| Sarcomere | |
|---|---|
 Image of sarcomere | |
| Details | |
| Part of | Striated muscle |
| Identifiers | |
| Latin | sarcomerum |
| MeSH | D012518 |
| TH | H2.00.05.0.00008 |
| FMA | 67895 |
| Anatomical terms of microanatomy | |
Asarcomere (Greek σάρξsarx "flesh", μέροςmeros "part") is the smallest functional unit ofstriated muscle tissue.[1] It is the repeating unit between two Z-lines.[further explanation needed]Skeletal muscles are composed of tubular muscle cells (called muscle fibers or myofibers) which are formed duringembryonicmyogenesis. Muscle fibers contain numerous tubularmyofibrils. Myofibrils are composed of repeating sections of sarcomeres, which appear under the microscope as alternating dark and light bands. Sarcomeres are composed of long, fibrous proteins as filaments that slide past each other when a muscle contracts or relaxes. Thecostamere is a different component that connects the sarcomere to thesarcolemma.
Two of the important proteins aremyosin, which forms the thick filament, andactin, which forms the thin filament. Myosin has a long fibrous tail and a globular head that binds to actin. Themyosin head also binds toATP, which is the source of energy for muscle movement. Myosin can only bind to actin when the binding sites on actin are exposed by calcium ions.
Actin molecules are bound to the Z-line, which forms the borders of the sarcomere. Other bands appear when the sarcomere is relaxed.[2]
The myofibrils ofsmooth muscle cells are not arranged into sarcomeres.
The sarcomeres giveskeletal andcardiac muscle theirstriated appearance,[2] which was first described byVan Leeuwenhoek.[3]
The relationship between the proteins and the regions of the sarcomere are as follows:
The proteintropomyosin covers the myosin-binding sites of the actin molecules in the muscle cell. For a muscle cell to contract, tropomyosin must be moved to uncover the binding sites on the actin. Calcium ions bind withtroponin C molecules (which are dispersed throughout the tropomyosin protein) and alter the structure of the tropomyosin, forcing it to reveal the cross-bridge binding site on the actin.
The concentration of calcium within muscle cells is controlled by thesarcoplasmic reticulum, a unique form ofendoplasmic reticulum in thesarcoplasm.
Muscle cells are stimulated when amotor neuron releases the neurotransmitteracetylcholine, which travels across theneuromuscular junction (the synapse between the terminal button of the neuron and the muscle cell).Acetylcholine binds to a post-synapticnicotinic acetylcholine receptor. A change in the receptor conformation allows an influx ofsodium ions and initiation of a post-synapticaction potential. The action potential then travels alongT-tubules (transverse tubules) until it reaches the sarcoplasmic reticulum.
Here, the depolarized membrane activates voltage-gatedL-type calcium channels, present in the plasma membrane. The L-type calcium channels are in close association withryanodine receptors present on the sarcoplasmic reticulum. The inward flow of calcium from the L-type calcium channels activates ryanodine receptors to release calcium ions from the sarcoplasmic reticulum. This mechanism is calledcalcium-induced calcium release (CICR). It is not understood whether the physical opening of the L-type calcium channels or the presence of calcium causes the ryanodine receptors to open. The outflow of calcium allows the myosin heads access to the actin cross-bridge binding sites, permitting muscle contraction.[5]
Muscle contraction ends when calcium ions are pumped back into the sarcoplasmic reticulum, allowing the contractile apparatus and, thus, muscle cell to relax.
Upon muscle contraction, the A-bands do not change their length (1.85 micrometer in mammalian skeletal muscle),[5] whereas the I-bands and the H-zone shorten. This causes the Z-lines to come closer together.
At rest, themyosin head is bound to anATP molecule in a low-energy configuration and is unable to access the cross-bridge binding sites on the actin. However, the myosin head can hydrolyze ATP intoadenosine diphosphate (ADP) and an inorganic phosphate ion. A portion of the energy released in this reaction changes the shape of the myosin head and promotes it to a high-energy configuration. Through the process of binding to the actin, the myosin head releases ADP and an inorganic phosphate ion, changing its configuration back to one of low energy. The myosin remains attached to actin in a state known asrigor, until a new ATP binds the myosin head. This binding of ATP to myosin releases the actin by cross-bridge dissociation. The ATP-associated myosin is ready for another cycle, beginning with hydrolysis of the ATP.
The A-band is visible as dark transverse lines across myofibers; the I-band is visible as lightly staining transverse lines, and the Z-line is visible as dark lines separating sarcomeres at the light-microscope level.
The structure of the sarcomere affects its function in several ways. The overlap of actin and myosin gives rise to thelength-tension curve, which shows how sarcomereforce output decreases if the muscle is stretched so that fewer cross-bridges can form or compressed until actin filaments interfere with each other. Length of the actin and myosin filaments (taken together as sarcomere length) affects force and velocity – longer sarcomeres have more cross-bridges and thus more force, but have a reduced range of shortening. Vertebrates display a very limited range of sarcomere lengths, with roughly the same optimal length (length at peak length-tension) in all muscles of an individual as well as between species.Arthropods, however, show tremendous variation (over seven-fold) in sarcomere length, both between species and between muscles in a single individual. The reasons for the lack of substantial sarcomere variability in vertebrates is not fully known.[citation needed]
