1
Physiological optics, information processing in the retina.
Perception of motion, depth, form and color
Readings
B&B: Chapter 13, pages 331 - 343
BLKS: Chapter 8
Physiology of vision
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 Functional anatomy of the eye
 Optical
 Neural
 Photoreceptors
 Rods
 Cones
 Phototransduction
 Mechanism
 Termination
 Light adaptation
 Colour Vision
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The auditory system is one of the engineering masterpieces of the human body.
At the heart of the system is an array of miniature acoustical detectors packed
into a space no larger than a pea. These detectors can faithfully transduce
vibrations as small as the diameter of an atom, and they can respond a
thousand times faster than visual photoreceptors. Such rapid auditory responses
to acoustical cues facilitate the initial orientation of the head and body to novel
stimuli, especially those that are not initially within the field of view. Although
humans are highly visual creatures, much human communication is mediated by
the auditory system; indeed, loss of hearing can be more socially debilitating
than blindness. From a cultural perspective, the auditory system is essential not
only to language, but also to music, one of the most aesthetically sophisticated
forms of human expression. For these and other reasons, audition represents a
fascinating and especially important aspect of sensation, and more generally of
brain function.
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Optical anatomy of the eye
4
Optical portion of eye focuses
light thru cornea and lens onto
the fovea.
 Cornea
 Thin, transparent epithelium devoid of
blood vessels
 Receives nutrients by diffusion from tear
fluid
 Major refraktory portion of the eya, has
unmyelinated nerve endings sensitive to
touch and pressure
 Aqueous humor
 Produced by ciliary epithelial cells. Protein
free watery liquid that supplies nutrients
to cornea and lens
 Maintains intraocular pressure and gives
shape to anterior portion of eyeBoron, Boulpaep: Medical Physiology, 2003
Cornea: major refractory portion of the eye (fixed refractory index). Has unmyelinated nerve endings
sensitive to touch and pressure. Receives nutrients thru diffusion from tear fluid. Laser eye surgery
reshapes the cornea to reduce the need for corrective lenses.
Aqueous humor: produced by ciliary epithelial cells. High rate of turnover. Gluacoma increases pressure in
eye due to increased production or decreased drainage. Canals of Schlemm drain the aqeuous humor.
The eye is a fluid-filled sphere enclosed by three layers of tissue (Figure 11.1). Most of the outer layer is
composed of a tough white fibrous tissue, the sclera. At the front of the eye, however, this opaque outer
layer is transformed into the cornea, a specialized transparent tissue that permits light rays to enter the
eye. The middle layer of tissue includes three distinct but continuous structures: the iris, the ciliary body,
and the choroid. The iris is the colored portion of the eye that can be seen through the cornea. It contains
two sets of muscles with opposing actions, which allow the size of the pupil (the opening in its center) to be
adjusted under neural control. The ciliary body is a ring of tissue that encircles the lens and includes a
muscular component that is important for adjusting the refractive power of the lens, and a vascular
component (the so-called ciliary processes) that produces the fluid that fills the front of the eye. The
choroid is composed of a rich capillary bed that serves as the main source of blood supply for the
photoreceptors of the retina. Only the innermost layer of the eye, the retina, contains neurons that are
sensitive to light and are capable of transmitting visual signals to central targets.
En route to the retina, light passes through the cornea, the lens, and two distinct fluid environments. The
anterior chamber, the space between the lens and the cornea, is filled with aqueous humor, a clear,
watery liquid that supplies nutrients to these structures as well as to the lens. Aqueous humor is produced
by the ciliary processes in the posterior chamber (the region between the lens and the iris) and flows into
the anterior chamber through the pupil. A specialized meshwork of cells that lies at the junction of the iris
and the cornea is responsible for its uptake. Under normal conditions, the rates of aqueous humor
production and uptake are in equilibrium, ensuring a constant intraocular pressure. Abnormally high levels
of intraocular pressure, which occur in glaucoma, can reduce the blood supply to the eye and eventually
damage retinal neurons.
