Dr. Michael P. Gillespie
Special Senses
 Receptors for the special senses – smell, taste, vision,
hearing, and equilibrium – are anatomically distinct
from one another.
 These receptors are concentrated in very specific
locations in the head.
 They are usually embedded in the epithelial tissue
within complex sensory organs such as the eyes and
ears.
 The neural pathways for the special senses are more
complex than those of the general senses.
Smell & Taste
 Smell and taste are chemical senses. They involve the
interaction of molecules with the receptors.
 Impulses from these sense propagate to the limbic
system and higher cortical areas. Consequently, they
evoke emotional responses and memories.
Chemosensor
 A chemosensor, also known as a chemoreceptor, is
a sensory receptor that transduces a chemical signal
into an action potential.
 A chemosensor detects certain chemicals in the
environment.
Olfactory Receptors
 There are between 10 – 100 million receptors for
olfaction (sense of smell).
 They are contained in the olfactory epithelium (5cm2)
Olfactory Receptors
 3 Kinds of cells



Olfactory receptors
 1st order neurons – bipolar
 Olfactory hairs – cilia that project from the dendrite – respond
to chemicals called odarants
Supporting cells
 Columnar epithelium of mucous membrane
 Support, nourish, detoxify chemicals
Basal cells
 Stem cells – produce new olfactory cells (live approx. 1 month)
Olfactory Epithelium
Olfactory Glands (Bowman’s)
 Bowman’s glands are in the connective tissue that
supports the epithelium.
 They produce mucous which moistens the epithelial
surface and dissolves the odorants.
Innervation
 Branches of the facial nerve (CN VII) innervate the
supporting cells and olfactory glands.
 They stimulate the olfactory glands and the lacrimal
glands.
 The lacrimal glands produce tears from pepper and
ammonia.
Physiology of Olfaction
 There are hundreds of “primary” odors.
 Humans can recognize about 10,000 different odors.
 Different combinations of olfactory receptors
stimulate different patterns of activity in the brain.
Odor Thresholds and
Adaptation
 All special senses have a low threshold including
olfaction.
 Methyl mercaptan can be detected with as little as 1/25
billionth of a milligram/ mL air.
 Adaptation is a decrease in sensitivity. It occurs
rapidly. 50% of the decrease occurs within the first
second or so and then very slowly after that.
Olfactory Pathway
 There are approximately 20 olfactory foramina on
either side of the nose in the cribiform plate of the
ethmoid bone.
 40 or so bundles of axons form right and left olfactory
nerves (CN I).
 They terminate in the olfactory bulbs – below the frontal
lobes of the cerebrum.
Olfactory Pathway
 Axons of the olfactory bulbs form the olfactory tract
which projects to the primary olfactory area of the
cerebral cortex.
 Some project into the limbic system and hypothalamus
(emotional and memory evoked responses.
 Olfactory sensations are the only sensations that reach
the cerebral cortex without first synapsing in the
thalamus.
Olfactory Pathway
 The primary olfactory area has axons that extend to
the orbitofrontal area (frontal lobe) – region for odor
identification.
Hyposmia
 Hyposmia is a reduced ability to smell.
 Women have a keener sense of smell than men,
especially at ovulation.
 Smoking impairs the sense of smell.
 Age deteriorates the olfactory receptors.
 Affects 50% over 65 and 75% over 80 years of age.
Hyposmia
 Neurological changes impair the receptors.
 Head injury, Alzheimer’s, Parkinson’s.
 Medications impair receptors.
 Antihistamines, analgesics, steroids.
Aromatherapy
 Effects of smells on our psychology have been claimed.
 Lavender, Orange Blossom, Rose, and Sage are said to
be calming.
 Sandlewood, Patachouli and Jasmine are said to
alleviate mild depression.
 Association areas of the brain.
Survival Function
 Our sense of smell serves a survival function to help us
select non-poisonous foods.
