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CHAPTER
16
The Special Senses
Accessory Structures of the Eye
•
Lacrimal apparatus—
keeps the surface of the
eye moist
• Lacrimal gland—
produces lacrimal
fluid
• Lacrimal sac—
fluid empties into
nasal cavity
Lacrimal sac
Lacrimal gland
Excretory ducts
of lacrimal glands
Lacrimal punctum
Lacrimal canaliculus
Nasolacrimal duct
Inferior meatus
of nasal cavity
Nostril
Figure 16.5
Extrinsic Eye Muscles
Superior oblique muscle
Trochlea
Trochlea
Superior oblique tendon
Superior rectus muscle
Superior
oblique
Superior
rectus
Medial
rectus
Lateral
rectus
Lateral rectus muscle
Inferior
oblique
Common
tendinous
ring
Inferior
rectus
muscle
Inferior
rectus
Inferior
oblique
muscle
(a) Lateral view of the right eye
(b) Anterior view of the right eye
Figure 16.6a, b
1
Anatomy of the Eyeball
• External wall consists of three layers (tunics)
•
•
•
Fibrous layer
Vascular layer
Sensory layer
• IInternall cavity
i contains
i fluids
fl id (humors)
(h
)
• Anterior segment- aqueous humor
• Posterior segment- vitreous humor
Medial View of the Eye
Ora serrata
Ciliary body
Ciliary zonule
(suspensory
ligament)
Cornea
Iris
P il
Pupil
Sclera
Choroid
Retina
Macula lutea
Fovea centralis
Posterior pole
Anterior
pole
Optic nerve
Anterior
segment
(contains
aqueous humor)
Lens
Central artery
Scleral venous
and vein of
sinus
the retina
Posterior segment
Optic disc
(contains vitreous humor)
(blind spot)
(a) Diagrammatic view. The vitreous humor is illustrated only in the bottom part of the eyeball.
Figure 16.7a
Fibrous Layer
•
•
•
Sclera- white of the eye (dura mater of
brain); provides anchoring for extrinsic
eye muscles
Cornea- light enters here, transparent
Cornea
Junction of sclera and cornea- limbus
2
•
•
Vascular Layer
Choroid-forms posterior 5/6 of vascular tunic,
continuous with ciliary body; prevents light scatter
Ciliary body- a thickened ring of tissue that
encircles the lens- consists of smooth muscle
(ciliary muscle) – focuses the lens.
•
•
•
•
•
Ciliary processes-generate suspensory ligament
Accomodation
Makes aqueous humor
Iris- visible, colored part of eye. Allows light to
enter. Has smooth muscle fibers
Pupil- opening of iris (pupillary light reflex)opening for light
The Vascular Layer
Ora serrata
Ciliary body
Ciliary zonule
(suspensory
ligament)
Cornea
Iris
P il
Pupil
Anterior
pole
Sclera
Choroid
Retina
Macula lutea
Fovea centralis
Posterior pole
Optic nerve
Anterior
segment
(contains
aqueous humor)
Lens
Central artery
Scleral venous
and vein of
sinus
the retina
Posterior segment
Optic disc
(contains vitreous humor)
(blind spot)
(a) Diagrammatic view. The vitreous humor is illustrated only in the bottom part of the eyeball.
