Learning Objectives
Understand the structure and function of smooth muscle tissue
By the end of this section, you will be able to:
- Understand the difference between single-unit and multi-unit smooth muscle
- Describe the microanatomy of a smooth muscle cell
- Explain the process of smooth muscle contraction
- Explain how smooth muscle differs from skeletal muscle
Smooth muscle, so-named because the cells do not have visible striations, is present in the walls of hollow organs (e.g., urinary bladder), lining the blood vessels, and in the eye (e.g., iris) and skin (e.g., erector pili muscle). Smooth muscle displays involuntary control and can be triggered via hormones, neural stimulation by the ANS, and local factors. In certain locations, such as the walls of visceral organs, stretching the muscle can trigger its contraction).
Smooth muscle fibers are spindle-shaped and, unlike skeletal muscle fibers, have a single nucleus; individual cells range in size from 30 to 200 μm. Smooth muscle fibers are often found forming sheets of tissue and function in a coordinated fashion due to the presence of gap junctions between the cells. Termed unitary smooth muscle or visceral muscle, this type of smooth muscle is the most common observed in the human body, forming the walls of hollow organs. Single-unit smooth muscle produces slow, steady contractions that allow substances, such as food in the digestive tract, to move through the body.
Multi-unit smooth muscle, the second type of smooth muscle observed, are composed of cells that rarely possess gap junctions, and thus are not electrically coupled. As a result, contraction does not spread from one cell to the next, but is instead confined to the cell that was originally stimulated. This type of smooth muscle is observed in the large airways to the lungs, in the large arteries, the arrector pili muscles associated with hair follicles, and the internal eye muscles which regulate light entry and lens shape.
External Website
View the University of Michigan WebScope at http://virtualslides.med.umich.edu/Histology/Digestive%20System/Intestines/169_HISTO_40X.svs/view.apml to explore the tissue sample in greater detail.
Although smooth muscle cells do not have striations, smooth muscle fibers do have actin and myosin contractile proteins which interact to generate tension. These fibers are not arranged in orderly sarcomeres (hence, no striations) but instead are anchored to dense bodies which are scattered throughout the cytoplasm and anchored to the sarcolemma. A network of intermediate fibers run between the dense bodies providing an internal framework for contractile proteins to work against.
A dense body is analogous to the Z-discs of skeletal muscle, anchoring the thin filaments in position. Calcium ions are supplied primarily from the extracellular environment. T-tubules are absent but small indentations, called calveoli, in the sarcolemma represent locations where there are a high density of calcium channel