Muscle Physiology II Flashcards
Muscle Twitch
Contraction produced in a muscle fiber in response to a single action potential • Twitch is an all-or-nothing event
Preload
Preload is the load on a muscle
in a relaxed state
Applying preload to the muscle
results in:
Stretching of the muscles
– Generation of passive tension
in the muscle
Afterload
Afterload is the load the muscle
works against during contraction.
tension
• Force exerted by contracting
muscle
load
• Force opposing contraction (such
as weight to be moved)
Isometric Contraction
•Length constant •Contractile elements contract, generating tension •When load > tension •Muscle does NOT shorten, load not lifted -Sarcomeres shorten while elastic elements stretch
Isotonic Contraction
•Constant tension
•When tension > load
•Load is lifted as muscle shortens
- sarcomeres shorten more but, because elastic elements
are already stretched, the muscles shorten
Concentric action
a shortening
action
Eccentric action
lengthening actio
Phases of Isotonic Contraction
Phase 1 Isometric contraction
Phase 2 Isotonic contraction
Phase 3 Isotonic relaxation
Phase 4 Isometric relaxation
bell-shaped curve (force/time)
Isometric contraction
flat-topped curve (force/time)
Isotonic contraction
Passive tension generation
The elastic component of skeletal
muscle creates a resistance to
stretch.
Re of Force
Preload and Passive Tension
Preload Length Passive
tension
Force Velocity Relation-Increase Preload + Increase Fmax:
Shifts curve to the right.
• Increase crossbridge overlap
• increases maximum weight that can be
moved, max force generated.
orce Velocity Relation-Increase Preload+Increase Velocity
Same weight/afterload can be moved
faster.
ATPase activity not altered
-No change Vmax
Vmax is Determined by
myosin ATPase
activity → how fast crossbridge cycle spins
Vmax and Different Muscle Types-Red (Type I, slow oxidative):
-less powerfu
-endurance
muscle
-Lower Myosin ATPase
-myoglobin
pigment
-High capacity for aerobic metabolism
-Slow twitch
Vmax and Different Muscle Types-White (Type II, fast glycolytic):
- powerful
- utilized short-term
- High Myosin ATPase
- Low myoglobin
- High capacity for anaerobic glycolysis
- Fast twitch
How do skeletal muscles regulate their force?
By Graded Muscle Contractions
- Temporal summation:
Tension produced by each fiber (due to high frequency stimulation)
Spatial summation (recruitment)
Number of fibers contracting
Factors Affecting Force of Individual Muscle Fiber
- Frequency of stimulation
- Fiber diameter
- Changes in fiber length
Basis of Summation
Increases in frequency of action potentials in muscle fibers increases tension -Action potential causes Ca2+ release -Contraction - Contractions can overlap and sum
Cause of Summation and Tetanus
-At high frequencies, release exceeds re-uptake
-Calcium increases in cytosol
– Eventually saturates system
-
Tetanus
-Clostridium tetani.
-toxin that is released blocks the release of inhibitory neurotransmitters,
γ-aminobutyric acid (GABA)
-increase in the resting firing rate of motor
neurons.
-Increased repetitive stimulation
Tetanus-Management
Antitoxin -neutralize circulating and unbound toxin.
Diazepam (GABA agonist)- to control spasm
Force developed by the whole
muscle depends on
More fibers contracting → greater tension -Recruit motor units • Activate motor neuron activates all muscle fibers in the motor unit • Increases in tension occur in steps proportional to size of motor unit
