Bullock (7), (10), (12), (13) has demonstrated the existence of
a third type of subthreshold response, which he calls the graded
response. While the postsynaptic membrane is quite often electrically
inexcitable, other regions of the somatic and dendritic membranes
appear to be moderately excitable. It is in these regions that Bullock
observes the graded response. If one applies a series of pulsed voltage
stimuli to the graded-response region, the observed responses would be
similar to those shown in Figure 4A. Plotting the peak response voltage
as a function of the stimulus voltage would result in a curve similar
to that in Figure 4B (see Ref. 3, page 4). For small values of input
voltage, the response curve is linear; the membrane is passive. As the
stimulus voltage is increased, however, the response becomes more and
more disproportionate. The membrane is actively amplifying the stimulus
potential. At even higher values of stimulus potential, the system
becomes regenerative; and a full action potential results. The peak
amplitude of the response depends on the duration of the stimulus as
well as on the amplitude. It also depends on the rate of application of
the stimulus voltage. If the stimulus potential is a voltage ramp, for
example, the response will depend on the slope of the ramp. If the rate
of rise is sufficiently low, the membrane will respond in a passive
manner to voltages much greater than the spike threshold for suddenly
applied voltages. In other words, the graded-response regions appear to
accommodate to slowly varying potentials.
In terms of functional operation, we can think of the synapse as a
transducer. The input to this transducer is a spike or series of spikes
in the presynaptic axon. The output is an accumulative, long-lasting
potential which in some way (perhaps not uniquely) represents the
pattern of presynaptic spikes. The pacemaker appears to perform the
function of a clock, producing periodic spikes or spike bursts or
producing periodic changes in the over-all excitability of the neuron.
The graded-response regions appear to act as nonlinear amplifiers and,
occasionally, spike initiators. The net result of this electrical
activity is transformed into a series of spikes which originate at
spike initiation sites and are propagated along axons to other neurons.
The electrical activity in the neuron described above is summarized in
the following outline (taken in part from Bullock (7)):
1. Synaptic Potentials
a. Excitatory or inhibitory
b. Facilitated, antifacilitated, or neither
c. With excitatory aftereffect, inhibitory aftereffect,
neither, or both
2. Pacemaker Potentials
a. Relaxation type, undulatory type, or none at all
b. Producing single spike, spike burst, or no spikes
c. Rhythmic or sporadic