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Smooth Muscle Contraction Excitation Contraction Coupling
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(USMLE Physiology) Compact summary of smooth muscle contraction mechanisms. • 👉 Related videos you may need: • Excitation Contraction Coupling in Cardiac vs Skeletal Muscle • • Muscle Physiology | Cardiac vs Skelet... • Cardiac muscle cells physiology and concepts • • Video • Skeletal muscle physiology | Skeletal muscle fibers • • Muscle Physiology | Skeletal muscle p... • Neurosmuscular junction | Cross-bridge cycle • • Video • Mechanism of Smooth Muscle Contraction: • In most cases, adjacent smooth muscle fibers exhibit slow, synchronized • contractions, the whole sheet responding to a stimulus in unison. This synchronization reflects electrical coupling of smooth muscle cells by gap junctions, specialized cell connections. Skeletal muscle fibers are electrically isolated from one another, each stimulated to contract by its • own neuromuscular junction. By contrast, gap junctions allow smooth muscles to transmit action potentials from fiber to fiber. Some smooth muscle fibers in the stomach and small intestine • are pacemaker cells: Once excited, they act as “drummers” to set the pace of contraction for the entire muscle sheet. These pacemakers depolarize spontaneously in the absence of external • stimuli. However, neural and chemical stimuli can modify both the rate and the intensity of smooth muscle contraction. • Contraction in smooth muscle is like contraction in skeletal • muscle in the following ways: • ● Actin and myosin interact by the sliding filament mechanism. • ● The final trigger for contraction is a rise in the intracellular • calcium ion level. • ● ATP energizes the sliding process. • During excitation-contraction coupling, the tubules of the SR release Ca2+, but Ca2+ also moves into the cell from the extracellular space via membrane channels. In all striated muscle types, • calcium ions activate myosin by binding to troponin. In smooth muscle, calcium activates myosin by interacting with a regulatory molecule called calmodulin, a cytoplasmic calcium-binding protein. Calmodulin, in turn, interacts with a kinase enzyme called myosin kinase or myosin light chain kinase which phosphorylates the myosin, activating it. As in skeletal muscle, smooth muscle relaxes when intracellular Ca2+ levels drop—but getting smooth muscle to stop contracting is more complex. Events known to be involved include calcium detachment from calmodulin, active transport of Ca2+into the SR and extracellular fluid, and dephosphorylation of myosin by a phosphorylase enzyme, which reduces the activity of the myosin ATPases. • In summary: • Step 1 - Calcium ions (Ca2+) enter the cytosol from the extracellular space via voltage-gated • or non-voltage-gated Ca2+ channels, or from the scant SR. • Step 2 - Ca2+ binds to and activates calmodulin. • Step 3 - Activated calmodulin activates the myosin light chain kinase enzymes. • Step 4 - The activated kinase enzymes catalyze transfer of phosphate to myosin, activating the myosin ATPases. • Step 5 - Activated myosin forms cross bridges with actin of the thin filaments. Shortening begins.
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