Structure of muscle

A muscle consists of groups of muscle bundles that join into a tendon at each end. The muscle bundles in the quadriceps are the vastus medialis, rectus femoris, vastus inter-medialis and vastus lateralis. Each bundle is separately wrapped in a sheath of connective tissue. Each muscle is composed of many fibres, packaged into bundles of about 

In a strap muscle there is little connective tissue within the muscle since most is concentrated in tendons at the end. Such muscles produce the tenderest meat fillet steak is the psoas muscle of a cow. Pennate muscles contain more connective tissue within the muscle itself and hence form the cheaper cuts of meat. 

cylindrical fibres within the bundles correspond to the cells of the other tissues. These muscle fibres vary enormously in length from about 0.4 to 10-15 cm and in diameter from about 0.01 to 0.1 mm. Each fibre contains mitochondria and many nuclei (since it develops from a number of cells). It is surrounded by a plasma membrane (the sarcolemma). 

A fibre is packed with longitudinally arranged myofibrils that contract. This is the contractile unit of the fibre it is 1-3 pm thick in diameter. Each myofibril consists of about 1000-2000 filaments, which are known as myofilaments. 

Muscles are attached to bone by tendons tough, strong strands of connective tissue which transmit the force from muscle to bone. In addition, because of their elastic properties, they convert some kinetic energy into potential energy which is then used during the next stroke. This reduces the amount of ATP and hence the amount of fuel required to power running (Box 13.1). 

Each muscle fiber, or cell, is enveloped by an elastic membrane (sarcolemma), immediately beneath which lie large numbers of ovoid, longitudinally oriented nuclei, leaving the interior of the fiber occupied by the sarcoplasm and myofibrils. In embryonic muscle, the nuclei occupy a central position. 

The motor unit consists of the motor neuron in the anterior horn of the spinal cord and the muscle fibers supplied by its branched axon closely applied at the motor end-plates. In this way a single motor neuron may supply a very large number of fibers, which tend to be grouped together but may be dispersed between different fasciculi. 

Sustained contraction of muscle fibers produces repetitive electrical potentials in them that increase in rapidity with strength of contraction and in number as more motor units take part. These potentials may be conducted from a coaxial needle electrode in the muscle, amplified, and their amplitude and frequency analyzed and recorded on a cathode-ray oscilloscope as the very typical full interference pattern of normal voluntary contraction. Characteristic departures from this normal electro-myogram are of very great importance in the diagnosis of muscle disease (B21a). 

The microscopic appearances of diseased muscle may sometimes be the only means by which a diagnosis is possible. 

In established neurogenic weakness and wasting due to lower motor neuron damage the characteristic appearances are those of groups of atrophic fibers, lately supplied by the damaged neuron, lying beside healthy fibers with intact innervation. This is in full accord with the structure of the motor unit, described above. In peripheral nerve disease, clinical evidence of sensory involvement, such as paraesthesias, may be present as weU. 

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Fig. 2. Macroscopic and microscopic structure of muscle (a) Entire muscle and its cross-section with fatty septa, (b) Fascicle with several muscle fibres (cells). A layer of fat along the fascicle is indicated, (c) Striated myofibre corresponding with one single muscle cell containing several nuclei. The lengths of a myofibre can be several tens of centimetres, (d) Myofibril inside a myocyte. It is one contractile element and contains actin and myosin and further proteins important for the muscular function, (e) Electron myograph of human skeletal muscle showing the band structure caused by the contractile myofilaments in the sarcomeres. One nucleus (Nu) and small glycogen granules (arrow, size <0.1 pm) are indicated.

THE EFFECT OF ISCHEMIA ON THE 3-DIMENSIONAL STRUCTURE OF MUSCLE/MEAT. 

In protein cross-linking studies, DMP has been used to examine the subunit structure of muscle pyruvate kinase (Davies and Kaplan, 1972), for the cross-linking of lactose synthetase (Brew et al., 1975), and to conjugate a fluorescent derivative of a-lactalbumin to glactosyltransferase (O Keefe et al., 1980). The reagent also has... 

Historically, this method of stepwise hydrolysis seems to have been first applied to a nucleotide by Levene and Jacobs, in determining the structure of muscle inosinic acid. ... 

Figure 34,12 Sarcomere. (A) Structure of muscle cell and myofibril containing sarcomeres. (B) Electron micrograph of a longitudinal section of a skeletal-muscle myofibril, showing a single sarcomere. (C) Schematic representations of cross sections correspond to the regions in the micrograph. [Courtesy of Dr. Hugh Huxley.]...

It is useful to first consider the structure of muscle s mechanochemical transduction elements, actin and the subfragment-1 (S-1) portion of myosin, before analyzing the effects of regulatory proteins that modulate interactions of the two and have the potential therefore to control force generation. Molecular details of the structure of skeletal muscle actin (Kabsch et al, 1990 Holmes and Kabsch, 1991 Lorenz et al., 1993) and S-1 (Rayment etal., 1993a) are known at the atomic level (Fig. 1), and given the general similarity of F-actin and S-1 in smooth and in skeletal muscle, it is reasonable to assume that they are closely related in the two... 

See also Actin and Myosin, The Structure of Muscle, The Sliding Filament Model, Tropomyosin, Troponins... 

The structure of muscle can be viewed at the electron micrograph level shown in Figure 8.9a, The muscle tissue is composed of bundles of muscle cells called muscle fibers. Within a muscle fiber are myofibrils, which are also arranged in bundles. Individual myofibrils contain the structurally distinct regions described below. Myofibrils have thin filaments composed of actin and thick filaments composed of myosin. Arrangement of the thick and thin filaments in a myofibril produces the distinctive pattern in Figure 8.9a and Figure 8.15. 

The Reaction of Myosin and Actin The Structure of Muscle (Figure 8.8, Figure 8.9a)... 

Y. Lu, C. M. Jeffries and J. Trewhella, Invited Review Probing the Structures of Muscle Regulatory Proteins Using SmaU-Angle Solution Scattering , Biopolymers, 2011, 95, 505. 

Kendrew, Sir John Cowdery (1917-97) English molecular biologist who established the three-dimensional structure of muscle hemoglobin (myoglobin) by X-ray crystallography in 1959. 

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