Which of the following statements is the most accurate description of the sliding filament mechanism?
a) thick filaments shorten while thin filaments remain unchanged
b) z discs slide over the thick and thin filaments
c) thin filaments slide past thick filaments towards the m line.
d) both thick and thin filaments shorten

c) thin filaments slide past thick filaments towards the M line

Explanation:

Each muscle fiber has many myofibrils composed of many sarcomeres which have thick and thin filaments. Thick filaments are made of myosin protein whereas thin filaments have actin protein. There are various bands and zones in a sarcomere like A band, I band etc. H zone consists of only thick filaments. It has a central vertical line called as M line where the accessory proteins hold together the thick filaments.

According to sliding filament mechanism, sarcomere must shorten for muscle to contract. However sarcomere shortening is not caused due to shortening of thick and thin filaments. Instead they slide over each other towards the M line which leads to shortening of sarcomere without changing the length of filaments. The sliding is caused by forming of cross bridges between actin and myosin proteins.

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