The space between the back of the lens and the surface of the retina is filled with a thick, gelatinous
substance called the vitreous humor, which accounts for about 80% of the volume of the eye. In addition
to maintaining the shape of the eye, the vitreous humor contains phagocytic cells that remove blood and
other debris that might otherwise interfere with light transmission. The housekeeping abilities of the
vitreous humor are limited, however, as a large number of middle-aged and elderly individuals with vitreal
“floaters” will attest. Floaters are collections of debris too large for phagocytic consumption that therefore
remain to cast annoying shadows on the retina; they typically arise when the aging vitreous membrane
pulls away from the overly long eyeball of myopic individuals.
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Optical anatomy of the eye
5
 Pupil
 Aperture of the eye
 Iris
 is the colored portion of the eye, than can be seen
through the cornea
 contains two sets of muscles
 Controls diameter of pupil
◼ Contraction of sphincter muscles → miosis
◼ Contraction of radial muscles → mydriasis
The eye is a fluid-filled sphere enclosed by three layers of tissue (Figure 11.1). Most of the outer
layer is composed of a tough white fibrous tissue, the sclera. At the front of the eye, however,
this opaque outer layer is transformed into the cornea, a specialized transparent tissue that
permits light rays to enter the eye. The middle layer of tissue includes three distinct but
continuous structures: the iris, the ciliary body, and the choroid. The iris is the colored portion of
the eye that can be seen through the cornea. It contains two sets of muscles with opposing
actions, which allow the size of the pupil (the opening in its center) to be adjusted under neural
control. The ciliary body is a ring of tissue that encircles the lens and includes a muscular
component that is important for adjusting the refractive power of the lens, and a vascular
component (the so-called ciliary processes) that produces the fluid that fills the front of the eye.
The choroid is composed of a rich capillary bed that serves as the main source of blood supply
for the photoreceptors of the retina. Only the innermost layer of the eye, the retina, contains
neurons that are sensitive to light and are capable of transmitting visual signals to central targets.
En route to the retina, light passes through the cornea, the lens, and two distinct fluid
environments. The anterior chamber, the space between the lens and the cornea, is filled with
aqueous humor, a clear, watery liquid that supplies nutrients to these structures as well as to
the lens. Aqueous humor is produced by the ciliary processes in the posterior chamber (the
region between the lens and the iris) and flows into the anterior chamber through the pupil. A
specialized meshwork of cells that lies at the junction of the iris and the cornea is responsible for
its uptake. Under normal conditions, the rates of aqueous humor production and uptake are in
equilibrium, ensuring a constant intraocular pressure. Abnormally high levels of intraocular
pressure, which occur in glaucoma, can reduce the blood supply to the eye and eventually
damage retinal neurons.
The space between the back of the lens and the surface of the retina is filled with a thick,
gelatinous substance called the vitreous humor, which accounts for about 80% of the volume of
the eye. In addition to maintaining the shape of the eye, the vitreous humor contains phagocytic
cells that remove blood and other debris that might otherwise interfere with light transmission.
The housekeeping abilities of the vitreous humor are limited, however, as a large number of
middle-aged and elderly individuals with vitreal “floaters” will attest. Floaters are collections of
debris too large for phagocytic consumption that therefore remain to cast annoying shadows on
the retina; they typically arise when the aging vitreous membrane pulls away from the overly long
eyeball of myopic individuals.
5
Optical anatomy of the eye
6
 Lens
 Dense, high protein structure that
adjusts optical focus
 Focus adjusted by process called
accommodation
◼ At rest, zonal fibers suspend lens
and keep it flat
◼ Focus on objects far away
◼ Contraction of ciliary muscles
releases tension in zonal fibers
◼ Lens becomes rounder
◼ Focus on near objects
The eye is a fluid-filled sphere enclosed by three layers of tissue (Figure 11.1). Most of the outer
layer is composed of a tough white fibrous tissue, the sclera. At the front of the eye, however,
this opaque outer layer is transformed into the cornea, a specialized transparent tissue that
permits light rays to enter the eye. The middle layer of tissue includes three distinct but
continuous structures: the iris, the ciliary body, and the choroid. The iris is the colored portion of
the eye that can be seen through the cornea. It contains two sets of muscles with opposing
actions, which allow the size of the pupil (the opening in its center) to be adjusted under neural
control. The ciliary body is a ring of tissue that encircles the lens and includes a muscular
component that is important for adjusting the refractive power of the lens, and a vascular
component (the so-called ciliary processes) that produces the fluid that fills the front of the eye.
