Spike Potentials. The spike potentials are true action potentials. They occur automatically when
the resting becomes more positive than about −40 millivolts (the normal resting membrane potential
in the smooth muscle fibers of the gut is between −50 and −60 millivolts). Note in Figure 62-3 that
each time the peaks of the slow waves temporarily become more positive than −40 millivolts, spike
potentials appear on these peaks. The higher the slow wave potential rises, the greater the frequency of
the spike potentials, usually ranging between 1 and 10 spikes per second. The spike potentials last 10
to 40 times as long in gastrointestinal muscle as the action potentials in large nerve fibers, each
gastrointestinal spike lasting as long as 10 to 20 milliseconds. Another important difference between
the action potentials of the gastrointestinal smooth muscle andthose of nerve fibers is the manner in
which they are generated.
