Med 6573 Nervous System
Dr. Janet Fitzakerley jfitzake@d.umn.edu
http://www.d.umn.edu/~jfitzake/Lectures/Teaching.html
Winter 2012
Vision
Page 1 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 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.
7.
PHOTOTRANSDUCTION occurs via a 4 step process that uses a 2nd
messenger cascade to amplify the signal.
In rods, activation of
Med 6573 Nervous System
Dr. Janet Fitzakerley jfitzake@d.umn.edu
http://www.d.umn.edu/~jfitzake/Lectures/Teaching.html
Winter 2012
Vision
Page 2 of 14
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 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,
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.
2.
3.
4.
5.
6.
7.
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.
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.
Med 6573 Nervous System
Dr. Janet Fitzakerley jfitzake@d.umn.edu
http://www.d.umn.edu/~jfitzake/Lectures/Teaching.html
Winter 2012
Vision
Page 3 of 14
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



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 lubrication
o
eye protection
o
visual information processing
Med 6573 Nervous System
Dr. Janet Fitzakerley jfitzake@d.umn.edu
http://www.d.umn.edu/~jfitzake/Lectures/Teaching.html

Winter 2012
Vision
Page 4 of 14
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 eyelids
2. 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
age
o
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 gland
2. 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
Med 6573 Nervous System
Dr. Janet Fitzakerley jfitzake@d.umn.edu
http://www.d.umn.edu/~jfitzake/Lectures/Teaching.html
Winter 2012
Vision
Page 5 of 14
2. 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
Med 6573 Nervous System
Dr. Janet Fitzakerley jfitzake@d.umn.edu
http://www.d.umn.edu/~jfitzake/Lectures/Teaching.html
Accomodation
Winter 2012
Vision
Page 6 of 14
Med 6573 Nervous System
Dr. Janet Fitzakerley jfitzake@d.umn.edu
http://www.d.umn.edu/~jfitzake/Lectures/Teaching.html
Winter 2012
Vision
Page 7 of 14
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.
Med 6573 Nervous System
Dr. Janet Fitzakerley jfitzake@d.umn.edu
http://www.d.umn.edu/~jfitzake/Lectures/Teaching.html
Winter 2012
Vision
Page 8 of 14
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
Med 6573 Nervous System
Dr. Janet Fitzakerley jfitzake@d.umn.edu
http://www.d.umn.edu/~jfitzake/Lectures/Teaching.html
Winter 2012
Vision
Page 9 of 14
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.
Med 6573 Nervous System
Dr. Janet Fitzakerley jfitzake@d.umn.edu
http://www.d.umn.edu/~jfitzake/Lectures/Teaching.html
Winter 2012
Vision
Page 10 of 14
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 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
Med 6573 Nervous System
Dr. Janet Fitzakerley jfitzake@d.umn.edu
http://www.d.umn.edu/~jfitzake/Lectures/Teaching.html
Winter 2012
Vision
Page 11 of 14
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.
Med 6573 Nervous System
Dr. Janet Fitzakerley jfitzake@d.umn.edu
http://www.d.umn.edu/~jfitzake/Lectures/Teaching.html
Winter 2012
Vision
Page 12 of 14
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
Retinitis Pigmentosa
Med 6573 Nervous System
Dr. Janet Fitzakerley jfitzake@d.umn.edu
http://www.d.umn.edu/~jfitzake/Lectures/Teaching.html
Winter 2012
Vision
Page 13 of 14
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.
Med 6573 Nervous System
Dr. Janet Fitzakerley jfitzake@d.umn.edu
http://www.d.umn.edu/~jfitzake/Lectures/Teaching.html
Vitamin A Deficiency
Macular Degeneration
Winter 2012
Vision
Page 14 of 14