Regulation and Integration of the Body
Comparison of Graded Potentials and Action Potentials
GRADED POTENTIAL (GP)
ACTION POTENTIAL (AP)
Cell body and dendrites, typically
Axon hillock and axon
Short distance—typically within cell body to axon
hillock (0.1–1.0 mm)
Long distance—from trigger zone at axon hillock through entire
length of axon (a few mm to over a meter)
Various sizes (graded); decays with distance
Always the same size (all-or-none); does not decay with distance
Chemical (neurotransmitter) or sensory stimulus
(e.g., light, pressure, temperature)
Voltage (depolarization, triggered by GP reaching threshold)
Voltage independent; occurs when stimulus is no
Voltage regulated; occurs when Na
channels inactivate and K
Stimulus responses can summate to increase
amplitude of graded potential
Does not occur; an all-or-none phenomenon
frequency of stimuli
Spatial: stimuli from
activity of a postsynaptic neuron. Otherwise, nerve impulses
would never result.
Two types of summation occur: temporal and spatial.
time) occurs when one
or more presynaptic neurons transmit impulses in rapid-ﬁre
order and bursts of neurotransmitter are released in quick
succession. ±e ﬁrst impulse produces a small EPSP, and
before it dissipates, successive impulses trigger more EPSPs.
±ese summate, causing the postsynaptic membrane to de-
polarize much more than it would from a single EPSP (Fig-
occurs when the postsynaptic neuron
is stimulated simultaneously by a large number of terminals
from one or, more commonly, many presynaptic neurons.
Huge numbers of its receptors bind neurotransmitter and
simultaneously initiate EPSPs, which summate and dramati-
cally enhance depolarization (Figure 11.19c).
Although we have focused on EPSPs here, IPSPs also sum-
mate, both temporally and spatially. In this case, the postsynap-
tic neuron is inhibited to a greater degree.
Most neurons receive both excitatory and inhibitory in-
puts from thousands of other neurons. Additionally, the
same axon may form diﬀerent types of synapses (in terms