Smooth muscle of the ureter is regarded as unitary because only pacemaker muscle cells in the renal pelvis are innervated and excitation spreads among individual myocytes by gap junctions. In contrast, detrusor contraction is innervation dependent (cell to cell coupling is limited to small collections of myocytes). Also, the contraction rate for detrusor smooth muscle is relatively fast among smooth muscles. Thus the detrusor muscle is characterized as a multiunit, phasic type of smooth muscle, as is bladder neck and urethral smooth muscle.

Additional information about detrusor myocytes: They have resting transmembrane potential ranging from 35 to 70 mV. Depolarization increases the frequency of spontaneous action potentials in myocytes, while hyperpolarization has the opposite effect. The rising phase of the myocyte action potential (AP) is due to Ca++ influx through voltage gated ion channels; the AP fall is driven by K+ efflux. The Ca++ influx associated with an AP leads to additional Ca++ release from sarcoplasmic reticulum. The combined increased cytoplasmic [Ca++] initiates smooth muscle contraction via a complex series of events involving Ca++—calmodulin formation, myosin-II phosphorylation, and myosin-actin interaction.

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