The muscular system consists of skeletal muscles, cardiac muscles, and smooth muscles. Individual muscles are the organs of the muscular system. Skeletal Muscles are divided and organized into fascicles. The orientation and organization of fascicles within a muscle determine the muscle's directionality and strength. Individual muscle cells are called muscle fibers. Muscle fibers contain specialized structures, including actin and myosin proteins, which are present in repeated units called sarcomeres. These proteins are responsible for the muscle's ability to generate force. When stimulated by a motor neuron, actin and myosin form cross-bridges that pull against one another to contract the muscle.
At A Glance
Skeletal muscles control movement, stability, heat production, and regulation of blood glucose levels, while cardiac muscles pump blood throughout the body, and smooth muscles are responsible for involuntary movements of internal organs.
- Connective tissues are used to subdivide and organize skeletal muscles.
- The orientation of the muscle fascicles, bundles of muscle fibers, determines the strength and directionality of a muscle contraction.
- Each muscle fiber is packed with myofibrils, which are long strands of proteins arranged concentrically and are responsible for generating force. Actin and myosin proteins give the striated appearance in skeletal muscle.
- When a muscle is stimulated, the contraction cycle occurs in three stages: excitation−contraction coupling, contraction, and relaxation.
Muscle contractions require the input of ATP to move the actin and myosin filaments across each other.
Muscle tension is the force output of a muscle.
Isotonic contractions occur when a muscle contracts while changing length, and isometric contractions occur when a muscle contracts but does not change length.
Muscles require ATP for the myosin head to release actin and begin a new cross-bridging cycle.
- There are three muscle fiber types (slow-twitch, fast-twitch, and intermediate) classified based on the type of activity they perform.
Muscles can become stronger and/or more resistant to fatigue with training.
- A motor unit is one motor neuron and all of its innervated muscle fibers.