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Special Senses with figures

Chapter 18 – Special Senses

Olfaction (Smell)

     Chemicals that
we perceive as smell are detected by olfactory organs that
consist of:

Olfactory epithelium –
This epithelium consists of olfactory receptor cells,
supporting cells,
and basal cells (stem cells).
Lamina propria – The
lamina propria of olfactory epithelium contains
olfactory glands
which produce mucus.
  Olfactory Pathway

olfactory receptor cells are highly modified bipolar
The axons of these cells collect into 20 or
more bundles that pass through the cribriform plate
and synapse with second-order neurons in the
olfactory bulb.
axons of the second-order neurons travel in the
olfactory tract
to reach the olfactory cortex,
the hypothalamus, and portions of the
limbic system.
Gustation (Taste)

receptors are distributed over the dorsal surface of the tongue
in taste buds. Taste buds are recessed into the
surrounding epithelium and contain gustatory cells,
transitional cells,
and basal cells. The gustatory
cells have microvilli (taste hairs) that extend through a
narrow opening of the taste bud called the taste pore.
     Taste buds are
found on the sides of three of four types of
found on the surface of the tongue:

1. Filiform papillae – are
“thread-shaped” papillae that do not have taste
buds but are responsible for the tongue’s roughness.
remaining three papillae have taste buds along their
side walls.
2. Fungiform papillae –
are “mushroom-shaped” papillae are scattered over the
anterior two-thirds of the tongue.
3. Foliate papillae – are
creases and folds along the posterior sides of the
4. Circumvallate papillae –
(circum – around; vallate – walled) are papillae in
which a central bump is surrounded by a “wall” of
tissue. The circumvallate papillae are arranged in an
inverted V formation at the boundary between the root
and body of the tongue.
Equilibrium and Hearing

     The ear is
divided into three anatomical regions:

  External Ear

The external ear collects and directs sound
waves to the eardrum.

The structures of the external ear include:

Auricle (pinna)
This is what is commonly known as
the ear. It is a flexible fold of skin
supported internally by elastic
that surrounds the
external acoustic meatus.
acoustic meatus (canal) –
This canal
permits sound waves to strike the
tympanic membrane.
glands –
These glands are found in
the skin lining the external acoustic
meatus. The waxy secretion of this gland
slows the growth of microorganisms.
membrane (tympanum) –
This is a
delicate, semi-transparent membrane that
separates the external and middle ears.
  Middle Ear

The middle ear consists of an air-filled space
called the tympanic cavity. The tympanic
cavity is connected with the nasopharynx by the
auditory tube (Eustachian tube). The
tympanic cavity also communicates with the
mastoid sinuses in the mastoid process. It is
through this connection that infections can
spread from the middle ear to the meninges of
the brain to cause meningitis.

Auditory Ossicles

Three tiny bones called ear ossicles are
found in the middle ear and transfer the
vibrations of the tympanic membrane to
the fluid-filled chambers of the inner
ear. The ear ossicles act as levers that
amplify the force of vibration delivered
to the inner ear.

The ear ossicles are called:

Malleus (Hammer) –
to the tympanum
Incus (Anvil) –
Stapes (Stirrup) –
The base
or, foot plate, of this bone
covers the oval window on the
inner ear.

The magnitude of the force that reaches
the inner ear can be reduced by two
that dampen the vibrations
produced by loud sounds.
Tensor Tympani M. –
to the malleus and by
contracting stiffens the
tympanic membrane reducing
Stapedius M. –
attaches to
the stapes and by contracting
reduces the vibrations of the
  Inner Ear

     The receptors
for equilibrium and hearing are located within fluid-filled
chambers and tubes that form the membranous labyrinth.
The fluid of the membranous labyrinth is called endolymph
and differs from extracellular fluid in having a high
and low sodium concentration.
     The membranous
labyrinth is surrounded and protected by a shell of bone called
the bony labyrinth. The contours of the bony labyrinth
closely resemble the membranous labyrinth. The space between the
membranous and bony labyrinth is filled with a fluid called
that closely resembles CSF.
     The bony
labyrinth can be divided into three regions the vestibule,
semicircular canals
and cochlea. The bony wall of the
bony labyrinth is solid except at two locations at the base of
the cochlea:

1. Round window – is a
round opening in the bone that is spanned by a thin,
flexible membrane.
2. Oval window – is an
oval opening that is sealed by the foot plate of the
stapes which is held in position by an annular
that attaches the outer edge of the
footplate to the edge of the oval window.
  Vestibular Complex and

vestibular complex is the part of the inner ear that
preserves physical equilibrium by detecting rotation,
and acceleration.

