What is the primary function of the sarcomere?

• A. Part of brainstem controlling respiration
• B. A blood vessel layer
• C. Functional contractile unit of muscle
• D. Outer layer of GI tract

Answer: (C)

The main idea is that a sarcomere is the basic functional unit that actually produces muscle contraction. It’s the segment between two Z-discs in a myofibril, and it contains overlapping thick (myosin) and thin (actin) filaments. When a muscle is activated, calcium released from the sarcoplasmic reticulum binds to troponin, shifting tropomyosin away from myosin-binding sites on actin. Myosin heads attach to these sites and pull the actin filaments toward the M-line, shortening the sarcomere in a process powered by ATP. This sliding filament mechanism is what generates the force of contraction; many sarcomeres shortening together along a muscle fiber yield overall muscle shortening and movement. The other options refer to unrelated structures (a brainstem region, a blood-vessel layer, or a GI tract layer), not to the contractile machinery of muscle.