1. Special Senses
• Have highly localized receptors that
provide specific information about the
environment
Five special senses
• Smell
Interaction of chemicals with sensory receptors
• Taste
Interaction of chemicals with sensory receptors
• Sight
Interaction of light with sensory receptors
• Hearing
Interaction of mechanical stimulation with
sensory receptors
• Balance
Interaction of mechanical stimulation with
sensory receptors
Olfaction
• Sense of Smell
• Response to airborne molecules, called
odorants, entering the nasal cavity
At least 7 (perhaps 50) primary odors exist
• Camphoraceous (e.g., moth balls)
• Musky-fish
• Floral
• Pepperminty
• Ethereal (e.g., fresh pears)
• Pungent-pepper
• Putrid-rotten
• Olfactory neurons have very low
thresholds and accommodate rapidly
• Olfactory Epithelium and Bulb
Olfactory neurons in the olfactory
epithelium are bipolar neurons
• Distal ends have olfactory hairs
Olfactory hairs have receptors that
respond to dissolved substances
• Approximately 1000 different odorant
receptors
• Receptors activate G proteins, which
results in ion channels opening and
depolarization
• guanosine nucleotide-binding proteins
Neuronal Pathways for Olfaction
• Axons from the olfactory neurons
extend as olfactory nerves to the
olfactory bulb, where they synapse with
interneurons
• Axons from interneurons form the
olfactory tracts, which connect to the
olfactory cortex
Olfactory bulbs and cortex accommodate
to odors.
Taste
Sensory structures that detect taste stimuli are
taste buds.
Most taste buds are located in the epithelium of
papillae.
Taste buds are found on the
• Tongue
• Palate
• Lips
• Throat
There are four types of papillae
• Three contain taste buds
• The fourth and most numerous has no
taste buds, but gives the tongue its
roughness
Histology of Taste Buds
Taste buds consist of
• Taste cells
Have taste hairs that extend into taste pores

2. Nonsensory cell types:
• Basilar cells
• Supporting cells
Function of Taste
Receptors on the hairs detect dissolved
substances
Five basic types of taste exist:
• Salty
• Sodium ions
• Sour
• Acids
• Sweet
• Sugars, some other carbohydrates, and
some proteins
• Bitter
• Alkaloids (bases)
• Umami
• Elicited by the amino acid glutamate
and related compounds
Function of Taste
All taste buds can sense the five primary tastes,
but tend to be most sensitive to one
Sensitivity to bitter substances is the highest
(Poisons)
• Taste is strongly influenced by olfactory
sensations
• Nasal congestion can dampen the taste
sensation
Tongue can detect other stimuli besides taste
• Temperature
• Texture
Neuronal Pathways for Taste
• The facial nerve carries taste sensations
from the anterior two-thirds of the
tongue.
• The glossopharyngeal nerve carries
taste sensations from the posterior
one-third of the tongue
• The vagus nerve carries taste sensations
from the epiglottis
The neural pathways for taste extend from the
medulla oblongata to the thalamus and to the
cerebral cortex
Visual System
Consists of
• Eye
• eyeball
• optic nerve
Accessory Structures
• eyebrows, eyelids, conjunctiva, lacrimal
apparatus, and extrinsic eye muscles
• Sensory Neurons
Accessory Structures
• Eyebrows
Prevent perspiration from entering the eyes
and help shade the eyes
• Eyelids
Consist of five tissue layers
Protect the eyes from foreign objects
Help lubricate the eyes by spreading
tears over their surface
Lubricating glands associated with the eyelids
• Meibomian glands and sebaceous
glands
• Ciliary glands lie between the hair
follicles
• Eyelashes
Project from the free margin of each eyelid
Initiate reflex blinking
• Conjunctiva
Covers the inner eyelid and the anterior part of
the eye

3. Accessory Structures
• Lacrimal Apparatus
Consists of the lacrimal gland, lacrimal
canaliculi, and a nasolacrimal duct
Lacrimal glands secrete tears
• Tears
• Contain mostly water, with some salts,
mucus, and lysozyme
• Enter the eye via superolateral
excretory ducts
• Exit the eye medially via the lacrimal
canaliculi
• Drain into the nasolacrimal duct
Accessory Structures
Extrinsic Eye Muscles
• Six strap-like muscles
Enable the eye to follow moving objects
Maintain the shape of the eyeball
Four rectus muscles originate from the annular
ring
Two oblique muscles move the eye in the
vertical plane
Anatomy of the Eye
Eyeball - A slightly irregular hollow sphere with
anterior and posterior poles
The eyeball is composed of three layers
Fibrous layer
• Sclera
• Cornea
Vascular layer
• Choroid
• Ciliary body
• Iris
Nervous layer
• Retina
The internal cavity is filled with fluids called
humors
Fibrous Layer
• Sclera
Posterior 4/5ths of the eye
White connective tissue that maintains the
shape of the eyeball
Provides a site for muscle attachment
Cornea
Anterior 1/5th of the eye
Transparent and refracts light that enters the
eye
Vascular Layer
Choroid
• A vascular network
• Many melanin-containing pigment cells
• Appears black in color
• Prevents the reflection of light inside
the eye
Ciliary body
Ciliary ring
A thickened ring of tissue surrounding the lens
Composed of smooth muscle bundles (ciliary
muscles)
Anchors the suspensory ligament that holds the
lens in place
Contraction of ciliary muscles changes the
shape of the lens
Ciliary process-produces aqueous humor
Pupil-light enters the eye through this.
Iris
Smooth muscle regulated by the autonomic
nervous system.
-Regulates the amount of light by controlling
the size of pupil
Two muscles of Iris
Sphincter pupillae
Close vision and bright light: pupils constrict.
Circular group.
Dilator pupillae- radial group.
• Distant vision and dim light: pupils
dilate

