Sensory & Motor Systems 4/13/2012
Chapter 9 The Eye
o Axons of retinal neurons are bundled into optic nerves, which distribute visual
information (in the form of Action Potentials) to several brain structures that
perform different functions.
Some targets of the optic nerves are involved in regulating biological
rhythms, which are synchronized with light-dark daily cycle; others are
involved in the control of eye position and optics.
The first synaptic relay in the pathway that serves visual perception occurs in a cell group
of the dorsal thalamus called the lateral geniculate nucleus (LGN).
- From the LGN, visual information ascends to the cerebral cortex where it is
interpreted and remembered
Properties of Light:
Light can be:
- Reflected: the bouncing of light rays off a surface.
o The manner in which a ray of light is reflected depends on the angle at which
it strikes the surface. A ray striking a mirror perpendicularly is reflected 180
degrees back upon itself, a ray striking the mirror at a 45 degree angle is
reflect 90 degrees.
- Absorbed: the transfer of light energy to a particle or surface. Some compounds
absorb light energy only in a limited range of wavelengths, and then reflect the
- Refracted (how images are formed in the eye): the bending of light rays that can
occur when they travel from one transparent medium to another. The bending of
light occurs because the speed of light differs across different media.
o The greater the differences between the speed of light in the two media, the
greater the angle of refraction.
o The transparent media in the eye bend light rays to form images on the
The Structure of the Eye:
I. Gross Anatomy of the Eye:
Pupil: the opening that allows light to enter the eye and reach the retina; it appears dark
because of the light absorbing pigments in the retina.
Iris (surrounds the pupil): pigmentation provides the eyes color. The iris contains two
muscles that can vary the size of the pupil; one makes it smaller when it contracts, the
other makes it larger.
The iris and the pupil are both surrounded by the cornea. The cornea is continuous with
the sclera, the white of the eye, which forms the tough wall of the eyeball. The eyeball
sits in a bony eye socket in the skull, also called the eyes orbit. Inserted into the sclera are
three pairs of extraocular muscles, which move the eyeball in the orbit. These muscles Sensory & Motor Systems 4/13/2012
Chapter 9 The Eye
normally are not visible because they lie behind the conjunctiva, a membrane that folds
back from the inside of the eyelids and attaches to the sclera. 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.
II. Cross-Sectional Anatomy of the Eye:
The Cornea lacks blood vessels and is nourished by the fluid behind it, the aqueous
humor. This view reveals the transparent lens located behind the iris. The lens is
suspended by ligaments (called zonule fibers) attached to the ciliary muscles which are
attached to the sclera and form a ring inside the eye.
*** The lens also divides the interior of the eye into two compartments containing slightly
- The aqueous humor is the watery fluid that lies between the cornea and the lens
- The more viscous, jellylike vitreous humor lies between the lens and the retina; its
pressure serves to keep the eyeball spherical.
Image Formation by the Eye:
Light rays that strike the curved surface of the cornea bend so that they converge on the
back of the eye; those that enter the center of the eye pass straight to the retina. The
distance from the refractive surface to the point where parallel light rays converge is called
the focal distance.
- Focal distance depends on the curvature of the cornea, the tighter the curve, the
shorter the focal distance.
Accommodation by the Lens:
- Lens is involved in forming crisp images of objects located closer than about 9 m
from the eye.
o As objects approach, the light rays originating at a point can no longer be
considered to be parallel. These rays diverge and greater refractive power is
required to bring them into focus on the retina.
This additional focusing power is provided by changing the shape of
the lens a process known as accommodation.
- During accommodation, the ciliary muscle contracts and swells in size, thereby
making the area inside the muscle smaller and decreasing the tension in the
o The lens becomes rounder and thicker because of its natural elasticity. This
rounding increases the curvature of the lens surfaces, thereby increasing
their refractive power. Conversely, relaxation of the ciliary muscle increases
the tension in the suspensory ligaments and the lens is stretched into a flatter
- The ability of the eye to distinguish two nearby points is called visual acuity.
- Acuity depends on several factors, but especially on the spacing of photoreceptors
in the retina and the precision of the eyes refraction.Sensory & Motor Systems 4/13/2012
Chapter 9 The Eye
Box 9.2 -Eye Disorders:
1. Imbalance in the extraocular muscles ( known as strabismus)
- Eyes will point in different directions.
- In most cases is congenital
- Treatment usually involves the use of prismatic glasses or surgery to the extraocular
muscles to realign the eyes.
- Without treatment, conflicting images are sent to the brain from the two eyes,
degrading depth perception, and more importantly, cause the person to suppress
input from one eye.
o The dominant eye will be normal but the suppressed eye will become
amblyopic, meaning that it has poor visual acuity.
- If medical intervention is delayed until adulthood, the condition cannot be
a. Type I: Esotropia: the directions of gaze of the two eyes cross and the person is
said to be cross-eyed.
b. Type II: Exotropia: the directions of gaze diverge and the person is said to be wall-
- Most common amongst adults
- If it significantly impairs vision, surgery is usually required.
o In a cataract operation, the lens is removed and replaced with an artificial
plastic lens. The plastic lens wi