 There are very few naturally occurring toxic vapors
that are odorless.
 Synthetic vapors often give false impressions to our
senses. Our natural preferences can no longer be
relied upon.
Gustation (Taste)
 Chemical sense.
 There are only 5 primary tastes that can be
distinguished.
 Sour, sweet, bitter, salty, umani (receptors stimulated by
MSG)
 All other flavors are combinations of the 5 primary
tastes and smell.
Taste Receptors
Taste Buds
Taste Receptor Cells
 Contrary to popular belief, there is no tongue ‘map’.
 Responsiveness to the five basic modalities – bitter,
sour, sweet, salty, and umami – is present in all areas of
the tongue.
 The taste receptor cells are tuned to detect each of the
five basic tastes.
 A given gustatory receptor may respond more strongly
to some tastants than others.
Taste Buds and Papillae
 There are approximately 10,000 taste buds.
 Most are on the tongue.
 There are some on the soft palate, pharynx, and
epiglottis.
 Each taste bud is an oval body with 3 kinds of
epithelial cells.
 Supporting cells.
 Gustatory receptor cells (life span of approx. 10 days).
 Basal cells.
Epithelial Cells on Taste Buds
 The Supporting Cells surround approximately 50
gustatory receptor cells in a taste bud.
 The Gustatory Receptor Cells synapse with 1st order
neurons. ! 1st order neurons contacts many gustatory
receptor cells.
 The Gustatory Hair (microvillus) projects through
the taste pore.
 The Basal Cells are stem cells at the periphery of the
taste bud. They produce supporting cells which will
develop into gustatory cells.
Taste Buds
 The taste buds are found in elevations on the tongue.
 Vallate (circumvallate) papillae
 Fungiform papillae
 Foliate papillae
 Filiform papillae
Vallate (Circumvallate) Papillae
 About 12 very large circular vallate papillae form an
inverted V-shaped row at the back of the tongue.
 Each of these papillae contains approximately 100-300
taste buds.
Fungiform Papillae
 The Fungiform (mushroom like) papillae are
mushroom shaped elevations scattered over the entire
surface of the tongue.
 They contain about 5 tastebuds each.
Foliate Papillae
 The foliate (leaflike) papillae are located in small
trenches on the lateral margins of the tongue, but
most of their taste buds degenerate in early childhood.
Filiform Papillae
 Filiform papillae cover the entire surface of the tongue.
 They are pointed, threadlike structures that contain
tactile receptors but no taste buds.
 They increase friction between the tongue and the
food, making it easier for the tongue to move food into
the oral cavity.
Papillae
Tastants
 Tastants are chemicals that stimulate gustatory
receptors.
 Tastants dissolve in saliva. They can then make
contact with the plasma membrane of the gustatory
hairs, which are the sites of taste transduction.
 This generates a receptor potential, which in turn
triggers nerve impulses with first-order sensory
neurons.
Tastant Stimulation of
Gustatory Receptors
 Different tastants stimulate the gustatory receptors in
different ways to generate the receptor potential.
 The sodium ions in salty foods enter the gustatory
receptor cells via Na+ channels in the membrane.
 The hydrogen ions in sour tastants flow in through H+
channels.
Tastant Stimulation of
Gustatory Receptors
 Other tastants (sweet, bitter, and umami) do not enter
the gustatory receptor cells. They bind to receptors on
the plasma membrane. They trigger second
messengers in the cell.
 All tastants ultimately result in the release of
neurotransmitters from the gustatory receptor cell.
 Different foods taste different because of the patterns
of nerve impulses in groups of first-order neurons that
synapse with the receptors.
Taste Thresholds
 The threshold for taste varies for each of the primary
tastes.
 The threshold for bitter substances (i.e. quinine) is the
lowest.
 Poisonous substances are often bitter.
 The threshold for sour substances (i.e. lemon) is
somewhat higher.
 The thresholds for salty substances and sweet
substances are similar and higher than the others.
