Chapter 16
The Special Senses
1
The Special Senses
 Chemical senses
 Taste (gustation)
 Smell (olfaction)
 Vision
 The ear
 Hearing
 Equilibrium
2
Touch
The sense of touch is
part of the General
somatic senses____
This chapter deals with the Special
category of the two left sensory boxes
3
TASTE
 Taste buds: mostly on tongue
 Two types
 Fungiform papillae (small, on entire surface of tongue)
 Circumvallate papillae (inverted “V” near back of tongue)
4
 Taste buds of 50-100
epithelial cells each
 Taste receptor cells
(gustatory cells)
 Microvilli through pore,
bathed in saliva
 Dissolved molecules
bind & induce receptor
cells to generate
impulses in sensory
nerve fibers
5
 Types of taste





Sweet
Sour
Salty
Bitter
Umami “beef taste”- elicited by Glutamine
 Gustatory (taste) pathway to brainstem &
cerebral cortex via two cranial nerves:
 VII (Facial n.) – anterior 2/3 of tongue
 IX (Glossopharyngeal n.) – posterior 1/3
tongue and pharynx
6
Smell
(olfaction)
Olfactory epithelium in roof of nasal cavity
 Has millions of bipolar neurons = olfactory receptor cells
Only neurons undergoing replacement throughout adult life
Olfactory hair (cilia) bind odor molecules
 Mucus captures & dissolves odor molecules
Each receptor cell has an axon - are bundled into “filaments” of olfactory nerve
 Penetrate cribriform plate of ethmoid bone & enter olfactory bulb
7
 Olfactory bulb is in forebrain
 In bulb nerve axons branch and synapse with
mitral cells (neurons in clusters of “glomeruli”)
 Mitral cells send signals via olfactory tract
Filaments of Olfactory nerve (CN I)
*
Olfactory bulb__
_______Olfactory tract
*
8
9
Anosmia: absence of the sense of smell






Trauma
Colds or allergies producing excessive mucus
Polyps causing blockage
1/3 are from zinc deficiency
Head injury
Aging
10
The Eye and Vision
 Vision is the dominant sense in humans
 70% of sensory receptors in humans are
in the eyes
 40% of the cerebral cortex is involved in
processing visual information
 The eye (or eyeball) is the visual organ




Diameter 2.5 cm (1 inch)
Only anterior 1/6 visible
Lies in bony orbit
Surrounded by a protective cushion of fat
11
Accessory structures of the eye
 Eyebrows
 Eyelids or palpebrae
 Upper & lower separated by palpebral fissure
 Corners: medial & lateral canthi
 Eyelashes
12
 Eyelid tarsal plates give structure
 Where orbicularis oculi muscles attach (close eyes)
 Levator palpebrae superioris muscle
 Lifts upper lid voluntarily (inserts on tarsal plate)
13
 Tarsal glands –
modified sebaceous
(oil) glands in tarsal
plates
 Conjunctiva transparent mucus
membrane of stratified
columnar epithelium
 Palpebral conjunctiva
 Bulbar conjunctiva
 Covers white of eye but
not the cornea
(transparent tissue over
the iris and pupil)
14
Lacrimal apparatus
 Responsible for tears
 The fluid has mucus,
antibodies and lysozyme
 Lacrimal gland in orbit
superolateral to eye
 Tears pass out through
puncta into canaliculi
into sac into
nasolacrimal duct
 Empty into nasal cavity
(sniffles)
15
Extraocular (extrinsic) eye muscles: 6 in #
 Four are rectus muscles (straight)
 Lateral rectus, medial rectus, superior rectus,
and inferior rectus.
 Two are oblique: superior and inferior
16
When Extrinsic Eye Muscles Contract
 Superior oblique- eyes look out and down
 Superior rectus- eyes looks up
 Lateral rectus- eyes look outward
 Medial rectus- eyes look inward
 Inferior rectus- eyes looks down
 Inferior oblique- eyes look in and up
18
Extraocular (extrinsic) eye muscles
Cranial nerve innervations:
 Lateral rectus: VI (Abducens nerve)
 Medial, superior, inferior rectus & inferior oblique:
III (Oculomotor nerve.)
 Superior oblique: IV (Trochlear n.)
19
3 Layers form the external wall of the eye
1.
(outer) Fibrous: dense connective tissue


