Review of anatomy & physiology of the Retina

Dr. Ayesha Abdullah
14.12.2012

Learning outcomes
By the end of this lecture the students would be
able to;
• Correlate the structural organization of the
retina with its functions and development.
• Identify structural landmarks on retinal
photographs.
• Name the investigations commonly employed
for the assessment of retinal disorders.

Some questions
• How do we see
• What are the similarities and differences between a camera
and the eye
• Name part of the nervous system that can be visualized
without any invasive procedure
• Why is the optic disc referred to as the blind spot
• Have you noticed tiny bright moving dots when looking into
the blue sky
• If the photoreceptor had been anteriorly placed, what
would have happened
• How do we know its day/ night
• How does the brain regulate circadian rhythms

Camera and the eye

• Unlike the image from a camera the resolution of the retinal image is not
uniform.
• Why
• What is the result
• There are about 100 times more photoreceptors than the Ganglion cells.
• Retina compresses images as unlike the camera.

Anatomical landmarks of the retina

Optic disc
Macula
Fovea
Foveola
Normal dimensions
Anatomical characteristics
Place where the optic nerve fibers leave
the retina. It is devoid of rods and cones
hence the blind spot. Contains the central
retinal artery and vein
It is the area where the ganglion cells are
two layered. Contains the xanthophyl
pigment giving it the pigmented look.
A depression in the inner retinal surface.
It contains cones only.
The inner nuclear layer and the ganglion
cell layer is absent.
Clinically Observable
characteristics
It’s a pale disc like
structure with vessels
emerging out of its center
called the cup. Its about
1.5 mm in size.
It is about 5.5 mm in
diameter (3.5 disc
diameter/ 18 0 of visual
angle). Roughly the area
between the arterial
arcades.
A concave central retinal
depression about the same
size as the disc (1.5mm)
Parafovea The thickest part of the retina Area surrounding the fovea

Histological structure of the retina

Development of the retina

Functions of the retina
• Light perception
• Brightness appreciation
• Contrast sensitivity
• Two point discrimination and appreciation of
details
• Colour perception
• Light and dark adaptation
• Circadian rhythms & hormonal balance

Some important facts
• There are about 150 million receptors and only 1 million optic nerve
fibers, there must be convergence and thus mixing of signals
• The horizontal action of the horizontal and amacrine cells can allow
one area of the retina to control another (e.g., one stimulus
inhibiting another). This inhibition is key to the sum of messages
sent to the higher centers of the brain.
• The response of cones to various wavelengths of light is called their
spectral sensitivity
• There are blue, green, and red cones but more accurately short,
medium, and long wavelength sensitive cone subgroupstrichromatic
vision
• The receptive field of a sensory neuron is a region of space in which
the presence of a stimulus will alter the firing of that neuron
• The receptive field of a Ganglion cell in the retina of the eye is
composed of input from all of the photoreceptors which synapse
with it, and a group of ganglion cells in turn forms the receptive
field for a cell in the brain. This process is called convergence.

RPE
vitreous

• Rod System
– Achromatic
– High convergence
– High light sensitivity
– Low visual acuity
• Cone System
– Chromatic
– Low convergence
– Low light sensitivity
– High visual acuity
Rods & Cones

Direction of visual
impulse
Direction of light

Investigations for retinal structural and
functional assessment
• Clinical assessment- Ophthalmoscopy

Ophthalmic investigations
• Ultrasound –B & A scans
• Ocular coherence tomography (OCT)
• Angiography
• Elctroretinogram
• Elctro-oculogram

OCT

Angiography

Electroretinogram

Homework
• What is the blood supply of the inner and
outer retinal layers
• What makes the inner and outer retinal blood
barriers and what is their significance
• E mail me at, the answer should not more
than 04 lines.
msqheartline@hotmail.com