Smooth muscle

Smooth muscle is one of the three main types of muscle in vertebrates, along with skeletal and cardiac muscle. It is found in both vertebrates and invertebrates and is controlled by the autonomic nervous system. Unlike skeletal muscle, smooth muscle is non-striated, meaning it doesn’t have bands or stripes.

Smooth muscle is located in the walls of many hollow organs, such as the stomach, intestines, bladder, and uterus. It is also in the walls of blood vessels and lymph vessels, where it is called vascular smooth muscle. Smooth muscle can also be found in the respiratory, urinary, and reproductive systems.

In the eyes, special smooth muscles like the ciliary muscles, iris dilator muscle, and iris sphincter muscle help control vision. The ciliary muscles change the shape of the lens to focus on objects, and the iris muscles adjust the size of the pupils. In the skin, smooth muscle cells such as those in the arrector pili make hair stand up when we feel cold or are scared.

Structure

Smooth muscle is found in the walls of internal organs, blood vessels, the urinary tract, and the digestive tract. It looks different from skeletal and cardiac muscle because it has no stripes, so it appears smooth under a microscope.

There are two types of smooth muscle: single-unit and multiunit. Single-unit smooth muscle works together as one group, so the whole muscle contracts or relaxes at the same time. Multiunit smooth muscle contracts in a less organized way. Smooth muscle can tighten and loosen without direct signals from nerves, helping organs like the intestines and bladder function properly.

Excitation-contraction coupling

Further information: Excitation-contraction coupling

Smooth muscle can tighten or relax based on signals it receives. These signals can come from special cells or substances like hormones.

When smooth muscle tightens, tiny parts inside called myosin and actin slide past each other. This needs energy from a substance called ATP. The tightening happens when myosin attaches to actin and pulls, making the muscle shrink.

Smooth muscle can tighten in two ways: quickly for a short time, or slowly for a longer time. Quick tightening happens in the digestive system. Slow tightening is found in blood vessels.

The tightening starts when calcium enters the muscle cell. Calcium helps activate a substance called MLCK, which makes the myosin move. Even after calcium levels drop, the muscle can stay tightened using special mechanisms. This helps muscles in blood vessels stay tightened for long periods.

Invertebrate smooth muscle

Invertebrate smooth muscle works when calcium attaches to a protein named myosin. This makes the muscle contract. Some animals, like clams, can stay closed for a long time using a special "catch phase." This phase uses very little energy. The catch phase uses proteins similar to those in vertebrate smooth muscle.

Specific effects

Smooth muscle in different parts of the body does different jobs, but it works in similar ways. In blood vessels, smooth muscle can tighten to make the vessels narrower. This helps control blood pressure and where blood flows. This tightening can last a long time in places like blood vessels, air passages in the lungs, and certain gates in the body.

In the digestive system, smooth muscle helps push food along by rhythmically contracting. This process is called peristaltic movement. Some smooth muscle in the kidneys helps release a substance called renin. Renin is part of the renin–angiotensin system that helps control blood pressure.

Growth and rearrangement

Scientists are still learning how things outside the body make smooth muscle grow and change shape. Many growth factors and signals from the nervous system affect how smooth muscle grows.

A special pathway called the Notch receptor pathway helps form blood vessels such as arteries and veins. This growth can sometimes lead to health problems like atherosclerosis. A substance called nitric oxide can help stop this extra growth.

Smooth muscle mostly comes from a part of the early embryo called the mesoderm. But, the smooth muscle in the heart’s main arteries, like the aorta and pulmonary arteries, comes from a different source called neural crest cells.

Related diseases

Multisystemic smooth muscle dysfunction syndrome is a condition where a developing baby does not make enough smooth muscle for the gastrointestinal system. Sadly, this condition cannot be survived.

Anti-smooth muscle antibodies (ASMA) can happen when the body’s immune system mistakenly attacks its own tissues. This is often linked to disorders like hepatitis, cirrhosis, or lupus.

Smooth muscle tumors are usually not cancerous. They are called leiomyomas. These tumors can form in many places in the body but are most often found in the uterus, small bowel, and esophagus. Some smooth muscle tumors are cancerous and are known as leiomyosarcomas. These are one of the more common types of soft-tissue sarcomas.