Cardiac and smooth muscles.pdf

Cardiac muscle ▪ Composed of elongated, branched cells. ▪ The cells are connected together by (intercalated discs). ▪ Cardiac contraction is involuntary, vigorous, and rhythmic.  Cardiac muscle ▪ The cardiac muscle cells are:...

Cardiac muscle ▪ Composed of elongated, branched cells. ▪ The cells are connected together by (intercalated discs). ▪ Cardiac contraction is involuntary, vigorous, and rhythmic.  Cardiac muscle ▪ The cardiac muscle cells are: 5-30 μm in diameter 85-120 μm in length. Striated (like the skeletal muscle). Has one centrally located nucleus (unlike skeletal muscle which is multinucleated).  Cardiac muscle ▪ The cardiac muscle is striated by transverse lines at irregular intervals. These lines represents the intercalated discs between adjacent cells. ▪ The intercalated discs are made of: ▪ Desmosomes These provide strong intercellular adhesion during contractions. ▪ Fascia adherens  Cardiac muscle ▪ In the intercalated disc, running longitudinally parallel to the myofibrils, are the gap junctions ▪ Gap junctions provide ionic continuity between the cells to serve as “electrical synapses” promoting rapid impulse conduction through many cardiac muscle cells simultaneously and contraction of many adjacent cells as a unit.  Cardiac muscle ▪ Contractile process in cardiac and skeletal muscle cells are the similar. ▪ Source of energy: mitochondria (occupy up to 40% of the cell volume), higher than skeletal muscle fibers. ▪ Fuel of energy: mainly fatty acids (which are stored as triglycerides in lipid droplets).  Cardiac muscle ▪ The ventricles are thicker than the atria (the ventricles pump blood through the entire cardiovascular system). ▪ The cardiac muscle transverse tubule system have the same basic functions as their counterparts in skeletal muscle fibers.  Cardiac muscle ▪ The ventricular T-tubules are large and penetrate the sarcoplasm to reach the Z discs. The atrial T-tubules are smaller or absent. ▪ Sarcoplasmic reticulum is less well-organized compared to skeletal muscle fibers. ▪ The junctions between its terminal cisterns and T-tubules typically involve only one structure of each type, forming profiles called dyads rather than triads.  The smooth muscle ▪ Specialized for slow and steady contraction (under the influence of autonomic nerves and hormones). ▪ Found in: a. Blood vessels. b. Digestive, respiratory, urinary, and reproductive tracts and their associated organs (also called visceral muscle).  The smooth muscle ▪ Each smooth muscle cell is: ▪ Elongated, tapering (fusiform shape), and unstriated ▪ Covered by external lamina and collagen fibers (of type I and type III ) comprising the endomysium. ▪ Contains single, central, and elongated nucleus.  The smooth muscle ▪ The narrow ends of each cell lie adjacent to the broad parts of neighboring cells. Therefore, cross sections show a range of cell diameters, with only the largest profiles containing a nucleus.  The smooth muscle ▪ SM cells are linked together by gap junctions. ▪ At the surface, there are numerous plasmalemma invaginations called (caveolae), which contain the major ion channels that control Ca2+ release from sarcoplasmic cisternae to initiate contraction. ▪ SM have sarcoplasmic reticulum, but lack T-tubules.  The smooth muscle ▪ The actin and myosin filaments crisscross the sarcoplasm obliquely. ▪ The myosin filaments have a less regular arrangement among the thin filaments and fewer cross-bridges than in striated muscle.  The smooth muscle ▪ The actin filaments are not associated with troponin and tropomyosin, using instead calmodulin and Ca2+-sensitive myosin light-chain kinase (MLCK) to produce contraction.  The smooth muscle ▪ The actin myofilaments are attached to dense bodies (functionally similar to the Z discs). Dense bodies serve as points for transmitting the contractile force not only within the cells, but also between adjacent cells.  The smooth muscle ▪ Smooth muscle cells also contain: a. Intermediate filaments, composed of desmin, which also attach to the dense bodies. b. The submembranous dense bodies include cadherins of desmosomes linking adjacent smooth muscle cells.  The smooth muscle ▪ Smooth muscle is non-voluntary and has no well-defined neuromuscular junctions. ▪ Contraction is stimulated by: a. Autonomic nerves (as in blood vessels smooth muscles). b. Hormones (as in the gastrointestinal tract by paracrine hormones and the uterus by oxytocin from the pituitary gland).  The smooth muscle ▪ Axons of autonomic nerves form varicosities in close contact with smooth muscle fibers. These varicosities form synapses with the smooth muscle and release a neurotransmitter, usually acetylcholine or norepinephrine, which diffuses and binds receptors in the plasma membrane smooth muscle cells.  The smooth muscle ▪ As in cardiac muscle, stimulation is propagated to more distant fibers via gap junctions that allow all the smooth muscle cells to contract synchronously or in a coordinated manner.  The smooth muscle ▪ Smooth muscle cells also supplement fibroblast activity by synthesizing collagen, elastin, and proteoglycans. ▪ Active synthesis of extracellular matrix (ECM) by the smooth muscle may reflect less specialization for strong contractions than in skeletal and cardiac muscle. The smooth muscle