Final answer:
Smooth muscle contains actin and myosin filaments not arranged in sarcomeres, causing the lack of striations and allowing for involuntary control of contractions, unlike the striated and multinucleated skeletal muscle which relies on regular sarcomere structures and voluntary neural signals for contraction.
Step-by-step explanation:
The contractile filaments in smooth muscle are composed of actin and myosin, similar to skeletal muscle. However, in smooth muscle, these filaments are not organized into sarcomeres, which gives skeletal muscle its striated appearance. Instead, the filaments in smooth muscle are arranged in a staggered pattern and are anchored to dense bodies, analogous to the Z-discs in skeletal and cardiac muscle fibers. The smooth muscle cells, or myocytes, are spindle-shaped with a single nucleus and can generate their own connective tissue known as endomysium.
One of the key differences between smooth and skeletal muscle is that smooth muscle does not exhibit striations due to the lack of regular sarcomere structure. Additionally, while skeletal muscle contractions rely on a rapid response to nervous system signals, many smooth muscle contractions are involuntary and controlled by a variety of stimuli, including hormonal, neural, and local factors. Moreover, the calcium ions necessary for smooth muscle contraction are supplied by the sarcoplasmic reticulum (SR) in the fibers and also by sequestration from the extracellular fluid through membrane indentations called calveoli.
In contrast, skeletal muscle fibers are cylindrical, have multiple nuclei, and contain regularly ordered sarcomeres that contribute to their striated appearance. Skeletal muscle contraction is also dependent on the sliding filament model, with the precise arrangement of actin and myosin in sarcomeres facilitating the contraction process through ATP consumption and the action of regulatory proteins like troponin and tropomyosin.