Download period of contraction

Skeletal Muscle
Contraction
as a Whole
Human Anatomy & Physiology
Energy Sources
for Contraction
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• 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.
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Oxygen Supply and
Cellular Respiration
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• 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.
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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
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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.
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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.
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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.
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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.
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Recording a
Muscular Contraction
• A myogram is the recording of an electricallystimulated 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.
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
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.
Two Types of Contractions