1. Visual Transduction
Once light waves have been successfully focused on the retina, the information “stored” in that electromagnetic energy must be changed by photopigments in the photoreceptors into signals our brain can interpret - a process called visual transduction
The single type of photopigment in rods is rhodopsin, whereas there are 3 different cone photopigments
Color vision results from different colors of light selectively activating the different cone photopigments

2. Visual Transduction
The first step in visual transduction is absorption of light by a photopigment, a colored protein that undergoes structural changes when it
absorbs light in the outer
segment of a photoreceptor
Light absorption initiates
a series of events that
lead to the production
of a receptor potential
(number 4 in the diagram)

3. Visual Transduction
All photopigments associated with vision contain two parts: a glycoprotein known as opsin and a derivative of vitamin A called retinal
Although there are 4 different opsins, retinal is the light-absorbing part of all visual photopigments
To simplify the process we can say that there is a cyclical bleaching and regeneration of photopigment
Bleaching is a term describing a conformational change in the retinal molecule in response to light

4. Visual Transduction
In darkness, retinal has a bent shape called cis-retinal
Absorption of a photon of light causes it to straighten into the trans-retinal form in a process called isomerization
Trans-retinal completely separates from the opsin; since the final products look colorless, this part of the cycle is called bleaching of photopigment
An enzyme converts trans-retinal→ cis-retinal
The cis-retinal regenerates the photopigment
In darkness, the neurotransmitter glutamate is released keeping Na+ channels open and inhibiting the bipolar cell: This inflow of Na+is called the “dark current”. Photons cause Na+ channels to close, and the rod hyperpolarizes. It’s strange that when your eyes are closed and you are asleep the rods are the most active.

5. Bleaching and regeneration of photopigments are summarized here
Horizontal cells transmit inhibitory signals to bipolar cells in the areas lateral to excited rods and cones.
Bleaching and regeneration of photopigments are summarized here

6. Visual Transduction
In daylight, regeneration of rhodopsin cannot keep up with the bleaching process, so rods contribute little to daylight vision. In contrast, cone photopigments regenerate rapidly enough that some of the cis form is always present, even in very bright light
As a consequence, light adaptation (from dark conditions  light conditions) happens in seconds; dark adaptation (from light  dark) takes minutes to occur (up to 40 minutes to fully adapt)
After complete bleaching, regeneration of half of the rhodopsin takes 5 minutes; half of the cone photopigments regenerate in only 90 seconds. Full regeneration of bleached rhodopsin takes 30 to 40 minutes.

7. Visual Transduction
Most forms of color blindness, an inherited inability to distinguish between certain colors, result from the absence or deficiency of one of the three types of cones
Most common type is red-green color blindness in which red cones or green cones are missing
Prolonged vitamin A deficiency and the resulting below-normal amount of rhodopsin may cause night blindness or nyctalopia, an inability to see well at low light levels

8. The Visual Pathway
The graded potentials generated by the photoreceptors undergo considerable processing at synapses among the various types of neurons in the retina (horizontal cells, bipolar cells, and amacrine cells)- certain features of visual
input are enhanced while others
are discarded
Overall, convergence pre-
dominates as 126 million
photo-receptors impinge on only
1 million ganglion cells
Horizontal cells transmit inhibitory signals to bipolar cells in the areas lateral to excited rods and cones. Horizontal cells also assist in the differentiation of various colors. Amacrine cells, which are excited by bipolar cells, synapse with ganglion cells and transmit information to them that signals a change in the level of illumination of the retina. When bipolar or amacrine cells transmit excitatory signals to ganglion cells, the ganglion cells become depolarized and initiate nerve impulses.

9. The Visual Pathway
The axons of retinal ganglion cells provide output that travels back “towards the light”, exiting the eyeball as the
optic nerve, which emerges from the vitreous surface of the retina
The axons then pass
through a crossover point
called the optic chiasm

10. The Visual Pathway
Some axons cross to the opposite side, while others remain uncrossed. Once through the
optic chiasm the axons enter the
brain matter as the optic tracts
(most terminate in thalamus)
Here they synapse with
neurons that project to the
1o visual cortex in the
occipital lobes

11. 1 2 4 5 3 1 2 3 4 5 6 1 2 4 3 1 2 3 1 2 1 Visual field of left eye
Temporal
half
right eye
Nasal
Midbrain
Left eye
retina
Optic
radiations
Left eye and its pathways
tract
Primary visual area of cerebral
cortex (area 17) in occipital lobe
Lateral geniculate nucleus
of the thalamus
Right eye
Right eye and its pathways
Nasal retina 1 2 4 5 3
Visual field of
left eye
Temporal
half
right eye
Nasal
Midbrain
Left eye
retina
Optic
radiations
Left eye and its pathways
tract
Primary visual area of cerebral
cortex (area 17) in occipital lobe
Lateral geniculate nucleus
of the thalamus
Right eye
Right eye and its pathways
Nasal retina 1 2 3 4 5 6
Visual field of
left eye
Temporal
half
right eye
Nasal
Midbrain
Left eye
retina
Optic
radiations
Left eye and its pathways
tract
Primary visual area of cerebral
cortex (area 17) in occipital lobe
Lateral geniculate nucleus
of the thalamus
Right eye
Right eye and its pathways
Nasal retina 1 2 4 3
Visual field of
left eye
Temporal
half
right eye
Nasal
Midbrain
Left eye
retina
Optic
radiations
Left eye and its pathways
Primary visual area of cerebral
cortex (area 17) in occipital lobe
Lateral geniculate nucleus
of the thalamus
Right eye
Right eye and its pathways
Nasal retina 1 2 3
Visual field of
left eye
Temporal
half
right eye
Nasal
Midbrain
Left eye
retina
Optic
radiations
Left eye and its pathways
Primary visual area of cerebral
cortex (area 17) in occipital lobe
Lateral geniculate nucleus
of the thalamus
Right eye
Right eye and its pathways
Nasal retina 1 2
Visual field of
left eye
Temporal
half
right eye
Nasal
Midbrain
Left eye
retina
Optic
radiations
Left eye and its pathways
Primary visual area of cerebral
cortex (area 17) in occipital lobe
Lateral geniculate nucleus
of the thalamus
Right eye
Right eye and its pathways
Nasal retina 1

