Question

Explain in detail the microscopic evidence indicating that thick and thin filaments slide past each other when a skeletal muscle fiber contracts.

Answer 1

There are 3 types of muscles in us: skeletal or striated or voluntary, smooth or visceral or unstriated, cardiac muscles which are striated muscles of our heart.

The skeletal muscles occur in bundles. They are closely associated with and attached to the skeleton of the body. The 2 ends of a skeletal muscle are typically attached to 2 different bones by tendons ( a structure that binds a muscle to a bone).

The skeletal muscles occur in the form of bundles called fasicles. The myofilaments are structural as well as functional units of muscle.

Each muscle is enclosed in a dense connective tissue sheath, called epimysium or fascia. It closes numerous small muscle bundles called muscle fasciculi with a connective tissue covering called perimysium. Each muscle fasciculum is formed of several muscle fibres. Each muscle fibre is covered with endomysium.

Each muscle cell is covered with a thin plasma membrane called sarcolemma enclosing multinucleate cytoplasm called sarcoplasm. In the sarcoplasm, numerous myofibrils are embedded. These are packed in parallel bundles, separated by thin sheets of cytoplasm.

Each myofibril has alternate dark and light bands arranged throughout the length. The light bands or I-bands are formed of thin actin filaments. Each light band is bisected at the mid-point by a thin dark-line called the Z-line.- bands The part of myofibril between 2 adjacent Z-lines is called sarcomere. The dark bands or A -bands are formed of thick myosin filaments. They are free at both ends and are thicker than I-bands. Each A-band is bisected at the mid-point by a thin paler line called H-zone.

Sliding Filaments:

The unit of muscle contraction is sarcomere which is formed of 1/2 I band, 1 full A band, another 1/2 I band.

During rest, the muscle is relaxed. the actin and myosin filaments lie parallel to each other partially overlapping at the 2 ends of myosin filaments.

During muscle contraction, the thin actin filaments from both the ends of sarcomere slide towards H-zone between thick myosin filaments till H-zone disappears completely and width of I-bands is reduced. The whole muscle filament shortens in size.

Events during muscle contraction:

1. Depolarisation of Sarcolemme: The neural signal from motor neuron reaches the neuromuscular junction of sarcolemma and causes the release of a neurotransmitter or acetylcholine from the vesicles into the nerve-muscle gap. Sarcolemma gets depolarised and action potential is set.

2. Release of calcium ions: Action potential of sarcolemma is transmitted to T- tubules of sarcoplasmic reticulum causing the release of Calcium ions.

3. Conformational changes in actin filaments: The released calcium ions bind to troponin and tropomyosin proteins. These are closely associated with actin filaments. The 3-D shape of troponin-tropomyosin-active complex is changed and the active site on actin filaments for myosin are exposed.

4. Activation of myosin heads: Calcium ions act on myosin heads, activating them to release energy from ATP. ATP is required to move the head of myosin filaments towards active sites of actin filaments.

5. Formation of actin-myosin complex: Myosin heads bind to the active sites on globular subunits of actin filaments. Cross-bridges are formed.

6. Sliding of actin filaments over myosin filaments: since cross bridges are formed between actin and myosin, the myosin heads rotate pulling actin filaments towards H-zone in the centre of A band.

7. Contraction of muscle: As the actin myofilaments from opposite ends of the sarcomere move towards each other, H-zone disappears.I band shortens, sarcomere contracts. Entire muscle fibre contracts.

8. Repolarisation of Sarcolemma and relaxation of muscles: When stimulation of muscle fibre stops, sodium ions stop moving in. Resting potential of sarcolemma is resumed. Calcium ions are pumped back into sarcoplasmic reticulum. The level of cacium ion falls resulting in the movement of troponin and tropomyosin back to their original positions. Both sets of actin filaments return to their original positions.