Transcript Myograms and Muscle Contraction

Myograms and Muscle
Contraction
Myogram of Muscle Twitch
Muscle Twitch Terminology
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Latent Period: period of time between when the stimulus is
applied and contraction begins. Action potential is sweeping
across the sarcolemma and Ca2+ is being released from the SR.
Contraction phase: Muscle begins contracting and continues
until it reaches it’s peak contraction. Calcium is binding to
troponin, moving the tropomyosin from the actin active sites,
cross bridges between the myosin heads and actin form, and
power stroke occurs.
Relaxation phase: when the action potential has passed and the
SR is collecting the Ca2+ from the sarcoplasm, ending
contraction and allowing muscle to return to resting tension.
Myograms
Myogram Terminology
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Muscle Twitch: One stimulus is being applied
to the muscle and therefore one contraction
cycle occurs.
 Treppe: A second stimulus is applied once the
muscle has ended its relaxation phase.
Results in a slightly more forceful contraction.
Multiple stimuli applied in this manner will
result in a higher and higher force of
contraction until a plateau is reached.
“Treppe” is German for step.
Myogram Terminology
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Wave Summation: A second stimulus is applied during the
relaxation phase of a contraction cycle. Results in a more
forceful second contraction.
Incomplete or Unfused Tetanus: Stimuli applied during
relaxation phase and applied frequently enough to raise the
force of contraction to near maximum force and to a level
approximately 4 times more forceful than during treppe.
Complete or Fused Tetanus: Stimuli applied so frequently that
muscle has no time to enter a relaxation phase and results in a
smooth, sustained contraction at maximum force.
Motor Unit
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A motor unit is comprised of a motor neuron and all
the muscle fibers under the control of that motor
neuron.
Each muscle contains many motor units.
The size of the motor unit is an indicator for how
finely controlled the movements are. Very small
motor units have very fine control while very large
motor units have more gross control.
Motor Unit Reinforcement
Recruitment
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When you decide to make a movement, first smaller, slower,
less powerful motor units are stimulated. If it is not enough to
result in the desired movement, larger, faster, more powerful
motor units will gradually be recruited until the movement can be
made. This occurs until peak force is attained.
When peak force is attained, and all motor units in a muscle are
contracting, the contraction is short lived because muscle
fatigue will set in quickly due to lack of energy and Calcium to
keep a sustained contraction.
When sustained contractions are needed, motor units are
stimulated in a sort of relay system: while some motor units are
contracting, others are resting, which allows for longer sustained
contractions and staves off muscle fatigue. This is called
asynchronous motor unit summation.
Muscle Tone: the resting tension
in a muscle.
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Some motor units in a skeletal muscle are always
contracting, but not forcefully enough to produce
movement.
Helps maintain constant tension on the tendons.
Stabilizes the position of the bones and joints to
maintain posture and body position.
Isotonic Contraction
Isotonic Contractions
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Isotonic Contractions: Contraction in which as tension
rises, the muscle shortens. (ex: lifting an object off
the desk, walking, running, etc)
Types of Isotonic Contractions
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Concentric Contraction
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Muscle tension exceeds resistance and muscle shortens.
Eccentric Contraction
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Peak tension is less than the load and the muscle elongates
owing to the contraction of another muscle or the pull of gravity.
Isometric Contraction
Isometric Contractions
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Isometric Contractions: the muscle as a whole never
changes length because the force of contraction
never overcomes the resistance.
Examples: carrying a bag of groceries, holding a
baby, holding your head up.
During isometric contractions, the muscle bulges as
tension rises, but never shortens. Cross bridges still
form and contract, but as the muscle contracts, the
tendons lengthen so the overall length does not
change.
Resistance and Speed of
Contraction
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The lower the resistance, the faster the speed of
contraction.
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The heavier the resistance, the slower the movement
because tension has to gradually build within the muscle as
more and more motor units are recruited.
Also at a molecular level, myosin head pivoting is slower
with heavy resistance, which means the contraction itself is
also slower.