Taste Adaptation
 Complete adaptation of a taste can occur in 1-5
minutes if continuous stimulation.
Gustatory Pathway
 Three cranial nerves contain axons for the gustatory
pathways.
 Facial Nerve (CN VII) – serves taste buds in anterior 2/3
of the tongue.
 Glossopharyngeal Nerve (CN IX) – serves taste buds in
the posterior 1/3 of the tongue.
 Vagus Nerve (CN X) – serves taste buds in the throat and
epiglottis.
 Impulses propagate to the gustatory nucleus in the
medulla oblongata.
Gustatory Pathway
 Some axons carrying taste signals project into the
limbic system and the hypothalamus.
 Others project to the thalamus and from there to the
primary gustatory area in the parietal lobe of the
cerebral cortex.
 This allows us to perceive taste.
Taste Aversion
 The taste projections to the hypothalamus and limbic
system account for the strong association between
taste and emotions.
 Sweet foods evoke reactions of pleasure, while bitter
foods can evoke reactions of disgust. This is true even
in newborn babies.
 Animals learn to avoid foods that upset the digestive
system. This is known as taste aversion.
 Certain medications cause upset stomach and can
cause taste aversion to all foods.
Vision
 Sight is extremely important for human survival.
 More than half of the sensory receptors in the human
body are located in the eyes.
Electromagnetic Radiation
 Electromagnetic radiation is energy in the form of
waves that radiate from the sun.
 Many types:
 Gamma rays, X-rays, UV rays
 Visible light
 Infrared radiation, Microwaves, Radio waves
 The range of electromagnetic radiation is known as the
electromagnetic spectrum.
Electromagnetic Spectrum
Wavelength
 The distance between two consecutive peaks of an
electromagnetic wave is the wavelength.
 Wavelengths range from short to long.
 Gamma rays – short than a nanometer.
 Radio waves – greater than a meter
 The eyes are responsible for the detection of visible
light.
Wavelength
 The color of visible light depends upon its wavelength.
 Wavelength of 400 nm is violet
 Wavelength of 700 nm is red
 An object will absorb certain wavelengths of light and
reflect others. It will appear the color of the
wavelengths it reflects.
 White – reflects all wavelengths of visible light.
 Black – absorbs all wavelengths of visible light.
Anatomy of the Eye
Accessory Structures of the Eye
 Eyelids
 Eyelashes
 Eyebrows
 Lacrimal apparatus
 Extrinsic eye muscles
Eyelids
 The palpebrae or eyelids (palprbra – singlular) shade
the eyes during sleep, protect the eyes from excessive
light and foreign objects, and spread lubricating
secretions over the eyeballs.
 The upper eyelid contains the levator palpebrae
superioris muscle and is more moveable than the
lower.
 The lacrimal caruncle is a small, reddish elevation on
the medial border and contains sebaceous (oil) and
sudoriferous (sweat) glands.
Eyelids
 The Meibomian glands are embedded in the eyelids
and secrete fluid that prevents the eyelids from
adhering to each other. Infection of the glands
produces a cyst known as a chalazion.
 The bulbar conjunctiva passes from the eyelids to the
surface of the eyeball and covers the sclera (“white” of
the eye). The conjunctiva is vascular. Irritation or
infection cause bloodshot eyes.
Eyelashes and Eyebrows
 The eyelashes and eyebrows help protect the eyeballs
from foreign objects, perspiration, and the direct rays
of the sun.
 Sebaceous ciliary glands are located at the base of
the hair follicles of the eyelashes. The release a
lubricating fluid into the follicles.
 Infection of these glands results in a sty.
Lacrimal Apparatus
 The lacrimal apparatus is a group of structures that
produces and drains lacrimal fluid or tears.
 The lacrimal ducts empty tears onto the surface of the
conjunctiva of the upper lid.
