5th lecture on the physiology of eye by dr. roomi
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PHYSIOLOGY OF EYE
BY
DR. MUDASSAR ALI ROOMI (MBBS, M. Phil.)
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Neural Circuitry of the Retina
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Neural Circuitry of the Retina
1. The photoreceptors
themselvesthe rods
and coneswhich
transmit signals to theouterplexiform layer,
where they synapse
with bipolar cells and
horizontal cells
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Neural Circuitry of the Retina
2. The horizontal cells,
which transmit signals
horizontally in the outer
plexiform layer from therods and cones to
bipolar cells
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Neural Circuitry of the Retina
3. The bipolar cells,
which transmit signals
verticallyfrom the rods,
cones, and horizontalcells to the inner
plexiform layer, where
they synapse with
ganglion cells andamacrine cells
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Neural Circuitry of the Retina
4. The amacrine cells,
which transmit signals in
two directions, either
directly from bipolar cells
to ganglion cells or
horizontally within the
inner plexiform layer from
axons of the bipolar cells
to dendrites of the
ganglion cells or to other
amacrine cells
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Neural Circuitry of the Retina
5. The ganglion cells,
which transmit output
signals from the retina
through the optic nerveinto the brain
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The Visual Pathway from the Cones to the Ganglion
Cells Functions
Differently from the Rod Pathway.
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Neurotransmitters Released by
Retinal Neurons
both the rods and the cones release glutamate attheir synapses with the bipolar cells. ***
amacrine cells secrete at least eight types oftransmitter substances, including gamma-aminobutyric acid, glycine, dopamine,acetylcholine, and indolamine, all of whichnormally function as inhibitory transmitters.
The transmitters of the bipolar, horizontal, and
interplexiform cells are unclear, but at least someof the horizontal cells release inhibitorytransmitters.
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Transmission of Most Signals Occurs in the Retinal
Neurons by Electrotonic Conduction, Not by Action
Potentials
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Lateral Inhibition to Enhance Visual Contrast
Function of the Horizontal Cells
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Lateral Inhibition to Enhance Visual Contrast
Function of the Horizontal Cells
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Excitation of Some Bipolar Cells and
Inhibition of OthersThe Depolarizing
and Hyperpolarizing Bipolar Cells
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Excitation of Some Bipolar Cells and
Inhibition of OthersThe Depolarizing
and Hyperpolarizing Bipolar Cells
There are two possible explanations for thisdifference.
1. One explanation is that the two bipolar cells are of
entirely different types
one responding bydepolarizing in response to the glutamateneurotransmitter released by the rods and cones,and the other responding by hyperpolarizing.
2. The other possibility is that one of the bipolar cells
receives direct excitation from the rods and cones,whereas the other receives its signal indirectlythrough a horizontal cell.
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Amacrine Cells and Their Functions
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Function of Amacrine Cells
About 30 different types
Some involved in the direct pathway from rods to bipolar to
amacrine to ganglion cells
Some amacrine cells respond strongly to the onset of the visualsignal, some to the extinguishment of the signal
Some respond to movement of the light signal across the retina
Amacrine cells are a type ofinterneuron that aid in the
beginning of visual signal analysis.
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Rods, Cones and Ganglion Cells
Each retina has 100 million rods and 3 million
cones and 1.6 million ganglion cells.
60 rods and 2 cones for each ganglion cell
At the central fovea there are no rods and theratio of cones to ganglion cells is 1:1.
May explain the high degree of visual acuity in the
central retina
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Three Types of Ganglion Cells
W cells(40%) receive most of their excitation from rod cells.
Large receptive field
sensitive to directional movement in the visual field
they are probably important for much of our cruderod vision under dark conditions
X cells (55%) small receptive field, discrete retinallocations, may be responsible for the transmission ofthe visual image itself, always receives input from at
least one cone, may be responsible for colortransmission.
Y cells (5%) large receptive field respond toinstantaneous changes in the visual field.
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Neural Organization of the Retina
Direction of
light Figure 50-11; Guyton & Hall X cells ?W cells ?
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Excitation of Ganglion Cells
spontaneously active with continuous action
potentials (basic 5-40 AP per sec)
visual signals are superimposed on this
background
many excited by changes in light intensity
respond to contrast borders, this is the way
the pattern of the scene is transmitted to the
brain
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Transmission of Color Signals by the
Ganglion Cells
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Processing in the Visual Cortex
separation of the signals from the two eyes is
lost in the primary visual cortex
signals from one eye enter every othercolumn, alternating with signals from the
other eye
allows the cortex to decipher whether the two
signals match
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Connections in the
Visual Cortex
In primary cortex Blobs receive
lateral signals from adjacent
columns respond to color vision
In secondary cortex color
blobs Decipher higher
meaning of color
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Analysis of the Visual Image
The visual signal in the primary visual cortex isconcerned mainly with contrasts in the visualscene.
The greater the sharpness of the contrast, thegreater the degree of stimulation.
Also detects the direction of orientation of eachline and border.
for each orientation of a line, a specificneuronal cell is stimulated.
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