1. The document discusses neuronal signalling in the retina, describing how light is detected by photoreceptors and converted into electrical signals. 2. It explains that photoreceptors contain light-sensitive proteins that undergo chemical changes when exposed to light, initiating a signalling cascade that ultimately produces action potentials. 3. The signalling causes photoreceptors to hyperpolarize in response to light, decreasing the flow of sodium ions, in contrast to most receptors that depolarize upon stimulation.

1. Neuronal signalling at
retina
Dr. Sai Sailesh Kumar G
Associate Professor
Department of Physiology
NRIIMS
Email: dr.goothy@gmail.com

2. Introduction
Light must pass through several retinal layers before
reaching the photoreceptors

3. Introduction
The major function of the eye is to focus light rays from the
environment on the rods and cones, the photoreceptor cells
of the retina.
The photoreceptors then transform the light energy into
electrical signals for transmission to the CNS.

4. Introduction
The receptor-containing portion of the retina is actually an
anatomic extension of the CNS, not a separate peripheral
organ.
During embryonic development, the retinal cells “back out” of
the nervous system, so the retinal layers, surprisingly, are
facing backward.

5. Introduction
 The neural portion of the retina consists of three layers of excitable cells
 (1) the outermost layer (closest to the choroid) containing the rods and cones, whose light-
sensitive ends face the choroid (away from the incoming light);
 (2) a middle layer of bipolar cells and associated interneurons;
 and (3) an inner layer of ganglion cells.
 Axons of the ganglion cells join to form the optic nerve, which leaves the retina slightly off-center.
 The point on the retina at which the optic nerve leaves and through which blood vessels pass is
the optic disc
 This region is often called the blind spot; no image can be detected in this area because it has no
rods and cones

7. Introduction
Light must pass through the ganglion and bipolar layers before
reaching the photoreceptors in all areas of the retina except the
fovea.
In the fovea, which is a pinhead-sized depression located in the
exact center of the retina,
 the bipolar and ganglion cell layers are pulled aside so that light

8. Introduction
Because of this feature, and because only cones have
greater acuity or discriminative ability than the rods) are
found here,
the fovea is the point of most distinct vision.
In fact, the fovea has the greatest concentration of cones in
the retina.

9. Introduction
 The pea-sized area immediately surrounding the fovea, the macula lutea, also
has a high concentration of cones and fairly high acuity
 Age-related macular degeneration (AMD) is the leading cause of blindness
 This condition is characterized by loss of photoreceptors in the macula lutea in
association with advancing age.
 Its victims have a “doughnut” vision.
 They suffer a loss in the middle of their visual field, which normally has the
highest acuity, and are left with only the less distinct peripheral vision

10. Phototransduction
Photoreceptors (rod and cone cells) consist of three parts
1. An outer segment, which lies closest to the eye’s exterior,
facing the choroid. It detects the light stimulus
2. An inner segment, which lies in the middle of the
photoreceptor’s length. It contains the metabolic machinery of
the cell

11. Phototransduction
3. A synaptic terminal, which lies closest to the eye’s interior,
facing the bipolar cells.
It varies its rate of neurotransmitter release, depending on the
extent of dark or light exposure detected by the outer
segment

13. Phototransduction
The outer segment, which is rod-shaped in rods and cone-shaped
in cones
consists of stacked, flattened, membranous discs containing an
abundance of light-sensitive photopigments.
Each retina contains more than 125 million photoreceptors,
more than 1 billion photopigments may be packed into the outer

14. Phototransduction
 Photopigments undergo chemical alterations when activated by light.
 Through a series of steps, this light-induced change and subsequent activation
of the photopigment bring about a receptor potential in the photoreceptor that
ultimately leads to the generation of action potentials in ganglion cells
 which transmits this information to the brain for visual processing.
 A photopigment consists of two components: opsin, an integral protein in the
disc plasma membrane; and retinal, a derivative of vitamin A.
 Retinal is the light-absorbing part of the photopigment.

15. Phototransduction
 the process of converting light stimuli into electrical signals,
 is basically the same for all photoreceptors,
 but the mechanism is contrary to the usual means by which receptors respond to their
adequate stimulus.
 Receptors typically depolarize when stimulated,
 but photoreceptors hyperpolarize on light absorption.
 Let us first examine the status of the photoreceptors in the dark, and then consider what
happens when they are exposed to light.
 We use rods as an example

16. Photoreceptor Activity in the Dark
 The photopigment in rods is rhodopsin.
 Retinal exists in different conformations in the dark and light.
 In the dark, it exists as 11-cis retinal,
 which fits into a binding site within the interior of the opsin portion of rhodopsin
 The plasma membrane of a photoreceptor’s outer segment contains chemically gated
 Na+ channels.
 Unlike other chemically gated channels that respond to extracellular chemical messengers,
 these channels respond to an internal second messenger, cyclic GMP, or cGMP (cyclic guanosine
monophosphate).
 Binding of cGMP to these Na+ channels keeps them open.

17. Photoreceptor Activity in the Dark
 In the absence of light,
 the concentration of cGMP is high
 Therefore, the Na+ channels of a photoreceptor, are open in the absence of stimulation, that is, in
the dark.
 The resultant passive inward Na+ leak,
 the so-called dark current,
 depolarizes the photoreceptor.
 The passive spread of this depolarization from the outer segment to the synaptic terminal
 Voltage-gated Ca2+ channels open.
 Ca2+ entry triggers the release of NT - glutamate

20. Photoreceptor Activity in the Light
 On exposure to light, the concentration of cGMP is
decreased through a series of biochemical steps triggered by
photopigment activation
When 11-cis retinal absorbs light, it changes to the all-trans
retinal conformation

22. Photoreceptor Activity in the Light
 On exposure to light, the concentration of cGMP is
decreased through a series of biochemical steps triggered by
photopigment activation
When 11-cis retinal absorbs light, it changes to the all-trans
retinal conformation

25. Further Retinal Processing of Light Input
 Each photoreceptor synapses with two side-by-side bipolar cells,
one an on-center bipolar cell and the other an off-center bipolar
cell.
These cells, in turn, terminate respectively on on-center ganglion
cells and off-center ganglion cells,
whose axons collectively form the optic nerve for transmission of

26. Further Retinal Processing of Light Input
 Each photoreceptor synapses with two side-by-side bipolar cells,
one an on-center bipolar cell and the other an off-center bipolar
cell.
These cells, in turn, terminate respectively on on-center ganglion
cells and off-center ganglion cells,
whose axons collectively form the optic nerve for transmission of

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