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Muscle :- “a machine for converting chemical
energy into mechanical work.”
The immediate source of chemical energy is ATP.
Basic metabolic systems are present in muscle as
in other parts of the body.
AdenosineTriphosphate.
The source of energy actually used to cause muscle
contraction is ATP.
Basic formula ofATP : Adenosine-PO3 ~ PO3 ~
PO3−
~ are high-energy phosphate bonds.
Each of these bonds stores 7300 calories of energy
per mole of ATP under standard conditions.
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When one phosphate radical is removed, more than
7300 calories of energy are released to energize the
muscle contractile process.
Removal of the first phosphate
ATP to Adenosine-PO3 ~ PO3 (ADP).
Removal of the second converts
ADP to Adenosine-PO3 (AMP).
Rudolph Blaze
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The amount ofATP present in the muscles is
sufficient to sustain maximal muscle power for only
about 3 seconds,
This might be enough for one half of a 50-meter
dash.
It is essential that newATP be formed continuously,
even during the performance of short athletic events.
The activities of three metabolic systems are exceedingly
important in understanding the limits of physical activity.
These systems are :
The phosphocreatine creatine system,
The glycogen–lactic acid system,
The aerobic system.
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Phosphocreatine creatine system
ATP is resynthesized fromADP by the addition of a
phosphate group.
Energy-rich phosphate compound that can supply energy
for short periods.This compound is
phosphorylcreatine,
It hydrolyze to creatine and phosphate groups with the
release of considerable energy.
Phosphocreatinecreatine system
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At rest, someATP in the mitochondria transfers its
phosphate to creatine, so that a phosphorylcreatine store
is built up.
During exercise, the phosphorylcreatine is hydrolyzed at
the junction between the myosin heads and actin,
formingATP fromADP and thus permitting contraction
to continue.
The high-energy phosphate bond of
phosphocreatine has more energy than the bond
ofATP.
10,300 calories per mole compared with 7300
for the ATP bond.
Phosphocreatine can easily provide enough
energy to reconstitute the high-energy bond of
ATP.
Most muscle cells have 2-4 times as much
phosphocreatine as ATP.
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Energy transfer from phosphocreatine toATP occurs
within a small fraction of a second.
The combined amounts of cellATP and cell
phosphocreatine are called the phosphagen
energy system.
These substances together can provide maximal muscle
power for 8 to 10 seconds.
It is enough for the 100-meter run.
Thus, the energy from the phosphagen system is used for
maximal short bursts of muscle power.
Glycogen–Lactic Acid System.
The stored glycogen in muscle can be split into
glucose, and the glucose can then be used for
energy.
The initial stage of this process, called glycolysis,
occurs without use of oxygen and, therefore, is
said to be anaerobic metabolism.
Each glucose molecule is split into two pyruvic
acid molecules,and energy is released to form four ATP
molecules for each original glucose molecule.
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If there is insufficient oxygen for the second stage
(the oxidative stage) of glucose metabolism to
occur, most of the pyruvic acid then is converted
into lactic acid.
Another characteristic of the glycogen–lactic acid
system is that it can form ATP molecules about
2.5 times as rapidly as can the oxidative
mechanism of mitochondria.
Glycogen lactic acid system is only about one half
as rapid as the phosphagen system.
The glycogen–lactic acid system can provide 1.3
to 1.6 minutes of maximal muscle activity.
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Aerobic System
The aerobic system is the oxidation of foodstuffs
in the mitochondria to provide energy.
Glucose, fatty acids, and amino acids from the
foodstuffs — after some intermediate
processing—combine with oxygen to release
tremendous amounts of energy that are used to
convert AMP and ADP into ATP.
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Explain the gradation of muscular
activity
The motor unit is the functional element of
muscle contraction
A typical skeletal muscle receives innervation from ~100
somatic motor neurons.
The motor unit consists of a single motor neuron and all
the muscle fibers that it activates.
When the motor neuron generates an action potential,
all the fibers in the motor unit fire simultaneously
The fineness of control for movement varies with the
innervation ratio—the number of muscle fibers
per motor neuron.
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The small motor units that are recruited during
sustained activity contain a high proportion of
type I muscle fibers, which are highly oxidative
and resistant to fatigue.
The large motor units that are recruited for brief
periods—for rapid, powerful activity—typically
consist of type IIa and IIb
Muscle force rises with the recruitment of
motor units and an increase in their firing
frequency.
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During contraction, the force exerted by a muscle
depends on
(1) how many motor units are recruited and
(2) how frequently each of the active motor neurons fire
action potentials.
Motor units are recruited in a progressive order, from
the smallest (i.e., fewest number of muscle fibers) and
therefore the weakest motor units to the largest and
strongest.
This intrinsic behavior of motor unit recruitment is
known as the size principle
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All or None Law
Individual muscle fibers either contract
maximally or do not contract at all.This is all or
non law.
This is applicable to all excitable tissues.
The graded response, which is seen by increasing
the strength of stimulus is due to more number of
fibers getting stimulated, while each fiber is
contracting maximally.
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Fatigue:
When repeatedly stimulated, muscle looses its excitability,
becomes gradually less excitable and ultimately ceases to
respond.This phenomenon is called muscular fatigue – which
is inability to do further work.
Causes of Fatigue
Exhaustion of source of energy in muscle.
Accumulation of end products of metabolism in muscle like
lactic acid.
Decrease in local synthesis of acetylcholine (neuromuscular
transmitter) during prolonged exercise.
Rise in intramuscular tension hampers passage of blood
through muscle.
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Seat of fatigue -
- Neuromuscular junction—when stimulated through
motor nerve.
- In vigorous muscular exercise – synapses.
In human subject fatigue is studied by instrument called
ergograph.
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Clonus:
When repeated stimuli are applied the type of response
will vary with frequency.
If the frequency is such that successive stimuli fall within
the period of relaxation of the previous curve, the record
will show series of oscillations.This is known as clonus.
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The relative contribution of motor unit recruitment and rate
coding varies among muscles.
Strength versus endurance training differentially
alters the properties of motor units
Sustained periods of low to moderate intensity performed
several times per week—endurance training—result in a
greater oxidative capacity of muscle fibers and are manifested
by increases in O2 delivery, capillary supply, and
mitochondrial content.
Brief sets of high-intensity contractions performed several
times per week—strength training— result in motor
units that can produce more force and can shorten against a
given load at greater velocity by increasing the amount of
contractile protein.
Summary
Motor unit
All or none law
Summation, staircase phenomena.
Fatigue.
Clonus.
