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Chapter 6: Contraction of Skeletal Muscle
Guyton and Hall, Textbook of Medical Physiology, 12th edition
Physiological Anatomy of Skeletal Muscle
•
Skeletal Muscle Fiber
a. Sarcolemma (plasma membrane) is a thin membrane
enclosing a muscle fiber
b. Myofibrils are composed of actin and myosin
c. Titin filaments keep the myosin and actin filaments
in place
d. Sarcoplasm is the intracellular fluid between
myofibrils
e. Sarcoplamic reticulum is a specialized endoplasmic
reticulum of muscle cells
Fig. 6.1
General Mechanism of Muscle Contraction
1.
2.
3.
4.
5.
6.
7.
8.
An action potential travels along a motor nerve to the
motor end plate
The nerve secretes acetylcholine
The AcH binds to sarcolemma and opens gated channels
Large amounts of Na+ enter the cell and initiates and AP
AP travels along the sarcolemma the same as in a nerve
cell
AP causes depolarization and triggers release of Ca++ from
the sarcoplasmic reticulum
Ca++ initiate the contraction cycle
After contraction, Ca++ ions are reabsorbed by the
sarcoplasmic reticulum
Fig. 6.3 Organization of proteins in a sarcomere
Molecular Mechanism of Muscle Contraction
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Sliding Filament Mechanism of Muscle Contraction
Fig. 6.5 Relaxed and contracted state of a myofibril
Molecular Mechanism (cont.)
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Molecular Characteristics of the Contractile Filaments
a. Myosin filaments are composed of multiple myosin
molecules
Fig. 6.6
Molecular Mechanism (cont.)
b. ATPase activity of the myosin hear
c. Actin filaments are composed of actin,
tropomyosin and troponin
Fig. 6.7 Actin filament
Molecular Mechanism (cont.)
d. Tropomyosin molecules-wrapped spirally around
the sides of the F-actin helix
e. Troponin and its role in muscle contraction-helps
attach the tropomyosin to the actin; strong affinity
for calcium during contraction
f. Inhibition of the actin filament by the troponintropomyosin complex and activation by calcium ions
g. Interaction between actin and myosin cross
bridges
h. Chemical events in the motion of the myosin heads
Molecular Mechanism (cont.)
Fig. 6.8 “Walk along” mechanism for contraction
•
Amount of Actin and Myosin Filament Overlap
Determines Tension Developed by the
Contracting Muscle
Fig. 6.9 Length-tension diagram
•
Effect of Muscle Length on Force of Contraction in
Whole Intact Muscle
Fig. 6.10 Relation of Muscle
Length to Tension
• Relation of Velocity of Contraction to Load-contracts
rapidly when it contracts against no load; velocity
decreases as load increases
Fig. 6.11 Relation of Load to
Velocity of Contraction
Energetics of Muscle Contraction
•
Work Output-when a muscle contracts against a load
it performs work. Energy is transferred from the
muscle to the external load to lift an object
•
Sources of Energy for Muscle Contraction
a. Phosphocreatine
b. Glycolysis (uses stored glycogen as energy source
c. Oxidative metabolism
Characteristics of Whole Muscle Contraction
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Muscle Twitch-demonstrated by eliciting single
muscle twitches
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Isotonic vs Isometric Contraction
a. Isometric-muscle does not shorten during contraction
b. Isotonic-muscle shortens but the tension remains
constant during the contraction
Muscle Contraction (cont.)
•
Characteristics of Isometric Twitches From
Different Muscles
Fig. 6.13
Muscle Contraction (cont.)
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Fast vs. Slow Muscle Fibers
a. Slow (Type I, Red Muscle)
1. Smaller fibers
2. Innervated by smaller nerve fibers
3. Extensive blood vessel system for oxygenation
4. Increased numbers of mitochondria
5. Large amounts of myoglobin (combines with
oxygen and stores it until needed
Muscle Contraction (cont.)
b. Fast (Type II, White Muscle)
1. Large fibers for great strength of contraction
2. Extensive sarcoplasmic reticulum for rapid
release of calcium
3. Large amounts of glycolytic enzymes for
glycolysis
4. Less extensive blood supply (anaerobic)
5. Fewer mitochondria
6. Low levels of myoglobin
Mechanics of Skeletal Muscle Contraction
•Motor Unit-all the muscle fibers innervated by a single
nerve fiber
a. Summation-adding together of individual
twitch contractions to increase the intensity
of the overall muscle contraction; can happen
two ways: (1) increasing the number of motor
units, or (2) increasing the frequency of
contraction
b. Multiple Fiber Summation
c. Frequency Summation and Tetanus
Mechanics (cont.)
Fig. 6.14 Frequency of Summation and Tetanus
Maximum Strength of Contraction
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The Staircase Effect (Treppe)-when a muscle
begins to contract after a long rest, its initial
strength may be as little as ½ its strength 10 to 50
muscle twitches later.
Thought to be due to a progressive increase of
calcium ions in the sarcoplasm.
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Muscle Tone-when muscles are at rest, a certain
degree of tautness remains. Due to a low rate of
impulses coming from the spinal cord; also
involves the muscle spindles (receptors)
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Muscle Fatigue-increases almost in the direct
proportion to the depletion of muscle glycogen;
also contributing is a loss of ATP, oxygen,
decreased blood flow to the muscle
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Lever Systems of the Body-muscles operate by
applying tension to their points of insertion;
analysis of the lever systems depends on
a.
b.
c.
d.
the point of muscle insertion
its distance from the fulcrum of the lever
the length of the lever arm
the position of the lever