Figure 16.7a
3
Sensory Layer (Retina)
•
•
•
Consists of two layers
Pigmented layer
Neural layer (external to internal)
• Photoreceptors (cones and rods)
• Bipolar neurons
• Ganglion cells
Posterior Aspect of the Eyeball
Neural layer of retina
Pigmented
layer of
retina
Choroid
Sclera
Pathway of light
Optic disc
Central artery
and vein of retina
Optic
nerve
(a) Posterior aspect of the eyeball
Figure 16.8a
4
Microscopic Anatomy of the Retina
Ganglion
cells
Bipolar
cells
Photoreceptors
Rod
Cone
Amacrine cell
Horizontal cell
Pathway of signal output
Pathway of light
Axons of
ganglion cells
Pigmented
layer of retina
(b) Cells of the neural layer of the retina
Choroid
Outer segments
of rods and cones
Nuclei of
ganglion
cells
Nuclei
Nuclei of
of bipolar rods and
cells
cones
Pigmented
layer of
retina
(c) Photomicrograph of retina
Figure 16.8b, c
Photoreceptors
Process
of bipolar
cell
Synaptic
terminals
Inner
fibers
Rod cell
body
Rod
cell
body
Nuclei
Cone
cell
body
Mitochondria
Outer
fiber
Outer segment
Pigmented layer
Inner segme
ent
Connecting
cilia
Apical
microvillus
Discs
containing
visual pigments
Discs being
phagocytized
Melanin
granules
Pigment
cell
nucleus
Basal lamina
(border with
choroid)
Figure 16.9
Blood Supply of the Retina
•
Retina receives
blood from two
sources
• Outer third of the
retina—supplied
by capillaries in
the choroid
• Inner two-thirds of
the retina—
supplied by central
artery and vein of
the retina
Central artery
and vein
emerging
from the
optic disc
Macula
lutea
Optic disc
Retina
Figure 16.10
5
Internal Chambers, Fluids and the Lens
• Posterior cavity- vitreous humor
• Anterior cavity- anterior chamber (aqueous humor) and
posterior chamber
• Aqueous humor- is renewed continuously and is in constant
•
motion- formed as filtrate of the blood from capillaries in
ciliary processes, flows through pupil into anterior chamber,
drains into veins via canal of Schlemm and returns to blood.
Lens- a thick transparent, biconvex disc that changes shape
to allow focusing of the light on the retina.
Internal Chambers and Fluids
Cornea
Posterior
segment
(contains
vitreous
humor)
Iris
Lens epithelium
Lens
Lens
Cornea
2
Corneal epithelium
Corneal endothelium
Aqueous humor
1 Aqueous humor is
formed by filtration from
the capillaries in the ciliary
processes.
Anterior
segment
(contains
aqueous
humor)
2 Aqueous humor flows
from the posterior chamber
through the pupil into the
anterior chamber. Some also
flows through the vitreous
humor (not shown).
3 Aqueous humor is
reabsorbed into the venous
blood by the scleral venous
sinus.
Anterior chamber
Ciliary zonule
(suspensory
ligament)
Posterior chamber
3
Scleral venous
sinus
Corneoscleral
junction
1
Ciliary
processes
Ciliary
muscle
Ciliary
body
Bulbar
conjunctiva
Sclera
Figure 16.11
Visual Pathways
•
•
•
Most visual information travels to the cerebral
cortex
Responsible for conscious “seeing”
Other pathways travel to nuclei in the midbrain
andd di
diencephalon
h l
6
Visual Pathways to the Cerebral Cortex
• Pathway begins at the retina
• Light activates photoreceptors
• Photoreceptors signal bipolar cells
• Bipolar cells signal ganglion cells
• Axons of ganglion cells exit eye as the optic
nerve
Visual Pathways to the Cerebral Cortex
•
Optic tracts send axons to:
• Lateral geniculate nucleus of the thalamus
• Synapse with thalamic neurons
• Fibers of the optic radiation reach the
primary visual cortex
Visual Pathways to the Brain and Visual Fields
Both eyes
Fixation point
Right eye
Left eye
Optic nerve
Suprachiasmatic
nucleus
Pretectal
nucleus
Lateral
geniculate
nucleus of
thalamus
Optic chiasma
Optic tract
Uncrossed
(ipsilateral) fiber
Crossed
(contralateral) fiber
Optic
radiation
Occipital
lobe
(primary visual
cortex)
(a) The visual fields of the two eyes overlap considerably.
Note that fibers from the lateral portion of each retinal field
do not cross at the optic chiasma.
Superior
colliculus
Figure 16.15a
7
Visual Pathways to Other Parts of the Brain
•
•
Some axons from the optic tracts
• Branch to midbrain
• Superior colliculi
• Pretectal nuclei
Other branches from the optic tracts
• Branch to the suprachiasmatic nucleus
The Ear and Equilibrium
•
•
•
The ear is divided into the outer, middle and inner
ear.
