Question

1. Outline how muscle contraction is initiated in the cell and include the role for Ca²⁺ in this process.
   

Answer 1

Muscle contraction is initiated by a signal sent by the central nervous system via a motor neuron. Motor unit is formed by constitution of motor neuron along with the muscle fibre. The junction that formed between the motor neuron and the sarcolemma of the muscle fibre is called neuromuscular junction or motor end plate. The neurotransmitter acetylcholine release when the neural signal reaching this junction which help in generating an action potential in the sarcolemma. This action potential spread throughout the muscle fibre that result in releasing of calcium ion into the sarcoplasm. Due to release of calcium ion the level of calcium increases. Now calcium ion bond with subunit of troponin on actin filaments that favors in removing the mask of active site for myosin. Now the exposed active site on actin filament bind with myosin head. This process is occur by utilizing energy from ATP hydrolysis. This form cross bridge. Attached actin filament pulled towards the centre of 'A' band that help in pulling the 'Z' line attached to this actin inward and help in causing a shortening of sarcomere i.e., contraction. 'I' band get reduced but 'A' band have same length during shortening of muscles.