jfitzakedumnedu httpwwwdumnedujfitzakeLecturesTeachinghtml Critical Facts There are two fundamental protective mechanisms for the eye Regulation of eyelid position including BLINKING ID: 930154
Download Presentation The PPT/PDF document "VISION Dr. Janet Fitzakerley" is the property of its rightful owner. Permission is granted to download and print the materials on this web site for personal, non-commercial use only, and to display it on your personal computer provided you do not modify the materials and that you retain all copyright notices contained in the materials. By downloading content from our website, you accept the terms of this agreement.
Slide1
VISION
Dr. Janet Fitzakerley
jfitzake@d.umn.edu
http://www.d.umn.edu/~jfitzake/Lectures/Teaching.html
Slide2Critical Facts
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.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.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.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.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.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.
Slide3Critical Facts (cont’d)
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.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.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.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.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).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).
Slide4Essential Material from Other Lectures
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)
Slide5Learning Objectives
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.
Slide6OPTICS
Slide7Protective 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.
Slide8Blinking
Slide9Tear Production
Slide10Focusing
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.
Slide11Refraction
Slide12Accomodation
Slide13Refractive Errors
Slide14Regulation 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.
Slide15Formation 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.
Slide16Slide17Glaucoma
Slide18PHYSIOLOGY OF THE RETINA
Slide19Slide20Visible Light
Slide21Photoreceptors
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.
Slide22Slide23RODS
CONES
Amount of photopigment
MoreLessPigment type1 = rhodopsin3 overlapping patterns of activity for colour (see page 15)
Sensitivity
High
(1 photon if dark adapted)
Saturated in daylight
Smaller dynamic range
Low
(multiple photons to activate)
Saturate in very intense light
Large DR
Temporal resolution
Low
Slow response
Responses are integrated
High
Fast response
Less integration
Spatial resolution
Poor
Respond to scattered light
Not in fovea
large amount of convergence onto
bipolar cells
Very good
Respond to narrow spots of light
In fovea
little amount of convergence onto
bipolar
cells
Slide24Visual 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.
Slide25Slide26Phototransduction
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.
Slide27Slide28Receptor Potential
Slide29Retinosis Pigmentosa
Slide30Retinopathy
Slide31Visual Cycle
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.
Slide32Slide33Vitamin A Deficiency
Slide34Macular Degeneration
Slide35Ganglion Cell Physiology
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.
Slide36Slide37VISUAL CORTEX PHYSIOLOGY
Slide38Slide39Colour Perception
Slide40Colour 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.
Slide41Edge Perception
Slide42Topographic 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).
Slide43Slide44Depth Perception
Slide45Development
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).