The choroid is composed of a rich capillary bed that serves as the main source of blood supply
for the photoreceptors of the retina. Only the innermost layer of the eye, the retina, contains
neurons that are sensitive to light and are capable of transmitting visual signals to central targets.
En route to the retina, light passes through the cornea, the lens, and two distinct fluid
environments. The anterior chamber, the space between the lens and the cornea, is filled with
aqueous humor, a clear, watery liquid that supplies nutrients to these structures as well as to
the lens. Aqueous humor is produced by the ciliary processes in the posterior chamber (the
region between the lens and the iris) and flows into the anterior chamber through the pupil. A
specialized meshwork of cells that lies at the junction of the iris and the cornea is responsible for
its uptake. Under normal conditions, the rates of aqueous humor production and uptake are in
equilibrium, ensuring a constant intraocular pressure. Abnormally high levels of intraocular
pressure, which occur in glaucoma, can reduce the blood supply to the eye and eventually
damage retinal neurons.
The space between the back of the lens and the surface of the retina is filled with a thick,
gelatinous substance called the vitreous humor, which accounts for about 80% of the volume of
the eye. In addition to maintaining the shape of the eye, the vitreous humor contains phagocytic
cells that remove blood and other debris that might otherwise interfere with light transmission.
The housekeeping abilities of the vitreous humor are limited, however, as a large number of
middle-aged and elderly individuals with vitreal “floaters” will attest. Floaters are collections of
debris too large for phagocytic consumption that therefore remain to cast annoying shadows on
the retina; they typically arise when the aging vitreous membrane pulls away from the overly long
eyeball of myopic individuals.
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Accommodation and associated
disorders9
 Accommodation of the lens is limited
and age dependent
 With age, lens becomes stiffer and
less compliant.
 Age related loss of
accommodation called
presbyopia
 Accommodation accompanied by
adaptive changes in size of pupil
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Accommodation and associated
disorders10
 Myopia
 Image focused in front of retina
 Far away objects appear blurry
 Hyperopia
 Image focused behind retina
 Close objects appear blurry
Each can be caused by abnormal shape of
the eye as well.
Myopia: lens is too round
Hyperopia: lens is too flat
Astigmatism is abnormal curvature of cornea, images are blurry both near and
far.
Myopia and hyperopia can be caused by abnormal shape of eyeball as well.
Eye that is longer than normal results in myopia, corrected with a concave lens
(focuses images at longer distance)
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Optical anatomy of the eye
11
 Vitreous humor
 Gel of extracellular fluid containing
collagen
 Choroid
 rich in blood vessels and supports the
retina
 Retina
 Neural portion that transduces light into
electrical signals that pass down the
optic nerve
 Optic nerve exits at optic disc. Devoid of
photoreceptors: blind spot
 Fovea is point on retina that has
maximal visual acuity
Retina is part of the CNS derived from diencephalon
Macula is yellow region at back of eye on retina. Contains fovea is broader
region responsible for central vision
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 Pigment epithelium
 Absorps light rays, prevention the reflection
of rays back through the retina
 Contains melanin to absorbs excess
light
 Stores Vitamin A
 Photoreceptors
 Transduce light energy into electrical
energy
 Rods and cones
 Ganglion cells
 Output cells of retina project via optic
nerve
Bipolar cells – 12 different types occur
Horizontal cells
Amacrine cells - 29types have been
described
 The neural elements of retina are bound
together by glial cells – Muller cells
Boron, Boulpaep, Medical Physiology, 2003
RETINA
Its organized on layers
Visual receptors+4types of neurons.
Many different synaptic transmitters
Inner segments
Despite its peripheral location, the retina or neural portion of the eye, is actually part of the
central nervous system.
