5th lecture on the physiology of eye by dr. roomi

7/28/2019 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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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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2. The horizontal cells,

which transmit signals

horizontally in the outer

plexiform layer from therods and cones to

bipolar cells


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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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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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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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5th lecture on the physiology of eye by dr. roomi

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