| A | B |
|---|
| When a depolarization signal arrives, | Ca2+ exits the SR via ryanodine-sensitive Ca2+ channel |
| Creatine kinase is an enzyme found in | sarcoplasm and as component of M-line of H band |
| Creatine kinase | catalyzes transfer of phosphate from creatine phosphate to ADP |
| Ca2+ binds troponin C and.. | causes conformational change in troponin-tropomyosin complex |
| When myelinated axons reach the perimysium, they lose their myelin sheath but... | the presynaptic buttons remain covered with Schwann cell processes |
| Synaptic buttons occupy a depression of the muscle fiber, called the | primary synaptic cleft |
| Acetylcholine receptors are located at the | crests of the deep junctional folds (secondary synaptic clefts) |
| voltage-gated Na+ channels are located | down into the secondary synaptic clefts |
| The basal lamina of the synaptic cleft contains... | acetylcholinesterase, which inactivates acetylcholine |
| The basal lamina covering the Schwann cell becomes | continuous with the basal lamina of the muscle fiber |
| T tubules.. | form rings around every sarcomere of every myofibril at the A-I junction |
| An excitation-contraction signal is generated by | acetylcholine |
| that the sarcoplasm of a skeletal muscle cell is packed with | myofibrils (each consisting of a linear repeat of sarcomeres) with abundant mitochondria between |
| The length of the thick and thin filaments does not change during muscle contract | the length of the A band and the distance between the Z disk and the adjacent H band are constant |
| The length of the sarcomere decreases because thick and thin filaments slide past each other | (the size of the H band and I band decrease) |
| Tropomyosin | two nearly identical à-helical polypeptides twisted around each other |
| Tropomyosin | runs in the groove formed by F-actin strands |
| Troponin | a complex of three proteins |
| Troponin I | inhibits the binding of myosin to actin |
| Troponin C | binds Ca2+ and is found only in striated muscle |
| Troponin T | binds the complex to tropomyosin |
| Myosin | the major component of the thick filament |
| Myosin | binds to F-actin-the major component of the thin filament-in a reversible fashion |
| Myosin | has adenosine triphosphatase (ATPase) activity (it hydrolyzes ATP) |
| Nebulin | is associated with thin (actin) filaments; it inserts into the Z disk |
| Titin | associates with thick (myosin) myofilaments and inserts into the Z disk, extending to the bare zone of the myosin filaments, close to the M line |
| Nebulin | acts as a template for determining the length of actin filaments |
| Titin | controls the assembly of the myosin myofilament by acting as a template |
| Titin | has a role in sarcomere elasticity by forming a spring-like connection between the end of the thick myofilament and the Z disk |
| Z disks | are the insertion site of actin filaments of the sarcomere |
| à-actinin | anchors the barbed end of actin filaments to the Z disk |
| Desmin | forms intermediate filaments that extend from the Z disk of one myofibril to the adjacent myofibril, forming a supportive latticework |
| Desmin filaments also extend from | the sarcolemma to the nuclear envelope |
| costameres | specialized sarcolemma-associated plaques |
| Costameres act in concert with the dystrophin-associated protein complex to | transduce contractile force from the Z disk to the basal lamina |
| plectin filaments | Link Desmin filaments to the Z disk and to each other |
| Desmin, plectin, and àB-crystallin | form a mechanical stress protective network at the Z-disk level |
| The heat shock protein àB-crystallin | protects Desmin filaments from stress-induced damage |
| The barbed end of actin filaments | inserts into the Z disk |
| G-actin monomers bind to each other in a head-to-tail fashion, thus... | giving the filament polarity, with barbed (plus) and pointed (minus) ends |
| F-actin is composed of | globular monomers (G-actin) |
| The Z disk forms a transverse sarcomeric scaffold that serves to ... | ensure the efficient transmission of the generated force. |
| Thin myofilaments | measure 7 nm in width and 1 ?m in length |
| Thin myofilaments | form the I band which is light |
| Thick filaments | measure 15 nm in width and 1.5 ?m in length |
| Thick filaments | found in the A band which is dark |
| The A band is bisected by | a light region called the H band |
| The major component of the H band | creatine kinase, which catalyzes the formation of ATP from creatine phosphate and adenosine diphosphate (ADP) |
| creatine phosphate | maintains steady levels of ATP during prolonged muscle contraction |
| Runs through the midline of the H band at the middle of each A band | the M line |
| M-line striations correspond to | The alignment of the lateral assembled tails of myosin |
| Thin filaments insert into | each side of the Z disk, whose components include à-actinin |
| Skeletal muscle cells are formed in the embryo by the fusion of myoblasts that produce | a postmitotic, multinucleated myotube |
| The site of contact of the T tubule with the sarcoplasmic reticulum cisternae | triad |
| Myofibril | chain of sarcomeres |
| The tendon anchors into a bone through | periosteal Sharpey's fibers |
| epimysium | a dense connective tissue layer ensheathing the entire muscle |
| perimysium | derives from the epimysium and surrounds bundles or fascicles of muscle cells |
| endomysium | a delicate layer of reticular fibers and extracellular matrix surrounding each muscle cell |
| Blood vessels and nerves use these connective tissue sheaths to | reach the interior of the muscle |
| at each end of a muscle the myotendinous junction is formed by the | Interdigitatation of radiating-muscle fascicles with regular dense connective tissue of the tendon |
| the terminal differentiation of the muscle cell precursor- | triggered by myogenin and MRF4. |
| attach to the surface of the myotubes before a basal lamina surrounds the satellite cell and myotube | Satellite cells |
| MyoD expression | induces the proliferation of satellite cells |
| Satellite cells | muscle maintenance, repair, and regeneration in the adult |
| Satellite cells | are mitotically quiescent in the adult, but can reassume self-renewal and proliferation in response to stress or trauma |
| receptor expressed on the surface of quiescent satellite cells encoded by a proto-oncogene | c-Met receptor |
| myogenic precursor cells | descendants of the activated satellite cells which undergo multiple rounds of cell division before they can fuse with existing or new myofibers |
| HGF-c-Met complex | upregulates a signaling cascade leading to proliferation of the satellite cells and the expression of Myf5 and MyoD |
| F-actin | the thin filament of the sarcomere, is double-stranded and twisted |
