Histology - Muscle

specialized for conducting movement; contractility of the cells; cells are elongated along the axis of contraction; excitable cell membrane – proprogation of stimuli; cells organized into bundles; connective tissue binds muscle cells together and carries blood vessels and nerves within it; rich vascularity of tissue pigment myoglobin gives fresh tissue a pink color

cytoplasm of muscle cell

plasma membrane of muscle cell

muscle cell itself – usually in striated muscle

T-tubule with 2 terminal cisternae (sarcoplasmic reticulum) on either side; 2 per sarcomere in striated muscle; Each skeletal muscle fiber has many thousands of triads, visible in muscle fibers that have been sectioned longitudinally. (This property holds because T tubules run perpendicular to the longitudinal axis of the muscle fiber.) Triads form the anatomical basis of excitation-contraction coupling, whereby a stimulus excites the muscle and causes it to contract. Cardiac muscle have analog diad structures

multinucleated muscle fibers made up of multiple fused embryonic cells, myoblasts (striated muscle) sarcoplasmic reticulum highly specialized form of endoplasmic reticulum, extensive in the sarcoplasm; calcium storage; forms an extensive continuous system of tubules and cisternae; surround the myofibrils; terminal cisternae are found near the A-1 junction, with tubular network along the A and I bands

produce strong, quick, discontinuous voluntary contraction multinucleated fibers striations due to regular, repeated arrangement of sarcomeres attached to bone or fascia, make up the sphincters; capable of rapid voluntary contraction upon nervous excitation; cells are elongated (1 to30 cm; avg. 3cm) and 10-100micrometers diameter each cell is independent; power of muscle depends of # of fibers contracting; easily fatigued – individual cells do not maintain contraction over time muscle cells increase in size following training - hypertrophy

produce weak + slow involuntary contraction spindle shaped, one centrally placed nucleus per cell; individual cells - organized in small clusters lining the walls of hollow organs (blood vessels, GI tract, respiratory tract, reproductive tract, ureter and bladder, iris of eye, hair follicles) closely related to connective tissue cells; arise from undifferentiated mesenchymal cells; can undergo proliferation during normal physiological conditions or tissue damage; create slow contraction motion – ex. peristaltic motion – to move substances in hollow organs

spindle-shaped, elongated cells variable size (1x20 micrometers in blood vessels, 12x600 micrometers in uterus) cells contain one elongated centrally placed nucleus no striation, since sarcomeres are not aligned within myofibrils actin & myosin filaments make up myofibrils (no specific ratio) cytoplasm stains homogenously with eosin

myofibrils not aligned within cytoplasml myofibrils do not run longitudinally or parallel to each other through the smooth muscle cells myofilaments are attached inside the cell to one another via DENSE BODIES; myofilaments are attached to plasmalemma via ATTACHMENT PLAQUES; both dense bodies and attachment plaques contain a-actinin, an anchor to actin filaments found in Z-disk in striated muscle, that links the actin filaments to plamslemma/cytoskeleton; pynocytotic vesicles (caveaolae) at the sarcolemma sequester calcium

cells arranged in bundles or sheets – forming contractile unit; bundles placed in cross directions to preserve the shape of hollow structures; during contraction, tensile force generated by individual muscle cells is transmitted to connective tissue by sheaths of reticular fibers; these reticular fibers are part of the basal lamina which is comprised of smooth muscle cells; cells linked by multiple gap junctions that allow Ca++ (second messenger) to move from one cell to the next; sarcoplasmic organelles around nucleus; smooth endoplasmic reticulum is found close to plasmalemma; cytoplasm filled with actin + intermediate filaments

smooth muscle cell nuclei appear elongated, finely granular chromatin; nuclei seen @ thickest part of the cell – not aligned with tissue; cells that comprise smooth muscle fibers are packed in close together making individual cells difficult to see – especially at low resolution; collagen fibers that connect muscle cells are visible

smooth cells secrete the connective tissue which connects the cells; basal lamina (collagen type IV) reticular fibers (collagen type III) elastin proteoglycans (ground substance)

when cells are contracted the nuclei have corkscrew shape, resulting from myofibril contraction pulling at the plasmalemma intracellular calcium levels rise, stimulates myosin-light-chain kinase that phosphorylates site on myosin filament; myosin head now able to react with actin – creating contraction; dephosphorylation – myosin dissociates from actin; phophorylation is a slow process, the link between the cells via gap junctions is slow, the muscle contraction may take up to a second, but can be sustained

smooth muscle cells appear with varying diameters (cut at different parts of the spindle); only small fraction of fibers display nuclei (recall that each smooth muscle is a single unit, each with its own nucleus), nucleus therefore occupies only a small portion of the cell’s total length