562
UNIT 3
Regulation and Integration of the Body
15
inhibited, bipolar cells depolarize and release neurotransmitter
onto ganglion cells. Once the signal reaches the ganglion cells,
it is converted into an AP. Tis AP is transmitted to the brain
along the ganglion cell axons that make up the optic nerve.
regulate neurotransmitter release at the synapse by opening or
closing voltage-gated Ca
2
1
channels.
As shown in the right panel of Figure 15.18, for example,
light hyperpolarizes photoreceptors, which then stop releas-
ing their inhibitory neurotransmitter (glutamate). No longer
Photoreceptor
cell (rod)
Bipolar
cell
Ganglion
cell
Light
Na
+
Ca
2
+
Ca
2
+
Ca
2
+
-
40 mV
-
40 mV
-
70 mV
-
70 mV
cGMP-gated channels
close, so cation influx
stops. Photoreceptor
hyperpolarizes.
cGMP-gated channels
open, allowing cation influx.
Photoreceptor depolarizes.
Voltage-gated Ca
2
+
channels close in synaptic
terminals.
No neurotransmitter
is released.
Lack of IPSPs in bipolar
cell results in depolarization.
Depolarization opens
voltage-gated Ca
2
+
channels;
neurotransmitter is released.
EPSPs occur in ganglion
cell.
Action potentials
propagate along the
optic nerve.
Voltage-gated Ca
2
+
channels open in synaptic
terminals.
Neurotransmitter is
released continuously.
Hyperpolarization closes
voltage-gated Ca
2
+
channels,
inhibiting neurotransmitter
release.
No EPSPs occur in
ganglion cell.
No action potentials occur
along the optic nerve.
Below, we look at a tiny column of retina.
The
outer segment of the rod, closest to the back
of the eye and farthest from the incoming light,
is at the top.
Neurotransmitter causes
IPSPs in bipolar cell.
Hyperpolarization results.
Light
In the dark
In the light
6
7
5
4
3
2
1
6
7
5
4
3
2
1
Figure 15.18
Signal transmission in the retina.
EPSP
5
excitatory postsynaptic potential;
IPSP
5
inhibitory postsynaptic potential.
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