Outer and middle ear participate in hearing only
Inner ear functions for both hearing and
equilibrium
ilib i
Structure of the Ear
External ear
Middle
ear
Internal ear
(labyrinth)
Auricle
(pinna)
Helix
Lobule
External
acoustic
meatus
(a) The three regions of the ear
Tympanic
membrane
Pharyngotympanic
(auditory) tube
Figure 16.17a
8
Tympanic Membrane
•
•
•
Separates outer from middle ear.
Shaped like a flattened cone
Transmits air vibrations to auditory ossicles
Structures of the Middle Ear
Oval window
(deep to stapes)
Entrance to mastoid
antrum in the
epitympanic recess
Malleus
(hammer)
Incus
(anvil)
Stapes
(stirrup)
Auditory
ossicles
Tympanic
membrane
Round window
Semicircular
canals
Vestibule
Vestibular
nerve
Cochlear
nerve
Cochlea
Pharyngotympanic
(auditory) tube
(b) Middle and internal ear
Figure 16.17b
Middle Ear
•
•
•
•
•
Tympanic cavity
Ossicles- malleus, incus, stapes (merge onto oval
window)- transmit sound from external ear to
internal ear. Increase force but not the amplitude
of vibrations transmitted by tympanic membrane
eustachian tube/ auditory tube- equalizes air
pressure
Oval window (stapes)
Round window – dissipates left-over energy in
cochlea
9
Middle Ear
•
•
Round window – dissipates left-over energy in
cochlea
Reflexive muscles that protect from loud sounds
(tympanic reflex)
• Stapedius
S
di ((stapes))
• Tensor tympani (malleus)
The Middle Ear
• Ear ossicles—smallest
View
bones in the body
•
•
•
Malleus—attaches to
the eardrum
Incus—between the
malleus and stapes
Stapes vibrates
Stapes—vibrates
against the oval
window
Malleus Incus Epitympanic recess
Superior
Lateral
Anterior
• Tensor tympani and
stapedius
•
Two tiny skeletal
muscles in the middle
ear cavity
Pharyngotympanic tube
Tensor
tympani
muscle
Tympanic Stapes
membrane
(medial view)
Stapedius
muscle
Figure 16.18
Inner Ear
•
Vestibule
•
Semicircular canals
•
Cochlea
•
•
•
Utricle and saccule- balance and
position of head; movement
Semicircular ducts-concerned
with movement; ampullae are
swellings near utricle
Cochlear duct- contains spiral
organ of Corti, converts
mechanical sound to nerve
impulses
10
Structure of the Ear
External ear
Middle
ear
Internal ear
(labyrinth)
Auricle
(pinna)
Helix
Lobule
External
acoustic
meatus
(a) The three regions of the ear
Tympanic
membrane
Pharyngotympanic
(auditory) tube
Figure 16.17a
The Internal Ear
Temporal
bone
Semicircular ducts in
semicircular canals
Anterior
Posterior
L t l
Lateral
Cristae ampullares
in the membranous
ampullae
Utricle in
vestibule
Saccule in
vestibule
Facial nerve
Vestibular nerve
Superior vestibular
ganglion
Inferior vestibular
ganglion
Cochlear nerve
Maculae
Spiral organ (of Corti)
Cochlear duct in
cochlea
Stapes in
oval window
Round window
Figure 16.19
Utricle and Saccule-vestibular apparatus
•
•
•
•
Concerned with balance and position of head
while stationary
Acceleration (linear)
Endolymph
Has special sensory epithelium called Macula
• Receptor hair cells
11
The Maculae in the Internal Ear
Macula of
utricle
Macula of
saccule
Kinocilium
Otoliths
Stereocilia
Otolithic
membrane
Hair bundle
Hair cells
Supporting
cells
Vestibular
nerve fibers
(a)
Figure 16.22a
The Maculae in the Internal Ear
Otolithic
membrane
Head upright
Otoliths
Hair
cell
Force of
gravity
Head tilted
(b)
Figure 16.22b
Semicircular Ducts &Vestibular apparatus
•
•
•
•
Rotational movement
Anterior, posterior, and lateral semicircular duct.
