Unit One: Introduction to Physiology: The Cell and General PhysiologyChapter 50: The Eye: II. Receptor and
Neural Function of the Retina
Guyton and Hall, Textbook of Medical Physiology, 12th edition
Anatomy and Physiology of the Retina
Layers of the Retina-functional components
arranged in layers from the outside to the inside
Pigmented layer
Outer nuclear layer containing the cell bodies of the
rods and cones
Outer plexiform layer
Inner nuclear layer
Inner plexiform layer
Inner limiting membrane
Layers of the Retina
Fovea- minute area in the center of the retina
(1 sq mm) capable of acute vision; contains
only cones
The outer segment
The inner segment
Fig. 50.3 Schematic drawing of the functional parts
of the rods and cones
Anatomy and Physiology of the Retina
Rods and Cones
outer segment
In cones, it is one of three color pigments which
function exactly like rhodopsin
Rods and Cones
In the outer segments of both rods and cones are
large numbers of discs (as many as 1000 per rod or
cone)
into the membranes of the discs
Inner segment contains the usual organelles and
cytoplasm
Rods and Cones
the horizontal and bipolar cells
Pigment Layer of the Retina
Melanin prevents light refraction throughout
the eyeball
Anatomy and Physiology of the Retina
Pigment Layer of the Retina
Vitamin A is an important precursor of the
photosensitive chemicals of rods and cones
Anatomy and Physiology of the Retina
Fig. 50.4 Membranuous structures of t he outer segments of a
rod and cone
Blood Supply of the Retina
Central retinal artery enters with the optic nerve
Branches to supply the entire retinal surface
Outermost layer is adherent to the choroid which
is also a highly vascular area
Photochemistry of Vision
Rhodopsin-Retinal Visual Cycle
Photochemistry of Vision
rhodopsin begins to decompose;
the retinal portion of rhodopsin, which converts
cis into a trans form and cannot bind to the active
site on the protein.
Photochemistry of Vision
Reformation of Rhodopsin
Requires energy and is catalyzed by retinal isomerase
Once formed it binds to the protein and is stable
Photochemistry of Vision
The trans-retinol is then converted to cis-retinal
Vitamin A is present in the pigment layer of the
retina and in the cytoplasm of rods
d. Excess retinal is converted to vitamin A
Photochemistry of Vision
by Light
depolarizing
in the outer segment of the rod
This causes hyperpolarization of the entire rod
membrane
Photochemistry of Vision
Fig. 50.6 Movement of sodium and potassium ions through the inner
and outer segments of the rod
Photochemistry of Vision
Fig. 50.7 Phototransduction in the outer segment of the photoreceptor membrane
Photochemistry of Vision
of the Receptor Potential to Light Intensity
Receptor potential occurs in 0.3 seconds and
lasts for about 1 second in the rods
In the cones it occurs four times as fast
Receptor potential is approx. proportional to the
logarithm of the light intensity which allows the
eye to discriminate light intensities through a range
many thousand times as great as would be otherwise
Photochemistry of Vision
Decreases Membrane Sodium Conductance
metarhodopsin
many molecules of transducin
phosphodiesterase
Decreases Membrane Sodium Conductance
allows the sodium channels to close
e. Within a second, rhopdopsin kinase inactivates
metarhodopsin and reversion back to the normal
state with open sodium channels
Photochemistry of Vision
Only one of three types of color pigments is present
in each of the different cones
Color pigments are blue, green, and red sensitive
pigments
Photochemistry of Vision
Fig. 50.8 Light absorption by the pigment of the rods and the three color receptive cones
Photochemistry of Vision
Light Adaptation- in bright light the
concentrations of photosensitive chemicals are
reduced
sensitive pigments
Photochemistry of Vision
Fig. 50.9 Dark adaptation, demonstrating he relation of cone adaptation to rod adaptation
Photochemistry of Vision
Change in pupillary size
Spectral sensitivities of the three types of cones
Interpretation of color in the Nervous System
Fig. 50.10 Demonstration of the degree of stimulation of the different color sensitive cones
by monochromatic lights of four colors: blue, green, yellow, and orange
Color Vision
Perception of White Light- equal stimulation of
the red, green, and blue cones gives the sensation of seeing white
Color Blindness- when a single group of cones is
missing, the person is unable to distinguish
some colors from others
Fig. 50.12 Neural organization of the retina; peripheral
area to the left, foveal area to the right
Neural Function of the Retina
Neural Circuitry of the Retina
Photoreceptors transmit signals to the outer plexiform layer where they synapse with bipolar cells and horizaontal cells
Horizontal cells which transmit signals horizontally in the outer plexiform layer from the rods and cones to bipolar cells
Bipolar cells which transmit signals vertically to the inner plexiform layer, where they synapse with ganglion cells and amacrine cells
Neural Function of the Retina
Neural Circuitry of the Retina
Amacrine cells transmit signals either directly from bipolar cells to ganglion cells or horizontally from axons of the bipolar cells to dendrites of the ganglion cells or other amacrine cells
Ganglion cells which transmit output signals from the retina through the optic nerve into the brain
Neural Function of the Retina
Visual Pathway from the Cones to the Ganglion Cells Functions Differently from the Rod Pathway
(Fig. 50.12) Visual pathway from the fovea has three neurons in a direct pathway: cones, bipolar cells, and ganglion cells
For rod vision there are four neurons in the direct pathway: rods, bipolar cells, amacrine cells, and ganglion cells
Neural Function of the Retina
Neurotransmitters
Amacrine cells release: GABA, glucine, dopamine,
acetylcholine, and indolamine; all of which are
inhibitory
Retinal Neurons by Electrtonic Conduction, Not
by Aps- direct flow of electric current in the
neuronal cytoplasm and nerve axons from the point of excitation all the way to the output synapses
Neural Function of the Retina
Lateral Inhibition- enhances visual contrast and is a function of the horizontal cells
Fig. 50.13 Excitation and inhibition of a retinal area caused by
a beam of light
Excitation and Inhibition- two sets of bipolar
cells provide opposing and inhibitory signals in the visual pathway
Depolarizing bipolar cells
Hyperpolarizing bipolar cells
Amacrine Cells and Their Functions- 30 types
identified and the functions of 6 have been
characterized
Responds strongly at the onset
Responds to changes in illumination
Movement of a spot across the retina
Neural Function of the Retina
Ganglion Cells and Optic Nerve Fibers
100 million rods, 3 million cones, and 1.6 million
ganglion cells (60 rods and 2 cones converge on
an individual ganglion cell)
light
d. Rods are 30-300x more sensitive to light than cones; 200 rods converge on a fiber in the periphery
Neural Function of the Retina
Excitation of the Ganglion Cells
Spontaneous continuous APs in the ganglion cells
Transmission of changes in light intensity- the
off-on response
Neural Function of the Retina
Transmission of Signals Depicting Contrasts in the Visual Scene: The Role of Lateral Inhibition
Fig. 50.15
Transmission of Color Signals by the Ganglion Cells
Single ganglion may be stimulated by several cones or by only a few