Transcript VISION

VISION
Dr. Janet Fitzakerley
[email protected]
http://www.d.umn.edu/~jfitzake/Lectures/Teaching.html
Critical Facts
1. There are two fundamental protective mechanisms for the eye. Regulation of eyelid position
(including BLINKING) involves striated (ACh; nicotinic) and smooth (NE; α1 adrenergic)
muscles. TEAR PRODUCTION occurs spontaneously (basal), reflexly or in response to
emotional stimuli, and is partially regulated by the parasympathetic nervous system (ACh;
muscarinic). EPIPHORA (overflow of tears) can be due to either overproduction or blocked
drainage.
2. The cornea and lens focus light on the retina; the cornea has greater refractive power but
the focusing power of the lens can be adjusted to allow near vision (accomodation).
Refractive errors include cataracts, hyperopia, myopia, presbyopia and astigmatism.
3. Light intensity is regulated by the PUPILLARY LIGHT REFLEX, which causes MIOSIS as a
result of parasympathetic stimulation of the sphincter pupillae muscles (muscarinic
receptors). MYDRIASIS results from sympathetic stimulation (α1 receptors) that activates
the dilator pupillae muscles.
4. Increased intraocular pressure causes loss of vision (potentially permanent). Open angle
glaucoma (the most common form) results from overproduction of the aqueous humor. Closed
angle glaucoma (typically the most rapidly evolving form) is caused by blockage of fluid
outflow.
5. RODS are responsible for SCOTOPIC vision (the monochromatic vision that occurs in low
light). The three types of cones (blue, green and red; or Short, Medium and Long
wavelength) have better temporal and spatial resolution than rods, making PHOTOPIC
VISION better for discrimination of surfaces and movement under bright light conditions.
6. The ability to discriminate fine details of the visual scene is termed VISUAL ACUITY.
Three types are recognized: SPATIAL, TEMPORAL and SPECTRAL. Visual acuity is
primarily a function of the cone system.
Critical Facts (cont’d)
7. PHOTOTRANSDUCTION occurs via a 4 step process that uses a 2nd messenger cascade to
amplify the signal. In rods, activation of rhodopsin ultimately results in the closure of
cyclic nucleotide gated Na+ channels, and hyperpolarization of the photoreceptor.
8. The VISUAL CYCLE consists of bleaching and recycling of 11-cis-retinol between the
photoreceptors and the retinal pigment epithelium (RPE). It is a key component of dark
adaptation in rods and is disrupted in vitamin A deficiency, and macular degeneration.
9. Ganglion cells (GCs) are like CNS neurons, in that their contrast-detecting capabilities are
enhanced by lateral inhibition provided by amacrine cells. On-center GCs produce more
action potentials when stimulated by a bright light in the center of their receptive field,
and inhibited by stimuli delivered to the surround. Off-center GCs are stimulated by
surround stimuli, and inhibited by center stimuli.
10. Perception of colour is a learned process which involves associating patterns of
photoreceptor activity with a particular hue. Even though the distribution of cones within
the retina is unique to each individual, the description of hue is standardized by teaching
people to associate specific words with their unique pattern of cone response.
11. Within primary visual cortex (V1), inputs from the fovea are overrepresented relative to the
periphery. The separate maps that are established for each visual field in primary V1 are
merged to form a single perceptual map of visual space. Due to OCULAR DOMINANCE,
cortical can extract depth cues based on the disparity in the images, providing the basis for
STEREOPSIS (depth perception).
12. STRABISMUS is a muscle imbalance that results in a misalignment of the visual axes of the
two eyes. Any type of strabismus that occurs after ~6 months of age causes DIPLOPIA
(perception of a single object as double) because the images fall on noncorresponding parts
of the retinas. In young children, suppression of the image in the weaker eye can cause a
permanent decrease in visual acuity (AMBLYOPIA).
Essential Material from Other Lectures
1.
2.
3.
4.
5.
6.
7.
Structure of the eyeball, including the innervation of the
levator palpebrae superioris and superior tarsal muscle, the
lacrimal gland, the cornea and the lens (Dr. Severson,
Applied Anatomy)
CSF formation (Dr. Drewes, Nervous System)
Pupillary reflex/innervation of the dilator and constrictor
muscles of the pupil (Dr. Forbes, Nervous System)
Anatomical structures associated with aqueous humor
formation and flow, including the ciliary body and the canal
of Schlemm (Dr. Severson, Applied Anatomy).
Histology of the retina (Dr. Downing, Nervous System).
Receptor potentials and lateral inhibition (Dr. Fitzakerley,
Nervous System)
Visual Fields (Dr. Forbes, Nervous System)
Learning Objectives
1.
2.
3.
4.
5.
6.
7.
8.
9.
Be able to describe the neurotransmitters involved in eyelid movements, and
characterize the purpose and types of blinking. Explain epiphora.
Explain the processes of refraction and accomodation as they apply to
transmission of light to the retina. Define the following refractive errors:
cataracts, hyperopia, myopia, presbyopia and astigmatism.
Describe the processes of mydriasis and miosis, including the
neurotransmitters involved.
Explain how the aqueous humor is formed and drains, and outline control
mechanisms for each part of the process. Detail the differences between
closed angle and open angle glaucoma.
Compare and contrast the physiology of rods and cones. Relate the
physiological differences between rods to the different forms of visual
acuity. Differentiate between retinopathy and retinitis pigmentosa.
List the steps in phototransduction, including the properties of the receptor
potential.
Describe the visual cycle, and understand the perturbations that occur to
this process during vitamin A deficiency and macular degeneration.
Outline how lateral inhibition contributes to the receptive field properties of
ganglion cells.
Explain how the primary visual cortex processes color and motion, and
generates depth perception. Describe how amblyopia develops from
stabismus and diplopia.
OPTICS
Protective Mechanisms
There are two fundamental protective mechanisms for the eye.
Regulation of eyelid position (including BLINKING) involves striated
(ACh; nicotinic) and smooth (NE; α1 adrenergic) muscles. TEAR
PRODUCTION occurs spontaneously (basal), reflexly or in response
to emotional stimuli, and is partially regulated by the parasympathetic
nervous system (ACh; muscarinic). EPIPHORA (overflow of tears)
can be due to either overproduction or blocked drainage.
Blinking
Tear Production
Focusing
The cornea and lens focus light on the retina; the cornea has
greater refractive power but the focusing power of the lens can
be adjusted to allow near vision (accomodation). Refractive errors
include cataracts, hyperopia, myopia, presbyopia and astigmatism.
Refraction
Accomodation
Refractive Errors
Regulation of Light Intensity
Light intensity is regulated by the PUPILLARY LIGHT REFLEX,
which causes MIOSIS as a result of parasympathetic
stimulation of the sphincter pupillae muscles (muscarinic
receptors). MYDRIASIS results from sympathetic stimulation
(α1 receptors) that activates the dilator pupillae muscles.
Formation of the Aqueous Humor
Increased intraocular pressure causes loss of vision
(potentially permanent). Open angle glaucoma (the most common
form) results from overproduction of the aqueous humor.
Closed angle glaucoma (typically the most rapidly evolving form)
is caused by blockage of fluid outflow.
Glaucoma
PHYSIOLOGY OF THE RETINA
Visible Light
Photoreceptors
Rods are responsible for SCOTOPIC vision (the monochromatic
vision that occurs in low light). The three types of cones (blue,
green and red; or Short, Medium and Long wavelength) have
better temporal and spatial resolution than rods, making
PHOTOPIC VISION better for discrimination of surfaces and
movement under bright light conditions.
Amount of
photopigment
Pigment type
RODS
CONES
More
Less
3 overlapping
 patterns of activity for colour (see
page 15)
Low
(multiple photons to activate)
1 = rhodopsin
High
(1 photon if dark adapted)
Sensitivity
Temporal resolution

