What is voluntary muscle?
Involuntary muscles are muscles that are not controllable consciously, and instead contract due to unconscious impulses sent by the autonomic nervous system or certain specialized cells or hormones. Both smooth muscle and cardiac muscle can be classified as involuntary muscles. Smooth muscle is comprised of spindle-shaped cells that have no striations and is found in numerous locations throughout the human body. Cardiac muscle is striated rather than smooth, and is found only within the walls of the heart. Smooth muscles are involuntary muscles composed of thick and thin protein filaments that are homologous to the organelles known as myofibrils found in skeletal muscles. The thin filaments are composed of a globular protein called actin, while the thick ones are made up of a motor protein called myosin. Smooth muscles require extracellular calcium ions to contract: the ions activate a nucleotide called Adenosine triphosphate (ATP), which then activates the myosin filaments. The myosin filaments attach to the actin filaments in a process known as the crossbridge cycle, which causes the thick and thin filaments to slide over each other and contract. When the myosin filaments release the actin filaments, the muscle relaxes.