscle tone
• Coordination
• Timing
• Learning
• Damage can result in impaired function of any of
  above attributes.

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BASAL GANGLIA

• Located in the forebrain
• Role in motor control discovered as a function of
  damage to the area
• Huntingtons Disease
• Uncontrollable movement, forgetfulness, dementia.
• Parkinsons Disease
• Shuffling gait (off meds on meds), resting
  tremor (off on) and action tremor (off stim
  on stim), slowed movements (off meds on meds),
  rigidity.
• Time course of brain stimulation effects
• Also shown to have some role in retrieval of
  memories

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MOTOR CORTEX

• Not involved in action planning
• One of the last regions to become active
• Muscle contracts 50 msec after motor cortex
  becomes active
• What is the neuron called that leaves the MC and
  causes muscle contraction to occur?

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PREMOTOR CORTEX

• Involved in more proximal muscle activation
• What types of muscles would become active
• What is the reason we activate these muscles
  prior to distal musculature?

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SUPPLEMENTARY MOTOR CORTEX

• More involved in planning and production of
  complex movement sequences

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NEXT WEEKS READINGS

• Shapiro Schmidt
• Motor program and motor schema theories
• Balco-Perry
• Clinical application of dynamic systems theory
• Clark
• Motor development perspective of dynamic systems
  theory

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Learning Objectives
w Learn the basic components of skeletal muscle,
the muscle fiber, and the myofibril.
w Note the cellular events leading to a basic
muscle action.
w Discover how muscle functions during exercise.
w Consider the differences in fiber types and
their impact on physical performance.
w Learn how muscles generate force and movement
by pulling on bones.
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Types of Muscles
Smooth w Involuntary muscle controlled
unconsciously w In the walls of blood vessels and
internal organs
Cardiac w Controls itself with help from nervous
and endocrine systems w Only in the heart
Skeletal w Voluntary muscle controlled
consciously w Over 600 throughout the body
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Key Points
The Muscle Fiber
w An individual muscle cell is called a muscle
fiber.
w A muscle fiber is enclosed by a plasma membrane
called the sarcolemma.
w The cytoplasm of a muscle fiber is called a
sarcoplasm.
w Within the sarcoplasm, the T tubules allow
transport of substances throughout the muscle
fiber and the sarcoplasmic reticulum stores
calcium.
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Key Points
The Myofibril
w Myofibrils are made up of sarcomeres, the
smallest functional units of a muscle.
w A sarcomere is composed of filaments of two
proteins, myosin and actin, which are responsible
for muscle contraction.
w Myosin is a thick filament with a globular head
at one end.
w An actin filamentcomposed of actin,
tropomyosin, and troponinis attached to a Z disk.
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Key Points
Muscle Fiber Action
w Muscle action is initiated by a nerve impulse.
w The nerve releases ACh, which allows sodium to
enter and depolarize the cell. If the cell is
sufficiently depolarized, an action potential
occurs which releases stored Ca2 ions.
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Key Points
Muscle Fiber Action
w Once myosin binds with actin, the myosin head
tilts and pulls the actin filament so they slide
across each other.
w Muscle action ends when calcium is pumped out
of the sarcoplasm to the sarcoplasmic reticulum
for storage.
w Energy for muscle action is provided when the
myosin head binds to ATP. ATPase on the myosin
head splits the ATP into a usable energy source.
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Muscle Biopsy
w Hollow needle is inserted into muscle to take a
sample.
w Sample is mounted, frozen, thinly sliced, and
examined under a microscope.
w Allows study of muscle fibers and the effects
of exercise and training on fiber composition.
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Key Points
Slow- and Fast-Twitch Muscle Fibers
w Skeletal muscles contain both ST and FT fibers.
w ATPase in FT fibers acts faster providing
energy for muscle action more quickly than ATPase
in ST fibers.
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Key Points
Slow- and Fast-Twitch Muscle Fibers
w Motor units supplying FT fibers are larger than
those supplying ST fibers thus, FT motor units
can recruit more fibers.
w ST fibers have high aerobic endurance and are
suited to low-intensity endurance activities.
w FT fibers are better for anaerobic or explosive
activities.