Skeletal Muscle Contraction as a Whole Human Anatomy &
Physiology
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Energy Sources for Contraction Energy for contraction comes
from molecules of ATP. This chemical is in limited supply and so
must often be regenerated Creatine phosphate, which stores excess
energy released by the mitochondria, is present to regenerate ATP
from ADP and phosphate. Whenever the supply of ATP is sufficient,
creatine phosphokinase promotes the synthesis of creatine
phosphate. As ATP decomposes, the energy from creatine phosphate
can be transferred to ADP molecules, converting them back to ATP. 2
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Oxygen Supply and Cellular Respiration The early phase of
cellular respiration yields few molecules of ATP, so muscle has a
high requirement for oxygen, which enables the complete breakdown
of glucose in the mitochondria. Hemoglobin in red blood cells
carries oxygen to muscle. The pigment myoglobin stores oxygen in
muscle tissue. 4 Copyright The McGraw-Hill Companies, Inc.
Permission required for reproduction or display.
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Oxygen Debt During rest or moderate activity, there is enough
oxygen to support aerobic respiration. Oxygen deficiency may
develop during strenuous exercise, and lactic acid accumulates as
an end product of anaerobic respiration. Lactic acid diffuses out
of muscle cells and is carried in the bloodstream to the liver.
Oxygen debt refers to the amount of oxygen that liver cells require
to convert the accumulated lactic acid into glucose, plus the
amount that muscle cells need to resynthesize ATP and creatine
phosphate to their original concentrations. Repaying an oxygen debt
may take several hours. After exercise, the oxygen debt is repaid
by deep breathing 6 Copyright The McGraw-Hill Companies, Inc.
Permission required for reproduction or display.
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Muscle Fatigue When a muscle loses its ability to contract
during strenuous exercise, it is referred to as fatigue. Muscle
fatigue usually arises from the accumulation of lactic acid in the
muscle. A lowered pH as a result of accumulated lactic acid
prevents the muscle from contracting. 7 Copyright The McGraw-Hill
Companies, Inc. Permission required for reproduction or
display.
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Muscle Cramp A muscle cramp occurs due to a lack of ATP
required to return calcium ions back to the sarcoplasmic reticulum
so muscle fibers can relax.
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Heat Production Contraction of skeletal muscle represents an
important source of heat for the body. 9 Copyright The McGraw-Hill
Companies, Inc. Permission required for reproduction or
display.
Slide 10
Muscular Responses One method of studying muscle function is to
remove a single fiber and connect it to a device that records its
responses to electrical stimulation. 10 Copyright The McGraw-Hill
Companies, Inc. Permission required for reproduction or
display.
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Threshold Stimulus A muscle fiber remains unresponsive to
stimulation unless the stimulus is of a certain strength, called
the threshold stimulus.
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All-or-None Response When a muscle fiber contracts, it
contracts to its full extent (all-or-none response); it cannot
contract partially. 12 Copyright The McGraw-Hill Companies, Inc.
Permission required for reproduction or display.
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Recording a Muscular Contraction A myogram is the recording of
an electrically- stimulated muscle contraction. A single, short
contraction involving only a few motor units is referred to as a
twitch. The time delay between when the stimulus is applied and
when the muscle contracts is called the latent period, which is
less than two milliseconds. The latent period is followed by a
period of contraction and a period of relaxation.
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Summation A muscle fiber receiving a series of stimuli of
increasing frequency reaches a point when it is unable to relax
completely and the force of individual twitches combine by the
process of summation. If the sustained contraction lacks any
relaxation, it is called a tetanic contraction. An increase in the
number of activated motor units within a muscle at higher
intensities of stimulation is called recruitment. Individual muscle
cells contract completely when adequately stimulated, but a muscle
responds to different degrees depending on HOW many muscle cells
are stimulated Summation and recruitment together can produce a
sustained contraction of increasing strength. Copyright The
McGraw-Hill Companies, Inc. Permission required for reproduction or
display.
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16 a. Series of twitches b. Summation c. Tetanic
contraction
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Muscle Tone Muscle tone is achieved by a continuous state of
sustained contraction of motor units within a muscle. It is a
result of a staggered series of nerve impulses delivered to
different cells within the muscle. Muscle tone keeps muscles
healthy and ready to react If the nerve supply is destroyed, the
muscle loses tone, becomes paralyzed, and atrophies
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Effect of Exercise on Muscles Inactive muscles atrophy. Muscles
challenged by resistance exercise to respond beyond their ability
increase in size and strength. Muscles subjected to regular aerobic
exercise become more efficient and stronger and can work longer
without tiring. Aerobic exercise also benefits other body
systems.
