Smooth Muscle

What is smooth muscle?

Smooth muscle is a type of muscle found in many places inside your body. It gets its name from how it looks, which is different from most other types of muscle. The smooth muscles throughout your body work without you thinking about them. They handle some of the most important automatic jobs throughout your body systems.

Your body has three types of muscle: skeletal, cardiac and smooth. Skeletal muscle cells are long and cylindrical, which is part of why they’re often called muscle fibers. These fibers form bundles. Bunched together, these fibers make the muscle’s surface look like it has grooves (striations).

Cardiac muscle also looks smooth, but it has key differences in its structure. It also only exists in one place in your body: your heart.

The main difference between smooth and skeletal muscles is that skeletal muscles are supposed to respond to your control. Smooth muscle movements happen without you thinking about them. Smooth muscle cells also have a different structure from skeletal muscle cells. They have narrow ends, a wider middle and are a lot shorter.

Smooth muscle is found in places throughout your body, like your:

• Airways
• Blood vessels
• Digestive system
• Eyes
• Lymph vessels
• Skin
• Urinary and reproductive systems

What does smooth muscle do?

Smooth muscle has a few main jobs:

• Passage width control. Some of your smooth muscles manage the width of passages inside your body. Examples of this include your blood and lymph vessels and airways.
• Transporting substances. Smooth muscles can flex together to push what’s inside a tubelike passage. This is how multiple sections of your digestive tract work.
• Gatekeeping. Most muscles stay relaxed and only flex when needed. The smooth muscle in sphincters, like in your bladder or butthole (anus), stays flexed and only relaxes when needed.

Smooth muscles in certain places can also have very specific jobs. Some examples include:

• Skin. Smooth muscle is what causes hairs to stand on end, like with goosebumps.
• Eyes. Smooth muscles in the ciliary body of your eye pull on fibers called zonules to control how your eyes focus. Smooth muscles in your iris also control how your pupils dilate or constrict.
• Uterus. Smooth muscles are where contractions happen during labor.

What are the common conditions and disorders that affect smooth muscle?

Smooth muscle is vulnerable to many of the same conditions that can affect any muscle, especially injuries or paralysis. Some other conditions that can affect smooth muscle include:

• Anti-smooth muscle autoantibodies. This is when your immune system turns on and damages your smooth muscle. This is more likely to happen in people with autoimmune hepatitis and similar conditions.
• Muscular dystrophies. These are conditions that gradually make your muscles stop working properly.
• Spasms. Smooth muscle can twitch or contract uncontrollably. Some specific examples include spasms in your blood vessels (vasospasm) or airways (asthma).
• Visceral myopathies. These are a group of smooth muscle diseases that interfere with how these muscles work. They can be deadly when they affect smooth muscle in places like your bladder or large intestine. They’re often genetic.

Common signs or symptoms of smooth muscle conditions

Smooth muscle conditions usually disrupt how body systems work. The symptoms depend on where the muscles are and what they do. Some examples of symptoms include:

• Blood pressure extremes. This can cause either high blood pressure or passing out from low blood pressure.
• Slowed or stalled digestion. If your digestive tract isn’t pushing digesting food through, you can have symptoms like abdominal pain or constipation.
• Difficulty peeing. You might not be able to pee (urinate) properly if your bladder muscles aren’t working properly.

Because smooth muscle is part of so many different body systems, there are a lot of possible smooth muscle symptoms and conditions. A healthcare provider can tell you more if you have concerns about symptoms or conditions, or refer you to a specialist who can help.