Figure 6. Asymmetrical membrane potential changes recorded from different cell types during the pacemaker cycle.
A, the equivalent circuit for a small segment of antral wall. The circular muscle layer is shown as a variable resistor (RC), initially being allotted the same value as the longitudinal muscle layer (RL; 9.3 MΩ). This simulation illustrates the difference between a symmetrically loaded ICCmy layer and a more realistic asymmetrically loaded one. Ba, a slow wave with an initial component of about 12 mV; the amplitude of the initial component, which was determined in the presence of caffeine, is shown as a dotted line. Bb, a 40 mV pacemaker potential recorded in the ICCmy layer, which generated the initial component of the slow wave shown in Ba. Bc, a follower potential recorded in the longitudinal muscle layer. C, simulations of slow waves (Ca), pacemaker potentials (Cb) and follower potentials (Cc) for four different values of RC (equal to RL, RL/2, RL/4 and RL/8; i.e. 9.3, 4.6, 2.3 and 1.2 MΩ, respectively). Note that large changes in RC result in moderate changes to the amplitude of the initial component of the slow wave and negligible changes to the amplitudes of the pacemaker potential and the follower potential. The membrane potentials of the circular smooth muscle layer, ICCmy and longitudinal layer were −64, −64 and −65 mV, respectively.