Mechanism of muscle contraction

Sliding – filament hypothesis

Hanson and Huxley proposed this hypothesis (1955). According to
this hypothesis, the contractile unit of muscle is made up of two types of
filaments i.e., myosin and actin. These filaments during contraction slide over
one another. It has been observed that both at the time of contraction and
stretching, the length of the ‘A band’ remains constant. The length of the ‘I
band’, on the other hand, changes in accordance with the length of the muscle.

Due to this, the Z discs are pulled together. Thus successive sarcomeres are
pulled. This leads to the contraction of the muscle.

ATP as the source of Energy for contraction

For a muscle in action, energy is required. This energy is released by cleaving ATP molecules to ADP+ Pi. Greater amount of work is performed by the muscle, with greater amount of ATP that is cleaved. This energy binds with active site of actin filament to contract.

Neuro muscular Transmission

The muscles are innervated by myelinated nerve fibres that originate from the spinal cord. The nerve ending makes a junction, called neuromuscular junction. The nerve is connected at the midpoint of muscle fibre so that action potential in the fibre travels in both directions. When a nerve impulse reaches the neuro-muscular junction, a substance called acetylcholine is released. The acetylcholine induces muscle fibre for contraction.

Initiation of muscle contraction

Initiation of contraction of skeletal muscle begins with action potentials in the muscle fibres. These elicit electrical currents that spread to the interior of the fibre, where Ca ions are released from the sarcoplasmic reticulum. The Ca ions inturn initiate the chemical events of the contractile process. This overall process of controlling muscle contraction is called excitation.

Maximum strength of contraction

The maximum strength of contraction of a muscle operating at a normal muscle length is about 3.5 Kg / Sq.cm. of muscle.

Changes in muscle strength

When a muscle begins to contract after a long period of rest, its initial strength of contraction may be as little as one half of its maximum strength. It gains strength after 30-50 muscle twitches. Weight lifters apply this to reach maximum capacity.

Muscle Tone

When the muscles are at rest, a certain amount of contraction usually
remains. This residual degree of contraction in skeletal muscles is called
muscle tone. The skeletal muscle tone is the result of nerve impulses from
the spinal cord. These in turn are partially controlled by impulses transmitted from the brain. To some extent, the muscle tone is due to the muscle fibres itself.

Muscle Fatigue ( muscle contraction is diminished)

Prolonged and strong contraction of muscle leads to the state of muscle
fatigue. This is due to muscle glycogen depletion and lactic acid accumulation.
Fatigue probably results from inability of the contractile process and deficient
metabolic process of the muscle fibre.
However, experiments show, that transmission of nerve signals through
the neuromuscular junction can occasionally diminish the prolonged muscle
activity, thus further reducing the power of muscle contraction. It is a well
known fact that non-supply of blood to the muscles leads to almost
complete fatigue within a minute. This is due to non-supply of O2 and
nutrients to the muscles.

Rigor Mortis

Several hours after death all the muscles of the body attain
a state of contracture called Rigor mortis. This is due to complete
depletion of ATP in muscle fibres. The muscle remains in rigor, until the
lysosome enzymes completely destroy all muscle proteins. This will take place
within 15-25 hours after death.

Muscle Pull

Muscle pull is actually a muscle tear. A traumatic pulling of the fibres
produces tear known as strain. This injury can occur due to sudden
stretching of muscle beyond their point of elasticity. While some strains are
the result of high velocity impacts, other types of pull can be due to repetitive movements.

When the muscles are being exerted over a long period of time, they become vulnerable to tearing from a simple change in the position of muscles. Back pain is a common problem, caused by muscle pull.

This may be due to improper posture at the chairs. In these cases, the muscles are tightened over a long time, but the muscles may not tear until something as
simple as reaching for a glass of water occurs.