Final answer:
The projections from the thick filaments in sarcomeres are known as myosin heads, which play a vital role in muscle contraction by interacting with actin thin filaments.
So, the correct answer is 3.
Step-by-step explanation:
The projections observed in longitudinal sections of sarcomeres from the thick filaments at regularly spaced intervals are known as myosin heads. These myosin heads are crucial for muscle contraction as they interact with the thin filaments, which are primarily composed of actin, along with regulatory proteins such as tropomyosin and troponin. During muscle contraction, the myosin heads bind to active sites on actin, forming cross-bridges that pull on the thin filaments and shorten the sarcomere, leading to muscle contraction.
So, the correct answer is 3.