Smooth muscle

Smooth muscle is composed of elongated, nonstriated cells, each of which is enclosed by a basal lamina and a network of reticular fibers. The last 2 components serve to combine the force generated by each smooth muscle fiber into a concerted action, eg, peristalsis in the intestine.

Fig.13. Smooth muscle tissue

Smooth muscle cells are spindle-shaped; ie, they are largest at their midpoints and taper toward their ends. They may be from 20 μm in small blood vessels to 500 μm in the pregnant uterus. Each cell has a single nucleus which is elongated and centrally located in the cytoplasm at the widest part of the cell.

To achieve closest packing, the narrow part of one cell lies adjacent to the broad part of neighboring cells (Fig.14). When such an arrangement is viewed in cross section, one sees a range of diameters with only the largest profiles containing a nucleus.

Fig.14

The borders of the cell become scalloped when smooth muscle contracts, and the nucleus becomes folded or has the appearance of a corkscrew (spiral-shaped) (Fig.15).

Fig.15.

Smooth endoplasmic reticulum is sparse but closely associated with sarcolemmal vesicles (caveolae), which present along the periphery of the cell (Fig.16, 1). Caveolae might function to take up and release calcium ions. T tubules are not present in smooth muscle cell.

Fig.16.

Concentrated at the poles of the nucleus are mitochondria, free ribosomes, cisternae of rER, and the Golgi complex. Organelles are involved in the synthesis of type III collagen, elastin, glycoproteins, the external lamina, and growth factors.

The characteristic contractile activity of smooth muscle is related to the structure and organization of its actin and myosin filaments.

In smooth muscle cells, bundles of myofilaments crisscross obliquely through the cell, forming a lattice-like network. Unlike the myosin filaments in skeletal muscle, which have a bare central region with myosin heads on each end, smooth muscle myosin has heads all along its length and bare regions at the ends of the filaments. The molecular organization of the myosin filaments allows much greater actin overlap and a greater degree of contraction in smooth muscle.

Both structural and biochemical studies reveal that smooth muscle actin and myosin contract by sliding filament mechanism similar to that which occurs in striated muscles.

Smooth muscle has an arrangement where bundles of contractile proteins criss-cross the cell being inserted into anchoring points (focal densities, dense bodies) within the cytoplasm as well as anchoring to the cell membrane (Fig.15).

Two types of dense bodies – membrane-associated and cytoplasmic – are seen in smooth muscle. Both contain α-actinin and are similar to the Z lines of striated muscles. Both thin and intermediate filaments (desmin; vimentin in vascular smooth muscle) insert into dense bodies that transmit contractile force to adjacent smooth muscle cells and their surrounding network of reticular fibers.

NERVE TISSUE

Structurally, nerve tissue consists of two groups of cell types:

• nerve cells, or neurons

• several types of glial cells, or neuroglia, which support and protect neurons and participate in neural activity, neural nutrition, and the defense processes of the central nervous system.

Дата добавления: 2015-12-07