12. The Ear
Audition, the process of hearing, is accomplished by the organs of the ear. The ear is an engineering marvel because its sensory receptors can transduce sound vibrations with amplitudes as small as the diameter of an atom of gold into electrical
signals 1000 times faster than
the eye can respond to light
The ear also contains
receptors for equilibrium

13. The Ear The ear has 3 principle regions
The external ear, which uses air to collect and channel sound waves
The middle ear, which uses a
bony system to amplify
sound vibrations
The internal ear, which
generates action potentials to transmit
sound and balance information to the brain
The pathways for sound transmission starts in the air, the to the solid bone of the middle ear, then to the endolymph of the inner ear.

14. The External Ear The anatomy of the external ear includes
The auricle (pinna), a flap of elastic cartilage covered by skin and containing ceruminous glands
A curved 1” long external auditory canal situated in the temporal bone leading from
the meatus to the tympanic
membrane (TM – or ear
drum) which separates the
outer ear from the cavity
of the middle ear

15. The Middle Ear
The middle ear is an air-filled cavity in the temporal bone. It is lined with epithelium and contains 3 auditory ossicles (bones)
The stapes (stirrup)
The incus (anvil)
The handle of
the malleus
(hammer) attaches
to the TM

16. The Middle Ear
Two small skeletal muscles (the tensor tympani and stapedius) attach to
the ossicle and
dampen vibrations
to prevent damage
from sudden,
loud sounds

17. The Middle Ear

18. The Middle Ear
The Eustachian (auditory) tube connects the middle ear with the nasopharynx (upper portion of the throat)
It consists of bone and hyaline
cartilage and is normally
passively collapsed. It opens to
equalize pressures on each side
of the TM
(allowing it
to vibrate freely)
The chamber of the middle ear is continuous through the eustachian tube with the nasopharynx, but also with the mastoid antrum and mastoid air cells. Infection of the mucosa lining the middle ear will extend to the mastoid air cells in the bone behind the ear.

19. The Inner Ear
The internal ear (inner ear) is also called the labyrinth because of its complicated series of canals
Structurally, it consists of two main divisions: an outer bony labyrinth that encloses an inner membranous labyrinth
the bony labyrinth is sculpted out of the petrous part of the temporal bone, and divided into three areas: (1) the semicircular canals, (2) the vestibule,
and (3) the cochlea
The oval window starts the inner ear. As the stapes rocks back and forth the oval window and round window oscillates (like pushing on the end of a waterbed).

20. The Inner Ear The vestibule is the middle part of the bony labyrinth
The membranous labyrinth in the vestibule consists of two sacs called the utricle and the saccule (both contain rc for static equilibrium)
The cochlea , located anterior to the
vestibule, contains rc for hearing
The three semicircular canals
are above the vestibule, each
ending in a swollen
enlargement called the ampulla
(for dynamic equilibrium)

21. The Inner Ear The snail shaped cochlea contains the hearing apparatus
Two types of fluid (perilymph and endolymph) fill its 3 different internal channels: The scala vestibuli, scala tympani, and cochlear duct
A section
through one
turn of the
cochlea is shown

22. The Inner Ear
Perilymph transmits the vibrations coming from the stapes in the oval window up and around the scala vestibuli, and then back down and around the scala tympani – causing the endolymph in the cochlear duct to vibrate
Pressure waves in the endolymph cause the basilar membrane of the cochlear duct to vibrate, moving the hair cells of the spiral organ of Corti against an overhanging flexible gelatinous membrane called the tectorial membrane

23. The Inner Ear
Note how the sound waves between the number 1 and number 2 in this diagram are shown impacting different parts of membranous labyrinth. This is a representation of sounds waves of different frequencies being transduced at the segment of the basilar membrane that
is “tuned” for
a particular
pitch

24. The Inner Ear
Movements of the hair cells in contact with the tectorial membrane transduce mechanical vibrations into electrical signals which generate nerve impulses along the cochlear branch of CN VIII

25. The Auditory Pathway
This graphic depicts the events in the stimulation of auditory receptors, from channeling sound waves into the external ear and onto the TM, to the transduction of those vibrations
into local
receptor
potentials
Sound waves enter the external auditory canal and strike the eardrum. The vibrations of the eardrum cause the ossicles to vibrate and the stapes pushes the membrane of the oval window in and out. The movement of the oval window sends fluid pressure waves into the perilymph of the scala vestibuli which then transmit them to the scala tympani and eventually to the round window (causing it to bulge outward into the middle ear). The pressure waves move into the endolymph of the cochlear duct and cause the basilar membrane to vibrate which moves the hair cells of the spiral organ against the tectorial membrane. This leads to bending of the stereocilia and ultimately to the generation of nerve impulses in first-order neurons in cochlear nerve fibers. Sound waves of various frequencies cause certain regions of the basilar membrane to vibrate more intensely than other regions.
Each segment of the basilar membrane is “tuned” for a