160 



DONALD M. MAYNARD 



Fig. 14. Inhibitory post-synaptic potentials in follower cells (intracellular 

 recordings, Puinilinis). a. Spontaneously active cell. Pacemaker potential is inter- 

 rupted by onset of inhibition (90/sec) which drives the internal potential negative 

 towards a maintained hyperpolarization. Note rebound. Calibration for upper 

 trace, 200 msec. Lower trace at faster film speed, 60/sec inhibitor stimulation. 

 The first stimuli produce only artifacts; clear response is seen beginning with the 

 seventh and is a brief depolarizing, facilitating synaptic potential followed by an 

 hyperpolarizing phase which summates. b. Follower with spontaneity. Synaptic 

 potential hyperpolarizes only, but holds high level of polarization following 

 burst discharge. Calibration; 400 msec, 20 mV. c. Simple follower without 

 spontaneity. Depolarizing synaptic potentials; inhibitor stimulated at 60/'sec. 

 In second trace e. p. s. p. from ganglion neurons and i. p. s. p. from inhibitor summate. 

 Note two abortive pacemaker discharges in first line. Calibration, top trace, 1 sec; 

 lower trace, 100 msec, 10 mV. (Courtesy, Terzuolo and Bullock, Arch. iral. Biol. 



96 : 117-134, 1958.) 



Figure 16 provides some bases for understanding the dual action of the 

 inhibitor. As with most i.p.s.p.'s in other systems, the direction of the potential 

 change depends upon the value of the membrane potential, lower membrane 

 potentials leading to hyperpolarizing i.p.s.p.'s, higher membrane potentials, 

 to depolarizing i.p.s.p.'s. A non-spontaneous ganglion neuron such as Unit 1 

 or 2 in Panulirus may consequently be converted into a spontaneous unit 

 with hyperpolarizing i.p.s.p.'s simply by presetting the membrane potential 

 at a lower level. This would mean that the dual nature of the inhibitor fiber 

 does not necessarily require qualitative differences in presynaptic termina- 

 tions, but rather simply that the postsynaptic elements be on different sides 

 of the reversal potential for the i.p.s.p. The confirmation of such a sup- 



