The unusual
microscopic anatomy of a muscle cell gave rise to its terminology. The
cytoplasm in a muscle cell is termed the
sarcoplasm; the
smooth endoplasmic reticulum of a muscle cell is termed the
sarcoplasmic reticulum; and the
cell membrane in a muscle cell is termed the
sarcolemma. The sarcolemma receives and conducts stimuli.
Skeletal muscle cells Skeletal muscle cells are the individual contractile cells within a muscle and are more usually known as muscle fibers because of their longer, threadlike appearance. Broadly there are two types of muscle fiber performing in
muscle contraction, either as slow twitch (
type I) or fast twitch (
type II). A single muscle, such as the
biceps brachii in a young adult human male, contains around 253,000 muscle fibers. Skeletal muscle fibers are the only muscle cells that are
multinucleated with the
nuclei usually referred to as
myonuclei. This occurs during
myogenesis with the
fusion of
myoblasts, each contributing a nucleus to the newly formed muscle cell or
myotube. Fusion depends on muscle-specific proteins known as
fusogens called
myomaker and
myomerger. A striated muscle fiber contains
myofibrils consisting of long protein chains of
myofilaments. There are three types of myofilaments: thin, thick, and elastic, that work together to produce a
muscle contraction. The
sarcoplasmic reticulum, a specialized type of
smooth endoplasmic reticulum, forms a network around each myofibril of the muscle fiber. This network is composed of groupings of two dilated end-sacs called terminal cisternae, and a single
T-tubule (transverse tubule), which bores through the cell and emerge on the other side; together these three components form the
triads that exist within the network of the sarcoplasmic reticulum, in which each T-tubule has two terminal cisternae on each side of it. The sarcoplasmic reticulum serves as a reservoir for calcium ions, so when an action potential spreads over the T-tubule, it signals the sarcoplasmic reticulum to release calcium ions from the gated membrane channels to stimulate muscle contraction. In skeletal muscle, at the end of each muscle fiber, the outer layer of the sarcolemma combines with tendon fibers at the
myotendinous junction. Within the muscle fiber pressed against the sarcolemma are multiply flattened
nuclei; embryologically, this
multinucleate condition results from multiple myoblasts fusing to produce each muscle fiber, where each myoblast contributes one nucleus. Cardiac muscle, like skeletal muscle, is also striated, and the cells contain myofibrils, myofilaments, and sarcomeres as the skeletal muscle cell. The cell membrane is anchored to the cell's
cytoskeleton by anchor fibers that are approximately 10 nm wide. These are generally located at the Z lines so that they form grooves, and transverse tubules emanate. In cardiac myocytes, this forms a scalloped surface. Smooth muscle cells are spindle-shaped with wide middles and tapering ends. They have a single nucleus and range from 30 to 200
micrometers in length. This is thousands of times shorter than skeletal muscle fibers. The diameter of their cells is also much smaller, which removes the need for
T-tubules found in striated muscle cells. Although smooth muscle cells lack sarcomeres and myofibrils, they do contain large amounts of the contractile proteins actin and myosin. Actin filaments are anchored by
dense bodies (similar to the
Z discs in sarcomeres) to the sarcolemma. ==Development==