Smooth muscle spontaneous depolarizations

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 ... 

A pacemaker potential involves gradual depolarization of the cell membrane to threshold. The subsequent generation of an action potential causes smooth muscle contraction. This type of spontaneous depolarization is referred to as a "pacemaker potential" because it creates a regular rhythm of contraction. 

Fig. 4.2. Effects of triphenylethylene SERMs on spontaneous and depolarization-induced contractions in visceral smooth muscle. Tamoxifen (a) and ethylbromide tamoxifen (EBTx, b) rapidly and reversibly inhibit spontaneous peristaltic activity in duodenal muscle. Both compounds also inhibit depolarization-induced tonic contraction of uterine muscle (c). The inhibition of L-type voltage-dependent calcium channels underlies the relaxing effects illustrated here. Drugs concentrations were 10 xM in all cases. %RA percent of activity related to maximal activity...

Cardiac glycosides affect all excitable tissues, including smooth muscle and the central nervous system. Inhibition of Na+/K+ ATPase in these tissues depolarizes and increases spontaneous activity both in neurons and in smooth muscle cells. The gastrointestinal tract is the most common site of digitalis toxicity outside the heart. The effects include anorexia, nausea, vomiting, and diarrhea. 

The Class IV drugs are calcium channel blockers. They decrease the inward current carried by calcium, resulting in a decrease in the rate of Phase 4 spontaneous depolarization and slowed conduction in tissues dependent on calcium currents, such as the AV node (Figure 17.12). Although voltage-sensitive calcium channels occur in many different tissues, the major effect of calcium-channel blockers is on vascular smooth muscle and the heart. 

Another principal means of action potential generation consists in spontaneous, rhythmic membrane depolarization. This occurs in specialized pacemaker cells in heart and smooth muscle. Therefore, while these tissues are modulated by neuronal and hormonal influences, they are capable of self-stimulation in the absence of any neuronal control. 

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. 

Arrhythmias arise from ectopic foci in the cardiac tissue. Recall that all cardiac myocytes have the potential for spontaneous depolarization because of their smooth muscle-like properties and the close proximity of the cell membrane to depolarization threshold. Normally, these foci are eliminated by the powerful depolarization of the SA nodal cells. Unopposed, these foci generate depolarizations that cause small premature contractions and inhibit full contraction of cardiac muscle in response to the normal SA nodal depolarization. This is of primary importance in ventricular tissue. 

Initiation of postjunctional activity. If an EPSP exceeds a certain threshold value, it initiates an action potential in the postsynaptic membrane by activating voltage-sensitive channels in the immediate vicinity. In certain smooth muscle types in which propagated impulses are minimal, an EPSP may increase the rate of spontaneous depolarization, cause Ca + release, and enhance muscle tone in gland ceds, the EPSP initiates secretion through Ca + mobdization. An IPSP, which occurs in neurons and smooth muscle but not in skeletal muscle, wdl tend to oppose excitatory potentials simultaneously initiated by other neuronal sources. The ultimate response depends on the summation of ad the potentials. 

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