4. • Changes in emotional state: pupils
dilate when the subject matter is
appealing or requires problem-solving
skills
• Controls the amount of light entering
the pupil
• Color is determined by the amount of
melanin present
• Large amounts of melanin: brown or
black
• Less melanin: light brown, green, or
grey
• Even less melanin: blue
Nervous Layer
Retina
• The inner layer of the eyeball
• Has over 126 million photoreceptor
cells, which respond to light
Macula (fovea centralis)
• Area of greatest sensitivity to light
• Highest concentration of photoreceptor
cells
Optic disc
• Location through which nerves exit and
blood vessels enter the eye
• No photoreceptor cells
• The “blind spot” of the eye
Chambers of the Eye
Composed of three chambers
Anterior chamber
Between the cornea and the iris
Posterior chamber
Between the iris and the lens
Viterous chamber
Much larger then the other two chambers
Posterior to the lens
Aqueous Humor
Fills the anterior and posterior chambers
Supports, nourishes, and removes wastes for
the cornea, which has no blood vessels
Produced by the ciliary processes as a blood
filtrate
Returned to the circulation through the scleral
venous sinus
Vitreous Humor
Fills the vitreous chamber
Contributes to intraocular pressure
Helps maintain the shape of the eyeball
Holds the lens and retina in place
Functions in the refraction of light in the eye
Lens
A biconvex, transparent, flexible, avascular
structure that:
Allows precise focusing of light onto the retina
Is composed of epithelium and lens fibers
Lens epithelium: anterior cells that
differentiate into lens fibers
Lens fibers: cells filled with the transparent
protein crystallin
With age, the lens becomes more compact and
dense and loses its elasticity
Functions of the Complete Eye
Properties of Light
Electromagnetic spectrum
All energy waves from short gamma rays to long
radio waves
Visible spectrum
Portion of the electromagnetic spectrum that
can be detected by the human eye
• Refraction- Bending of light
• Light striking a concave surface refracts
outward (divergence)
• Light striking a convex surface refracts
inward (convergence)

5. • Converging light rays meet at the focal
point and are said to be focused
Functions of the Complete Eye
Focusing system of the Eye (light refracting)
A.Cornea
Responsible for most of the convergence
B. Aqueous humor
C. Lens
Adjusts the convergence by changing
shape
D. Vitreous humor
Distant and Near Vision
Distant vision: looking at objects 20 feet or
more from the eye
Near vision: looking at objects less than 20 feet
from the eye
• Relaxation of the ciliary muscles causes
the lens to flatten, producing the
emmetropic eye
Emmetropia - Normal resting condition of
the lens
Far point of vision
• Point at which the lens does not have to
thicken for focusing to occur
• Normally 20 feet or more from the eye
Near point of vision
Closest point an object can come to the eye and
still be focused
When an object is less than 20 feet from the
eye, the image falling on the retina is no longer
in focus
Three events must occur to bring the image
into focus
Accommodation by the lens
• Contraction of the ciliary muscles
causes the lens to become more
spherical
• Change in the lens shape enables the
eye to focus on objects that are less
than 20 feet away
Constriction of the Pupil
Increases the depth of focus
Convergence of the eyes
Medial rotation of the eyes
Structure and Function of the Retina
Pigmented layer of the retina provides a black
backdrop for increasing visual acuity
• Rods and cones synapse with bipolar
cells
• Bipolar cells synapse with ganglion cells,
which form the optic nerve
Rods
• Responsible for non-color vision and
vision in low illumination (night vision)
• Rod-shaped photoreceptive part of the
rods contains about 700 double-layered
membranous discs
• Discs contain rhodopsin
• Rhodopsin -A purple pigment consisting
of the protein opsin covalently bound
to a yellow photosensitive pigment
called retinal (derived from Vit. A)
• Exposure to light activates rhodopsin
• Rhodopsin is split by light into retinal
and opsin, eventually resulting in an
action potential
• Light adaptation is caused by a
reduction of rhodopsin
• Dark adaptation is caused by rhodopsin
production
Cones
• Responsible for color vision and visual
acuity

