Muscle and Muscle Cell Anatomy

Anatomy of Muscles and Muscle Cells

The anatomy of muscles and muscle cells is crucial for understanding muscular function. The term "sark" means flesh, which often refers to the muscle tissue that is consumed, while the prefix "my" generally denotes muscle-related structures. Most skeletal muscles are anchored to bones via tendons, which are dense regular connective tissue connecting muscles to bones. It is essential to distinguish between tendons and ligaments; tendons attach muscles to bones, whereas ligaments connect one bone to another.

Within skeletal muscles, a cross-section reveals them divided into individual chambers known as fascicles. Each fascicle is a bundle of muscle cells, and each muscle cell is surrounded by a layer of connective tissue called the endomysium. The fascicle itself is enveloped by perimysium, and the entire muscle has a protective outer layer called epimysium, made of dense irregular connective tissue. This structure is resistant to forces and protects the muscle.

Skeletal muscle cells, or muscle fibers, differ significantly from typical cellular models. They are remarkably long, with some being over a foot in length, which led to their original classification as muscle fibers rather than individual cells. Each muscle cell comprises smaller bundles called myofibrils, which are essential for muscle contraction. Myofibrils contain two types of protein filaments: myosin (thick filament) and actin (thin filament).

The plasma membrane of muscle cells is termed the sarcolemma, while their cytoplasm is known as sarcoplasm. Muscle cells are multinucleate, featuring multiple nuclei within a single cell. Additionally, the smooth endoplasmic reticulum in muscle cells is specialized and termed the sarcoplasmic reticulum, which plays a vital role in muscle contraction by storing calcium ions necessary for the contraction process.

The sarcoplasmic reticulum comprises enlarged portions known as terminal cisternae on either side, connected by the transverse tubules, which are inward folds of the sarcolemma. Together, these structures form a triad, essential for effectively transmitting electrical signals into the muscle cell to trigger contraction. Muscle contraction itself generates force along the myofibrils, which have a repeating pattern of thin (actin) and thick (myosin) filaments. While actin and myosin are critical in muscle contraction, they also play roles in nearly every cell in the human body, contributing to cellular shape and movement, as observed in cytoskeleton dynamics and processes such as cell division. The focus on their role in muscle contraction will be addressed in the next segment of study.