Muscle contractile state, regulation

The smooth muscle cell does not respond in an all-or-none manner, but instead its contractile state is a variable compromise between diverse regulatory influences. While a vertebrate skeletal muscle fiber is at complete rest unless activated by a motor nerve, regulation of the contractile activity of a smooth muscle cell is more complex. First, the smooth muscle cell typically receives input from many different kinds of nerve fibers. The various cell membrane receptors in turn activate different intracellular signal-transduction pathways which may affect (a) membrane channels, and hence, electrical activity (b) calcium storage or release or (c) the proteins of the contractile machinery. While each have their own biochemically specific ways, the actual mechanisms are for the most part known only in outline. 

The gastrointestinal (GI) tract is in a continuous contractile, absorptive, and secretory state. The control of this state is complex, with contributions by the muscle itself, local nerves (i.e., the enteric nervous system, ENS), the central nervous system (CNS), and humoral pathways. Of these, perhaps the most important regulator of physiological gut function is the ENS (Figure 37-1), which is an autonomous collection of nerves within the wall of the Gl tract, organized into two connected networks of neurons the myenteric (Auerbach s) plexus, found between the circular and longitudinal muscle layers, and the submucosal (Meissner s) plexus, found below the epithehum. The former is responsible for motor control, while the latter regulates secretion, fluid transport, and vascular flow. 

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