Lacrimal Apparatus
 The fluid passes into through the lacrimal puncta, into
lacrimal canals, to the lacrimal sac, and then into the
nasolacrimal duct.
 The lacrimal glands are supplied by parasympathetic
fibers of the facial nerves (VII).
 Tears are cleared away by either evaporation or by
passing into the lacrimal ducts.
Flow of Tears
Lacrimal Gland
Lacrimal Ducts
Superior or Inferior
Lacrimal Canal
Lacrimal Sac
Nasolacrimal Duct
Nasal Cavity
Lacrimal Apparatus
Extrinsic Eye Muscles
 Six extrinsic eye muscles move each eye:
 Superior rectus, inferior rectus, medial rectus, inferior
oblique (CN III - Oculomotor).
 Superior oblique (CN IV – Trochlear).
 Lateral rectus (CN VI – Abducens).
 They are supplied by cranial nerves III, IV, VI. SO4LR6.
 Motor units in these muscles tend to be small with
each motor neuron serving only 2 or 3 muscle fibers.
This permits smooth, precise, and rapid movements.
Accessory Structures of the Eye
Anatomy of the Eyeball
 The adult eyeball is about 2.5 cm (1 inch) in diameter.
 Only 1/6 of the surface area is exposed. The remainder
is protected by the orbit.
 The wall of the eyeball consists of three layers:
 Fibrous tunic
 Vascualr tunic
 Retina
Fibrous Tunic
 The fibrous tunic is the superficial layer of the eyeball
and consists of the anterior cornea and posterior
sclera.
Cornea
 The cornea is a transparent coat that covers the
colored iris.
 It is curved and helps to focus light.
 The central part of the cornea receives oxygen from the
outside air.
Sclera
 The sclera (“white” of the eye) covers the entire eyeball
except the cornea.
 The sclera gives shape to the eyeball, makes it more
rigid, protects its inner parts, and serves as a site of
attachment for the extrinsic eye muscles.
Canal of Schlemm
 At the junction of the sclera and cornea is an opening
known as the scleral venous sinus (canal of
Schlemm).
 The aqueous humor drains into this sinus.
Vascular Tunic
 The vascular tunic or uvea is the middle layer of the
eyeball.
 It is composed of three parts:
 Choroid
 Ciliary body
 Iris
Choroid
 The choroid is highly vascularized.
 It provides nutrients to the posterior surface of the
retina.
 It contains melanocytes which produce the pigment
melanin.
Choroid
 The melanin absorbs stray light rays, which prevents
reflection and scattering of light within the eyeball.
 Consequently, the image cast on the retina by the
cornea and the lens remains sharp and clear.
 Albinos lack melanin in all parts of the body, therefore
bright light is perceived as a bright glare due to
scattering.
Ciliary Body
 In the anterior portion of the vascular tunic, the
choroid becomes the ciliary body.
 It contains melanin producing melanocytes.
 The ciliary processes produce aqueous humor.
 Zonular fibers extend from the ciliary processes and
attach to the lens.
Ciliary Body
 The ciliary muscle is a circular band of smooth
muscle that controls the tightness of the zonular
fibers.
 Contraction or relaxation of the ciliary muscle changes
the tightness on the zonular fibers, which alters the
shape of the lens, adapting it for near or far vision.
Iris
 The iris (= rainbow) is the colored portion of the
eyeball.
 It is shaped like a flattened doughnut.
 It is suspended between the cornea and the lens.
 It contains melanocytes. The amount of melanin
produced determines eye color.
 It contains circular and radial smooth muscle fibers.
Iris
 A principle function is to regulate the amount of light
entering the eyeball through the pupil, the hole in the
center of the iris.
 When bright light stimulates the eye, parasympathetic
fibers of the oculomotor nerve (CN III) stimulate the
circular muscles (sphincter pupillae) to contract
causing a decrease in pupil size (constriction).
 In dim light, sympathetic neurons stimulate the radial
muscles (dilator pupillae) to contract, causing an
increase in the pupil’s size (dilation).
