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Cardiac Muscle

Contract spontaneously and display a rhythmic beat. Are attached end to end by intercalated disks. May branch at their ends to form connections with adjacent fibers. Contain one or two nuclei located centrally within the cell. Thick and thin filaments make up poorly defined myofibrils. Same crossbinding pattern as skeletal muscle.

Cardiac T-tubules

Larger than those of skeletal muscle, lined by external (basal) lamina. Located at the Z lines in mammalian cardiac muscle (rather than at A1 junctions)

Cardiac Sarcoplasmic Reticulum

Is poorly developed and contributes to the formation of diads (structures consisting of one T-tubule and one profile of sarcoplasmic reticulum).

Cardiac Mitochondira

Lie parallel to the I band and glycogen is common in the cell, especially at either pole of the nucleus.

Atrial Cardiac Muscle Cells

Contain atrial granules in addition to other features.

Atrial granules

Source of two polypeptide hormones. Cardiodilatin, a vascular smooth muscle relaxant that facilitates vasodilation. Cardionatrin, a potent natriuretic and diuretic hormone.

Intercalated Disks

Elaborate stepwise junctions forming the end-to-end attachments of adjacent cardiac muscle cells. Two specializations make up the transverse portion of the intercalated disk; The fascia adherens (an extensive region of adhesion analogous to the zonula adherens) and desmosomes. Longitudinal portion contains a gap junction. The transverse portion substitutes at the Z line, to which actin filaments attach to it.

Purkinje Fibers

Specialized conducting cells of the atrioventricular bundle. Very large modified muscle cells filled with glycogen and containing many mitochondria. Contains a few myofibrils, which are located peripherally. Make contact with cardiac muscle cells through gap junctions, desmosomes and fasciae adherentes (but not through typical intercalated disks).

Smooth Muscle

Fusiform in shape, with single, centrally placed nucleus. May be arranged in layers, small bundles, or in helical patterns in arteries. Surrounded by reticular fiber network, divide and are important in regeneration.

Smooth Muscle nuclei

Not observed in each cell when cross-sectioned, since many lie outside of plane of section. In contracted state, smooth muscle cells are sectioned longitudinally, their nuclei appear accordion pleated and deeply indented.

Smooth muscle organelles

Involved in the synthesis of type III collagen, elastin, GAG's glycoproteins, the external lamina and growth factors.

Cytoplasmic and Peripheral Densities

In smooth muscle cells are believe to be analogous to Z line. Contain alpha-actinin and function as myofilament attachment sites. Actin, myosin and intermediate filaments are also present. In nonvascular smooth muscle cells, the intermediate filaments are desmin, but in vascular smooth muscle they are vimentin.

Gap junction (nexus)

Between smooth muscle cells facilitate spread of excitation.

Sarcolemmal Vesicles (cavolae)

Believed to be similar to the smooth endoplasmic reticulum.

Smooth muscle Innervation

Sympathetic and parasympathetic. Extent depends on function and size of bundle. NErve terminals are observed close to the muscle within the endomysium.

Smooth muscle contraction

Initiated through nerve impulses, stretching within muscle itself, or via hormones.

Vascular smooth muscle contraction

Contraction usually initiated by a nerve impulse and there is little spread of conduction from cell to cell.

Visceral smooth muscle contraction

Contaction Is myogenic (stretch) and the signal spreads from cell to cell via gap junctions.

Uterine smooth muscle contraction

Smooth muscle cells contact in response to oxytocin.

Myoepithelial cells

Originate from ectoderm, basket-like shape and have seveal radiating processes. Located between the epithelium and the basal lamina in certain glands (mammary, submandibular) and ducts. Contains hemidesmosomes, actin, myosin, and intermediate (cytokeratin) filaments as well as cytoplasmic and peripheral densities that serve as attachment sites for them. Their contraction forces secretory material from the glandular epithelium into the ducts and out of the gland.

Duchenne Muscular Dystrophy

Most common inherited muscle disease and characteristically affects male children. Due to defect in the gene coding for dystrophin which links the actin cytoskeleton of muscle to the external lamina. Its absence leads to abnormal muscle fiber fragility.

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