Smooth muscle cells, also known as unstriated muscle cells, play a vital role in the structure of hollow organs such as the gastrointestinal tract, airways, blood vessels, and urogenital tract. Their main function is to change the shape and dimensions of these organs, either by propelling the organ's contents through peristalsis or by increasing the resistance to flow, as seen in vasoconstriction. The mechanism of smooth muscle contraction involves the interaction between myosin and actin, similar to striated muscle, but with some important differences. In smooth muscle, contraction is regulated by thick filaments and requires a modification in myosin before it can bind to actin. On the other hand, in striated muscle, contraction is regulated by thin filaments, and the troponin-tropomyosin complex needs to move on the actin filament for myosin to bind to actin. Smooth muscle can be stimulated to contract by electrical or hormonal signals and has the ability to maintain prolonged contractions with low energy consumption, which is crucial for functions like maintaining vascular tone and blood pressure. The regulation of smooth muscle contraction is complex and involves multiple intracellular signaling pathways in some cases. In this chapter, we aim to explore the mechanisms underlying the diverse regulation of smooth muscle contraction and compare them, where applicable, to those observed in striated muscle. We will also discuss how alterations in smooth muscle function and regulation have been implicated in various pathological conditions.

Overview of Smooth Muscle:
Smooth muscle can be categorized into two types: single-unit and multiunit. In single-unit smooth muscle, the cells are electrically connected, allowing for the transmission of electrical stimulation from one cell to adjacent cells. This results in a wave of contraction, as seen in peristalsis. Additionally, this wave of electrical activity and subsequent contraction in single-unit smooth muscle can be initiated by pacemaker cells, which display spontaneous depolarization. In contrast, multiunit smooth muscle cells are not electrically connected, so the stimulation of one cell does not necessarily activate neighboring cells. Examples of multiunit smooth muscle include the vas deferens in the male reproductive tract and the iris of the eye. However, smooth muscle displays even more diversity, and the single-unit and multiunit classifications represent opposite ends of a spectrum. It is important to note that the terms single-unit and multiunit oversimplify the complexity of smooth muscle, as many smooth muscles are modulated by a combination of neural elements, with varying degrees of cell-to-cell coupling, and locally produced activators or inhibitors that contribute to a coordinated response in smooth muscle. Another important factor to consider when discussing types of smooth muscle is the activity pattern.

Types of Smooth Muscle:
Smooth muscle can exhibit different patterns of activity.
Upon completion of this chapter the student should be able to answer the following questions: 1.
Describe the organization of smooth muscle in various tissues, and how it meets the demands of each tissue/ organ.
2.
Discuss the mechanisms that promote contraction and relaxation of smooth muscle in the vasculature and various organs.
3.
Describe the autoregulatory mechanism by which an artery can maintain relatively constant blood flow to a tissue over a broad range of perfusion pressures.
4.
Describe the basis and utility of a transition from phasic contraction to tonic contraction.
5.
Discuss the length-tension curves and force-velocity curves for smooth muscle, and the molecular basis for each of these curves.
If the student has already completed on skeletal muscle and cardiac muscle, comparison of all three tissues for each of the learning objectives listed should be possible.
onstriated, or smooth, muscle cells are a major component of hollow organs such as the alimentary canal, airways, vasculature, and urogenital tract. Contraction of smooth muscle serves to alter the dimensions of the organ, which may result in either propelling the contents of the organ (as in peristalsis of the intestine) or increasing the resistance to flow (as in vasoconstriction). The basic mechanism underlying contraction of smooth muscle involves an interaction of myosin with actin (as in striated muscle), although there are some important differences. Specifically, contraction of smooth muscle is thick-filament regulated and requires an alteration in myosin before it can interact with actin, whereas contraction of striated muscle is thin-filament regulated and requires movement of the troponin-tropomyosin complex on the actin filament before myosin can bind to actin. Smooth muscle can contract in response to either electrical or hormonal signals and exhibits the ability to remain contracted for extended periods at low levels of energy consumption, which is important for functions such as maintaining vascular tone and hence blood pressure. Thus regulation of contraction of smooth muscle is complex, sometimes involving multiple intracellular signaling cascades. In the present chapter, effort is made to identify mechanisms underlying this diverse regulation of smooth muscle contraction and, when appropriate, compare these regulatory mechanisms with those observed in striated muscle. Alterations in smooth muscle function/ regulation that have been implicated in various pathological conditions are also discussed.
Overview of Smooth Muscle
Types of Smooth Muscle
Smooth muscle has been subdivided into two groups: single unit and multiunit. In single-unit smooth muscle the smooth muscle cells are electrically coupled such that electrical stimulation of one cell is followed by stimulation of adjacent smooth muscle cells. This results in a wave of contraction, as in peristalsis. Moreover, this wave of electrical activity, and hence contraction, in single-unit smooth muscle may be initiated by a pacemaker cell (i.e., a smooth muscle cell that exhibits spontaneous depolarization). In contrast, multiunit smooth muscle cells are not electrically coupled, so stimulation of one cell does not necessarily result in activation of adjacent smooth muscle cells. Examples of multiunit smooth muscle include the vas deferens of the male genital tract and the iris of the eye. Smooth muscle, however, is even more diverse, with the single-unit and multiunit classifications representing ends of a spectrum. The terms single-unit and multiunit represent an oversimplification, however; many smooth muscles are modulated by a combination of neural elements with at least some degree of cell-to-cell coupling and locally produced activators or inhibitors that also promote a somewhat coordinated response of smooth muscles.
A second consideration when discussing types of smooth muscle is the activity pattern (