There are five types of neurons in the retina: photoreceptors, bipolar cells, ganglion cells,
horizontal cells, and amacrine cells.
Absorption of light by the photopigment in the outer segment of the photoreceptors initiates a
cascade of events that changes the membrane potential of the receptor, and therefore the
amount of neurotransmitter released by the photoreceptor synapses onto the cells they contact.
At first glance, the spatial arrangement of retinal layers seems counterintuitive, since light rays
must pass through the non-light-sensitive elements of the retina (and retinal vasculature!) before
reaching the outer segments of the photoreceptors, where photons are absorbed. The reason for
this curious feature of retinal organization lies in the special relationship that exists between the
outer segments of the photoreceptors and the pigment epithelium. The outer segments contain
membranous disks that house the light-sensitive photopigment and other proteins involved in the
transduction process. These disks are formed near the inner segment of the photoreceptor and
move toward the tip of the outer segment, where they are shed. The pigment epithelium plays an
essential role in removing the expended receptor disks; this is no small task, since all the disks in
the outer segments are replaced every 12 days. In addition, the pigment epithelium contains the
biochemical machinery that is required to regenerate photopigment molecules after they have
been exposed to light. It is presumably the demands of the photoreceptor disk life cycle and
photopigment recycling that explain why rods and cones are found in the outermost rather than
the innermost layer of the retina. Disruptions in the normal relationships between pigment
epithelium and retinal photoreceptors such as those that occur in retinitis pigmentosa have
severe consequences for vision
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Periphery of retina
 High degree of convergence → large
receptive field
 High sensitivity to light, low spatial
resolution
Fovea
 Low convergence → small receptive fields
 Lower sensitivity to light, high resolution
(visual acuity)
light
Electricalsignal
At the periphery of the retina there is convergence of synaptic input from many
photoreceptors onto bipolar and ganglion cells, reducing spatial resolution
because receptive fields are larger, but increasing sensitivity because more
photoreceptors collect light
Outside fovea density of cones drops and density of rods rises; there are no
photoreceptors at optic disc where ganglion cell axons leave retina (blind spot).
Fovea is region 300-700 m in diameter located in center of retina and contains
the highest density of cones
Over most of retina, light must travel through several layers to reach
photoreceptors; at fovea layers of neurons are shifted aside, reducing distortion
due to light scatter
Most photoreceptors in fovea synapse on only one bipolar cell which in turn
synapses on only one ganglion cell, resulting in smallest receptive fields and
greatest resolution
Fovea
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Visual acuity of fovea enhanced by:
 One to one ratio of photoreceptor to ganglion
cell
 Lateral displacement of neurons to minimize
scattering of light
 High density of cones
Cones are narrower and can pack more densley
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Photoreceptors
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Rods
 Responsible for monochromatic, darkadapted
vision
 Inner segment contains nucleus and
metabolic machinery
 Produces photopigment
 Outer segments is transduction site
 Consists of high density of stacks
of disk membranes: flattened,
membrane bound organelles
 contain the photopigment
rhodopsin
v
v
Photoreceptors consist of synaptic terminal connected by short axon to inner
segment (contains nucleus and metabolic machinery) and outer segment.
Outer segments of rods consist of stacks of membrane discs rich in
photopigment rhodopsin.
Inner segment synthesizes photopigments and inserts them into membrane of
vesicles which move from inner to outer segment.
In rods vesicles become incorporated into new discs, which move up the stack
until they reach apex where they are shed and recycled by pigment epithelium.
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16
Photoreceptors
16
Cones
 3 subtypes responsible for colour
vision
 Inner segment produces
photopigments similar to rhodopsin
 Outer segments is transduction site
 consist of infolded stack
membranes that are continuous
with the outer membrane
◼ vesicles containing pigment are
inserted into the membrane
folds of the outer segment
Photopigments contain same retinal, just different forms of opsin
Outer segments of cones consist of folded, stacked membrane containing other
photopigments (opsins) but in lower concentration than rods therefore less
sensitive to light.
As with rods, the inner segment synthesizes photopigments and inserts them
into membrane of vesicles which move from inner to outer segment.
