retina

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

28. THANK YOU