Question

1. Give a brief detail of the physiologic steps to muscle contraction.

Answer 1

Myofibrils (of the muscle) are the aggregates of myofilaments, which are responsible for muscle contraction. Myofilaments are composed of two types of protein filaments called actin and myosin.

Actin and myosin filaments are the structural components of the sarcomere, which is the actual contractile part of the skeletal and cardiac muscle. The interaction (cross-bridge formation) between the actin and myosin filaments produce muscle contraction.

Sliding filament theory: The following sequence of events describes the contraction mechanism in a skeletal muscle cell (muscle shortening).

1. At the neuromuscular junction, acetylcholine is released into the synaptic cleft at motor neurons, which binds to the postsynaptic receptors present on neuromuscular fibers. The influx of sodium ions causes depolarization, and the action potential is generated.

2. The action potential, carried deep into the cell by the T-tubules, triggers the release of calcium ions from the sarcoplasmic reticulum.

3. The calcium ion concentrations at the myofilaments increase. The calcium tends to bind with troponin C and move the tropomyosin and expose the myosin-binding sites on actin.

4). Each myosin molecule contains a globular head and tail portion. The binding of ATP (adenosine triphosphate) to the head region of myosin hydrolyzes the ATP to ADP (adenosine diphosphate), releasing the phosphate bond; this hydrolysis provides the energy for muscle contraction. Then the thick myosin filaments slide over the thin stationary actin filaments and cross-bridges are formed.

5. Binding of another ATP molecule to myosin detaches the actin and myosin cross-bridges. The concentration of the calcium ions at the myofilaments decreases as they are actively transported into the sarcoplasmic reticulum.

6. The muscle cell relaxes and lengthens.