Microfilaments are found inside cells and are tiny strands made of a protein called actin. They are sometimes referred to as actin filaments, or thin filaments, and they help support the structure of the cell, or cytoskeleton. In addition, microfilaments are involved in changing the shape of the cell, cell movement and division. Inside muscle, microfilaments are arranged together with thick filaments made of myosin protein, and together they enable muscle contraction to occur.
Eukaryotic cells, which are found in animals and plants, generally contain microfilaments. They are contained within the substance that fills the cell, known as cytoplasm. Each microfilament is made up of many tiny molecules of a protein called actin, which is manufactured inside the cell. Individual actin molecules are rounded and they are stranded together into long chains. Two long chains twist around each other to form an elongated spiral known as a helix, and this creates one microfilament with a diameter of around five nanometers.
In the cell, microfilaments are usually found gathered around the periphery, just beneath the outer surface. Here they can regulate the shape of the cell, responding to changes in the surrounding environment. The thin filaments play a part in forming tiny projections from the cell surface, known as microvilli. They can also form a larger protrusion, enabling a cell to move in an amoeba-like way across a surface. Microfilaments are also involved in extending the surfaces of some immune cells to engulf unwanted substances.
Inside muscle, actin filaments combine with myosin filaments in a way that gives muscles their strength and ability to contract. The myosin filaments are bundled together to form what are called thick filaments, with a diameter of about 15 nanometers. Stacks of thick filaments and stacks of thin filaments are arranged alternately along the length of a muscle fiber, with their ends slightly overlapping one another. During muscle contraction, links between the thin and thick filaments are made and broken, causing the filaments to slide past one another in a ratchet-like movement.
As the eukaryotic cells in animals rely on microfilaments to function normally, some fungi and plants produce poisons to target them, as protection against being eaten. Many of the toxins in such poisons work by binding to actin molecules, interfering with the production and behavior of microfilaments. One example is the phalloidin poison, found in death cap mushrooms, and it has been suggested that eating large amounts of raw meat can help counteract the effect of this toxin, by providing lots of extra actin for the phalloidin to bind to.