Summary

This document provides a comprehensive overview of cardiac and smooth muscles, including their structure, function, and differences. It details the cellular components and mechanisms involved in muscle contraction, and describes the location of these muscle types within the body, from cardiovascular to digestive systems. Useful for students studying biology and anatomy at the undergraduate level.

Full Transcript

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

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