<P> There are three types of muscles--cardiac, skeletal, and smooth . Smooth muscles are used to control the flow of substances within the lumens of hollow organs, and are not consciously controlled . Skeletal and cardiac muscles have striations that are visible under a microscope due to the components within their cells . Only skeletal and smooth muscles are part of the musculoskeletal system and only the skeletal muscles can move the body . Cardiac muscles are found in the heart and are used only to circulate blood; like the smooth muscles, these muscles are not under conscious control . Skeletal muscles are attached to bones and arranged in opposing groups around joints . Muscles are innervated, to communicate nervous energy to, by nerves, which conduct electrical currents from the central nervous system and cause the muscles to contract . </P> <P> In mammals, when a muscle contracts, a series of reactions occur . Muscle contraction is stimulated by the motor neuron sending a message to the muscles from the somatic nervous system . Depolarization of the motor neuron results in neurotransmitters being released from the nerve terminal . The space between the nerve terminal and the muscle cell is called the neuromuscular junction . These neurotransmitters diffuse across the synapse and bind to specific receptor sites on the cell membrane of the muscle fiber . When enough receptors are stimulated, an action potential is generated and the permeability of the sarcolemma is altered . This process is known as initiation . </P> <P> A tendon is a tough, flexible band of fibrous connective tissue that connects muscles to bones . The extra-cellular connective tissue between muscle fibers binds to tendons at the distal and proximal ends, and the tendon binds to the periosteum of individual bones at the muscle's origin and insertion . As muscles contract, tendons transmit the forces to the relatively rigid bones, pulling on them and causing movement . Tendons can stretch substantially, allowing them to function as springs during locomotion, thereby saving energy . </P> <P> Joints are structures that connect individual bones and may allow bones to move against each other to cause movement . There are two divisions of joints, diarthroses which allow extensive mobility between two or more articular heads, and false joints or synarthroses, joints that are immovable, that allow little or no movement and are predominantly fibrous . Synovial joints, joints that are not directly joined, are lubricated by a solution called synovial fluid that is produced by the synovial membranes . This fluid lowers the friction between the articular surfaces and is kept within an articular capsule, binding the joint with its taut tissue . </P>

What is the main organs in the muscular system