Download ana-phy-ret2015

Dr. Ayesha Abdullah
17.12.2015
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 various categories of
retinal disorders.
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
Normal dimensions
Anatomical characteristics
Clinically Observable
characteristics
Optic disc
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’s a pale disc like
structure with vessels
emerging out of its center
called the cup. Its about
1.5 mm in size.
Macula
It is the area where the ganglion cells are
two layered. Contains the xanthophyl
pigment giving it the pigmented look.
It is about 5.5 mm in
diameter (3.5 disc
diameter/ 180 of visual
angle). Roughly the area
between the arterial
arcades.
Fovea
A depression in the inner retinal surface.
It contains cones only.
The inner nuclear layer and the ganglion
cell layer is absent.
A concave central retinal
depression about the same
size as the disc (1.5mm)
The thickest part of the retina
Area surrounding the fovea
Foveola
Parafovea
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
1. There are about 150 million receptors and only 1
million optic nerve fibers, there must be convergence
and thus mixing of signals
2. 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.
3. The response of cones to various wavelengths of light
is called their spectral sensitivity
4. There are blue, green, and red cones but more
accurately short, medium, and long wavelength
sensitive cone subgroups- trichromatic vision
Some important facts
5. 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
6. 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.
vitreous
RPE
Rods & Cones
• Rod System
–
–
–
–
Achromatic
High convergence
High light sensitivity
Low visual acuity
• Cone System
–
–
–
–
Chromatic
Low convergence
Low light sensitivity
High visual acuity
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?
• Why light has to travel through all the layers
before generating the sensation of vision?