Smooth Muscle

Smooth Muscle Definition

A smooth muscle is a type of muscle tissue that applies pressure to vessels and organs. Sheets or strands of smooth muscle cells make up smooth muscle. Actin and myosin fibers run through these cells and are supported by a framework of other proteins. 

When certain stimuli are applied, smooth muscle contracts as ATP is freed for use by myosin. By releasing ATP at different intensities, smooth muscle can have graded contractions rather than skeletal muscle’s “on-or-off” contractions.

Smooth Muscle Structure

The cells of smooth muscle tissue lack clearly defined striations, unlike those of skeletal or cardiac tissues. Unlike other muscle cells, smooth muscle cells are organized differently. In smooth muscle, actin and myosin filaments are stacked 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. 

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

Due to its specialized function of contracting for long periods and retaining force, smooth muscles have been adapted to a variety of body parts. The smooth muscle lines many parts of the circulatory and digestive systems, as well as raising the hair on your arms.

Smooth muscles play an important role in maintaining and controlling blood pressure and oxygen flow in the circulatory system. Smooth muscle lines every vein and artery, even though the heart applies most of the pressure. 

A contraction of these small muscles can apply pressure to the system or a relaxation of them can allow more blood to flow. These smooth muscles are stimulated by oxygen levels, and when oxygen levels are low, they modify the veins to provide enough oxygen.

Similarly, smooth muscle lines most of the digestive system. Cells in the digestive system are stimulated differently than those in the circulatory system. When you swallow, smooth muscle tissue in the gut reacts. As you swallow, tension is applied to one side of the sheet. The cells on that side contract in reaction, causing a wave to propagate down your digestive tract. Peristalsis is responsible for moving food through the twists and turns of the gut.

Since smooth muscles can contract and hold, they are used in a variety of body functions. Besides those functions listed above, smooth muscle also contracts the irises, raises the small hairs on your arm, contracts the sphincters in your body, and even moves fluids through organs by applying pressure. Although smooth muscle does not contract or release as quickly as skeletal or cardiac muscle, it provides consistent, elastic tension.


What is smooth muscle?

Smooth muscle is a type of muscle tissue that is found in the walls of hollow organs, such as the intestines, uterus, and blood vessels. It is called “smooth” because it lacks the striations, or stripes, that are characteristic of skeletal muscle.

How does smooth muscle differ from skeletal muscle?

Smooth muscle differs from skeletal muscle in several ways, including its structure, function, and mode of control. Smooth muscle is non-voluntary, meaning that it is controlled by the autonomic nervous system and not under conscious control. It is also more resistant to fatigue and can maintain a state of sustained contraction, whereas skeletal muscle is more prone to fatigue and is used primarily for rapid, forceful movements.

What are some functions of smooth muscle?

Smooth muscle is responsible for a variety of functions in the body, including the regulation of blood pressure, the movement of food through the digestive system, the dilation and constriction of blood vessels, and the contraction of the uterus during childbirth.

What are some disorders that affect smooth muscle?

Disorders that affect smooth muscle include asthma, which involves the contraction of smooth muscle in the airways, and hypertension, which involves the contraction of smooth muscle in the walls of blood vessels. Other disorders that affect smooth muscle include gastrointestinal disorders such as irritable bowel syndrome and inflammatory bowel disease.

How is smooth muscle affected by aging?

As the body ages, smooth muscle may become less effective at contracting and may undergo structural changes that can contribute to various age-related diseases, such as atherosclerosis. Additionally, the loss of smooth muscle function can contribute to issues such as urinary incontinence.

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