Which structures are the attachment sites for actin and myosin in smooth muscle cells?

In smooth muscle cells, actin and myosin, which are the primary contractile proteins, are anchored to structures known as dense bodies. These dense bodies serve as critical attachment points for the filaments, allowing for effective contraction when the proteins interact. During contraction, the actin filaments pull on the dense bodies, which then transmit the force throughout the muscle cell, resulting in a coordinated shortening of the muscle.

Dense bodies are functionally analogous to Z discs found in striated muscle; however, smooth muscle lacks the organized sarcomeres that create the striated appearance. Instead, smooth muscle fibers are arranged in a way that distributes tension across the entire cell, facilitated by the presence of these dense bodies.

The other options do not serve the same function in smooth muscle. Z discs are specific to skeletal and cardiac muscle, where they serve as the boundaries of sarcomeres. Neuromuscular junctions refer to the points of communication between motor neurons and skeletal muscle fibers, influencing voluntary muscle contraction but not playing a direct role in the contraction mechanism of smooth muscle. Fascia is connective tissue that surrounds and supports muscles, but it does not have a direct role in the attachment of contractile proteins within smooth muscle cells.