However, in cones the vesicles are inserted into membrane folds of outer
segment
17
Phototransduction: Dark current
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 Partially active guanylyl cyclase keeps
cytoplasmic [cGMP] high in the dark
 Outer segment contains cGMP-gated
cation channels
 Influx of Na+ and Ca2+
 Inner segment contains non-gated K+
selective channels
 K+ efflux
 Resting, or dark Vm is -40 mV
 concentration gradients maintained by
Na+/K+ pump and NCX
Guanylyl cyclase synthesizes cGMP from GTP
Outer segment membrane has cation channels which remain open in the dark
whereas inner segment has K+ channels that are not regulated by light.
Na+ (90%) and Ca++(10%) enter through cation channels in outer segment and
K+ leaves inner segment, resulting in hyperpolarization (resting membrane
potential of rods is ~ – 40 mV ) and ionic current called dark current.
Na-K pump removes Na+ from inner segment and Na-Ca exchanger removes
Ca++ from outer segment to maintain concentration gradients.
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Phototransduction
19
Photoreceptors hyperpolarize in response to light and release less
neurotransmitter
 In darkness, the Vm of -40 mV keeps CaV channels in the synaptic
terminal open
 photoreceptors continuously release neurotransmitter glutamate
 absorption of light by photopigment ’s [cGMP]
 cation channels close
 K+ efflux predominates, hyperpolarizes cell (-70mV)
 CaV channels close, decreased release of glutamate
Depolarized state of membrane keeps voltage-gated Ca++ channels open
in synaptic terminals, resulting in constant release of neurotransmitter
(glutamate)
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Phototransduction: mechanism
20
 Photopigment rhodopsin is the light receptor in rods
 opsin
◼ G-protein coupled membrane receptor
 Retinal= retinene1
◼ Light absorbing compound
◼ the aldehyde
form of retinol or
Vitamin A
B&B Figure 13-11
 retinal changes conformation from 11-cis to alltrans
after absorbing a photon
 isomerization of retinal activates opsin
opsin
Aldehyde is r-c=o
Retinol contains only an C-OH
Trans form is more stable
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Phototransduction: mechanism
22
1. Absorption of a photon isomerizes retinal
a) Converts opsin to metarhodopsin II
2. Metarodophsin II activates the G-protein transducin
a) Activates cGMP phosphodiesterase (PDE)
3. PDE hydrolyzes cGMP to GMP
a) Decreased [cGMP] closes cGMP gated cation channels
b) Photoreceptor hyperpolarizes, less glutamate released
light
transducin exchanges GDP for GTP
activated transducin (G protein) → activates cGMP phosphodiesterase →
hydrolyzes cGMP to GMP (5’-guanylate monophosphate)→ ↓ [cGMP]i → closes
cGMP-gated cation channels → hyperpolarization → ↓ neurotransmitter release
all-trans retinal separates from opsin (bleaching)
converts to retinol
translocates to the pigment epithelium where it is converted back to 11-cis
retinal
returns to the outer segment and recombines with opsin
recycling process takes several minutes
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23
Phototransduction: termination
23
 Activated rhodopsin is a target for phosphorylation
by rhodopsin kinase
 Phosphorylated rhodopsin inactivated by cytosolic
protein arrestin
 All-trans retinal transported to the pigment
epithelium where it is converted back to 11-cis
retinal, and recycled back to the rod
 Activated transducin inactivates itself by
hydrolyzing GTP to GDP
Ca++ entry through cation channels inhibits guanylyl cyclase, which synthesizes
cGMP, and stimulates phosphodiesterase to regulate [cGMP]I
closure of cGMP-gated channels → ↓ [Ca++]i, reducing inhibition of guanylyl
cyclase and inhibiting phosphodiesterase to increase [cGMP]i
24
Phototransduction: light adaptation
24
Eyes adapt to increased light intensity and remain
sensitive to further changes in light
❑ Optic adaptation:
❑ Constriction of pupils to allow in less light
❑ Photoreceptor adaptation:
❑ The closure of cGMP gated channel reduces inward
