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Chapter 9

NROC64H3 Chapter Notes - Chapter 9: Brainstem, Trichromacy, Visual Phototransduction

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Matthias Neimier

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NROC64 - Chapter #9:
The Eye:
Sensitivity to light allows us to detect prey, predators and mates
Light is electromagnetic energy that is emitted in the form of waves
About half the human cerebral cortex is involved with analyzing the visual world
At the back of the eye is the retina contains photoreceptors specialized to convert light energy
into neural activity
Eye has the ability to track moving objects and the ability to keep its transparent surfaces clean
(by blinking)
The retina is actually a part of the brain
Each eye has 2 overlapping retinas: one specialized for low light levels that we encounter from
dusk to dawn and another for higher light levels and for the detection of colour from sunrise to
Axons of retinal neurons are bundled into optic nerves which distribute visual information to
several brain structures that perform different functions
The first synaptic relay that serves perception occurs in a cell group of the dorsal thalamus
called the lateral geniculate nucleus (LGN) cerebral cortex interpreted and remembered
Properties of Light:
The visual system uses light to form images of the world around us
Electromagnetic radiation that is visible to our eyes can be described as a wave of energy
Wavelength the distance b/w successive peaks or troughs
Frequency the # of waves per second
Amplitude the diff b/w wave trough and peak
Energy content of electromagnetic radiation is proportional to its frequency
Radiation emitted at high frequency (short wavelengths) has the highest energy content
Lower frequencies = longer wavelengths = less energy
Only a small part of the electromagnetic spectrum is visible to our eye
Visible light: 400-700nm
Colours themselves are coloured by the brain
Light rays travel in straight lines until they interact with atoms and molecules of the atmosphere
and objects on the ground
These interactions include: reflection, absorption and refraction
Reflection the bouncing of light rays off a surface
o Strikes at 180, reflected back; strikes at 45, reflected at 90
Absorption transfer of light energy to a particle or surface
o black surfaces absorb all visible wavelengths
o blue absorbs long wavelengths but reflects short ones
light sensitive photoreceptor cells in the retina contain pigments and use the energy absorbed
from light to generate changes in membrane potential
refraction the bending of light rays that can occur when they travel from one transparent
medium to another
o light passes through air more rapidly than water
The Structure of The Eye:
organ specialized for detection, localization and analysis of light
Gross Anatomy of the Eye:
pupil the opening that allows light to enter the eye and reach the retina; appears dark b/c of
the light-absorbing pigments of the retina
Iris surround the pupil, pigmentation provides what we call the eye’s colour
o Contains 2 muscles that can vary in size: one makes it smaller when it contracts and the
other larger
Cornea glassy transparent external surface of the eye, surround the pupil and iris
Sclera continuous with the cornea, “white of the eye” which forms the tough wall of the
Eye’s orbit bony socket in the skull in which the eye sites
Extraocular muscles inserted into the sclera are three pairs of these; which move the eyeball
in the orbit
Conjuctiva a membrane that folds back from the inside of the eyelids and attaches to the
Optic nerve carrying axons from the retina, exits the back of the eye, passes through the orbit,
and reaches the base of the brain near the pituitary gland
Opthalamoscopic Appearance of the Eye:
Opthalmoscope device that enable one to peer into the ye through the pupil of the retina
Optic disk pale circular region from which the retinal blood vessels originate
o Also where the optic fibers exit the retina
Sensation of light cannot occur at the optic disk b/c there are no photoreceptors nor can it occur
where the blood vessels are
Our brain fills in any holes of our vision
Macula in the middle of each retina, darker-coloured region with a yellowish hue, part of the
retina for central vision
Vessels arc from the optic disk to the macula
Macula has a relative absence of blood vessels which improves the quality of central vision
Fovea a dark spot about 2 mm in diameter
o Latin for the word pit
o Retina is thinner in the fovea than elsewhere
o Marks the centre of the retina
o Part that lies near the nose = nasal, near temple = temporal, above = superior, below =
Cross-Sectional Anatomy of the Eye:
The cornea lacks blood vessels and is nourished by the fluid behind it, the aqueous humor
Lens behind the iris
Lens suspended by ligaments (zonule fibers) attached to the ciliary muscles, which are
attached to the sclera and form a rind inside the eye
Changes in the shape of the lens enable our eyes to adjust their focus to different viewing
Lens also divides the interior of the yes into 2 compartments containing slightly diff fluids:
o Aqueous humor watery fluid that lies b/w the cornea and the lens
o Vitreous humor more viscous, jelly-like, lies b/w the lens and the retina, its pressure
serves to keep the eyeball spherical
Box 9.1 Demonstrating the Blind Regions of your Eye:
Hole in the retina = regions where the optic nerve axons exit the eye and the retinal blood
vessels enter the eye, the optic disk is completely devoid of photoreceptors
Blood vessels coursing across retina are opaque and block the light from falling on
photoreceptors beneath them
Mechanisms in the visual cortex appear to “fill in” the missing regions
Image Formation by the Eye:
Eye collects the light rays emitted by or reflected off objects in the environment and focuses
them onto the retina to form images
Cornea is the site of most of the refractive power of the eyes
Refraction by the Cornea:
Light rays that strike the curved surface of the cornea bend so that they converge on the back of
the eye
Focal distance the distance from the refractive surface to the point where parallel light rays
converge, depends on the curvature of the cornea, the tighter the curve the shorter the focal
Dipoter - measurement for the focal distance in meters
Cornea has a refractive power of 42 dipoters, parallel light rays striking the corneal surface will
be focused 0.024 (2.4cm) behind it
Refractive power depends on the slowing of light at the air-cornea interface
Water-cornea interface has very little focusing power
Accommodation by the Lens:
The lens also contributes to the formation of a sharp image at a distant point
Lens is involved more in the formation of crisp images of objects located closer than about 9 m
from the eye
Accommodation additional focusing power provided by changing the shape of the lens
Ciliary muscle forms a ring around the lens
During accommodation the ciliary muscle contracts and swells in size thereby making the area
inside the muscles smaller and decreasing the tension in the suspensory ligaments
o Lens become rounder and thicker (b/c of its natural elasticity) rounding increases the
curvature of the lens increasing refractive power
Conversely the relaxation of the cilary muscle increases the tension in the suspensory ligaments
and the lens is stretched into a flatter shape
Ability to accommodate changes with age
Box 9.2 Eye Disorders:
Imbalance in the extraocular muscles in the eyes, the eyes will point in different directions =
strabismus (2 types of this)
o Esotropia directions of gaze of the 2 eyes cross and the person is said to be cross-eyed
o Exotropia directions of gaze diverge and person is said to be wall-eyed
Strabismus is congenital it can and should be corrected in early childhood
Treatment involved the use of prismatic glasses or surgery to the extraocular muscles to realign
the eyes
Without treatment one eye is dominant the other is suppressed
The supressed eye will become amblyopic poor visual accuracy
If medical attention delayed, the damage may be permanent
Cataract clouding of the lens
o Many people over 65 have some degree of cataract
o Surgery may be required
o In the operation the lens is removed and replaced with an artificial plastic lens
o Cannot adjust the same but vision will be good