PMC

A growing list of Z-band proteins in the mature myofibrils in skeletal muscle cells. The diagram is arranged to indicate the central role of alpha-actinin in the Z-bands that directly and indirectly binds to the major elements of the muscle cell, i.e., thin and thick filaments, intermediate filaments, costameres, and membranous structures like the sarcolemma, transverse tubules and the sarcoplasmic reticulum.

Electron micrographs of the intercalated disc and neural innervation of cardiac muscle cells in N. clavata. a: Intercalated disc (cd) is formed by deep penetration of the sarcolemma. Coupling of transverse tubules (T) and terminal cisternae of sarcoplasmic reticulum (sr) are observed. b: Intercalated discs are composed of dense layer of sarcolemma, two plasma membranes (arrowheads), and a regular interspace (asterisk). c: Gap junctions (ga), fascia adherens (fa), and macula adherens (arrows) junctions are seen. d, e. Cardiac muscle cell (mc) is innervated by a branch of external nerves (ne). A bundle of axons (ax) extend across the heart and penetrate the myocardium. Scale bars indicate 0.25 μm (a-c), 5 μm (d), and 2.5 μm (e), respectively

The mechanism of WHC variation in skeletal muscle contraction and relaxation. When the muscles receive external stimulation, the influx of external Ca²⁺ via channels induces the depolarization of the sarcolemma, thus resulting in the following events. (1) The depolarization of transverse tubules (T tubules) allows cytoplasmic Ca²⁺ to be released into the sarcoplasm. (2) The depolarization is transmitted via the T tubules to the SR and acts on protein complexes such as ryanodine receptors and inositol triphosphate (IP3) receptors in the SR, contributing to the release of Ca²⁺ from the SR into the sarcoplasm. (3) Ca²⁺ binds to the troponin C subunit (TnC) and then induces the tropomyosin to shift deeper into the grooves of the actin. Exposure of actin active sites allows for myosin head binding. Concurrently, the release of ATP from the inert Mg-ATP complex activates the myosin head ATPase. (4) Myosin binds with actin to form contractile actomyosin. Contraction of myofibrils ultimately leads to the movement of water out of the muscle cell into the extracellular space. (5) When the action potential disappears, Ca²⁺ is extruded to the extracellular space by the NCX and the PMCA and transported to the SR via the SERCA. Actomyosin is dissociated due to the recombination of tropomyosin and actin caused by the separation of Ca²⁺ and troponin, which allows for expansion of the myofibril and makes more room for water.

(A) Diagram. (B) C2 cells exhibit an electron dense cytoplasm with abundant rough endoplasmic reticulum and glycogen deposits. These cells rest on a thick and electron dense basal lamina (arrowheads). There are extensive intercellular spaces and smaller canaliculi, also seen under light microscopy (). (C) Transverse section of a cilium shows the typical 9 + 2 microtubule arrangement and membrane blebs (arrows). (D) Membrane-bound bundles of electron-dense tubules/filaments in the apical domain of a C2 cell. Scale bars represent: (B) 1 μm; (C) 50 nm; (D) 250 nm. Abbreviations: adh, adherent junction; bla, basal lamina; cil, cilia; dtu, bundles of electron-dense tubules; gly, glycogen deposit; icc, intercellular canaliculi; ics, intercellular space; msc, muscle cell; mtb, microtubules; mvi, microvilli; neu, neurite bundle; rer, rough endoplasmic reticulum; sep, septate junction; tep, thin epithelial projections.