I. Smooth Muscle<\/p>\n
A. General features –not striated, no sarcomeres, found in wall of tubular viscera, involuntary<\/em><\/p>\n B. Fusiform shape, single nucleus, can undergo mitosis<\/p>\n C. Myosin present<\/p>\n D. Actin –attaches to dense bodies and to the cell membrane. The dense bodies, in turn, are linked by fibers that form the cytoskeleton<\/em><\/p>\n E. Poor SR, no T-tubules, innervated by autonomic nervous system<\/p>\n II. Smooth muscle contraction<\/p>\n A. Neurogenic –originates from one or more autonomic neurons; multi-unit smooth muscle has each muscle cell receive its own nerve supply and each cell contracts independently<\/em><\/p>\n B. Myogenic –originates by electrical communication via gap junction from its neighbor; single-unit smooth muscle has one neuron pass through the muscle cells without synapsing with any specific cell and the muscle cells are coupled by electrical gap junctions – the muscles contract as a unit.<\/em><\/p>\n C. Regulation –from binding and unbinding of thick and thin filaments regulated by calcium.<\/em><\/p>\n D. Intracellular calcium concentrations –regulates slow contraction and relaxation and can come from mainly the ECF or some from the SR<\/em><\/p>\n E. As calcium increases it binds to free calmodulin– a soluble intracellular protein with a calcium binding site that is analogous to troponin in skeletal muscle-\u00a0<\/em>which unblocks the binding sites on actin\u00a0(via a different protein)<\/em>\u00a0& permits cross-bridging between actin and myosin<\/p>\n F. Calcium-Calmodulin complex also leads to energizing the myosin head by combining with the myosin light chain kinase(MLCK – an enzyme that adds a phosphate group on the myosin head)\u00a0<\/em>which results in actin and myosin binding, hydrolysis of ATP and repetitive power strokes. Another enzyme (a phosphatase) removes phosphate on myosin head for relaxation.<\/p>\n III. Electrophysiology in Smooth Muscle<\/p>\n A. RMP –can change in response to neural, mechanical, or hormonal stimulation<\/em><\/p>\n B. APs –can be a spike or a spike followed by a plateau. The plateau accounts for prolonged smooth muscle contractions.<\/em><\/p>\n C. Depolarization –due to opening of voltage-gated calcium channels; calcium then also aids the mechanics of contraction<\/em><\/p>\n D. Pacemaker Potential –some smooth muscle cells can initiate spontaneous electrical activity and therefore show pacemaker potential due to membrane permeability changes (probably to calcium) Note: this contraction may occur with or without an AP. If this activity oscillates slowly, the term “slow waves” is used.<\/em><\/p>\n","protected":false},"excerpt":{"rendered":" I. Smooth Muscle A. General features –not striated, no sarcomeres, found in wall of tubular viscera, involuntary B. Fusiform shape, single nucleus, can undergo mitosis C. Myosin present D. Actin –attaches to dense bodies and to the cell membrane. The … Continue reading