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Comprises: a) optics, primary role is to focus image onto receptor cells.
b) information transfer, converts image into neural discharges.
c) processing, transforms neural information into image.
Principles of Light
Wavelength: distance between 2 successive wave peaks.
Frequency: cycles per second. Unit = Hertz (Hz).
Receptors in the eye are sensitive only the visible spectrum: 400 – 700 nm. Within
this range, light of different wavelengths is perceived as having different colors.
Anatomical Structure of the eye
The human eye
Retina: thin layer of neural tissue lining the back of the eyeball. Contains the light
sensitive receptor cells (photoreceptors), and neurons. Sensitive enough to respond to
a single photon of light
Fovea centralis: area of the retina that provides the greatest visual clarity because
photoreceptors are placed in direct alignment with incoming light, and because cones
are in the highest density here. Absent of blood vessels to allow the incident light to
project directly onto photoreceptors.
Lens and cornea: focus light rays on the retina, particularly the fovea. The cornea
actually plays the most important role in bending the light due to the refractive index
mismatch between the air and the aqueous humor. Lens is important for adjusting
focus for objects of different distances (termed “accommodation”). The image that is
focused on the retina is actually upside-down and reversed left-to-right (see below).
Ciliary muscles and zonular fibers (also called “suspensory ligaments”): control the
shape of the lens.
Iris: a pigmented tissue composed of smooth muscle and innervated by autonomic
nerves that regulates the amount of light entering the eye through the pupil (the hole in
the iris). Pupil size changes primarily in response to variations in incident light, but
also with emotion or pain. Stimulation of sympathetic nerves contracts radial muscle
fibers to enlarge the iris. Stimulation of parasympathetic nerves contracts circular
muscle fibers making the pupil smaller.
To focus on distant objects the zonular fibers pull the lens into a flattened shape.
For focusing on near objects, the natural elasticity of the lens maintains it in a rounded
shape, which provides more bending of the light. Also, parasympathetic innervation
contracts the ciliary muscles, removing tension on the zonular fibers and pushing the
lens into a rounded shape. Note: contraction moves the ciliary muscle nearer the lens.
In addition, to focus on very near objects, the lens moves slightly toward the back of
the eye, the eyes move inward (convergence), and the pupil constricts. All these
mechanisms are examples of accommodation.
The shape of the lens
The ciliary
of the eye
muscle of
The eye
Common eye disorders and clinical correction
Cataract: cells of the lens are absent of organelles, and thus, they are transparent. The
only cells that divide are located on the exterior of the lens. As new cells are formed
the older cells are covered, leading to an increase in lens stiffness and a discoloration
of the lens (eventually becomes black). The discoloration is responsible for cataract,
which is the most common eye disorder. The opaque lens can be treated with laser
surgery or replaced with an artificial lens.
Presbyopia: age-induced lens stiffening. Reduces ability to focus on near objects.
Treated with reading glasses.
Nearsighted (myopic) and farsighted (hyperopic): occurs due to changes in the shape
of the eyeball. Image is not focused on the retina. Corrected with eyeglasses.
Astigmatism: lens or cornea develop non-spherical surface. Treated with eye glasses.
Glaucoma: increased pressure in the eye caused by faster formation of aqueous humor
than removal. Leads to death of axons in optic nerve. Leading cause of irreversible
blindness. Treated by reducing intraocular pressure with eye drops (that increase
humor removal and decrease humor production) or surgery.
Organization of the retina
The retina
Light passes through all cell layers of retina before reaching the tips of
photoreceptors, which face away from incoming light. This arrangement of the retina
causes distortion of the image which is filtered out in the higher centers of the brain.
The purpose of this anatomical arrangement is to allow adequate removal of used
photoreceptor pigment. The choroid (see earlier figure), a pigmented layer, absorbs
light to prevent it from reflecting back onto the retina and re-stimulating the
2 types, located in the retina:
Cones. Adapted for color vision. 3 subtypes are found that are based on the visual
pigment in the cell: i) most sensitive to blue light, ii) most sensitive to green light,
and iii) most sensitive to red/yellow light. Perception of color comes from relative
stimulation of each of these 3 receptor subtypes. Most concentrated in fovea to
provide high resolution. Adapted for day vision – not responsive in low light
conditions (hence black and white vision at night) and saturate in intense light.
Photochemical cascade v. fast providing rapid response time.
