Smooth muscle

Smooth muscle is one of the three main types of muscle in vertebrates, along with skeletal and cardiac muscle. It is found not only in vertebrates but also in 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 because it lacks structures called sarcomeres.

Smooth muscle is located in the walls of many hollow organs, such as the stomach, intestines, bladder, and uterus. It is also present in the walls of blood vessels and lymph vessels, where it is known as vascular smooth muscle. Additionally, smooth muscle can 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, while 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 is different from skeletal and cardiac muscle because it lacks stripes, making it look smooth under a microscope.

Smooth muscle comes in two types: single-unit and multiunit. Single-unit smooth muscle works together as one unit, allowing the whole muscle to contract or relax at the same time. Multiunit smooth muscle contracts in a less coordinated way. Smooth muscle can tighten and loosen without direct nerve signals, helping organs like the intestines and bladder work properly.

Excitation-contraction coupling

Further information: Excitation-contraction coupling

Smooth muscle can tighten up or relax based on signals it receives. These signals can come from special cells in the body or from substances like hormones and nerve chemicals. Different parts of the body have smooth muscle that reacts in unique ways to these signals.

When smooth muscle tightens, it’s because tiny parts inside the muscle called myosin and actin slide past each other. This movement needs energy from a substance in cells called ATP. The tightening happens when the myosin parts attach to the actin and pull, creating tension that makes the muscle shrink. Smooth muscle tightens in two ways: quickly and for a short time, or slowly and for a longer time. The quick tightening is used in places like the digestive system, while the slow, steady tightening is found in places like blood vessels.

The tightening process starts when a substance called calcium enters the muscle cell. This calcium helps activate another substance called MLCK, which then triggers the myosin parts to start moving. Even after the calcium levels drop, the muscle can still stay tightened using special mechanisms that don’t need much energy. This helps muscles like those in blood vessels stay tightened for long periods without getting tired.

Invertebrate smooth muscle

Invertebrate smooth muscle works when calcium binds to a protein called myosin, causing the muscle to contract. This process uses cycling cross-bridges to generate force. Some animals, like clams, can stay closed for a long time using a special "catch phase" that uses very little energy. This catch phase involves proteins similar to those in vertebrate smooth muscle.

Specific effects

Smooth muscle in different parts of the body does slightly different jobs, even though it works in similar ways. In blood vessels, smooth muscle can tighten to make the vessels narrower, which helps control blood pressure and where blood flows. This slow 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, a process called peristaltic movement. Some smooth muscle in the kidneys helps release a substance called renin, which is part of the renin–angiotensin system that also helps control blood pressure.

Growth and rearrangement

Scientists are still learning how outside influences cause smooth muscle to grow and change its structure. Many growth factors and signals from the nervous system affect how smooth muscle grows and develops. A special pathway called the Notch receptor pathway is important for forming blood vessels like arteries and veins. This growth process can play a role in health problems such as atherosclerosis, and a substance called nitric oxide can stop this growth.

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

Related diseases

Multisystemic smooth muscle dysfunction syndrome is a genetic condition where a developing embryo does not create enough smooth muscle for the gastrointestinal system. This condition is not survivable.

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

Smooth muscle tumors are usually non-cancerous and are called leiomyomas. They 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.