Semicircular Canals

The anterior, posterior and
lateral semicircular canals
designed to detect rotation. Each
semicircular canal contains a
semicircular duct.
Each semicircular
canal has an expanded portion which
contains within it an expanded portion
of the semicircular duct called the
which contains the hair

The hair cells are the receptor cells.
They are associated with supporting
in a raised portion of the
lining of the ampulla called a
The apical surface of the
hair cell has long microvilli
called stereocilia (hence, “hair”
cell) and one cilium called a
The kinocilium and
stereocilia are embedded in a gelatinous
substance called the cupula which
nearly fills the space within the

Rotational movements in different planes
are detected by the mechanical
distortion of the stereocilia as
fluid moves within the semicircular
  Utricle and Saccule

The utricle and saccule are interconnected membranous
sacs of the membranous labyrinth found in the vestibule.
The endolymph of the utricle is confluent with that of
the semicircular ducts and the endolymph of the saccule
is confluent with that of the cochlear duct. The utricle
and saccule are interconnected by a narrow
endolymphatic duct
that ends in a blind pouch called
the endolymphatic sac.
     Both the
utricle and saccule contain hair cells similar to those
found in the semicircular canals. In each sac the hair
cells are concentrated on a oval spot in the wall called
a macula (“spot”). The kinocilia and stereocilia
of the hair cells are embedded in a gelatinous mass that
has crystals of calcium carbonate embedded on its
surface. This gelatinous mass with its crystals is
called an otolith and the crystals are called
difference in density between the crystals and the
gelatinous matrix causes a mechanical distortion of the
stereocilia of the hair when the head is tilted or the
body experiences acceleration. Hence, the hair cells
detect the position of the head in space and linear
  Cochlea and Hearing

cochlea coils about 2.5 turns around a central hub
called the modiolus. The sensory neurons that
form the cochlear nerve have their cell bodies in the
modiolus in a ganglion called the spiral ganglion.
cochlear duct
is present within the cochlea flanked
by the vestibular duct and the tympanic duct
which contains perilymph. At the base of the cochlea
the oval window is in contact with the vestibular
duct and the round window is in contact with the
tympanic duct. The vestibular and tympanic ducts are
confluent at the tip of the cochlea at a place called
the helicotrema.
  Organ of Corti

The hair cells that are responsible for hearing
are within the organ of Corti, or
spiral organ.
The organ of Corti rests on a
basilar membrane which separates the
cochlear duct from the tympanic duct. The hair
cells are arranged in an inner row and outer
rows that follow the turns of the cochlear duct.

The stereocilia (a kinocilium is lacking) of the
the hair cells are in contact with an
overhanging tectorial (“roof”) membrane
that is attached to the inner wall of the
cochlear duct.
  Sound Detection

Sound is detected when a pressure wave of a
frequency between 20 – 20,000 Hz strikes the
tympanic membrane causing it to vibrate. The
vibration is transferred to the oval window by
the ear ossicles and creates a pressure wave
within the fluid of the cochlea because of the
presence of the round window. The pressure wave
moves the basilar membrane relative to the more
rigid tectorial membrane causing the stereocilia
to bend.

The basilar membrane is relatively narrow and
stiff at the base and wide and loose at the tip
of the cochlea. As a consequence, the maximum
vibration any location along the length of the
basilar membrane depends upon the frequency of
the sound waves. By comparing the relative
movement of stereocilia along the length of the
basilar membrane the brain perceives frequency,
or pitch. The loudness of a sound is perceived
when the greater force of a louder sound causes
an increase in the range of movement of the
stereocilia of the hair cells.
  Accessory Structure of the Eye

Eyelids – (Palpebrae)

     The free margins of
the eyelids form the palpebral fissure. The edges where
the upper and lower eyelids meet are called the lateral and
medial canthus. The palpebral margins are lined by eyelashes.
     Glands associated with
the eyelids include:

Glands of Zeis – These are
sebaceous glands associated with the eyelashes.
Tarsal glands – (Meibomian glands)
– These glands line the inner margin of the lid and
produce a lipid-rich product that prevents the lids from
sticking together.
Lacrimal caruncle – This is a mound
of tissue in the medial canthus that produces thick
     The eyelids are
supported internally by sheets of connective tissue that form
the tarsal plate.
     The conjunctiva is
the epithelium that lines the inner surface of the eyelids and
continues onto the outer surface of the eye. The palpebral
lines the inner surface of the eyelid and the bulbar
lines the anterior surface of the eye. The
conjunctiva  consists of a specialized stratified
squamous epithelium
that is thick until it lines the
transparent cornea where it is relatively thin.
Lacrimal Apparatus

     The lacrimal apparatus
produces, distributes and removes tears. Tears are continually
produced and reduce friction, remove debris, prevent bacterial
infection and provide nutrients and oxygen for the
     The lacrimal apparatus
consists of:

1. Lacrimal Gland

  The lacrimal, or tear gland
produces a secretion that is watery, slightly
alkaline and contains the enzyme lysozyme which
has an antimicrobial function. The gland is
located in a depression in the frontal bone in the
superior lateral orbit. Ten to 12 ducts release
the tears into the superior, lateral region of the
fornix which is the pocket formed where the
palpebral and bulbar conjunctiva meet.
2. Superior and Inferior Lacrimal

  Blinking sweeps the tears
over the surface of the eye toward the medial
The tears drain into canals called
the superior and inferior lacrimal canaliculi
through pores called the superior and inferior
lacrimal puncta,
3. Lacrimal Sac

  The tears drain next into
the lacrimal sac which is found on a groove of the
lacrimal bone.
4. Nasolacrimal Duct

  The nasolacrimal duct is
found within a passage created by the lacrimal
bone and maxilla and opens into the nasal cavity
near the inferior meatus. 

