This document describes the structure and function of muscle cells and fibers. It discusses three layers of wrappings (endomysium, perimysium, and epimysium) that surround muscle fibers. It also details the structures within muscle cells, including nuclei, mitochondria, sarcoplasmic reticulum, myofibrils, and sarcomeres. Finally, it explains how muscles contract through motor neuron stimulation of muscle fibers and the role of the neuromuscular junction in transmitting nerve impulses to cause contraction.