Chapter 50: The Eye: II. Receptor and Neural Function of the Retina Guyton and Hall, Textbook of...
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Unit Ten: The Nervous System: B. Special Senses Chapter 50: The Eye: II. Receptor and Neural Function of the Retina Guyton and Hall, Textbook of Medical Physiology, 12 th edition
Chapter 50: The Eye: II. Receptor and Neural Function of the Retina Guyton and Hall, Textbook of Medical Physiology, 12 th edition
Chapter 50: The Eye: II. Receptor and Neural Function of the
Retina Guyton and Hall, Textbook of Medical Physiology, 12 th
edition
Slide 2
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
Slide 3
Anatomy and Physiology of the Retina Layers of the Retina Fig.
50.1 Layers of the retina
Slide 4
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.The outer segment b.The inner segment c.The
nucleus d.The synaptic body
Slide 5
Anatomy and Physiology of the Retina Fig. 50.3 Schematic
drawing of the functional parts of the rods and cones
Slide 6
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
Slide 7
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
Slide 8
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
Slide 9
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
Slide 10
Anatomy and Physiology of the Retina Fig. 50.4 Membranuous
structures of t he outer segments of a rod and cone
Slide 11
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
Slide 12
Photochemistry of Vision Rhodopsin-Retinal Visual Cycle Fig.
50.5 Rhodopsin-retinal visual cycle in the rod
Slide 13
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
Slide 14
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
Slide 15
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
Slide 16
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
Slide 17
Photochemistry of Vision Fig. 50.6 Movement of sodium and
potassium ions through the inner and outer segments of the rod
Slide 18
Photochemistry of Vision Fig. 50.7 Phototransduction in the
outer segment of the photoreceptor membrane
Slide 19
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
Slide 20
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
Slide 21
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
Slide 22
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
Slide 23
Photochemistry of Vision Fig. 50.8 Light absorption by the
pigment of the rods and the three color receptive cones
Slide 24
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
Slide 25
Photochemistry of Vision Fig. 50.9 Dark adaptation,
demonstrating he relation of cone adaptation to rod adaptation
Slide 26
Photochemistry of Vision Other Mechanisms of Light and Dark
Adaptation a.Change in pupillary size b.Neural adaptation
Slide 27
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
Slide 28
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
Slide 29
Neural Function of the Retina Fig. 50.12 Neural organization of
the retina; peripheral area to the left, foveal area to the
right
Slide 30
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
Slide 31
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
Slide 32
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
Slide 33
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
Slide 34
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
Slide 35
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
Slide 36
Neural Function of the Retina Amacrine Cells and Their
Functions- 30 types identified and the functions of 6 have been
characterized a.Part of the direct pathway for rod vision
b.Responds strongly at the onset c.Responds to changes in
illumination d.Movement of a spot across the retina
Slide 37
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
Slide 38
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
Slide 39
Neural Function of the Retina Transmission of Signals Depicting
Contrasts in the Visual Scene: The Role of Lateral Inhibition Fig.
50.15
Slide 40
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
