Transcript Anatomy and Physiology of the Retina

Chapter 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
a. Pigmented layer
b. Layer of rods and cones
c. Outer nuclear layer containing the cell bodies of the
rods and cones
d. Outer plexiform layer
e. Inner nuclear layer
f. Inner plexiform layer
g. Ganglionic layer
h. Layer of optic nerve fibers
i. Inner limiting membrane
Anatomy and Physiology of the Retina
• Layers of the Retina
Fig. 50.1 Layers of the retina
Anatomy and Physiology of the Retina
• Fovea- minute area in the center of the retina
(1 sq mm) capable of acute vision; contains
only cones
• Rods and Cones- the major functional segments
of either a rod or cone are:
a.
b.
c.
d.
The outer segment
The inner segment
The nucleus
The synaptic body
Anatomy and Physiology of the Retina
Fig. 50.3 Schematic drawing of the functional parts
of the rods and cones
Anatomy and Physiology of the Retina
• Rods and Cones
a. Light sensitive photochemicals are found in the
outer segment
b. In rods, it is rhodopsin
c. In cones, it is one of three color pigments which
function exactly like rhodopsin
Anatomy and Physiology of the Retina
• Rods and Cones
d. In the outer segments of both rods and cones are
large numbers of discs (as many as 1000 per rod or
cone)
e. Pigments are conjugated proteins incorporated
into the membranes of the discs
f. Inner segment contains the usual organelles and
cytoplasm
Anatomy and Physiology of the Retina
• Rods and Cones
g. Synaptic body connects with the neuronal cells,
the horizontal and bipolar cells
•
Pigment Layer of the Retina
a. Melanin prevents light refraction throughout
the eyeball
b. Stores large quantities of vitamin A
Anatomy and Physiology of the Retina
•
Pigment Layer of the Retina
c. 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
Anatomy and Physiology of the Retina
• Blood Supply of the Retina
a. Central retinal artery enters with the optic nerve
b. Branches to supply the entire retinal surface
c. Outermost layer is adherent to the choroid which
is also a highly vascular area
Photochemistry of Vision
• Rhodopsin-Retinal Visual Cycle
Fig. 50.5 Rhodopsin-retinal visual cycle in the rod
Photochemistry of Vision
• Rhodopsin-Retinal Visual Cycle-The Decomposition
by Light Energy
a. When light energy is absorbed by rhodopsin, the
rhodopsin begins to decompose;
b. The cause of this is photoactivation of electrons in
the retinal portion of rhodopsin, which converts
cis into a trans form and cannot bind to the active
site on the protein.
c. This leads to unstable intermediates
Photochemistry of Vision
• Reformation of Rhodopsin
a. First step is re-convert to cis form of retinal
b. Requires energy and is catalyzed by retinal isomerase
c. Once formed it binds to the protein and is stable
Photochemistry of Vision
• Role of Vitamin A
a. Second pathway converts the trans-retinal to
trans-retinol (one form of vitamin A)
b. The trans-retinol is then converted to cis-retinal
c. 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
• Excitation of the Rod When Rhodopsin is Activated
by Light
a. The rod receptor potential is hyperpolarizing, not
depolarizing
b. When rhodopsin decomposes, it decreases the
rod membrane conductance for sodium ions
in the outer segment of the rod
c. 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
• Duration of the Receptor Potential and Log Relation
of the Receptor Potential to Light Intensity
a. Receptor potential occurs in 0.3 seconds and
lasts for about 1 second in the rods
b. In the cones it occurs four times as fast
c. 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
• Mechanism by Which Rhodopsin Decomposition
Decreases Membrane Sodium Conductance
(Excitation Cascade)
a. Photon activates an electron in the cis-retinal portion
of rhodopsin and leads to the formation of
metarhodopsin
b. Activated rhodopsin acts as an enzyme to activate
many molecules of transducin
c. Activated transducin activates many mcles of
phosphodiesterase
Photochemistry of Vision
• Mechanism by Which Rhodopsin Decomposition
Decreases Membrane Sodium Conductance
(Excitation Cascade)
d. Activated phosphodiesterase hydrolyzes cGMP which
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
• Photochemistry of Color Vision by the Cones
a. Only one of three types of color pigments is present
in each of the different cones
b. 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
• Automatic Regulation of Retinal Sensitivity
a. Light Adaptation- in bright light the
concentrations of photosensitive chemicals are
reduced
b. Dark Adaptation- in darkness, the retinal and
opsins are converted back into the light
sensitive pigments
Photochemistry of Vision
Fig. 50.9 Dark adaptation, demonstrating he relation of cone adaptation to rod adaptation
Photochemistry of Vision
• Other Mechanisms of Light and Dark Adaptation
a. Change in pupillary size
b. Neural adaptation
Color Vision
• Tricolor Mechanism of Color Detection
a. Spectral sensitivities of the three types of cones
b. 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
a. Red-green
b. Blue weakness
Neural Function of the Retina
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
a. Photoreceptors transmit signals to the
outer plexiform layer where they synapse
with bipolar cells and horizaontal cells
b. Horizontal cells which transmit signals
horizontally in the outer plexiform layer
from the rods and cones to bipolar cells
c. 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
d. 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
e. 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
a. (Fig. 50.12) Visual pathway from the fovea has
three neurons in a direct pathway: cones, bipolar
cells, and ganglion cells
b. 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
a. Rods and cones release glutamate
b. Amacrine cells release: GABA, glucine, dopamine,
acetylcholine, and indolamine; all of which are
inhibitory
•
Transmission of Most Signals Occurs in the
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
Neural Function of the Retina
•
Excitation and Inhibition- two sets of bipolar
cells provide opposing and inhibitory
signals in the visual pathway
a. Depolarizing bipolar cells
b. Hyperpolarizing bipolar cells
Neural Function of the Retina
•
Amacrine Cells and Their Functions- 30 types
identified and the functions of 6 have been
characterized
a.
b.
c.
d.
Part of the direct pathway for rod vision
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
a. 100 million rods, 3 million cones, and 1.6 million
ganglion cells (60 rods and 2 cones converge on
an individual ganglion cell)
b. Central fovea has 35,000 cones and no rods
c. Greater sensitivity of the peripheral retina to weak
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
a. Spontaneous continuous APs in the ganglion cells
b. Transmission of changes in light intensity- the
off-on response
Fig. 50.14 Responses of a ganglion to light
Neural Function of the Retina
•
Transmission of Signals Depicting Contrasts in
the Visual Scene: The Role of Lateral Inhibition
Fig. 50.15
Neural Function of the Retina
•
Transmission of Color Signals by the Ganglion
Cells
a. Single ganglion may be stimulated by several
cones or by only a few
b. Some cells may be stimulated by one type but
inhibited by another
c. Importance of color contrast mechanisms is
that the retina itself begins to differentiate
colors