Membrane potential current generated

The sinoatrial (SA) node is located in the wall of the right atrium near the entrance of the superior vena cava. The specialized cells of the SA node spontaneously depolarize to threshold and generate 70 to 75 heart beats/ min. The "resting" membrane potential, or pacemaker potential, is different from that of neurons, which were discussed in Chapter 3 (Membrane Potential). First of all, this potential is approximately -55 mV, which is less negative than that found in neurons (-70 mV see Figure 13.2, panel A). Second, pacemaker potential is unstable and slowly depolarizes toward threshold (phase 4). Two important ion currents contribute to this slow depolarization. These cells are inherently leaky to sodium. The resulting influx of Na+ ions occurs through channels that differ from the fast Na+ channels that cause rapid depolarization in other types of excitable cells. Toward the end of phase... 

Studied in both an experimental and theoretical manner. The link between glycolytic oscillations and the pulsatile secretion of insulin in pancreatic p cells [53] is another topic of current concern. Models for the latter phenomenon rely on the coupling between intracellular metabolic oscillations and an ionic mechanism generating action potentials. Such coupling results in bursting oscillations of the membrane potential, which are known to accompany insulin secretion in these cells [54, 55]. 

For the first time, the use of artificial enzyme membranes allows the study of the interaction between enzyme activity and membrane potential in a well-defined context. Before the recent progress in manufacturing artificial membrane bearing immobilized enzyme, Blumenthal et al.21 described a system in which a papain solution was sandwiched between two cation and anion exchange membranes. Under short-circuit conditions the system was able to generate a current. A nonequiLbrium thermodynamic analysis was developed by the authors. 

The classical experimental work in this area is that of Cole (1949) and in particular that of Hodgkin and Huxley (1952). Their work concentrated upon the potential difference generated across the wall of the axon, which they regarded as a partly permeable membrane. A schematic of their apparatus is shown in Fig. 14.14. This equipment allowed current pulses to be sent across any section of the wall of an axon that was regarded as a membrane. 

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