Opens onto utricle
Ampulla
• Crista ampullaris- (crest with hair cells)
• Cupula- jelly-like pointed cap
12
Structure and Function of the Crista Ampullaris
Cupula
Crista
ampullaris
Endolymph
Hair bundle
(kinocilium
plus stereocilia)
Hair cell
Crista
ampullaris
Membranous
labyrinth
Fibers of vestibular nerve
Supporting
cell
(a) Anatomy of a crista ampullaris in a semicircular canal
(b) Scanning electron micrograph
of a crista ampullaris (45X)
Figure 16.23a, b
Structure and Function of the Crista Ampullaris
Section of
ampulla,
filled with
endolymph
Cupula
Fibers of
vestibular
nerve
At rest, the cupula stands upright.
Flow of endolymph
During rotational acceleration, endolymph
moves inside the semicircular canals in the
direction opposite the rotation (it lags
behind because of inertia). Endolymph flow
bends the cupula and excites the hair cells.
As rotational movement slows,
endolymph keeps moving in the
direction of the rotation, bending the
cupula in the opposite direction from
acceleration and inhibiting the hair cells.
(c) Movement of the cupula during rotational acceleration and deceleration
Figure 16.23c
Cochlear Duct
•
•
•
•
•
A spiral blind tube
Perilymph-filled chambers- scala vestibuli (starts
from oval window) and scala tympani Merge onto
round window
•
•
Vestibular membrane
Basilar membrane
Endolymph
Organ of Corti
•
•
Hair cells- stereocilia
Supporting cells
Tectorial membrane
13
The Cochlea
Modiolus
Cochlear nerve,
division of the
vestibulocochlear
nerve (VIII)
Spiral ganglion
Osseous spiral
lamina
Vestibular
membrane
Cochlear duct
(scala media)
Helicotrema
(a)
Osseous spiral lamina
Vestibular membrane
Tectorial membrane
Spiral
ganglion
Scala
vestibuli
(contains
perilymph)
Cochlear duct
(scala media;
contains
endolymph)
Stria
vascularis
Spiral organ
(of Corti)
Scala
tympani
(contains
perilymph)
Basilar
membrane
(b)
Figure 16.20a, b
The Cochlea
Vestibular membrane
Tectorial membrane
Cochlear duct
(scala media;
contains
endolymph)
Osseous spiral lamina
Scala
vestibuli
(contains
perilymph)
Spiral
ganglion
Stria
vascularis
Spiral organ
(of Corti)
Basilar
membrane
(b)
Scala
tympani
(contains
perilymph)
Tectorial membrane
Inner hair cell
Hairs (stereocilia)
Afferent nerve
fibers
Outer hair cells
Supporting cells
Fibers of
cochlear
nerve
Basilar
membrane
(c)
Figure 16.20b, c
14
Hearing Pathway
•
•
•
•
•
•
•
•
•
Sound vibrations
Ossicles- stapes and oval window
Perilymph of scala vestibuli and tympani
Endolymph of scala media
Tectorial membrane stays still, hair cells on basilar
membrane (stereocilia) have mechanically gated ion
channels
h
l (K+) leads
l d to
t
Hair cells stimulated (excess released via round
window)
AP in CNVIII to thalamus (via medulla and pons)
Thalamus
Primary auditory cortex in temporal lobe
The Role of the Cochlea in Hearing
Auditory ossicles
Malleus
Incus
Stapes
Cochlear nerve
Oval
window
1 Sound waves vibrate the
tympanic membrane.
Scala vestibuli
Helicotrema
4a
Scala tympani
Cochlear duct
2
3
4b
1
Tympanic
membrane
Basilar
membrane
2 Auditory ossicles vibrate.
Pressure is amplified.
3 Pressure waves created by the
stapes pushing on the oval window
move through fluid in the scala
vestibuli.
4a Sounds with frequencies below
ea g ttravel
a e tthrough
oug tthe
e
hearing
helicotrema and do not excite hair
cells.
4b Sounds in the hearing range go
through the cochlear duct,
vibrating the basilar membrane and
deflecting hairs on inner hair cells.
Round
window
Figure 16.21
15