graded potentials


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 .




Bipolar 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



Light now fills the excitatory region of the receptive field.

Number of




If light falls in the inhibitory region, the firing rate of cell is reduced.

Number of




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)