VISION

Dr. Janet Fitzakerleyjfitzake@d.umn.edu

http://www.d.umn.edu/~jfitzake/Lectures/Teaching.html

Critical Facts1. 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. 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)

2. CSF formation (Dr. Drewes, Nervous System)3. Pupillary reflex/innervation of the dilator and constrictor

muscles of the pupil (Dr. Forbes, Nervous System)4. Anatomical structures associated with aqueous humor

formation and flow, including the ciliary body and the canal of Schlemm (Dr. Severson, Applied Anatomy).

5. Histology of the retina (Dr. Downing, Nervous System).6. Receptor potentials and lateral inhibition (Dr. Fitzakerley,

Nervous System)7. Visual Fields (Dr. Forbes, Nervous System)

Learning Objectives1. Be able to describe the neurotransmitters involved in eyelid movements, and

characterize the purpose and types of blinking. Explain tear production and how it is regulated.

2. 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.

3. Describe the processes of mydriasis and miosis, including the neurotransmitters involved.

4. 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.

5. 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.

6. List the steps in phototransduction, including the properties of the receptor potential.

7. Describe the visual cycle, and understand the perturbations that occur to this process during vitamin A deficiency and macular degeneration.

8. Outline how lateral inhibition contributes to the receptive field properties of ganglion cells. Describe the function of bipolar, horizontal and amacrine cells.

9. Explain how the primary visual cortex processes color and motion, and generates depth perception. Describe how amblyopia develops from stabismus and diplopia.

OPTICS

Protective MechanismsThere 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

• eyelid movements are mediated by the orbicularis oculi (OO) and levator palpebrae superioris (LPS) muscles, as well as by the superior tarsal muscle (ST)o OO and LPS are striated muscles (ACh acts on nicotinic receptors to cause

contraction)o the superior tarsal muscle is a smooth muscle (sympathetic innervation via α1

receptors)

• three types of motions:

1. maintaining ocular opening tonic activation of LPS and ST;inactivation OO

2. gentle opening/closing, adjustment to changes in globe position

activation/inactivation of LPS;inactivation OO

3. blinking, firm closure of eyes OO activation; inhibition of LPS

Blinking• blinking serves a number of functions, including:

o corneal lubricationo eye protectiono visual information processing

• blinking can be spontaneous or reflex o spontaneous blinking:

• is precisely conjugated, periodic, symmetrical, brief and occurs in the absence of external stimuli or internal effort

• show a wide variation in rate (typically 10-20 blinks/minute in adults; lower in children)

• originates in premotor brainstem structures that are highly influenced by dopaminergic activity

– decreased in Parkinson's disease, and increased in schizophrenia and Huntington's disease, for example

o the blink reflex: • can be initiated by either touch to the cornea (afferents in the trigeminal nerve)

or by bright light/rapidly approaching objects (afferents in the optic nerve)• is faster than spontaneous blinking

Tear Production

• the tear film that covers the suface of the eye has 3 layers: o lipid secred by oil glands in the eyelidso aqueous-based solution from lacrimal gland (contains lysozyme

and other enzymes that provide protection against infection)o mucous from the conjunctiva

• the composition of the tear layer varies depending upon the stimulus and with ageo emotional tears contain more hormones, such as prolactin,

ACTH and enkephalin o basal tear production decreases with age

Tear Production

• tear flow occurs via evaporation and drainage through the nasolacrimal ducts into the nasal cavity o parasympathetic stimulation produces epiphora (overflow of

tears) by: 1. increasing tear production by the lacrimal gland2. decreasing outflow by facilitating closure of the lacrimal duct

passage o epiphora can be induced by:

• stimulation of the cornea (cranial nerve V) which produces reflex tears • strong emotional responses (mediated by the limbic system, especially the

hypothalamus) which produce psychic tears (crying or weeping) • strong parasympathetic stimulation is accompanied by other

symptoms, like reddening of the face and convulsive breathing

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.

RODS CONESAmount of

photopigment More Less

Pigment type 1 = 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

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-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.

Vitamin A Deficiency

Macular Degeneration

Ganglion 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.

VISUAL CORTEX PHYSIOLOGY

Colour Perception

Colour PerceptionPerception 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

DevelopmentSTRABISMUS 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).