6. • Three types, each with a different type
of iodopsin photopigment
• Pigments are most sensitive to blue,
red, and green light
• Perception of many colors results from
mixing the ratio of the different types of
cones that are active at a given moment
• Most visual images are focused on the
fovea centralis and macula
• Fovea centralis has a very high
concentration of cones
• In the remaining macula there are more
cones than rods
• Most rods are in the periphery of the
retina
• Bipolar and ganglion cells in the retina
can modify information sent to the
brain
• Interneurons in the inner layers of the
retina enhance contrast between the
edges of objects
Neuronal Pathways for Vision
• Ganglion cell axons form the optic
nerve, optic chiasm, and optic tracts
• Extend to the thalamus and synapse
• Then the neurons form the optic
radiations that project to the visual
cortex
• Depth perception is the ability to judge
relative distances of an object from the
eyes and is a property of binocular
vision
• Binocular vision results because a
slightly different image is seen by each
eye
Hearing and Balance
Three parts of the ear are
• External ear
Extends from the outside of the head to the
tympanic membrane
• Middle ear
Air-filled chamber medial to the tympanic
membrane
• Inner ear
Set of fluid-filled chambers medial to the middle
ear
The external and middle ear are involved with
hearing
The inner ear functions in both hearing and
equilibrium
Auditory Structures and Their Functions
External Ear
Auricle
-Fleshy part of the external ear
External acoustic meatus
-Passageway that leads to the tympanic
membrane
-Lined with hairs and ceruminous glands.
-Ceruminous glands produce cerumen (earwax)
Tympanic membrane (eardrum)
-Thin connective tissue membrane that vibrates
in response to sound
-Transfers sound energy to the middle ear
ossicles
-Boundary between outer and middle ears
Middle Ear
-A small, air-filled, mucosa-lined cavity
-Flanked laterally by the eardrum
-Flanked medially by the oval and round
windows
Contains three small bones: the malleus, incus,
and stapes
Transmit vibratory motion of the eardrum to
the oval window
Dampened by the tensor tympani and stapedius
muscles

7. Auditory tube (pharyngotympanic or
eustachian tube)
-Connects the middle ear to the pharynx
-Equalizes pressure in the middle ear cavity with
the external air pressure
Inner Ear
Bony labyrinth
Interconnecting, fluid-filled tunnels and
chambers within the temporal bone
Contains
Vestibule and semicircular canals: primarily
involved in balance
Cochlea: involved in hearing
Membranous labyrinth
Series of membranous sacs within the bony
labyrinth
Filled with a potassium-rich fluid called
endolymph
Space between the bony labyrinth and
membranous labyrinth is filled with perilymph
Cochlea
• Spiral-shaped canal within the temporal
bone
• Divided into three compartments by the
vestibular and basilar membranes
• Scala vestibuli and scala tympani
contain perilymph
• Cochlear duct contains endolymph and
the spiral organ
• Spiral organ consists of inner hair cells
and outer hair cells, which attach to the
tectorial membrane
• Hair cells have hairlike projections at
their apical ends, which are very long
microvilli called stereocilia
Auditory Function
Pitch is determined by the frequency of sound
waves
Volume is determined by the amplitude of
sound waves
Timbre is the resonant quality (overtones) of
sound
Auditory Function
• Hearing involves
• Sound waves funneled by the auricle
down the external acoustic meatus
cause the tympanic membrane to
vibrate
• Tympanic membrane vibrations pass
along the auditory ossicles to the oval
window of the inner ear
Movement of the stapes in the oval window
causes the perilymph, vestibular membrane,
and endolymph to vibrate and produces
movement of the basilar membrane
Movement of the basilar membrane causes
bending of the stereocilia of inner hair cells in
the spiral organ
Hearing involves
• Bending of the stereocilia pulls on
gating springs and opens K+ channels
• K+ ions enter the hair cell and result in
depolarization of the cell
• Depolarization causes the release of
glutamate, generating action potentials
in the sensory neurons associated with
hair cells
• The round window dissipates sound
waves and protects the inner ear from
pressure buildup
Neuronal Pathways for Hearing
• Axons from the vestibulocochlear nerve
synapse in the medulla
• Neurons from the medulla project
axons to the inferior colliculi, where
they synapse

8. • Neurons from this point project to the
thalamus and synapse
• Thalamic neurons extend to the
auditory cortex
• Efferent neurons project to cranial
nerve nuclei responsible for controlling
muscles that dampen sound in the
middle ear
Static Balance
Evaluates the position of the head relative to
gravity and detects linear acceleration and
deceleration
Vestibule contains
The utricle and saccule in the inner ear
Contain maculae made of hair cells
Hairs are embedded in an otolithic membrane
Consists of a gelatinous mass and crystals called
otoliths
Moves in response to gravity
Dynamic Balance
• Evaluates movements of the head
Semicircular Canals
• Three semicircular canals at right angles
to one another are present in the inner
ear
• The ampulla of each semicircular canal
contains the crista ampullaris
• Has hair cells with hairs embedded in a
gelatinous mass, the cupula
• When the head moves, endolymph
within the semicircular canals moves
the cupula
Neuronal Pathways for Balance
• Axons from the maculae and the cristae
ampullares extend to the vestibular
nucleus of the medulla
• Fibers from the medulla run to the
spinal cord, cerebellum, cortex, and
nuclei that control the extrinsic eye
muscles
• Balance also depends on proprioception
and visual input