Pupil Response to Light
Retina
 The retina is the inner layer of the eyeball.
 It is the beginning of the visual pathway.
 We can view the anatomy of the retina through an
ophthalmoscope.
 Landmarks visible through the ophthalmoscope:
 Optic disc – the site where the optic nerve (CN II) exits
the eyeball.
 Central retinal artery and central retinal vein.
 Macula lutea.
 Fovea centralis.
Retina
Photoreceptors
 Photoreceptors are specialized cells that begin the
process by which light rays are converted to nerve
impulses.
 Two types:
 Rods (approximately 120 million per retina)


Allow us to see in dim light.
Do not provide color vision.
 Cones (approximately 6 million per retina)


Stimulated in brighter light.
Produce color vision.
Three Types of Cones
 There are three types of cones in the retina:
 Blue cones – sensitive to blue light.
 Green cones – sensitive to green light.
 Red cones – sensitive to red light.
 Color vision results from the stimulation of various
combinations of these three types of cones.
Rods and Cones
 Most of our experiences are mediated by the cone
system, the loss of which produces legal blindness.
 A person who loses rod vision mainly has difficulty
seeing in dim light.
Blind Spot
 The optic disc is the site where the optic nerve exits
the eyeball.
 The optic disc is also called the blind spot.
 There are no rods or cones where the blind spot is.
 We are typically not aware of having a blind spot
because the two eyes compensate for one another.
Microscopic Structure Retina
Detached Retina
 A detached retina may occur due to trauma, such as a
blow to the head, in various eye disorders, or as a result
of age-related degeneration.
 Detachment occurs between the neural portion of the
retina and the pigment epithelium.
 Fluid accumulates between these layers and forces the
retina outward.
 This results in distorted vision and blindness in the
corresponding fields.
 Laser surgery or cryosurgery can correct this.
Macula Lutea
Maculae Receptors
Age-related Macular
Degeneration (AMD)
 Age-related macular disease (AMD), also known as
macular degeneration, is a degenerative disorder of
the retina in persons 50 years of age or older.
 Abnormalities occur in the region of the macula lutea,
which is ordinarily the most acute area of vision.
 Victims of AMD retain their vision but lose the ability
to look straight ahead.
 They cannot see facial features to identify a person in
front of them.
Age-related Macular
Degeneration (AMD)
 AMD is the leading cause of blindness in those over
age 75, afflicting 13 million Americans.
 AMD is 2.5 times more common in 1 pack / day
smokers.
Lens
 The lens is behind the pupil and the iris, within the
cavity of the eyeball.
 Proteins called crystallins, arranged like layers of an
onion, make up the refractive media of the lens.
 The lens is normally perfectly transparent and lacks
blood vessels.
 The lens helps focus images on the retina to facilitate
clear vision.
Interior of the Eyeball
 The lens divides the interior of the eyeball into two
cavities: the anterior cavity and vitreous chamber.
 The anterior cavity – the space anterior to the lens –
consists of two chambers.
 Anterior chamber – lies between the cornea and iris.
 Posterior chamber – lies behind the iris and in front of
the lens.
 Both chambers of the anterior cavity are filled with
aqueous humor, a transparent watery fluid that
nourishes the lens and the cornea.
Iris Chambers
Interior of the Eyeball
 The posterior cavity of the eyeball is the vitreous
chamber, which lies between the lens and the retina.
 The vitreous body lies within the vitreous chamber.
The vitreous body is a transparent jellylike substance
that holds the retina flush against the choroid, giving
the retina an even surface for the reception of clear
images.
Interior of the Eyeball
 Occasionally, collections of debris may cast a shadow
on the retina and create the appearance of specks that
dart in and out of the field of vision. These are known
as vitreous floaters.
Intraocular Pressure
 The pressure of the eye is referred to as intraocular
pressure.
