In studying the recent literature in neurophysiology, one is
immediately struck by the diversity in form of both elicited and
spontaneous electrical activity in the single nerve cell. This applies
not only to the temporal patterns of all-or-none action potentials
but also to the graded somatic and dendritic potentials. The synaptic
membrane of a neuron, for example, is often found to be electrically
inexcitable and thus incapable of producing an action potential; yet
the graded, synaptically induced potentials show an amazing diversity
in form. In response to a presynaptic impulse, the postsynaptic
membrane may become hyperpolarized (inhibitory postsynaptic potential),
depolarized (excitatory postsynaptic potential), or remain at the
resting potential but with an increased permeability to certain ions
(a form of inhibition). The form of the postsynaptic potential in
response to an isolated presynaptic spike may vary from synapse to
synapse in several ways, as shown in Figure 1. Following a presynaptic
spike, the postsynaptic potential typically rises with some delay to
a peak value and then falls back toward the equilibrium or resting
potential. Three potentially important factors are the delay time
(synaptic delay), the peak amplitude (spatial weighting of synapse),
and the rate of fall toward the equilibrium potential (temporal
weighting of synapse). The responses of a synapse to individual spikes
in a volley may be progressively enhanced (facilitation), diminished
(antifacilitation), or neither (1), (2), (7), (8). Facilitation may be
in the form of progressively increased peak amplitude, or in the form
of progressively decreased rate of fall (see Figure 2). The time course
and magnitude of facilitation or antifacilitation may very well be
important synaptic parameters. In addition, the postsynaptic membrane
sometimes exhibits excitatory or inhibitory aftereffects (or both) on
cessation of a volley of presynaptic spikes (2), (7); and the time
course and magnitude of the aftereffects may be important parameters.
Clearly, even if one considers the synaptic potentials alone, he is
faced with an impressive variety of responses. Examples of the various
types of postsynaptic responses may be found in the literature, but for
purposes of the present discussion the idealized wave forms in Figure 2
will demonstrate the diversity of electrical behavior with which one is
faced.
[Illustration: A. EXCITATORY POSTSYNAPTIC POTENTIAL FROM APLYSIA (SEE
REF. 2)]
[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