GI smooth muscle differs from skeletal muscle in having two mechanisms that initiate the processes leading to contractile shortening and development of tension. In both skeletal muscle and GI smooth muscle, depolarization of the membrane electrical potential leads to the opening of voltage-gated calcium channels, followed by the elevation of cytosolic calcium, which, in turn, activates the contractile proteins. This mechanism is called electromechanical coupling. Smooth muscles have an additional mechanism in which the binding of a ligand to its receptor on the muscle membrane leads to the opening of calcium channels and the elevation of cytosolic calcium without any change in the membrane electrical potential. This mechanism is called pharmacomechanical coupling. The ligands may be chemical substances released as signals from nerves (neuro-crine), from nonneural cells in close proximity to the muscle (paracrine), or from endocrine cells as hormones delivered to the muscle by the blood.
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This ebook provides an introductory explanation of the workings of the human body, with an effort to draw connections between the body systems and explain their interdependencies. A framework for the book is homeostasis and how the body maintains balance within each system. This is intended as a first introduction to physiology for a college-level course.