Muscle tissue is responsible for all movement, whether voluntary or involuntary. The human body contains three classifications of this tissue: skeletal, cardiac, and smooth muscle. Skeletal muscle moves bones, and cardiac muscle pumps blood in the heart. Smooth muscle is located in the walls of hollow internal organs, such as the stomach, intestines, and blood vessels, where it regulates the passage of substances. The internal arrangement of these tissues determines whether a muscle is classified as striated or non-striated.
What Defines Striated Muscle
The term “striated” refers to the alternating light and dark bands or stripes visible when muscle tissue is viewed under a microscope. This striped pattern results from the precise, repeating organization of the contractile proteins actin and myosin within the muscle cell. These filaments are assembled into units called sarcomeres, which are the fundamental mechanical units of muscle contraction. Skeletal and cardiac muscle exhibit this characteristic appearance because their actin and myosin filaments are stacked in an orderly, parallel fashion. The overlap of thick myosin and thin actin filaments creates the distinct banding pattern, allowing for rapid and forceful muscle contraction.
The Internal Organization of Smooth Muscle
Smooth muscle does not possess striations because it lacks the organized, repeating sarcomeres found in skeletal and cardiac muscle. While smooth muscle cells still contain the contractile proteins actin and myosin, their internal arrangement is different. The filaments are scattered throughout the cell’s interior rather than bundled into a uniform, parallel pattern. Actin filaments are anchored to structures called dense bodies, which are scattered throughout the cytoplasm and attached to the cell membrane. These dense bodies act as anchor points for the thin filaments, similar to the Z-discs in striated muscle. The myofilaments span the cell in a diagonal or crisscross network. This non-linear organization allows the entire muscle cell to shorten in a corkscrew fashion during contraction.
How Smooth Muscle Contraction Differs
The structural difference in smooth muscle dictates a distinct functional profile compared to its striated counterparts. Because the contractile filaments are arranged in a crisscross pattern anchored to dense bodies, smooth muscle cells can contract over a much wider range of lengths. This ability, known as plasticity, is necessary for organs like the stomach and urinary bladder, which must expand significantly to hold contents and then contract forcefully to expel them. Smooth muscle contraction is much slower and can be sustained for long periods without fatigue. The regulatory mechanism also differs, relying on the protein calmodulin to initiate the process rather than the troponin complex found in striated muscle. This slower, sustained, and involuntary contraction is regulated by the autonomic nervous system, suiting its role in maintaining tension in blood vessel walls and moving substances through tubular organs.