Smooth muscle single-unit

Feature Skeletal muscle Multiunit smooth muscle Single unit smooth muscle... 

Distinguish between multiunit smooth muscle and single-unit... 

Action potentials are generated in single-unit smooth muscle. Simultaneous depolarization of 30 to 40 smooth muscle cells is required to generate a propagated action potential the presence of gap junctions allows this to occur readily. Because single-unit smooth muscle is self-excitable and capable of generating action potentials without input from the autonomic nervous system, it is referred to as myogenic. In this muscle, the function of the autonomic nervous system is to modify contractile activity only. Input is not needed to elicit contraction. 

The ability to depolarize spontaneously is related to the unstable resting membrane potentials in single-unit smooth muscle. Two types of spontaneous depolarizations may occur ... 

Many factors influence the contractile activity of smooth muscle. The strength of contraction of multiunit smooth muscle may be enhanced by stimulation of a greater number of cells, or contractile units. This mechanism is directly comparable to motor-unit recruitment employed by skeletal muscle. As the number of contracting muscle cells increases, so does the strength of contraction. However, this mechanism is of no value in single-unit smooth muscle. Due to the presence of gap junctions, all of the muscle cells in the tissue are activated at once. 

Anderson (1978) recommended the rabbit detrusor muscle as an unique in vitro smooth muscle preparation. Rabbit detrusor muscles are thin and devoid of underlying submucosal tissue with parallel fiber orientation. The tissue exhibits autorhythmicity, characteristic of most single unit type smooth muscle preparations and can be employed in either isometric or isotonic organ bath recording systems. 

In contrast to many other smooth muscle types, ASM is regarded as being multi-unit rather than single unit, which means that each smooth muscle cell is individually innervated. This is associated with a relative lack of gap junctions which normally provide a low resistance electrical pathway for cell to cell communication. Spontaneous electrical activity is rarely observed in ASM cells and action potentials only occur in exceptional circumstances, the usual electrical response to spasmogens being graded depolarization without the overall muscle bundle acting as a syncytium. 

It is Intriguing to speculate whether this same kind of ultra-structural differentiation (different repeating units) underlies the recognized ability of a single type of smooth muscle cell, for example, to respond to stimulation by a number of different hormones and humoral substances, each in a chemically precise but different way. Hokin has already suggested the existence of "membrane patches of active llproproteln... 

Worley, J.F., Deitmer, J.W., and Nelson, M.T. (1986) Single nisoldipine-sensitive calcium channels in smooth muscle cells isolated from rabbit mesenteric artery. Proceedings of the National Academy of Sciences of the United States of America, 83 5746-5750. 

One of the possible causes of the receptor reserve is demonstrated by the all or none response, e.g. in smooth-muscle cells. A single effector unit. Le. a single smooth-muscle cell, either contracts maximally or does not contract at all. This reaction of a single unit is evoked at a certain drug concentration, and thus at a certain RA concentration and at a definite stimulus value. Even if the stimulus is increased no further enhancement of the reaction of the single unit is possible. 

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