 It is produced mainly by the aqueous humor and partly
by the vitreous body.
 It is normally about 16 mmHg.
 It helps to maintain the shape of the eyeball and
prevent it from collapsing.
 Punctures of the eyeball can cause a loss of aqueous
humor and the vitreous body, thereby decreasing
intraocular pressure, a detached retina, and sometimes
blindness.
Image Formation
 In many ways the eye operates like a camera.
 It has optical elements which focus as image on the
retina.
 Three processes help to focus the image:
 The refraction or bending of light by the lens and
cornea.
 The change in shape of the lens (accommodation).
 The constriction or narrowing of the pupil.
Refraction of Light Rays
 When light rays pass from one substance (air) to
another substance with a different density (water),
they bend at the junction between the two substances.
 This bending is known as refraction.
 Light is refracted at both the cornea and the lens so
that it comes into exact focus on the retina.
Refraction of Light Rays
 Images focused on the retina are inverted (upside
down). They also undergo light to left reversal.
 75% of the refraction occurs at the cornea.
 25% occurs at the lens, which also changes the focus to
view either distant or near objects.
Refraction
Accomodation and the Near
Point of Vision
 Convex – surface that curves outward.
 When a lens is convex, it will refract incoming light
rays towards one another.
 Concave – surface that curves inward.
 When a lens is concave, it refract incoming light rays
away from each other.
 The lens of the eye is convex on both the anterior and
posterior surfaces.
Accomodation and the Near
Point of Vision
 As the curvature becomes greater, its focusing power
increases.
 When the eye is focusing on a close object, the lens
becomes more curved, causing greater refraction of
the light rays.
 This increase in the curvature of the lens is called
accommodation.
 The near point of vision is the minimum distance
from the eye that an object can be clearly focused with
the maximum accommodation.
Accomodation and the Near
Point of Vision
 When viewing distant objects, the ciliary muscle is
relaxed and the lens is flatter because the taught
zonular fibers are stretching it in all directions.
 When viewing close objects, the ciliary muscle
contracts, which pulls the ciliary processes towards the
lens. This releases tension on the lens and zonular
fibers. The lens is elastic and then becomes more
spherical.
 Parasympathetic fibers of CN III (oculomotor)
innervate the ciliary muscle.
Refraction Abnormalities
 Emmetropic eye – normal eye – can sufficiently
refract light rays from objects 6 m (20 ft) away so that a
clear image is focused on the retina.
 Myopia – nearsightedness – can see close objects
clearly, but not distant objects.
 Hyperopia (hypermetropia) – farsightedness – can
see distant objects clearly, but not close ones.
 Astigmatism – either the cornea or the lens has an
irregular curvature. Parts of the image are out of
focus.
Refraction Abnormalities &
Corrections
Constriction of the Pupil
 Constriction of the pupil is a narrowing of the
diameter of the hole through which light enters the
eye due to contraction of the circular muscles of the
iris.
 This occurs automatically during accommodation to
prevent light from entering at the periphery of the
lens.
 The pupil also constricts in bright light.
Pupil Response To Light
Convergence
 Binocular vision is focusing on one set of objects with
both eyes.
 This allows us to perceive depth and the three
dimensional nature of objects.
 When we look ahead at an object, light is refracted to
comparable spots on the retinas of both eyes.
 As we move closer to an object, the eyes must rotate
medially towards the object being viewed.
 Convergence is the medial movement of the two
eyeballs so that both are pointed towards the object.
Physiology of Vision
Photoreceptors and
Photopigments
 Rods and cones were named for the different
appearance of the outer segment – the distal end next
to the pigmented layer.
 The outer segment of rods are cylindrical or rodshaped; those of cones are tapered or cone-shaped.
 Transduction of light energy into a receptor potential
occurs in the outer segment.
 Photopigments are integral proteins in the plasma
membrane.
Photoreceptors and
Photopigments
 In cones, the plasma membrane is folded back and
forth in a pleated fashion.
