Structure of Muscle
Striated muscle fibres are packed into bundles enclosed in a tough connective tissue. These bundles are grouped to form a muscle. Every skeletal muscle is also enclosed in a thin connective tissue. The ends of muscles connect to bones through another kind of connective tissue called tendon. So, tendon joins a bone to a muscle.
The muscle cell, also called muscle fibre because of its long shape, is multinucleated and contains myofibrils made of myofilaments. Myofilaments are proteins which are of two types:
- thick filaments made of myosin protein
- thin filaments made of actin protein
Myosin and actin proteins are contractile proteins and responsible for muscular contraction.
The functional unit of the myofibril is called sarcomere. It lies between two successive dense linear structure called Z lines. The thin filaments also contain two other proteins, tropomyosin and troponin. Troponin is the switch, which in the presence of calcium ions controls muscle contraction.
Sliding Model of Muscle Contraction
Striated muscle contraction is explained by Sliding Filament Theory. This theory can be explained through the following steps:
- The thick and thin filaments myosin and actin are linked by cross bridges of troponin and tropomyosin.
- These crossbridges, on contraction, pull the thin filaments back over thick filaments.
- As a result, the thin filaments slide over the thick filaments. Calcium and ATP are required for attaching and releasing Troponin.
- Because of this sliding action, Z lines come closer and sarcomere shortens.
- All sarcomeres shorten together so the entire muscle contracts.
- The muscle relaxes when crossbridges relax and sarcomere regains original position.
Stimulation of Muscle Contraction
Muscles cannot contract on their own unless stimulated by a nerve. The nerve branches on a muscle and this area of the muscle fibre is called myoneural junction (myo: muscle; neuro: nerve).