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Med 6573 Nervous System Winter 2012Dr. Janet Fitzakerley [email protected] Visionhttp://www.d.umn.edu/~jfitzake/Lectures/Teaching.html of 14

VISION

CRITICAL FACTS(if med school is a Minnesota forest with millions of trees, these are the red pines).

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 a potentially permanent loss of vision. Closed angle glaucoma is the most rapidly advancing form (it constitutes a medical emergency) and is caused by blockage of fluid outflow. Open angle glaucoma (the most common form) results from either the overproduction of the aqueous humor or a 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.

7. PHOTOTRANSDUCTION occurs via a 4 step process that uses a 2nd

messenger cascade to amplify the signal. In rods, activation of

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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.

ESSENTIAL MATERIAL FROM OTHER LECTURES1. 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, Neurological Medicine)

2. CSF formation and Treatment of Hydrocephalus (Drs. Drewes and Trachte, Nervous System)

3. Pupillary reflex/innervation of the dilator and constrictor muscles of the pupil (Dr. Severson, Neurological Medicine)

4. Anatomical structures associated with aqueous humor formation and flow, including the ciliary body and the canal of Schlemm (Dr. Downing, Neurological Medicine).

5. Histology of the retina (Dr. Downing, Neurological Medicine).

6. Receptor potentials and lateral inhibition (Dr. Fitzakerley, Neurological Medicine)

LEARNING OBJECTIVES

1. 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.



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.


OPTICS

Protective Mechanisms

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 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: 1. lipid secred by oil glands in the eyelids2. aqueous-based solution from lacrimal gland (contains lysozyme and other

enzymes that provide protection against infection)3. 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 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:

1. stimulation of the cornea (cranial nerve V) which produces reflex tears 2. strong emotional responses (mediated by the limbic system, especially

the hypothalamus) which produce psychic tears (crying or weeping)



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.


strong parasympathetic stimulation is accompanied by other symptoms, like reddening of the face and convulsive breathing

FocusingRefraction




Accomodation



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.


Refractive Errors

Regulation of Light Intensity



Increased intraocular pressure causes a potentially permanent loss of vision. Closed angle glaucoma is the most rapidly advancing form (it constitutes a medical emergency) and is caused by blockage of fluid outflow. Open angle glaucoma (the most common form) results from either the overproduction of the aqueous humor or a blockage of fluid outflow..


Formation of the Aqueous Humor

Glaucoma



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.


PHYSIOLOGY OF THE RETINA

Visible Light

Photoreceptors

RODS CONES

Amount of photopigment More Less

Pigment type 1 = rhodopsin 3 overlapping




patterns of activity for colour

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 resolutionLow

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



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.


Visual Acuity



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.


Phototransduction

Receptor Potential

Retinitis Pigmentosa



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.


Retinopathy

Visual Cycle




Vitamin A Deficiency

Macular Degeneration



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