Overall structure of a muscle cell and neuromuscular junction:
1. Axon
2. Neuromuscular junction
3. Muscle fiber (Myocyte)
4. Myofibril

A myocyte (also known as a muscle cell or muscle fiber)^[1] is the type of cell found in muscle tissue. Myocytes are long, tubular cells that develop from myoblasts to form muscles in a process known as myogenesis.^[2] There are various specialized forms of myocytes: cardiac, skeletal, and smooth muscle cells, with various properties. Cardiac myocytes are responsible for generating the electrical impulses that control the heart rate, and are centronucleated. Skeletal muscle fibers help support and move the body and tend to have peripheral nuclei. Smooth muscle fibers control involuntary movements such as the peristalsis contractions in the stomach.^[3][4]

Terminology[edit]

The unusual microstructure of muscle cells has led cell biologists to create specialized terminology. However, each term specific to muscle cells has a counterpart that is used in the terminology applied to other types of cells:

Every muscle is surrounded by fascia. Fascia is composed of three layers of connective tissue, the epimysium, perimysium, and endomysium. Each muscle fiber is surrounded by a layer of connective tissue called the endomysium.^[5]

The sarcoplasm is the cytoplasm of a muscle fiber. Most of the sarcoplasm is filled with myofibrils, which are long protein cords composed of myofilaments. The sarcoplasm is also composed of glycogen, a polysaccharide of glucose monomers, which provides energy to the cell with heightened exercise, and myoglobin, the red pigment that stores oxygen until needed for muscular activity.^[5]

There are three types of myofilaments:^[5]

• Thick filaments, composed of protein molecules called myosin. In striations of muscle bands, these are the dark filaments that make up the A band.
• Thin filaments are composed of protein molecules called actin. In striations of muscle bands, these are the light filaments that make up the I band.
• Elastic filaments are composed of titin, a large springy protein; these filaments anchor the thick filaments to the Z disc.

Together, these myofilaments work 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 transverse tubule, or 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 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 a muscle contraction.^[5][6]

The sarcolemma is the plasma membrane of a muscle fiber and is designed to receive and conduct stimuli. At the end of each muscle fiber, the outer layer of the sarcolemma combines with tendon fibers.^[7] Within the muscle fiber pressed against the sarcolemma are multiple flattened nuclei; this multinuclear condition results from multiple myoblasts fusing to produce each muscle fiber, where each myoblast contributes one nucleus.^[5]

Structure of myocyte[edit]

The plasma membrane, plasmalemma, of myocyte has several specialized regions, which may include the intercalated disk and the transverse tubular system. The plasma 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.^[8]

The plasma 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.^[8]

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 negative prognostic impact.^[8]

Movement of myocyte[edit]

When contracting thin and thick filaments slide with respect to each other by using adenosine triphosphate. This pulls the Z disc closers together in a process called sliding filament mechanism. The contraction of all the sarcomeres results in the whole muscle fiber. This contraction of the myocyte is triggered by the action potential over the plasma membrane of the myocyte. The action potential uses transverse tubules to get from the surface to the interior of the myocyte, which are continuous within the plasma membrane. Sacroplasmic reticulum are membranous bags that which transverse tubules touches, though remains separate from. These wrap themselves around the each sacromere and are filled with Ca2+.^[9]

Excitation of myocyte causes depolarization at its synapses, the neuromuscular junctions, which triggers 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 it the release the neurotransmitter acetylcholine.^[10]

When the acetylcholine is released it diffuses across the synapse and binds to a receptor on sarcolemma, a unique term to muscle cells that refers to the plasma membrane. This initiates an impulse which travels across the sacrolemma.^[11]

When the action potential reaches the sacroplasmic reticulum it triggers the release of Ca2+ from the Ca2+ channels. The Ca2+ flows from the sacroplasmic reticulum into the sacromere with both of its filaments. This causes the filaments to start sliding and sacromeres 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 sacroplasmic reticulum which blocks the interaction between the thin and thick filament. This in turn cause the muscle cell to relax.^[11]

Kinds of myocyte contraction[edit]

There are four main different types of muscle contraction; twitch, treppe, tetanus and isometric/isotonic. Twitch contraction is the process previously described, in which a single stimulus signals for a single contraction. In twitch contraction the length of the contraction may vary depending on the size of the muscle cell. During treppe (or summattion) muscles do not start at maximum efficiency, instead they achieve increased strength of contraction due to repeated stimuli. Tetanus involves a sustained contraction of muscles due to a series of rapid stimuli, which can continue until the muscles fatigue. Isometric are skeletal muscle contractions that do not cause movement of the muscle. However isotonic are skeletal muscles contractions that do cause movement.^[11]

Myocyte growth[edit]

Myocyte grow when exercised and shrink when not in use. This is due to the fact that exercise stimulates the increase in myofibrils which increase the overall size of muscle cells. Well exercised muscles can not only add more size but can also develop more mitochondria, myoglobin, glycogen and a higher density of capillaries. However muscle cells cannot not divide to produce new cells, and as a result we have fewer muscle cells as an adult than a newborn.^[11]

Myofibrils[edit]

Portion of a myofibril, showing sarcomere structure:
S = Sarcomere, unit of muscle function
A = A-band, region of myosin
I = I-band, region of actin
H = H-zone, region of just myosin
Z = Z-line, sarcomere boundary
M = M-line, sarcomere center

Each myocyte contains myofibrils, which are very long chains of sarcomeres, the contractile units of the cell. A cell from the biceps brachii muscle may contain 100,000 sarcomeres.^{[12][verification needed]} The myofibrils of smooth muscle cells are not arranged into sarcomeres. The sarcomeres are composed of thin and thick filaments. Thin filaments is made of the protein actin and attach at Z lines which help them line up correctly with each other.^[13] Troponins are found at intervals along the thin filaments. Thick filaments are made of the elongated protein myosin.^[9] The sarcomere does not contain organelles or a nucleus. Sacromere are marked by Z lines which show the beginning and the end of a sacromere. Individual myocytes are surrounded by endomysium.

Myocytes are bound together by perimysium into bundles called fascicles; the bundles are then grouped together to form muscle tissue, which is enclosed in a sheath of epimysium. The perimysium contains blood vessels and nerves which provide for the muscle fibers. Muscle spindles are distributed throughout the muscles and provide sensory feedback information to the central nervous system. Myosin is a shaped like long shaft with a rounded end pointed out towards the surface. This structure forms the cross bridge that connects with the thin filaments.^[9]

Functional control[edit]

Kindlin-2 plays a role in elongation.^[14]

GATA4 and GATA6 play a role in differentiation.^[15]

Myoblasts[edit]

A myoblast is a type of embryonic progenitor cell that differentiates to give rise to muscle cells.^[16] Differentiation is regulated by myogenic regulatory factors, including MyoD, Myf5, myogenin, and MRF4.^[17]

Skeletal muscle fibers are made when myoblasts fuse together; muscle fibers therefore have multiple nuclei (each nucleus originating from a single myoblast). The fusion of myoblasts is specific to skeletal muscle (e.g., biceps brachii) and not cardiac muscle or smooth muscle.

Myoblasts that do not form muscle fibers dedifferentiate back into satellite cells. These satellite cells remain adjacent to a muscle fiber, situated between the sarcolemma and the endomysium (the connective tissue investment that divides the muscle fascicles into individual fibers). To re-activate myogenesis, the satellite cells must be stimulated so they differentiate into new fibers.

See also[edit]

• Sinoatrial node

References[edit]

External links[edit]

• Structure of a Muscle Cell