Smooth Muscle Definition
Smooth muscle is a type of muscle tissue which is used by various systems to apply pressure to vessels and organs. Smooth muscle is composed of sheets or strands of smooth muscle cells. These cells have fibers of actin and myosin which run through the cell and are supported by a framework of other proteins. Smooth muscle contracts under certain stimuli as ATP is freed for use by the myosin. The amount of ATP released depends on the intensity of the stimuli, allowing smooth muscle to have a graded contraction as opposed to the “on-or-off” contraction of skeletal muscle.
Smooth Muscle Structure
Smooth muscle tissue, unlike skeletal or cardiac tissues, does not have clearly defined striations visible on the cells. This is because smooth muscle cells are organized in a different way than other muscle cells. As seen in the image below, the actin and myosin filaments in smooth muscle are arranged in a stacked pattern across the cell. This “staircase” arrangement of actin and myosin is much different than the structure in skeletal and cardiac muscle. The actin filaments (red lines) in smooth muscle run from one side of the cell to the other, connecting at dense bodies and at the cell membrane. In skeletal and cardiac muscle, the actin filaments are attached to Z plates, which hold many actin filaments and show up as dark bands under the microscope. In smooth muscle, the actin and myosin fibers are arranged an angles to each other as they run through the cell. This can be seen in the image below.
Function of Smooth Muscle
Like all muscle tissue, the function of smooth muscle is to contract. The image above shows how the actin and myosin fibers shorten, effectively shrinking the cell. However, there are some important differences in how the smooth muscle contracts, compared to other types of muscle. In skeletal muscle, a signal from the somatic nervous system traverses to the muscle, where it stimulates organelles in the muscle cell to release calcium. The calcium causes a protein to detach from actin, and myosin quickly binds to the opening on actin. Since there was always available ATP, the myosin uses it to quickly contract the cell.
The same is not true in smooth muscle tissue. In smooth muscle, the contraction is not controlled voluntarily by the somatic nervous system, but by signals from the autonomous nervous system, such as nerve impulses, hormones, and other chemicals released by specialized organs. Smooth muscle is specialized to contract persistently, unlike skeletal muscle which much contract and release quickly. Instead of a calcium trigger which sets off a contraction reaction, smooth muscle has more of a throttle, like in a car.
A nerve impulse or outside stimulus reaches the cell, which tells it to release calcium. Smooth muscle cells do not have a special protein on actin which prevents myosin from binding. Rather, actin and myosin are constantly binding. But, myosin can only hold on and crawl forward when given energy. Inside smooth muscle cells is a complex pathway which allows the level of calcium to control the amount of ATP available to myosin. Thus, when the stimulus is removed, the cells do not relax right away. Myosin continues to bind to actin and crawl along the filaments until the level of calcium falls.
Smooth Muscle Location
This specialized function of contracting for long periods and hold that force is why smooth muscle has been adapted to many areas of the body. Smooth muscle lines many parts of the circulatory system, digestive system, and is even responsible for raising the hairs on your arm.
In the circulatory system, smooth muscle plays a vital role in maintaining and controlling the blood pressure and flow of oxygen throughout the body. While the majority of the pressure is applied by the heart, every vein and artery is lined with smooth muscle. These small muscles can contract to apply pressure to the system or relax to allow more blood to flow. Tests have shown that these smooth muscles are stimulated by the presence or absence of oxygen, and modify the veins to provide enough oxygen when it is low.
Smooth muscle also lines the majority of the digestive system, for similar reasons. However, the cells in the digestive system have different stimuli than those in the circulatory system. For instance, sheets of smooth muscle tissue in the gut react to you swallowing. When you swallow, tension is applied to one side of the sheet. The cells on that side contract in reaction, a wave begins to propagate itself down your digestive tract. This phenomena is known as peristalsis, and is responsible for moving food through the many twists and turns of the gut.
Smooth muscle, because of its ability to contract and hold, is used for many function in many places of the body. Besides those listed above, smooth muscle is also responsible for contracting the irises, raising the small hairs on your arm, contracting the many sphincters in your body, and even moving fluids through organs by applying pressure to them. While smooth muscle doesn’t contract or release as quickly as skeletal or cardiac muscle, it is much more useful for providing consistent, elastic tension.