Sclera – white of the eye
Cornea




2.
Clear because regular alignment
Role in light bending
Avascular but DOES have pain receptors
Regenerates
(middle) Vascular:



3.
Choroid – blood rich, dark pigmented
Ciliary body – attaches lens
Iris (colored part: see next slide)
(inner) Sensory

Retina and optic nerve
20
1.
(outer layer) Fibrous:
dense connective tissue


2.
Sclera – white of the eye
Cornea
(middle) Vascular: uvea


Choroid – blood rich, has
dark pigmented that
prevents light scattering
Ciliary body




3.
Muscles – control lens
shape
Processes – secrete
aqueous humor
Zonule (attaches lens)
Iris
(inner layer) Sensory

Retina and optic nerve
21
Layers of external wall of eye continued
1.
(outer) Fibrous: dense connective tissue


2.
Sclera – white of the eye
Cornea
(middle) Vascular: uvea


Choroid – posterior, pigmented
Ciliary body
 Iris
 Opening is called PUPIL: lets in light
 Acts like the diaphragm of a camera lens.
 Regulates the amount of light that enters by
contracting or dilating to see clearly.
 Dark to dim light = dilation
 Bright light and close vision = contraction
(inner) Sensory
3.

Retina
22
Layers of external wall of eye continued
1. (outer) Fibrous: dense connective tissue


