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.
SINGLE MUSCLE TWITCH
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.
MUSCLE TONUS (TONE)
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.
MECHANISM OF MUSCLE CONTRACTION
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.
ENERGY FOR MUSCLE CONTRACTION
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