Muscle cell

A muscle cell, also known as a myocyte, is a mature contractile cell in the muscle of an animal. In humans and other vertebrates there are three types: skeletal, smooth, and cardiac. A skeletal muscle cell is long and threadlike with many nuclei and is called a muscle fiber. Muscle cells develop from embryonic precursor cells called myoblasts.
Skeletal muscle cells form by fusion of myoblasts to produce multinucleated cells in a process known as myogenesis. Skeletal muscle cells and cardiac muscle cells both contain myofibrils and sarcomeres and form a striated muscle tissue.
Cardiac muscle cells form the cardiac muscle in the walls of the heart chambers, and have a single central nucleus. Cardiac muscle cells are joined to neighboring cells by intercalated discs, and when joined in a visible unit they are described as a cardiac muscle fiber.
Smooth muscle cells control involuntary movements such as the peristalsis contractions in the esophagus and stomach. Smooth muscle has no myofibrils or sarcomeres and is therefore non-striated. Smooth muscle cells have a single nucleus.

Structure

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 or fast twitch.
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 thin myofilaments are filaments of mostly actin and the thick filaments are of mostly myosin, and they slide over each other to shorten the fiber length in a muscle contraction. The third type of myofilament is an elastic filament composed of titin, a very large protein.
In striations of muscle bands, myosin forms the dark filaments that make up the A band. Thin filaments of actin are the light filaments that make up the I band. The smallest contractile unit in the fiber is called the sarcomere, which is a repeating unit within two Z bands. The sarcoplasm also contains glycogen which provides energy to the cell during heightened exercise, and myoglobin, the red pigment that stores oxygen until needed for muscular activity.
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, 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 cells

The cell membrane of a cardiac muscle cell has several specialized regions, which may include the intercalated disc, and transverse tubules. The cell membrane is covered by a lamina coat which is approximately 50 nm wide. The laminar coat is separable into two layers; the lamina densa and lamina lucida. In between these two layers can be several different types of ions, including calcium.
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.
The cytoskeleton is what the rest of the cell builds off of and has two primary purposes: the first is to stabilize the topography of the intracellular components, and the second is to help control the size and shape of the cell. While the first function is important for biochemical processes, the latter is crucial in defining the surface-to-volume ratio of the cell. This heavily influences the potential electrical properties of excitable cells. Additionally, deviation from the standard shape and size of the cell can have a negative prognostic impact.

Smooth muscle cells

are so-called because they have neither myofibrils nor sarcomeres and therefore no striations. They are found in the walls of hollow organs, including the stomach, intestines, bladder and uterus, in the walls of blood vessels, and in the tracts of the respiratory, urinary, and reproductive systems. In the eyes, the ciliary muscles dilate and contract the iris and alter the shape of the lens. In the skin, smooth muscle cells such as those of the arrector pili cause hair to stand erect in response to cold temperature or fear.
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 to the sarcolemma.

Development

A myoblast is an embryonic precursor cell that differentiates to give rise to the different muscle cell types. Differentiation is regulated by myogenic regulatory factors, including MyoD, Myf5, myogenin, and MRF4. GATA4 and GATA6 also play a role in myocyte differentiation.
Skeletal muscle fibers are made when myoblasts fuse together; muscle fibers therefore are cells with multiple nuclei, known as myonuclei, with each cell nucleus originating from a single myoblast. The fusion of myoblasts is specific to skeletal muscle, and not cardiac muscle or smooth muscle.
Myoblasts in skeletal muscle that do not form muscle fibers dedifferentiate back into myosatellite cells. These satellite cells remain adjacent to a skeletal muscle fiber, situated between the sarcolemma and the basement membrane of the endomysium. To re-activate myogenesis, the satellite cells must be stimulated to differentiate into new fibers.
Myoblasts and their derivatives, including satellite cells, can now be generated in vitro through directed differentiation of pluripotent stem cells.
Kindlin-2 plays a role in developmental elongation during myogenesis.

Function

Muscle contraction in striated muscle

Skeletal muscle contraction

When contracting, thin and thick filaments slide past each other by using adenosine triphosphate. This pulls the Z discs closer together in a process called the sliding filament mechanism. The contraction of all the sarcomeres results in the contraction of the whole muscle fiber. This contraction of the myocyte is triggered by the action potential over the cell membrane of the myocyte. The action potential uses transverse tubules to get from the surface to the interior of the myocyte, which is continuous within the cell membrane.
Sarcoplasmic reticula are membranous bags that transverse tubules touch but remain separate from. These wrap themselves around each sarcomere and are filled with Ca²⁺.
Excitation of a myocyte causes depolarization at its synapses, the neuromuscular junctions, which triggers an action potential. With a singular neuromuscular junction, each muscle fiber receives input from just one somatic efferent neuron. Action potential in a somatic efferent neuron causes the release of the neurotransmitter acetylcholine.
When the acetylcholine is released, it diffuses across the synapse and binds to a receptor on the sarcolemma, a term unique to muscle cells that refers to the cell membrane. This initiates an impulse that travels across the sarcolemma.
When the action potential reaches the sarcoplasmic reticulum, it triggers the release of Ca²⁺ from the Ca²⁺ channels. The Ca²⁺ flows from the sarcoplasmic reticulum into the sarcomere with both of its filaments. This causes the filaments to start sliding and the sarcomeres to become shorter. This requires a large amount of ATP, as it is used in both the attachment and release of every myosin head. Very quickly, Ca²⁺ is actively transported back into the sarcoplasmic reticulum, which blocks the interaction between the thin and thick filaments. This, in turn, causes the muscle cell to relax.
There are four main types of muscle contraction: isometric, isotonic, eccentric, and concentric. Isometric contractions are skeletal muscle contractions that do not cause movement of the muscle, and isotonic contractions are skeletal muscle contractions that do cause movement. Eccentric contraction is when a muscle moves under a load. Concentric contraction is when a muscle shortens and generates force.