Actin filaments, also known as microfilaments, are the smallest protein fiber in the cytoskeleton, measuring about 7 nm in diameter. They are especially abundant just below the plasma membrane, where they form an extensive network that performs several crucial functions including: 

• Forming a dynamic cytoskeleton that provides cells with structural support 
• Determining cell shape
• Enabling and supporting cell motility, such as through the formation and function of Lamellipodia or Filopodia
• Linking the cell’s interior to its surroundings 
• Facilitating movement of the cell surface, allowing cells to migrate, engulf particles and divide 
• Supporting muscle contraction in muscle cells 

In addition, motor proteins use actin cables as pathways to transport intracellular organelles to the daughter cells during mitosis.