Skeletal Muscle Organization
• Muscle fibers (cells)
Muscle Physiology
Lab #9
– elongate cells, parallel
arrangement
– sarcolemma – cell membrane
– sarcoplasmic reticulum (SR)
• Internal membraneous network
• Stores Ca2+
– transverse tubules
• Connect SR to sarcolemma
– myofibrils – protein bundles
composed of:
• thick filaments
• thin filaments
Myofibril Structure
• Composed of sarcomeres
– smallest functional unit of
muscle
– repeating units of thin and
thick protein filaments
• Thick Filament = Myosin
• Thin Filament = Actin,
Troponin, Tropomyosin
Thin Filament Structure
• Actin
– primary structural protein
– spherical protein subunits connected
in long, double strand
– Contains myosin binding site
• Tropomyosin
– threadlike proteins
– normally cover myosin binding sites
• Troponin
– Ca2+ Binding Protein
– holds tropomyosin in place
Thick Filament Structure
• Bundles of several hundred
myosin molecules
– intertwining tails + globular heads
• heads contain:
– actin binding sites
– ATP-hydrolyzing sites
• project outward towards actin
• form crossbridges
– bonds with actin
– Important during contraction
Skeletal Muscle Contraction
• Somatic motor neuron induces
action potential in muscle fiber
• AP travels down sarcolemma
• Induces depolarization in the Ttubules
• Depolarization conducted to
the SR
• Voltage-gated Ca2+ channels
open in the SR
– releasing Ca2+ into the cytosol
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Skeletal Muscle Contraction
• Ca2+ binds to troponin
• Troponin undergoes shape
change
– pulls on tropomyosin
• tropomyosin shifts position
– uncovers myosin binding
sites on the actin filaments
Skeletal Muscle Contraction
• Movement of thin
filaments over thick
• sarcomere shortening
• length of the filaments do
not change
All or None
• Individual muscle fibers respond to a single
stimulus in an all or none fashion
– undergo action potential
– action potential triggers contraction
– subthreshold stimulus = no contraction
– superthreshold stimulus = maximal contraction
Skeletal Muscle Contraction
• myosin head binds to actin
• crossbridge bends
– power stroke
– pulls thin filaments toward center
of the sarcomere
• crossbridge link breaks
• myosin head returns to original
configuration
• binds to next actin molecule
Muscle Relaxation
• Ca2+ pumped back into the SR by active carriermediated transport
– troponin releases Ca2+
– tropomyosin covers myosin binding sites on the actin
molecules
• Membrane-bound enzyme (acetylcholinesterase)
– breaks down ACh released at the NMJ
Motor Units
• Multiple muscle fibers are
enervated by a single motor
neuron
• Motor Unit
– motor neuron + all muscle
fibers it innervates
– muscle fibers in a motor unit
contract as a single unit
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Motor Unit Recruitment
• Individual motor units contract in an all-or-none
fashion
• Differences in contractile strength are due to
differences in the number of contracting motor
units
• Motor Unit Recruitment
– increasing the number of contracting motor units to
increase the overall strength of contraction
Twitch Properties
•
•
•
Latent Period
– Time btw AP production and
development of tension.
– Excitation-contraction coupling
Contraction Time
– Time btw beginning of contraction
and peak tension
– Shortening of muscle
Relaxation Time
– Time btw peak tension and return to
resting tension
– Ca2+ re-uptake and stretching of
muscle back to resting length
Muscle Mechanics
• Twitch
– single contraction and relaxation of muscle in
response to a single action potential
• Tetany (Tetanus)
– sustained tension exerted by a muscle due to
continuous contraction
Temporal Summation
• AP duration much shorter than
contraction duration
– several APs can occur in a muscle
fiber during the course of a
contraction
Muscle contraction
Action potentials
• Multiple APs can have a
summation effect on tension
generation
Latent Period
Contraction Time
– induce release of Ca2+ from SR
before muscle is fully relaxed
– increase tension
Relaxation Time
Tetany
• High rate of AP
generation in fibers
• no time for muscle to
relax
• sustained level of
tension
Experiments:
Frog gastrocnemius recordings
• Follow instructions in manual
CAREFULLY!!!
– Threshold
– Recruitment in muscle organs
– Twitch time measures
• Latent period, contraction time, relaxation time
– Repeated stimulation and stimulus frequency
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Electromyograms (EMGs)
• Muscles undergo action potentials
• Electrical signals conducted through body
fluids
• Can be measured from the surface of the
body
• EMGs can indicate muscle activity
Types of Muscle Contractions
• Isotonic Contractions
– Muscle shortens in length
– Generated same strength for
a given load throughout the
contraction process
• Isometric Contractions
– Muscle does not shorten,
remains at the same length
– Load = strength of
contraction
Experiment: EMGs
• Motor unit recruitment on forearm
• Isotonic vs. Isometric contractions
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