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The phenotype of each smooth muscle cell is relatable to the function of the particular organ it is in, whether the myocyte is parenchymal or is simply coordinated (as in a blood vessel) to the organ function. The functional diversity of smooth muscle, therefore, correlates with the functional diversity of every peripheral organ system. As reviewed here, this diversity is also expressed ontogenetically flowing downward to every level of tissue organization within an organ, from the pattern of the autonomic innervation to the phenotype of the parenchymal cell. A primary consideration herein is that much of the diversity in smooth myocyte contractile function is related to which signal transduction pathway(s) is activated. More specifically, different cells have different complements of membrane receptors (i.e., serotonergic, adrenergic, cholinergic, etc.), transducers (i.e., G proteins), and effectors (i.e., enzymes and ion channels), and therefore, heterogeneous pathways may be activated. This review also focuses on the mechanisms by which distinct neural pathways may differentially modulate/coordinate smooth muscle cell function(s), as well as taking into account the characteristics and distribution of the autonomic neurons themselves. Perhaps most importantly, this review examines the concept that understanding of neural control of smooth muscle tone needs to add a new component, that is, the syncytial nature of smooth muscle. Therefore, we address the newer evidence concerning the distribution and function of gap junctions between smooth muscle cells in organ systems throughout the body. This evidence points toward a dominating role for intermyocyte communication in coordinating smooth muscle responses in the various tissues in different organs. |
