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Muscle III. Smooth Muscle. Smooth Muscle. Not striated. Lack sarcomeres. Found in the walls of hollow organs. Not under voluntary control. Each cells is spindle shape. Diameter 2-10 um. Single nucleus. Capacity to divide through life. Thick filament: Myosin. Thin filament: actin.
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Smooth Muscle. • Not striated. • Lack sarcomeres. • Found in the walls of hollow organs. • Not under voluntary control.
Each cells is spindle shape. • Diameter 2-10 um. • Single nucleus. • Capacity to divide through life.
Thick filament: Myosin. • Thin filament: actin. • Thin filaments attach to dense bodies and to the membrane. • Dense bodies linked by fibers that form cytoskeleton.
Poorly developed or no SR. • No T-tube system. • Innervated by autonomic neurons. • Neurons make multiple or minimal contact with smooth muscle cells.
Multi-unit: each cell receives synaptic input from one or more neurons. Each cell may contract independently. • Neurogenic.
Single-unit (unitary): gap junctions permit electrical communication between cells. • Automatic (myogenic)
Regulation of Smooth Muscle Contraction. • Cyclic binding and unbinding of thick and thin filaments regulated by Ca++. • Contraction and relaxation are slow and reflects changes in [Ca++]I.
Electro Physiology in Smooth Muscle. • Vm in smooth muscle can change in response to neural, mechanical or hormonal stimulation. • Some develop APs that can be: • a spike. • a spike followed by a plateau.
APs in Smooth Muscle. • Upstroke of AP reflects the opening of voltage gated Ca++ channels. • Ca++ not only depolarizes the fiber but also contributes to the Ca++ necessary for contraction.
Some smooth muscle cells can initiate spontaneous electrical activity. • Automatic: Show pacemaker potential. • Changes in permeability of the membrane to Ca++, K+ (?)
If the upstoke of the slow wave reaches threshold APs can result. • APs are not necessary to generate contraction in smooth muscle cells.