Components of muscle . A muscle fiber consists of many myofibrils , which are tiny thread-like structures. A single muscle fiber is wrapped in a thin connective tissue layer called the endomysium . Many muscle fibers are bundled together by another layer of connective tissue called the perimysium . That forms a fascicle .
Most skeletal muscles consist of several fascicles.
A group of fascicles is wrapped together by the epimysium to form a muscle. Skeletal muscles receive impulses from motor neurons, which are branches of large nerves that lie outside the spinal cord and are part of the peripheral nervous system. Inside each fiber is a network of channels called the sarcoplasmic reticulum . That network contains high concentrations of positively charged calcium ions . Depolarization releases calcium into the sarcomere and triggers the myosin and actin to respond to one another by sliding together in contraction.