412
UNIT 3
Regulation and Integration of the Body
11
Table 11.2
Comparison of Graded Potentials and Action Potentials
 
GRADED POTENTIAL (GP)
ACTION POTENTIAL (AP)
Location
of event
Cell body and dendrites, typically
Axon hillock and axon
Distance
traveled
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)
Amplitude
(size)
Various sizes (graded); decays with distance
Always the same size (all-or-none); does not decay with distance
Stimulus for
opening ion
channels
Chemical (neurotransmitter) or sensory stimulus
(e.g., light, pressure, temperature)
Voltage (depolarization, triggered by GP reaching threshold)
Positive feed-
back cycle
Absent
Present
Repolarization
Voltage independent; occurs when stimulus is no
longer present
Voltage regulated; occurs when Na
1
channels inactivate and K
1
channels open
Summation
Stimulus responses can summate to increase
amplitude of graded potential
Does not occur; an all-or-none phenomenon
Temporal: increased
frequency of stimuli
Spatial: stimuli from
multiple sources
Cell body
Dendrites
Axon
Axon hillock
Short distance
Long distance
Axon hillock
activity of a postsynaptic neuron. Otherwise, nerve impulses
would never result.
Two types of summation occur: temporal and spatial.
Temporal summation
(
temporal
5
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-
ure 11.19b).
Spatial summation
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
previous page 446 Human Anatomy and Physiology (9th ed ) 2012 read online next page 448 Human Anatomy and Physiology (9th ed ) 2012 read online Home Toggle text on/off