     The eyeball is hollow and is
divided into two cavities:

1. Posterior cavity

  This cavity takes up most of the
internal space and extends anteriorly to the lens. It
contains a gelatinous vitreous body and is also
called the vitreous chamber.
2. Anterior cavity

  This cavity is in the front of the
lens and is filled with a clear liquid called aqueous
It is further subdivided into anterior
and posterior chambers by the iris.
     The wall of the eye has
three layers called tunics:

1. Fibrous Tunic

outermost layer of the eye is the fibrous tunic. Most
of the fibrous tunic is the sclera. The sclera
consists of dense fibrous connective tissue and provides
mechanical support and physical protection. The fibrous
connective tissue of the sclera also serves as
attachment sites for the extraocular muscles. 
     The anterior
portion of the fibrous tunic is transparent because of
the precise alignment of collagen fibers and the absence
of blood vessels. This portion is called the cornea. The
cornea is primarily responsible for focusing light rays
onto the retina.
2. Vascular Tunic

     As its name implies,
this tunic contains numerous blood vessels. It is also
the location of the intrinsic eye muscles.
     The vascular
tunic includes:


  The iris contains
blood vessels, pigment cells and smooth
muscles. The smooth muscles control the
diameter of the central opening of the iris
called the pupil.
Ciliary body

  The iris is attached
to the ciliary body which begins at the
junction between the cornea and sclera and
extends to the ora serrata. The
ciliary body is primarily the ciliary
the mass of which projects into
the interior of the eye. The epithelium that
covers the ciliary body is thrown into folds
called ciliary processes. Suspensory
are attached to the anterior
rim of the ciliary body and holds the lens
in position.

  The remaining vascular
tunic contains extensive capillaries and is
called the choroid. The choroid
supplies oxygen and nutrients to the outer
portion of the retina.
3. Neural Tunic

     The neural tunic
is the retina. The retina consists of two layers:

1. Pigmented layer

  The pigmented layer is
a single-cell layer of pigmented cells that
absorb light after it passes through the
retina. The pigmented layer provides vital
metabolic support for the photoreceptors of
the retina. It extends anteriorly to cover
the ciliary body and the iris.
2. Neural layer

  The neural layer
consists of three layers of cells that
include the photoreceptors, cells that
initiate the processing of visual
information and blood vessels that supply
the neural layer. The neural layer extends
anteriorly to the boundary called the ora
     The layers of
the neural layer from the outermost layer to the
innermost are:

Visual receptors

  There are two types of

  Rods are
more sensitive to light but there is
only one type of rod and color
discrimination is not possible with
  Cones are
less sensitive to light but there are
three types of cones with
sensitivities in different regions of
the light spectrum. Cones provide
color discrimination and greater
  The cornea and lens
focuses light on a spot of the retina that
contains only cones and appears as a yellow
spot called the macula lutea. In the
center of the macula lutea there is a
depression called the fovea which
provides the greatest visual acuity.
Bipolar cells

  The photoreceptors
synapse with bipolar cells. The transmission
of visual information through the bipolar
cells is modulated by horizontal cells found
in this layer.
Ganglion cells

  The bipolar cells
synapse with the ganglion cells whose axons
carry impulses to the brain in the optic
nerves and tracts. The transmission at this
layer is modulated by amacrine cells. The
axons of the ganglion cells leave the retina
by converging on the optic disc and
penetrating through the wall of the eye. The
central retinal artery and vein emerge
onto the surface of the retina at the optic
  Chambers of the Eye

     The anterior
and posterior chambers of the anterior cavity
of the eye are filled with aqueous humor. Aqueous humor
is continually produced at the ciliary processes of the
ciliary body. The aqueous humor passes through the pupil into
the anterior chamber and drains into the canal of Schlemm (scleral
venous sinus)
at the limbus of the cornea.
     The posterior
of the eye is behind the lens and is filled with a
gelatinous body called the vitreous body. The vitreous
body maintains the shape of the eye, supports the lens, and
presses the neural layer against the pigmented layer.
  Visual Pathways

     The visual
information received and initially processed in the retina
travels to the lateral geniculate nucleus of the
The fibers carrying information from the medial
of the retina crosses over (decussates) to the other
side (contralateral) of the brain at the optic chiasm.
The information is then relayed from the thalamus to the
visual cortex.
     The partial
decussation of the fibers from the retina result in the
information from the right visual field going to the left
occipital cortex and the information from the left visual field
going to the right occipital cortex. A comparison of the
information coming from each eye in the overlapping visual
fields gives us depth perception.



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