Rods. Mainly adapted for low light conditions. Contain more photopigment than
cones. Photopigment in rods called “rhodopsin”. Rods have a longer outer segment
than cones, allowing them to respond to a broader angle of incident light. Respond
more slowly that cones, and thus can summate more light responses. Absent in fovea,
more concentrated in peripheral retina. These features are responsible in part for the
high sensitivity and lower resolution of images formed from rods, when compared
with cones.
Photopigments and the phototransduction cascade
4 different photopigments found in the retina: one in rods, called rhodopsin, and 3
others in different cone types (discussed earlier). Each photopigment is made up of an
opsin, which is a membrane bound molecule that surrounds retinal, the chromophore
which is light sensitive and is a derivative of vitamin A. The 4 different
photopigments have a different type of opsin, but all have retinal. Each opsin binds
retinal differently and this property endows the photopigments sensitivity to a
different band of the visible spectrum.
Arrangement of the opsin and retinal
Sensitivities of the photopigments
Color blindness which occurs in 8 % of males and 1 % of females, results from a lack
of either the red or green cone pigment and is manifest as a difficulty perceiving red
from green.
Photopigments lie in specialized membranes that are arranged in ordered stacks. Light
activates retinal, triggering a conformational change that, via a series of biochemical
pathways, leads to graded hyperpolarization of the photoreceptor. Following
activation, enzymes revert retinal back to its original shape.
Rhodopsin, found in rods, is completely inactivated under normal light conditions.
Thus, it takes minutes to become adjusted to low light conditions, because rhodopsin
must first be regenerated by enzymes, so it can respond to low levels of light.
Neural pathways of vision
Light-induced hyperpolarization of photoreceptors decreases glutamate release from
the cell, which in turn causes a graded hyperpolarization of bipolar cells that synapse
with the photoreceptor. Bipolar cells subsequently activate:
1) amacrine cells, which influence nerve activity within the retina.
2) ganglion cells. Once activated, ganglion cells produce action potentials. Axons
from ganglion cells enter the optic nerve (cranial nerve II) and carry information to the
higher centers of the brain for information processing. The optic nerves meet at the
optic chiasm, where some fibers cross to the opposite side of the brain. The visual
pathway transmits information pertaining not only to the intensity and spectral
properties of the light, but also the location of the light source.
There are actually 2 types of ganglion cells. One type of ganglion cell receives input
from all 3 types of cones. Another type, called opponent color cells, receive
excitatory input from only one type of cone, and inhibitory input from another cone.
Single opponent color ganglion cell
Example of an opponent color
cell. This cell increases firing
rate when exposed to blue light,
but decreases when exposed to
red light. White light, which
contains blue and red light,
produces a weak response.
Optic fibers go to:
• the lateral geniculate nucleus of the thalamus, where information is sent to the
visual areas of the cortex for processing. This is the primary pathway.
•suprachiasmatic nucleus, which functions as a biological clock.
•brainstem and cerebellum, to coordinate eye and head movements, fixation of gaze,
and pupil size.
QuickT ime™ and a
TI FF (Uncompressed) decompressor
are needed to see t his picture.
Eye movement
6 skeletal muscles on each eyeball control fast and slow eye movements.
Small, jerking eye movements, called saccades, allow the eye to rapidly move from
one object to another. Saccades also allow the eye to constantly move the visual
image over the receptors, to prevent adaptation to the light.
Slow eye movements allow the tracking of moving objects and compensate for head
movement while focusing on an object.
Superior view of muscles
that move the eye
Detects changes in motion and position of the head.
Vestibular apparatus: a series of fluid filled tubes that lie in canals in the temporal
bone on each side of the head. Connect with the cochlear duct. Consist of three
semicircular ducts and 2 saclike swellings, called the utricle and the saccule. The
canals that house the semicircular ducts are called the semicircular canals.
The vestibular apparatus
The two sets of
Semicircular canals
The semicircular canals
Contain receptor cells like those in the organ of Corti. The stereocilia are covered in a
gelatinous mass, the cupula, which extend along the lumen of each ampulla, the bulge
in the wall of each duct.
Detect angular acceleration during head rotation along three axes. Head movement
causes displacement of the semicircular canals, but inertia leaves the fluid behind.
The moving ampulla causes bending of the stereocilia, which ultimately alters the
release of glutamate, a neurotransmitter, from the cell. Glutamate activates nerve
terminals that synapse with the hair cells.
