Although all cells are capable of some sort of movement, the dominant function of several cell types is to generate force through contraction. In these specialised contractile cells, movement is generated by interaction of the proteins actin and myosin (contractile proteins). Certain forms of contractile cell function as single-cell contractile units:

Other forms of contractile cells function by forming multicellular contractile units termed muscles. Such muscle cells can be divided into three types:

• Skeletal muscle is responsible for the movement of the skeleton as well as organs such as the globe of the eye and the tongue. Skeletal muscle is often referred to as voluntary muscle since it is capable of voluntary (conscious) control. The arrangement of the contractile proteins gives rise to the appearance of prominent cross-striations in some histological preparations and so the name striated muscle is often applied to skeletal muscle. The highly developed functions of the cytoplasmic organelles of muscle cells has led to the use of a special terminology for some muscle cell components: plasma membrane or plasmalemma = sarcolemma; cytoplasm = sarcoplasm; endoplasmic reticulum = sarcoplasmic reticulum.

• Smooth muscle is so named because, unlike other forms of muscle, the arrangement of contractile proteins does not give the histological appearance of cross-striations. This type of muscle forms the muscular component of visceral structures such as blood vessels, the gastrointestinal tract, the uterus and the urinary bladder, giving rise to the alternative name of visceral muscle. Since smooth muscle is under inherent autonomic and hormonal control, it is also described as involuntary muscle.

• Cardiac muscle has many structural and functional characteristics intermediate between those of skeletal and smooth muscle and provides for the continuous rhythmic contractility of the heart. Although striated in appearance, cardiac muscle is readily distinguishable from skeletal muscle and should not be referred to by the term ‘striated muscle’.

Muscle cells of all three types are surrounded by an external lamina (see Ch. 4). In all muscle cell types, contractile forces developed from the internal contractile proteins are transmitted to the external lamina via link proteins which span the muscle cell membrane. The external lamina binds individual muscle cells into a single functional mass.

Skeletal Muscle

Skeletal muscles have a wide variety of morphological forms and modes of action; nevertheless all have the same basic structure. Skeletal muscle is composed of extremely elongated, multinucleate contractile cells, often described as muscle fibres, bound together by collagenous supporting tissue. Individual muscle fibres vary considerably in diameter from 10 to 100 µm and may extend throughout the whole length of a muscle and, in some sites, may be many centimeters in length.

Skeletal muscle contraction is controlled by large motor nerves, individual nerve fibres branching within the muscle to supply a group of muscle fibres, collectively described as a motor unit. Excitation of any one motor nerve results in simultaneous contraction of all the muscle fibres of the corresponding motor unit. The structure of neuromuscular junctions is described in Fig. 7.12. The vitality of skeletal muscle fibres is dependent on maintenance of their nerve supply which, if damaged, results in atrophy of the fibres (see below). Skeletal muscle contains highly specialised stretch receptors known as neuromuscular spindles which are shown in Fig. 7.23.



FIG. 6.3 Skeletal muscle
(a) H&E, TS (HP) (b) H&E, LS (HP)
These micrographs show skeletal muscle from human limb muscles. Micrograph (a) in transverse section shows the muscle to be made up of numerous small fasciculi. The spaces between the fasciculi are filled with loose collagenous tissue, the perimysium P, which is continuous with the delicate endomysium En, separating individual muscle fibres in each fasciculus. The supporting tissue of skeletal muscle also contains elastin fibres (not distinguishable in this preparation) which are most numerous in muscles attached to soft tissues as in the tongue and face. Note the rich network of capillaries C in the endomysium. Small blood vessels B and nerves run in the perimysium.
Micrograph (b) demonstrates the characteristic histological features of skeletal muscle fibres in longitudinal section. Skeletal muscle fibres are extremely elongated, unbranched cylindrical cells with numerous flattened nuclei located at fairly regular intervals just beneath the sarcolemma (plasma membrane).
Each muscle fibre has multiple nuclei arranged at the cell periphery. In transverse section, as in micrograph (a), most muscle fibre profiles appear to contain only a single nucleus, while some do not include any because the plane of section has cut between the zones containing a nucleus.
In routine histological preparations stained with H&E, it is often possible to see the striations in skeletal muscle when cut in longitudinal section. Special stains are required for better resolution of these structures (see Fig. 6.6a).


