Muscle structure and theories of contraction.

Collagen molecules are structural macro-molecules of the extracellular matrix that include in their structure one or several domains that have a characteristic triple helical conformation. They have been classified by types that define distinct sets of polypeptide chains that can form homo- and heterotrimeric assemblies. All the collagen molecules participate in supramolecular aggregates that are stabilized in part by interactions between triple helical domains. Fourteen collagen types have been defined so far. They form a wide range of structures. Most notable are 1) fibrils that are found in most connective tissues and are made by alloys of fibrillar collagens (types I, II, III, V, and XI) and 2) sheets constituting basement membranes (type IV collagen), Descemet's membrane (type VIII collagen), worm cuticle, and organic exoskeleton of sponges. Other collagens, present in smaller quantities in tissues, play the role of connecting elements between these major structures and other tissue components. The f...

https://faseb.onlinelibrary.wiley.com/doi/pdf/10.1096/fasebj.5.13.1916105

Collagen family of proteins.

The biochemical and biophysical properties of the extracellular matrix (ECM) dictate tissue-specific cell behaviour. The molecules that are associated with the ECM of each tissue, including collagens, proteoglycans, laminins and fibronectin, and the manner in which they are assembled determine the structure and the organization of the resultant ECM. The product is a specific ECM signature that is comprised of unique compositional and topographical features that both reflect and facilitate the functional requirements of the tissue.

/pdf/extracellular-matrix-assembly-a-multiscale-deconstruction-3ucggnbaz1.pdf

Extracellular matrix assembly: a multiscale deconstruction

Several types of striated muscle have been examined by the technics of electron microscopy and the findings in myotome fibers of Amblystoma larvae, the sartorius, and cardiac muscle of the rat are reported on in some detail. Particular attention has been given to structural components of the interfibrillar sarcoplasm and most especially to a finely divided, vacuolar system known as the sarcoplasmic reticulum. This consists of membrane-limited vesicles, tubules, and cisternae associated in a continuous reticular structure which forms lace-like sleeves around the myofibrils. It shows a definable organization which repeats with each sarcomere of the fiber so that the entire system is segmented in phase with the striations of the associated myofibrils. Details of these repetitive patterns are presented diagrammatically in Text-figs. 1, 2, and 3 on pages 279, 283, and 288 respectively. The system is continuous across the fiber at the H band level and largely discontinuous longitudinally because of interruptions in the structure at the I and Z band levels. The structure of the system relates it to the endoplasmic reticulum of other cell types. The precise morphological relation of the reticulum to the myofibrils, with specializations opposite the different bands, prompts the supposition that the system is functionally important in muscle contraction. In this regard it is proposed that the membrane limiting the system is polarized like the sarcolemma and that the corresponding potential difference is utilized in the intracellular distribution of the excitatory impulse.

/pdf/studies-on-the-endoplasmic-reticulum-iii-its-form-and-23qtpksb3j.pdf

Studies on the endoplasmic reticulum : iii. its form and distribution in striated muscle cells

Molecular Structure of Collagen

Electron microscope investigations of the structure of collagen

1. Electron micrographs were made of myofibrils isolated from frog and other skeletal muscles fixed in formalin. The structure with respect to the location and disposition of the principal cross striations is in good agreement with that previously determined from histological studies.2. The myofibrils are composed of bundles of myosin filaments ranging in width from about 50 to 250 A and extending continuously and in relatively straight lines through the isotropic and anisotropic bands in both the extended and contracted states. The anisotropic bands also contain material of high electron scattering power and affinity for phosphotungstic acid. The distribution of this "A substance" changes with contraction in characteristic fashion. The evidence indicates that the myosin filaments are the contractile units.3. While the myosin filaments have an indefinite length in the intact fibril, they are fragmented extensively during extraction in weakly alkaline salt solutions (method of Greenstein and Edsall). Filam...

An investigation of cross striations and myosin filaments in muscle.

Electron microscope observations of collagen

The structure and properties of the trichocysts of Paramecium

In the study of the mechanism of muscle contraction, skeletal muscle has been the material most wideIy investigated. There are many physiological and chemical reasons for this, but the structural aspects are also important. The complex cytoarchitecture of cross-striated muscle and its alterations in various physiological states furnish visible clues regarding the contractile mechanism. The anisotropy of the alternate bands early led to a recognition of the importance of molecular orientation in contraction (Engelmann, 1875). The demonstration that myosin solutions show double refraction of flow (Muralt and Edsall, 1930) led to a localization of this protein in the anisotropic ( A ) 'bands. However, subsequent, more or less indirect lines of evidence indicate that the entire myofibril is composed of myosin, while the relative isotropy of the I bands was ascribed to a lack of preferred orientation of myosin in these regions (Schmidt, 1937). With the discovery of the enzymic relationship between myosin, adenosinetriphosphate (ATP) , and the mineral constituents, attention has again been directed to the localization of these substances in the muscle fibril. Explanation of contraction and relaxation is currently sought in topochemical reactions between the structural protein, myosin, and ATP activated by mineral constituents.

Electron microscope and X-ray diffraction studies of muscle structure.

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