All or none Principle in Muscle Contraction

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    A nerve fibre connects to a muscle fibre by one or more points called the motor end plates. It is at
    the motor end plate that a nerve impulse stimulates a muscle fibre. The motor units function on the
    “all-or-none principle” when stimulated, i.e., the minimal stimulus which causes contraction causes a
    complete contraction, a stronger stimulus will not cause a greater contraction. However, the entire
    muscle does not obey this law, because the extent of its contraction depends upon the number of its
    motor units which are contracting at any particular time. A few motor units in action cause a feeble
    contraction, many units in operation produce a stronger contraction. Thus, a gross muscle may show
    many grades of contraction, depending upon the amount of stimulation. It is a common observation
    that less exertion is needed to lift a sheet of paper than a book.
            The ability of the nervous system to progressively increase the strength of contraction by
    activating more and more of the motor neurons controlling the muscle is called recruiting of motor

    A specific minimum strength of the nerve impulse or some artificial stimulus required for exciting a
    muscle fibre to contract is called threshold stimulus of that muscle fibre. A nerve impulse or other
    stimulus below the threshold intensity of a muscle fibre fails to bring about its contraction. The
    threshold stimulus varies from fibre to fibre even in the same muscle.

    In a living animal, the muscles contract on stimulation by nerve impulses. In the laboratory, artificial
    stimuli, such as electric shock, application of acid, contact with a flame, bring about their
    contraction. A striated muscle responds to a single stimulus, say a single electric shock, with a single
    quick contraction. This single isolated contraction of a muscle fibre caused by a single nerve impulse
    or artificial stimulus is called muscle twitch. Immediately after a twitch, the muscle fibre relaxes.

    The simple muscle twitch is a laboratory phenomenon. In a living animal, the muscles do not
    normally contract in single twitches. Instead, they receive continuous trains of nerve impulses in
    rapid succession and undergo a smooth, sustained contraction as relaxation cannot occur between
    successive contractions. This state of sustained contraction due to many quickly repeated stimuli is
    known as tetanus, or tetanization. The various motor units are stimulated in rotation, and this makes
    the various muscle fibres contract and relax in rotation. With the result, the muscle as a whole
    remains partially contracted. The strength of the contraction depends on the number of muscle fibres
    that contract at any given time. It is because of tetanus that we can hold a book for a long time to
    read, and can carry a cup of tea across a room without a mishap.

   All apparently relaxed skeletal muscles always remain in partial contraction as long as their nerves
   are intact. This state of sustained partial contraction is called muscle tonus or tone. It is a sort of
   mild tetanus. The tone is maintained by a constant flow of nerve impulses to the muscle fibres. A
   muscle under slight tension can react more rapidly and can contract more strongly than one which is
   completely relaxed. Almost all our daily activities are carried out by titanic contractions of muscles.
   Tetanus is necessary to maintain posture and form of the body.
   Main events : In contraction, the laterally projecting heads (cross bridges) of the thick myocin
   myofilaments come in contact with the thin action myofilaments and rotate on them. This pulls the
   thin myofilaments towards the middle of the sarcomere past the thick myofilaments. the Z lines
   come closer together and the sarcomere becomes shorter. Length of the A band remains constant.
   Myofilament stay the same length. Free ends of actin myofilaments move closer to the centre of the
   sarcomere, bringing Z lines closer together. I bands shorten and H zone narrows. A similar action in
   all the sarcomeres results in shortening of the entire myofibril, and thereby of the whole fibre and the
   whole muscle. A contracted muscle becomes shorter and thicker, but its volume remains the same.
   Sliding Filament Theory. The above view about the contraction of striated muscle is called the
   sliding filament theory. It states that muscle contraction results from sliding of thin myofilaments
   past the thick myofilaments rather than by shrinking of myofilaments. This theory was proposed
   independently by A.F. Huxley and R. Niedergerke and by H.E. Huxley and Jean Manson in
   England in 1954. It is well supported by experimental evidence.

    Muscle fibres of humans contain two organic phosphates, namely, adenosine triphophate and
    phosphocreatine, also called creatine phosphate ,with high-energy phosphate bonds. Energy for
    muscle contraction is provided by hydrolysis of adenosine triphophate by myosin ATPase enzyme.
    This hydrolysis produces adenosine disphophate and inorganic phosphate. The store of ATP is
    quickly exhausted as a muscle cell typically stores only enough ATP for a few contractions. Muscle
    cell also stores glycogen between the myofibrils. They store the phosphagens too. A phosphagen of
    vertebrate muscle cells is phosphocreatine. The used up adenosine triphosphate must be restored for
    additional contractions. This is done by phosphocreatine. It donates its high energy-phosphate bond
    to ADP, producing ATP. This reaction is catalysed by an enzyme creatine kinase. When creatine
    phosphate is used up, new ATP is generated by aerobic respiration in the muscle cells. If ATP is
    used faster than the muscles fibres can produce it aerobically, the muscle fibres start anaerobic
    respiration (glycolysis) to replenish ATP. This produces lactic acid as a byproduct. Lactic acid
    accumulates in the muscle cells and deffuses out into the blood. A small portion of it reenters the
    relaxing muscle cells and is changed into glycogen. Major part of lactic acid passes into the liver.
    Here 1/5th of lactic acid is oxidized to CO2 and H2O. The energy (ATP) derived from this oxidation
    is used in changing the remaining 4/5th of liver lactic acid in to glycogen. Creatine phosphate is
    regenerated in relaxing muscle by using ATP produced by carbohydrate oxidation. The chemistry of
    muscle contraction is summarized in figure.
    All these reactions are catalysed by specific enzymes. The first 3 steps do not involve oxygen and
    can occur under anaerobic conditions. Fourth step needs oxygen. This means contraction of muscle
    and part of its recovery from contraction occur without oxygen. This is of great importance as it
    enables us to undertake great exertion. Oxygen is needed only in t
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