A myocyte (also known as a muscle cell) 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. There are various specialized forms of myocytes: cardiac, skeletal, and smooth muscle cells, with various properties. The striated cells of cardiac and skeletal muscles are referred to as muscle fibers. Cardiomyocytes are the muscle fibres that form the chambers of the heart, and have a single central nucleus. Skeletal muscle fibers help support and move the body and tend to have peripheral nuclei. Smooth muscle cells control involuntary movements such as the peristalsis contractions in the oesophagus and stomach.
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:
|Muscle cell||Other organismal cells|
|sarcoplasmic reticulum||smooth endoplasmic reticulum (SER)|
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.
There are three types of myofilaments:
- 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.
The sarcolemma is the cell membrane of a striated 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. 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.
Structure of myocyte
The cell membrane of a myocyte has several specialized regions, which may include the intercalated disk and the transverse tubular system. 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.
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 negative prognostic impact.
Each muscle fiber 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.[verification needed] The myofibrils of smooth muscle cells are not arranged into sarcomeres. The sarcomeres are composed of thin and thick filaments. Thin filaments are made of actin and attach at Z lines which help them line up correctly with each other. Troponins are found at intervals along the thin filaments. Thick filaments are made of the elongated protein myosin. The sarcomere does not contain organelles or a nucleus. Sarcomeres 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.
Formation from myoblasts
A myoblast is a type of embryonic progenitor cell that differentiates to give rise to muscle cells. 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 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 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 (the connective tissue investment that divides the muscle fascicles into individual fibers). To re-activate myogenesis, the satellite cells must be stimulated to differentiate into new fibers.
Muscle fiber growth
Muscle fibers 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 divide to produce new cells, and as a result we have fewer muscle cells as an adult than a newborn.
Movement of myocyte
When contracting, thin and thick filaments slide with respect to each other by using adenosine triphosphate. This pulls the Z discs closer together in a process called 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 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 filament. This in turn causes the muscle cell to relax.
Kinds of contraction
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 summation) 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.
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