Sclera – white of the eye
Cornea
2. (middle) Vascular: uvea



Choroid – posterior, pigmented
Ciliary body
Iris
3. (inner) Sensory

Retina -------will cover after the chambers
and lens
23
some pictures…
24
Chambers and fluids
(see previous pics)
 Vitreous humor in posterior segment
 Jellylike
 Forms in embryo and lasts life-time
 Anterior segment filled with aqueous
humor – liquid, replaced continuously
 Anterior chamber between cornea and iris
 Posterior chamber between iris and lens
 Glaucoma when problem with drainage
resulting in increased intraocular pressure
25
Lens: thick, transparent biconvex disc
 Changes shape for precise focusing of light on
retina
 Onion-like avascular fibers, increase through life
 Cataract if becomes clouded
Note lens below,
but in life it is clear
Cataract below: the lens is milky
and opaque, not the cornea
26
Cataract (opaque lens)
27
The eye is an optical
device: predominantly
the lens
(to a lesser degree, not
shown here, the cornea also)
Note: images are
upside down and
reversed from left to
right, like a camera
a.
Resting eye set for distance vision: parallel light focused on retina
b.
Resting eye doesn’t see near objects because divergent rays are
focused behind retina
Lens accommodates (becomes rounder) so as to bend divergent rays
28
more sharply, thereby allowing convergence on the retina
c.
Lens Accommodation
 Light must be focused to a
point on the retina for
optimal vision
 The eye is set for distance
vision
(over 20 ft away)
 20/20 vision- at 20 feet,
you see what a normal eye
would see at 20 feet
(20/100- at 20, normal
person would see at 100)
 The lens must change
shape to focus for closer
objects
Retina: develops as part of the brain
Remember
the 3 layers of
the external
eye?
1. (outer layer) Fibrous: dense connective tissue
Sclera – white of the eye
Cornea
2. (middle layer) Vascular: uvea
Choroid – posterior, pigmented
Ciliary body
Iris
3. (inner layer) Sensory
Retina and optic nerve
Retina is 2 layers
 Outer thin pigmented layer:
 Melanocytes (prevent light scattering)
 Inner thicker neural layer
 Plays a direct role in vision
 Three type of neurons: 1. Photoreceptors
2. Bipolor cells
3. Ganglion cells
30
Light passes through pupil in iris, through vitreous
humor, through axons, ganglion cells and bipolar cells, to
photoreceptors next to pigmented layer
31
Photoreceptor neurons signal bipolar cells, which signal
ganglion cells to generate (or not) action potentials: axons
run on internal surface to optic nerve which runs to brain
*Know that axons from the retina form the optic nerve, CN II
32
Photoreceptors: 2 types
 Rod cells
 More sensitive to light - vision permitted in
dim light but only gray and fuzzy
 Only black and white and not sharp
 Cone cells
 High acuity in bright light
 Color vision
 3 sub-types: blue, red and green light cones
*Know that rods are for B & W and cones are for color
33
http://www.yorku.ca/eye/rod-cone.gif
http://www.secretbeyondmatter.com/ourbrains/theworldinourbrains_files/11-1.jpg
Cone Sensitivity
 There are three
types of cones
 Different cones are
sensitive to different
wavelengths
- red- long
- green- medium
- blue- short
 Color blindness is
the result of lack of
one or more cone
type
COLORBLINDNESS
- An inherited trait that
is transferred on the
sex chromosomes
(23rd pair)- sex-linked
trait
- Occurs more often in
males
- Can not be cured or
corrected
•Comes from a lack of one
or more types of color
receptors.
•Most are green or red or
both and that is due to a
lack of red receptors.
•Another possibility is to
have the color receptors
missing entirely, which
would result in black and
white vision.
One of the Ishihara charts for color blindness
Commonly X-linked recessive: 8% males and 0.4% females
37
38
If you want more
detail, it’s
fascinating…
39
Retina through
ophthalmoscope
 Macula: at
posterior pole
 Fovea: maximal
visual acuity
(most
concentrated
cones)
 Optic disc: optic
nerve exits
 Vessels
40
Images Formed on the Retina
If the image is focused at the spot where the optic disk is located,
nothing will be seen. This is known as the blind spot. There are no
photoreceptors there, as nerves and blood vessels pass through this
point.
Green is area seen by both eyes, and is
the area of stereoscopic vision
Visual pathways
At optic chiasm, medial fibers from each eye
(which view lateral fields of vision) cross to
opposite side of the brain. Optic tracts (of
crossed and uncrossed fibers, sensing
opposite side of visual field of both eyes)
synapse with neurons in the thalamus. These
axons form the optic radiation and terminate in
the primary visual cortex in the occipital lobe.
Left half of visual field perceived by right
cerebral cortex, and vice versa.
42
Visual Pathway
 Photoreceptors of
the retina
 Optic nerve
 Optic nerve crosses
at the optic chiasma
 Optic tract
 Thalamus
 Visual Cortex of
Occipital Lobe
Visual field defects
print this out and follow from the fields to the visual cortex using 4 colors
remember: fields are reversed and upside down
Visual fields
Location of lesion:
1. Optic nerve
1.
2.
ipsilateral (same side) blind eye
2. Chiasmatic (pituitary tumors
classically)
lateral half of both eyes gone
1.
3.
3.
3. Optic tract
opposite half of visual field gone
2.
4.
5.
4. & 5. Distal to geniculate
ganglion of thalamus:
homonymous superior field (4)
or homonymous inferior field (5)
defect
5.
Visual cortex
4.
44
 Double vision: diplopia (what the patient experiences)
 Eyes do not look at the same point in the visual field
 Misalignment: strabismus (what is observed when shine a
light: not reflected in the same place on both eyes) – can be a
cause of diplopia




