graded potentials

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Physiology of Photoreceptors
• Vertebrate photoreceptors
hyperpolarize and produce
graded potentials
• Photoreceptors use
glutamate as transmitter
• Bipolar cells can both
hyperpolarize and
depolarize producing both
ON and OFF responses
• ON bipolar – glutamate is
inhibitory
• OFF bipolar – glutamate is
excitatory
Receptive Fields
• Record from a single ganglion cell in the retina
• Using small spots of light activate the portion of the
visual field that activates the neuron
Schematic Retina Showing a
Receptive Field
Orange are excitatory inputs into the receptive field.
Blue are inhibitory inputs into the receptive field.
Receptors
Horizontal
Cells
Bipolar Cells
Amacrine
Cells
Ganglion Cells
+
-
The excitatory connection
stimulates and increase in
the firing rate in the
ganglion cell.
Light strikes a cone
with an excitatory
connection to the
ganglion cell
Number of Action
Potentials:
1
Light now fills
the excitatory
region of the
receptive field.
Number of
Action
Potentials:
12
If light falls
in the
inhibitory
region, the
firing rate of
cell is
reduced.
Number of
Action
Potentials:
8
Lateral Inhibition – Variations in the
On/Off Structure
• Lights on the edge of
the field cause a
reduction in the
background activity of
the cell
• On and off neurons
• Center-surround
structure – need to
examine in light of
different channels of
information direct to
the cerebral cortex
Receptive Fields in
the Retina
• Two types of ganglion
cells:
– on and off dependent
upon the bipolar neurons
• Center Surround
structure of the
receptive field
described by Kuffler
• Best activated by
central illumination
• Best inhibited by
annular illumination
Different View of Center-Surround
Organization: Parallel Pathways
• Transformation of
visual information is
evident in the ganglion
cells of the retina
• X cells – sustained
linear responses
• Y cells – transient,
excitatory non-linear
responses
P and M Projections to LGN:
Different Physiologic Channels
• P cells in the retina (also known as midget ganglion cells) project to
the parvocellular layers (3-6) of LGN
• M cells in the retina (also known as parasol cells) project to the
magnocellular (ventral most) layers (1-2) of the LGN
• Intercalated layers are termed koniocellular (dustlike or tiny cells)
Physiology of V1
• Originally studied by
Hubel and Wiesel
who demonstrated
two types of neurons
– Simple cells –
constructed from
LGN on/off cells
– Complex cells –
constructed from
simple cells
Cortical Simple Cell
Cortical Complex Cells: Example
of Hierarchy of V1
• Strong orientation
selectivity in cells
• Moving bars in a
specific direction
• NO on/off areas
like in simple cells
• Receptive fields
were not elongated
• Located in layers
2,3, and 5 which
receive input from
layer 4 (from ?
simple cells)
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