FIG. 6.6 Skeletal muscle
(a) Heidenhain’s haematoxylin (HP) (b) EM ×2860 (c) EM ×18 700
This series of micrographs shows the arrangement of the contractile proteins within skeletal muscle and explains the striations seen with light microscopy.
Micrograph (a) shows the striations of a skeletal muscle fibre at a magnification close to the limit of resolution. They are composed of alternating broad light I bands (isotropic in polarised light) and dark (anisotropic) A bands. Fine dark lines called Z lines (Zwischenscheiben) Z can be seen bisecting the I bands. Note the nucleus N at the periphery of the cell.
Micrograph (b) shows the electron microscopic appearance of muscle with a nucleus N situated in a similar position. The sarcoplasm is filled with myofibrils My oriented parallel to the long axis of the cell. These are separated by a small amount of sarcoplasm containing rows of mitochondria Mt in a similar orientation. Each myofibril has prominent regular cross-striations arranged in register with those of the other myofibrils and corresponding to the I, A and Z bands seen in light microscopy. The Z bands are the most electron-dense and divide each myofibril into numerous contractile units called sarcomeres, arranged end to end.
With further magnification in micrograph (c), the arrangement of the contractile proteins (myofilaments) may be seen in each sarcomere. The dark A band is bisected by the lighter H (Heller) band, which is further bisected by a more dense M (Mittelscheibe) line. Irrespective of the degree of contraction of the muscle fibre, the A band remains constant in width. In contrast, the I and H bands narrow during contraction, and the Z lines are drawn closer together. These findings are explained by the sliding filament theory (Fig. 6.7). Mitochondria Mt and numerous glycogen granules G provide a rich energy source in the scanty cytoplasm between the myofibrils. The mature muscle cell contains little rough endoplasmic reticulum; it contains, however, a smooth membranous system S which is involved in activation of the contractile mechanism (see Figs 6.8 to 6.10).


FIG. 6.7 The arrangement of myofilaments in the sarcomere
Muscle function is intimately linked to its microscopic structure. The sarcomere consists of two types of myofilaments, thick filaments and thin filaments. Each type remains constant in length irrespective of the state of contraction of the muscle. The thick filaments, which are composed mainly of the protein myosin, are maintained in register by their attachment to a disc-like zone represented by the M line. Similarly, the thin filaments, which are composed mainly of the protein actin, are attached to a disc-like zone represented by the Z line. The I and H bands, both areas of low electron density, represent areas where the thick and thin filaments do not overlap one another.
The sliding filament mechanism of muscle contraction proposes that contraction occurs through a sliding movement of the thick and thin filaments over one another as illustrated above. Myosin molecules possess ATP-activated side projections or head groups which can bind to actin to form cross-bridges, temporary physical linkages between the thick and thin filaments. Once bound to the adjacent actin molecules, these myosin head groups generate movement by a change in protein configuration, triggered by energy from the hydrolysis of ATP, pulling the myosin thick filaments over the thin actin filaments and so shortening the length of the sarcomere. When another ATP molecule binds to the myosin head group, it detaches from actin, breaking the cross-bridge, and the head group then moves back to its original configuration, ready for the next cross-bridge cycle. This process can be likened to multiple small strokes from the oars of a boat steadily producing movement of the craft.
A large number of accessory proteins are also present in the sarcomere where they play roles in filament alignment and in regulation of contraction.

Aug 22, 2016 | Posted by in HISTOLOGY | Comments Off on Muscle
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