 In rods, the outer segment contains a stack of about
1000 discs, piled up like coins in a wrapper.
 The inner segment contains the cell nucleus, Golgi
complex, and many mitochondria.
 The proximal end expands into synaptic terminals
filled with synaptic vesicles.
Photoreceptors and
Photopigments
 The photopigment undergoes a structural change
when it absorbs light, which leads to a receptor
potential.
 Rods contain the pigment rhodopsin.
 Cones contain three different photopigments, one for
each of the three types of cones.
 Different colors of light activate different cone
pigments.
 All photopigments contain the glycoprotein opsin and
a derivative of vitamin A called retinal.
Rod and Cone Structure
Rods and Cones
Light and Dark Adaptation
 When you emerge from dark surroundings into the
light, light adaptation occurs. Your visual system
adjusts within seconds to the brighter surroundings.
 When you enter a darkened room, dark adaptation
occurs. Your sensitivity increases slowly over several
minutes.
Rods Do Not See Red
 The light response of the rods peaks sharply in blue
light. They respond little to red light.
 In bright light, the color sensitive cones predominate.
At twilight, the less-sensitive cones begin to shut down
and most of the vision comes from the rods.
 The attainment of optimum night vision can take up to
a half hour.
 You can view things with red light at night without
activating the cones and therefore, you will not lose
your night vision.
Color Blindness and Night
Blindness
Release of Neurotransmitter by
Photoreceptors
 A ligand known as cyclic GMP (guanosine
monophosphate) or cGMP allows the inflow of Na+
ions to depolarize the photoreceptor.
 Light causes a hyperpolarizing receptor potential in
photoreceptors, which decreases release of an
inhibitory neurotransmitter (glutamate).
 The cGMP channels close.
 The photoreceptor cells become excited and stimulate
the ganglion cells to form action potentials.
Visual Pathway
 Visual signals from the retina exit the eyeball as the
optic nerve (CN II) and proceed to the brain.
Processing of Visual Input in the
Retina
 Visual input is processed in the retina before
proceeding to the optic nerve.
 There are 126 million photoreceptors in the human
eye, but only 1 million ganglion cells.
 Some features of visual input are enhanced, while
others are discarded.
Processing of Visual Input in the
Retina
 Between 6 and 600 rods synapse with a single bipolar
cell; a cone more often synapses with a single bipolar
cell.
 Convergence of many rods onto a single bipolar cell
increases the light sensitivity, but may slightly blur the
image.
 Cone vision is less sensitive, but sharper due to the one
to one synapse with the bipolar cell.
Brain Pathway and Visual Fields
 1. Axons of all retinal ganglion cells in one eye exit the
eyeball at the optic disc and form the optic nerve on
that side.
 2. At the optic chiasm, axons from the temporal half
of each retina do not cross but continue directly to the
lateral geniculate nucleus of the thalamus on the same
side.
 3. Axons from the nasal half of each retina cross the
optic chiasm and continue to the opposite
hypothalamus.
Brain Pathway and Visual Fields
 4. Each optic tract consists of crossed and uncrossed
axons that project from the optic chiasm to the
thalamus on one side.
 5. Axon collateral extend to the midbrain to govern
pupil constriction and to the hypothalamus to govern
patterns of sleep and other circadian rhythms relevant
to light and darkness.
 6. Axons of thalamic neurons form the optic
radiations and project to the primary visual area of the
cortex on the same side.
Visual Pathway
Hearing and Equilibrium
 The ear can transduce sound vibrations with
amplitudes as small as the diameter of an atom of gold
(0.3 nm) into electrical signals 1000 times faster than
photoreceptors can respond to light.
 The ear also contains receptors for equilibrium.
Anatomy of the Ear
 The ear is divided into three main regions:
 External ear – collects sound waves and channels them
inward.
 Middle ear – conveys sound vibrations to the oval
window.
