[Illustration: B. EXCITATORY POSTSYNAPTIC POTENTIAL FROM PANULIRUS (SEE REF. 1)] [Illustration: C. EXCITATORY POSTSYNAPTIC POTENTIAL FROM MAMMALIAN MOTONEURONE (SEE REF. 24)] [Illustration: D. PRESYNAPTIC SPIKE Figure 1—Excitatory postsynaptic potentials in response to a single presynaptic spike] [Illustration: A. TEMPORAL FACILITATION] [Illustration: B. AMPLITUDE FACILITATION] [Illustration: C. ANTIFACILITATION] [Illustration: D. INHIBITORY POSTSYNAPTIC POTENTIALS EXHIBITING BIPHASIC REBOUND] [Illustration: E. PRESYNAPTIC SPIKE BURST Figure 2—Idealized postsynaptic potentials] In addition to synaptically induced potentials, low-frequency, spontaneous potential fluctuations have been observed in many neurons (2), (7), (9), (10), (11). These fluctuations, generally referred to as pacemaker potentials, are usually rhythmic and may be undulatory or more nearly saw-toothed in form. The depolarizing phase may be accompanied by a spike, a volley of spikes, or no spikes at all. Pacemaker frequencies have been noted from ten or more cycles per second down to one cycle every ten seconds or more. Some idealized pacemaker wave forms are shown in Figure 3. [Illustration: A. PERIODIC BURSTS] [Illustration: B. PACEMAKER POTENTIALS WITHOUT SPIKES] [Illustration: C. PACEMAKER POTENTIALS WITH SINGLE SPIKES ON DEPOLARIZING PHASE Figure 3—Idealized pacemaker potentials] [Illustration: A. FORM OF VOLTAGE STIMULI AND RESULTING MEMBRANE POTENTIAL CHANGES.] [Illustration: B. RESPONSE CURVE OF TYPICAL GRADED RESPONSE REGION. Figure 4—Graded response]