Saturated in daylight

Saturate in very intense light

Smaller dynamic range
Low

Large DR

Slow response

Fast response

Responses are integrated

Less integration
High
Poor
Spatial resolution
Very good

Respond to scattered light


Not in fovea  large amount of 
convergence onto bipolar cells
Respond to narrow spots of light
In fovea  little amount of
convergence onto bipolar cells
Visual Acuity
The ability to discriminate fine details of the visual scene is
termed VISUAL ACUITY.
Three types are recognized:
SPATIAL, TEMPORAL and SPECTRAL. Visual acuity is primarily a
function of the cone system.
Phototransduction
PHOTOTRANSDUCTION occurs via a 4 step process that uses a
2nd messenger cascade to amplify the signal. In rods, activation
of rhodopsin ultimately results in the closure of cyclic nucleotide
gated Na+ channels, and hyperpolarization of the photoreceptor.
Receptor Potential
Retinosis Pigmentosa
Retinopathy
Visual Cycle
The VISUAL CYCLE consists of bleaching and recycling of 11-cisretinol between the photoreceptors and the retinal pigment
epithelium (RPE). It is a key component of dark adaptation in rods
and is disrupted in vitamin A deficiency, and macular degeneration.
Vitamin A Deficiency
Macular Degeneration
Ganglion Cell Physiology
Ganglion cells (GCs) are like CNS neurons, in that their contrastdetecting capabilities are enhanced by lateral inhibition provided by
amacrine cells. On-center GCs produce more action potentials when
stimulated by a bright light in the center of their receptive field,
and inhibited by stimuli delivered to the surround. Off-center GCs
are stimulated by surround stimuli, and inhibited by center stimuli.
VISUAL CORTEX PHYSIOLOGY
Colour Perception
Colour Perception
Perception of colour is a learned process which involves
associating patterns of photoreceptor activity with a particular
hue. Even though the distribution of cones within the retina is
unique to each individual, the description of hue is standardized
by teaching people to associate specific words with their unique
pattern of cone response.
Edge Perception
Topographic Maps
Within primary visual cortex (V1), inputs from the fovea are
overrepresented relative to the periphery. The separate maps
that are established for each visual field in primary V1 are
merged to form a single perceptual map of visual space. Due to
OCULAR DOMINANCE, cortical can extract depth cues based
on the disparity in the images, providing the basis for
STEREOPSIS (depth perception).
Depth Perception
Development
STRABISMUS is a muscle imbalance that results in a
misalignment of the visual axes of the two eyes. Any type of
stabismus that occurs after ~6 months of age causes DIPLOPIA
(perception of a single object as double) because the images fall
on noncorresponding parts of the retinas. In young children,
suppression of the image in the weaker eye can cause a
permanent decrease in visual acuity (AMBLYOPIA).