 Internal ear – houses the receptors for hearing and
equilibrium.
External Ear
 The external (outer) ear consists of the auricle,
external auditory canal, and eardrum.
 The tympanic membrane (eardrum) is a thin,
semitransparent partition between the external
auditory canal and the middle ear.
 Tearing of the tympanic membrane is called a
perforated eardrum.
 Pressure from a cotton swab, trauma, or a middle ear
infection can cause perforation. It usually heals within 1
month.
External Ear
 The membrane can be examined using an otoscope.
 Ceruminous glands secrete cerumen (earwax).
 Cerumen and hairs help prevent dust and foreign
particles from collecting in the ear.
 Impacted cerumen can impair hearing.
Middle Ear
 The middle ear contains the auditory ossicles.
 The malleus attaches to the internal surface of the
tympanic membrane.
 The head of the malleus articulates with the incus.
 The incus articulates with the head of the stapes.
 The stapes fits into the oval window which is enclosed
by a secondary tympanic membrane.
 The stapedius muscles (supplied CN VII) dampens
vibrations of the stapes due to loud noises.
Middle Ear (Eustachean Tube)
 The auditory (pharyngotympanic) tube, also known as
the eustachian tube connects the middle ear to the
nasopharynx.
 It helps to equalize pressure in the middle ear.
 If pressure is not equalized, intense pain, hearing
impairment, ringing in the ears, and vertigo can
develop.
 It is a route for pathogens to enter and cause otitis
media.
Internal Ear
 The internal ear is also called the labyrinth because of
its complicated series of canals including the
semicircular canals and cochlea.
 The spiral organ or Corti contains supporting cells
including approximately 16,000 hair cells, which are
the receptors for hearing.
Anatomy of the Ear
Nature of Sound Waves
 Sound waves are alternating high and low pressure
regions traveling in the same direction through some
medium (such as air).
 The frequency of a sound wave is the pitch.
 The human ear most acutely detects sounds waves
between 500 and 5000 hertz (Hz).
Nature of Sound Waves
 The audible range extends between 20 and 20,000 Hz.
 Sounds of speech are between 100 and 3000 Hz.
 The larger the intensity (size or amplitude) of the
vibration, the louder the sound.
 Sound intensity is measured in units called decibels
(dB).
Physiology of Hearing
 1. The auricle directs sound waves into the external
auditory canal.
 2. Sound waves strike the tympanic membrane causes
it to vibrate back and forth. The distance it moves
depends upon the intensity and frequency of the
waves.
 3. The central eardrum connects to the malleus, which
also starts to vibrate. This vibration is then
transmitted to the incus and stapes.
 . As the stapes moves back and forth, it pushes the
membrane of the oval window in and out.
Physiology of Hearing
 5. The movement of the oval window sets up fluid
pressure waves in the perilymph of the cochlea.
 6. Pressure waves are transmitted to the round
window, causing it to bulge outward.
 7. These waves in turn create pressure waves in the
endolymph of the cochlear duct.
 8. This causes the basilar membrane to vibrate, which
moves the hair cells leading to receptor potentials and
ultimately nerve impulses.
Stimulation of Auditory
Receptors
Auditory Pathway
 Bending of the stereocilia of the hair cells of the spiral
organ causes the release of a neurotransmitter, which
causes nerve impulses in the sensory neurons.
 These nerve impulses pass along the axons to form the
cochlear branch of the vestibulocochlear nerve (CN
VIII).
 They synapse in the cochlear nuclei in the medulla
oblongata on the same side.
Auditory Pathway
 Some axons decussate in the medulla and terminate in
the midbrain on the opposite side.
 Other axons continue to the pons on the same side.
 Slight differences in the timing of the impulses allow
us to locate the source of the sound.
 The axons are then conveyed to the thalamus and
ultimately to the primary auditory area of the cerebral
cortex in the temporal lobe.
Auditory Pathway