flux of Ca2+ →decreased [Ca2+]i
❑ Ca2+ induced inhibition of guanylyl cyclase removed
❑ More cGMP made → reopening of some cGMP gated
channels → influx of cations → slight depolarization
Photoreceptor can once again be stimulated
(hyperpolarized) by photons
Ca++ entry through cation channels inhibits guanylyl cyclase, which synthesizes
cGMP, and stimulates phosphodiesterase to regulate [cGMP]I
closure of cGMP-gated channels → ↓ [Ca++]i, reducing inhibition of guanylyl
cyclase and inhibiting phosphodiesterase to increase [cGMP]i
Colour Vision
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❑ 3 types of cones, each contain
photopigment with different
absorption spectra
❑ 420 nm – blue
❑ 530 nm – green
❑ 560 nm - red
❑ Colour interpreted by ratio of
cone stimulation
❑ Orange (580nm) light
stimulates:
❑ Blue cone – 0%
❑ Green cone – 42%
❑ Red cone – 99%
❑ 0:42:99 ratio of cone
stimulation interpreted by
brain as orange Guyton Figure 50-8
Rod
Cones actually respond to violet, yellow-green, and yellow-red but called blue
green red by convention
Rod peak wavelength at 500nm
Red green colour blindness: red or green cones missing, therefore cannot
distinguish red from green because the colour spectra overlap.
25
Colour Vision: Disorders
26
 Malfunction of one group of cones
leads to colour blindness
 Most common form is red-green
colour blindness
 Either red or green cones are
missing
 Difficulty distinguishing red
from green because the colour
spectra overlap (ratio of cone
stimulation is affected →
impaired neural interpretation
of colours)
The spots are arranged so that a normal vision person sees a 74, whereas a
red-green colour blind person sees a 21
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27
Retinal circuitry: review of cell types
27
 rods and cones synapse on bipolar cells
and horizontal cells
 horizontal cells make lateral inhibitory
synapses with surrounding bipolar cells
or photoreceptors
 bipolar cells make synaptic connections
with ganglion cells and amacrine cells
 amacrine cells transmit signals from
bipolar cells to ganglion cells or to other
amacrine cells
 ganglion cells transmit action potentials
to the brain via the optic nerve
B&L Figure 8-7
Interplexiform cells: transmit signals in the retrograde manner from the inner
plexiform layer to the outer plexiform layer. Signals are inhibitory and control
lateral spread of visual signals by horizontal cells in the outer plexifrom layer.
Role may be to help control the degree of contrast in the visual image.
Amacrine cells help analyze visual signals before they leave the retina.
There are two type of bipolar cells:
•“on type” have excitatory receptors
•“off-type” have inhibitory receptors
Amacrine cells:
•transform sustained bipolar cell output into transient responses of
ganglion cells
•act as interneurons in pathway from rod bipolar cells to ganglion cells
Retinal circuitry: key features
28
 2 types of bipolar cells
 On center: hyperpolarized by
glutamate
 Off center: depolarized by
glutamate
 Bipolar and horizontal cells play
a role in lateral inhibition
 Important for increasing visual
contrast
 Set up “surround” arrangement
of ganglion cell receptive fields
B&L Figure 8-7
Direct path:
Photoreceptor → bipolar cell → ganglion
cell
Indirect path:
Photoreceptor → horizontal, amacrine,
bipolar cells → ganglion cells
cones in center of ganglion cell receptive field
influence ganglion cell activity by direct pathway
cones in surround of ganglion cell receptive
field influence ganglion cell activity by indirect
pathway
28
29
Receptive fields
29
 Photoreceptor receptive fields include retinal area that, when
stimulated by light, results in hyperpolarization of individual
photoreceptor
 Small and circular
 Ganglion cell receptive field size determined by
 ganglion cell type
 degree of convergence of photoreceptors and bipolar cells
and field type by retinal circuitry (lateral inhibition)
◼ On-center/off-surround
◼ Off-center/on-surround
Where in the retina is there is there a
high degree of convergence?
