Scheme showing the distribution of the major cytoskeletal components in a muscle fiber. Actin filaments (thin, purple) appear either in myofibrils or close to the sarcolemma, interacting with adhesion sites (pink). Intermediate filaments (blue) are distributed all over the cell, particularly around Z-lines, around the nuclei and mitochondria, and sometimes terminate in adhesion regions. Microtubules (green) are parallel to the major cell axis and have no conspicuous organizing center. Z-lines (red) alternate with M lines (yellow) and are aligned in separated myofibrils. Nuclei (blue) are aligned in the cell periphery, while mitochondria (orange) remain close to the myofibrils. Adhesion sites appear at the extremities of the myofibrils and periodically along the Z-line, forming costameres. Extracellular matrix molecules (black) are indicated vaguely, as they interact with several types of adhesion proteins. The nerve (checkered) contacts the muscle at a specialized locus, the neuromuscular junction.
