Smooth muscle

Some muscles work all by themselves.

They are inside your body. They help your tummy work. They also help your eyes see.

These muscles can stretch a lot. This helps your organs hold more stuff. They are in your blood tubes, too.

These muscles can move in a group. They work together like one big muscle. This helps things move through your body.

Some muscles can even make hair stand up. This happens when you are cold or scared.

Your body uses these muscles every day. They work even when you are asleep.

Smooth muscle is a special kind of tissue. It is found in many animals. You can find it in your stomach and your bladder. It is also in your blood vessels.

These muscles are different from your heart or arm muscles. They do not have stripes. Because of this, scientists call them non-striated. They are very good at stretching. This helps organs like your intestines hold more food.

There are two main types. One type is called single-unit muscle. These cells work together as one big group. They can even contract on their own. This is called myogenic. This means they do not always need a signal from the brain. The other type is multiunit muscle. These work more like separate parts.

Inside the cells, tiny parts work to make movement. There are proteins called actin and myosin. These two proteins work together to make the muscle pull.

Smooth muscle also helps your eyes. It changes the shape of your lens to help you see. It also helps hair stand up when you are cold. This happens because of small muscles in your skin.

Smooth muscle is one of three major types of muscle in animals with backbones. The other types are skeletal muscle and cardiac muscle. You can find smooth muscle in many places, like the walls of your stomach and bladder. It is also found in your intestines and uterus. This muscle type is called non-striated. This is because it does not have stripes or bands.

This muscle works in two different ways. Most smooth muscle is called single-unit muscle. In these groups, the cells work together as one big team. They use gap junctions to send signals to their neighbors. This allows the whole bundle to contract at once. This is how the uterus works during childbirth. Some of these cells act as pacemakers. They can create their own electrical signals to start a movement. This is called being myogenic.

There is also a second type called multiunit smooth muscle. This type is found in your trachea and your eyes. It is also in the large elastic arteries. Unlike single-unit muscle, these cells are neurogenic. This means they need a signal from the nervous system to move. They do not usually contract on their own. Some scientists say these two types are actually more linked than they seem. Most smooth muscle is influenced by many different signals at once.

Inside the cells, tiny proteins do the hard work of moving. These proteins are called actin and myosin. The myosin has two heavy chains that look like two snakes wrapped together. It also has light chains that help it stay stable. The actin proteins help the muscle pull. In smooth muscle, the ratio of actin to myosin is between 2:1 and 10:1.

These proteins are attached to small anchors called dense bodies. These anchors help the cell pull against its own structure.

Smooth muscle is very good at stretching. This is important for organs like the bladder that must hold much liquid. The cells can stretch and still pull with strength. This is part of a special ability called elasticity. You can see this muscle working in your eyes every day. The ciliary muscles change the shape of your lens to help you focus. Other muscles in your iris change your pupil size. Even your skin uses it to make hair stand up when you are cold.

Smooth muscle is a fundamental type of muscle tissue found in vertebrates and many invertebrates. It is one of three major muscle categories, alongside skeletal and cardiac muscle. Unlike the other two, smooth muscle is non-striated, meaning it lacks the visible bands or stripes called striations. This unique structure allows it to perform vital roles in many internal systems. It is controlled by the autonomic nervous system, which manages involuntary body functions. Because of its special properties, it is essential for the movement of many internal organs.

Smooth muscle is categorized into two primary subgroups: single-unit and multiunit. Most smooth muscle is single-unit, also known as visceral smooth muscle. These cells are often myogenic, meaning they can contract without needing a signal from a nerve. Within these bundles, cells are connected by gap junctions. These junctions allow electrical signals to pass quickly between neighbors. This creates a syncytium, where the entire sheet of cells contracts as one coordinated unit. This mechanism is critical for processes like uterine contractions during childbirth.

Multiunit smooth muscle functions differently and is considered neurogenic. This means the contraction must be started by an autonomic nervous system neuron. You can find this type of muscle in the trachea and the iris of the eye. It also lines the large elastic arteries. While scientists use these two categories, the distinction is sometimes an oversimplification. Most smooth muscle is actually influenced by a complex mix of neural elements and local chemical signals. These various factors help coordinate the muscle's response even in multiunit types.

At the microscopic level, a smooth muscle cell is a spindle-shaped myocyte. It has a wide middle and ends that taper to a point. Each cell contains a single nucleus and is quite small, measuring between 30 and 200 micrometers in length. While they lack myofibrils, the cytoplasm is filled with contractile proteins. The two most important proteins are actin and myosin. In smooth muscle, the ratio of actin to myosin is quite high, ranging from 2:1 to 10:1. This is very different from skeletal muscle, where myosin is much more dominant.

Myosin II is the primary motor protein in these cells. Each myosin molecule has two heavy chains that form a head and a tail. The tails are coiled together like two snakes wrapped around each other. Myosin also includes light chains that provide structural stability. The regulatory light chain, called MLC20, actively participates in the contraction process. Actin filaments, mostly composed of alpha-actin, work with myosin to create movement. These filaments are anchored to structures called dense bodies. These dense bodies act like anchors, allowing the cell to exert force against its own internal framework.

Smooth muscle is highly valued for its incredible elasticity. This allows the tissue to stretch significantly and still maintain its ability to contract. This quality is vital for hollow organs like the intestines and the urinary bladder. These organs must expand to hold contents but still need to push them along. To support this, cells secrete an extracellular matrix made of collagen and elastin. These fibers interact with specific receptors on the smooth muscle cells. This relationship helps the tissue manage the tension required for its various tasks.

We see smooth muscle working in many specialized ways throughout the body. In the eyes, ciliary muscles change the lens shape to help us focus through accommodation. The iris contains muscles that dilate or constrict the pupil to control light. In the skin, the arrector pili muscles cause hair to stand erect during cold temperatures or fear. Smooth muscle is also found in the walls of blood vessels, known as vascular smooth muscle. It is present in the respiratory, urinary, and reproductive tracts. This widespread presence makes it a cornerstone of vertebrate physiology.