i.e., response in center of receptive field is opposite to response in surround,
due to opposite effects of
direct and lateral pathways
depolarized by glutamate (opening of Na+ channels)
hyperpolarized by glutamate (opening of K+ channels or closing of Na+
channels)
Receptive fields
30
 On-center/off-surround
 Light shines on center of ganglion
cell receptive field → ganglion
cell increases AP firing
 Light on surround region →
decreased AP firing
 Off-center/on-surround
 Light on center → decreased AP
firing
 Light on surround → increased AP
firing
B&L Figure 8-8
Always have a tonic release of AP, but their frequency is mediated by
center/surround receptive fields
30
Neural circuits of retinal receptive
fields31
centresurround surround
Ganglion cell
receptive field
P PP
B B
G G
H H
_ _
On-center
bipolar and
ganglion cells
Off-center
bipolar and
ganglion cells
On center bipolar cells hyperpolarized by glutamate
Off center bipolar cells depolarized by glutamate
Center photoreceptors always synapse onto bipolar cells of each type, on center
and off center
Surround photoreceptors synapse on horizontal cells which mediate signals via
lateral inhibitory connections
31
Neural Circuits of Retinal
Receptive Fields
32
Light stimulus on center:
 ↓ glu release from central
photoreceptor
 ↓ inhibition of on-center bipolar
cell → depolarization
 ↑ NT release → on-center
ganglion cell excited
 less glu available to excite offcentre
bipolar cell →
hyperpolarization
 ↓NT release→ off-center
ganglion cell inhibited
light
On center bipolar cells hyperpolarized by glutamate
32
Neural Circuits of Retinal
Receptive Fields
33
light light
Light stimulus on surround:
 ↓ glu release from surround
photoreceptor
 ↓ excitation of horizontal cells →
↓ inhibitory NT released
 ↓ inhibition of central
photoreceptor → ↑ glu released
 ↑ glu hyperpolarizes on-center
bipolar cell and depolarizes offcenter
bipolar cell
 On-center ganglion cell
inhibited, off-center ganglion
cell excited
33
Retinal receptive fields: outcome
34
 Surround arrangement and lateral inhibition allows
ganglion cells to respond best to contrast borders in a
visual scene
 Ex. Reading dark letters against a white background
 Respond only weakly to diffuse illumination
B&L Figure 8-8
Light impinging on both center and surround of bipolar cell may result in
cancellation of center and surround effects.
Responses of amacrine cells depend on pattern of convergence from on-center
and off-center bipolar cells (response involves increase or decrease in firing
rate).
Firing rate of ganglion cells is determined by input from bipolar and amacrine
cells
•dominant input from amacrine cells can produce uniform or mixed responses
across receptive field
•dominant input from bipolar cells produces center-surround responses
34
Ganglion cell types and projections
35
 P cells
 Project to parvocellular layer of LGN
 Tonic firing, small surround receptive fields,
 Important for colour detection, form and
detail of visual image
 M cells
 Project to magnocellular layer of LGN
 Transient activity, large surround receptive
fields
 Convey information about illumination and
movement
 W cells
 Resemble M cells, large diffuse receptive
fields
 Function is less clear
Lateral geniculate nucleus
35
Visual
pathway
36
 Light from binocular zone strikes
retina in both eyes
 Monocular zone only strikes retina
on same side as light
The right visual field is projected to the
___________________ and
___________________ hemiretina
The optic nerves segregate and carry
information from
______________________
Each ___________________ crosses at
the optic chiasm
The optic tracts carry information from
______________________ to the
brain
Right
temporal
hemiretina
Left
temporal
hemiretina
Left/right
nasal
hemiretina
Optic
nerves
Optic
tracts
B&L Figure 8-9
Left visual field Right visual field
Fibers from the nasal hemiretina of each eye cross
to the opposite side at the optic chiasm, whereas fibers from
the temporal hemiretina do not cross. In the illustration, light
from the right half of the binocular zone falls on the left temporal
hemiretina and right nasal hemiretina. Axons from these
hemiretinas thus contain a complete representation of the right
hemifield of vision (see Figure 27-6).
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