Anatomy & Physiology

Anatomy & Physiology Study Guide

Structural Characteristics Smooth muscle cells are relatively long and slender. Each cell is spindle shaped and has a single, centrally located nucleus. A smooth muscle fiber has no T tubules, and the sarcoplasmic reticulum forms a loose network throughout the sarcoplasm. Smooth muscle cells also lack myofibrils and sarcomeres. As a result, this tissue is called non-striated muscle because it lacks striations. Thick filaments are scattered throughout the sarcoplasm of a smooth muscle cell. The thin filaments in a smooth muscle cell are attached to dense bodies, structures distributed throughout the sarcoplasm in a network of filaments composed of the protein desmin. The sarcolemma has some of the dense bodies attached to it. The dense bodies and intermediate filaments anchor the thin filaments such that, when sliding occurs between thin and thick filaments, the cell shortens. Dense bodies have a non-linear arrangement, therefore, the muscle cell twists like a corkscrew with each contraction. Adjacent smooth muscle cells, bound together at dense bodies, transmit the contractile forces from cell to cell. Functional Characteristics • Excitation–contraction coupling: The trigger for smooth muscle contraction is the appearance of free calcium ions in the cytoplasm. On stimulation, a blast of calcium ions enters the cell from the extracellular fluid, and additional calcium ions are released from the sarcoplasmic reticulum. Calcium ion concentrations increase throughout the cell. Once in the sarcoplasm, the calcium ions interact with calmodulin, a calcium-binding protein. Calmodulin then activates the myosin, which in turn enables the attachment of myosin heads to actin. • Length–tension relationships: Because the thick and thin filaments are scattered and are not organized into sarcomeres in smooth muscle, tension development and resting length are not directly related. A stretched smooth muscle soon adapts to its new length and retains the ability to contract on demand. This ability to function over a wide range of lengths is called plasticity . Smooth muscle can contract over a range of lengths four times greater than that of skeletal muscle.

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