A cupula
and ampulla
The speed and magnitude of head movement determine the direction in which the hair
cells are bent. The frequency of evoked action potentials in the afferent nerve fiber is
related both to direction and to force of movement. The direction in which the hair
cells are bent determines if the cell depolarizes or hyperpolarizes. Glutamate is
released from the hair cells at rest, and if the cell depolarizes glutamate release
increases. If the cell hyperpolarizes, glutamate release decreases. At a constant
velocity, the fluid in the duct moves at the same rate as the head, and the stereocilia
return to resting position. Thus, hair cells in the ampulla of the semicircular canals
only respond to acceleration or deceleration.
Activity in afferent neurons
Utricle and Saccule
Detect linear acceleration and changes in head movement relative to gravity.
In utricle, hair cells are not bent
In saccule, hair cells are bent
in a standing person
Stereocilia in the utricle and saccule are covered with a gelatinous substance in which
tiny stones made of calcium carbonate, called otoliths, are embedded. Otoliths are
heavier than the gelatinous material and therefore, move according to gravity.
Vestibular information and dysfunction
Information from hair cells is relayed to the brainstem via cranial nerve VIII. Then to
the vestibular centers in the parietal lobe, where it is integrated with information from
joints, tendons and skin to provide a complex picture of posture and movement.
Uses of vestibular information:
1) Control eyes to maintain fixed point, even during head movement.
2) Provide input to reflex mechanisms that control body posture. Although vestibular
system is considered to be the most important in maintaining balance, the majority of
information actually comes from other sources.
3) Conscious awareness of acceleration and position of the body, perception of space
around the body and memory of spatial information.
Vertigo: caused by stroke, irritation or infection of the ducts, or loose otoliths.
Induces illusion of movement, causing nausea and lightheadedness.
Motion sickness: occurs when adaptation to unfamiliar movements does not occur.
Ménière’s disease: caused by increased fluid pressure in one inner ear: input from ears
is unbalanced. Dizziness, ringing in the ears, hearing loss.
Receptors called chemoreceptors respond to chemical changes.
Functions to distinguish between food and potential toxins. Pleasant sensations
maintain appetite and initiate digestive responses, whereas unpleasant sensations
initiate coughing, gagging, and vomiting.
Approx. 10,000 taste buds found mainly on tongue. 50 - 100 receptor cells within
each taste bud are arranged like segments of an orange. Receptor cells are modified
epithelial cells rather that true neurons. Taste buds also contain stem cells that
produce new receptor cells every 1 – 2 weeks. Afferent neurons synapse onto receptor
Taste sensations are divided into five basic classes: salt, sour, sweet, bitter, and
umami*. The sensation of particular flavors is based on combinations of these 5
basic classes. Different tastes activate different signal transduction cascades in the
receptor cells:
i) direct passage of ions through ion channels.
ii) block of ion channels.
iii) activation of ion channels.
iv) activation of second messenger systems.
* Umami is recognized as a basic taste that causes increased release of
neurotransmitter. These receptors are stimulated by glutamate, which can be
found in food as MSG.
Taste transduction mechanisms
80 % of the flavor of food is detected by the olfactory system. The odor of a chemical
is directly related to its chemical structure. Humans can recognize thousands of
different odors due to our ability to encode, store in neural circuits, and recognize
patterns associated with different chemical structures.
3 major cell types are found in the olfactory epithelium, which occurs in patches in the
upper part of the nasal cavity:
1) olfactory receptor cells. Specialized afferent neurons that that have a single
enlarged dendrite that extends to the surface of the epithelium. Non-motile cilia,
which contain the receptor proteins, extend from the dendrite into the nasal cavity
where they are bathed in mucus. The axons from the the receptor cells form the
olfactory nerve (cranial nerve I).
2) stem cells, that produce new cells.
3) supporting cells, that provide a supportive matrix.
Also interspersed in the epithelium are Bowman’s glands, which produce a layer of
mucus that lines the nasal cavity. Nasal mucus contains mucopolysaccharides, salts,
antibodies, and odorant binding proteins, which increase the sensitivity to odorants
1000 times.
Humans recognize ~10,000 separate odors, but this can increase to 100,000 with
experience, e.g. wine tasters. This sensitivity occurs because more than 1000 different
receptor proteins can be expressed. However, each receptor cell expresses only a few
types of each receptor protein.
Axons from the receptor cells synapse onto olfactory bulbs. Specific receptor cells
activate certain olfactory bulb neurons, allowing decoding of which receptors were
activated. Information is transmitted to the olfactory cortex in the limbic system.