Cross eyed
Gaze & movements not conjugate (together)
Medial or lateral, fixed or not
Many causes
 Weakness or paralysis of extrinsic muscle of eye
– Surgical correction necessary
 Oculomotor nerve problem, other problems
 Lazy eye: amblyopia
 Cover/uncover test at 5 yo
 If don’t patch good eye by 6, brain ignores lazy eye and visual pathway
degenerates: eye functionally blind
NOTE: some neurological development and connections have a
window of time - need stimuli to develop, or ability lost
45
Geometrical illusions
Successive contrast : afterimages ...
fixate the black dot in the center for 60 seconds ...
… and then look at a the black dot in the right panel !
what do you see?
Terminology, remember…
 Optic – refers to the eye
 Otic – refers to the ear
 Getting eyedrops and ear drops mixed up
is probably not a good idea
50
Anatomy of the Ear
 The ear is divided into three areas
 Outer
(external)
ear
 Middle
ear
 Inner
ear
 (Add C. “INNER
EAR” to notes)
The External Ear
 Involved in
hearing only
 Structures of
the external ear
 Pinna (auricle)collects sound
 External
auditory canalchannels
sound inward
The External Auditory Canal
 Narrow chamber in the temporal bonethrough the external auditory meatus
 Lined with skin
 Ceruminous (wax) glands are present
 Ends at the tympanic membrane
(eardrum)
The Middle Ear or Tympanic Cavity
 Air-filled cavity within the temporal bone
 Only involved in the sense of hearing
The Middle Ear or Tympanic Cavity
 Two tubes are associated with the inner
ear
 The opening from the auditory canal is
covered by the tympanic membrane
(eardrum)
 The auditory tube connecting the middle ear
with the throat (also know as the eustacian
tube)
 Allows for equalizing pressure during yawning
or swallowing
 This tube is otherwise collapsed
Bones of the Tympanic Cavity
 Three bones
span the cavity
 Malleus
(hammer)
 Incus (anvil)
 Stapes (stirrip)
http://medicine.wustl.edu/~oto/bbears/images/ossic.jpg
http://www.ghorayeb.com/files/STAPES_on_a_Penny_375_SQ.jpg
Bones of the Tympanic Cavity
 Vibrations from
eardrum move
the malleus
 These bones
transfer sound
to the inner ear
Inner Ear or Bony Labyrinth
 Also known as
osseous labyrinthtwisted bony
tubes
 Includes sense
organs
for hearing and
balance
 Filled with
perilymph
Inner Ear or Bony Labryinth
 3 Subdivisions
 Cochlea
 Upper chamber
is the scala
vestibuli
 Lower chamber
is the scala
tympani
 Vestibule
 Semicircular
canals
Chochlea
 Spiral organ of Corti
 Receptors = hair cells on the basilar membrane
Scala vestibuli
Scala tympani
Organ of Corti
 Gel-like tectorial membrane is capable of
bending hair cells (endolymph in the
membranous labyrinth of the cochlear duct
flows over it and pushes on the membrane)
Scala vestibuli
Scala tympani
Organs of Hearing
 Organ of Corti
 Cochlear nerve attached to hair cells
transmits nerve impulses to auditory cortex
on temporal lobe
Scala vestibuli
Scala tympani
Mechanisms of Hearing
 Vibrations from
sound waves
move tectorial
membrane (pass
through the
endolymph fluid
filling the
membranous
labyrinth in the
cochlear duct)
 Hair cells are bent
by the membrane
65
Mechanisms of Hearing
 An action potential
starts in the cochlear
nerve
 The signal is
transmitted to the
midbrain (for
auditory reflexes
and then directed to
the auditory cortex
of the temporal
lobe)
Mechanisms of Hearing
Continued stimulation can lead
to adaptation (over stimulation
to the brain makes it stop
interpreting the sounds)
Organs of Equilibrium
 Receptor cells are in two structures
 Vestibule
 Semicircular canals
Organs of Equilibrium
 Equilibrium has two functional parts
 Static equilibrium- in the vestibule
 Dynamic equilibrium- in the semicircular
canals
Static Equilibrium
 Maculae –
receptors in
the vestibule
 Report on
the position
of the head
 Send
information
via the
vestibular
nerve
Static Equilibrium
 Anatomy of
the maculae
 Hair cells are
embedded in
the otolithic
membrane
 Otoliths (tiny
stones) float in
a gel around
the hair cells
Function of Maculae
Movements cause otoliths to bend the hair
cells (gravity moves the “rocks” over
and pulls the hairs)
http://neuromedia.neurobio.ucla.edu/campbell/eyeandear/wp_images/177_macula_HP.gif
Dynamic Equilibrium
 Whole structure is the
ampulla
 Crista ampullaris –
receptors in the
semicircular canals
 Tuft of hair cells
 Cupula (gelatinous cap)
covers the hair cells
Dynamic Equilibrium
 Action of angular head
movements
 The cupula stimulates the hair
cells
 Movement of endolymph
pushes the
cupula over
and pulls the
hairs
 An impulse is
sent via the
vestibular nerve
to the cerebellum
DYNAMIC EQUILIBRIUM STRUCTURES
http://www.faculty.une.edu/com/abell/histo/CristaAmp.jpg
http://neuromedia.neurobio.ucla.edu/campbell/eyeandear/wp_images/177_macula_crista.gif
Hearing loss- due to disease (ex. meningitus),
damage, or age related
Conduction deafness- prevention or blocking sounds
from entering inner ear.
Ex. ear wax, ruptured ear drum, middle ear
inflammation (otis media), and otosclerosis
(hardening of the ossicles of the ear)
Sensoneural deafness- damage to the neural
structures from any point from the cochlear hair
cells to and including the auditory cortical cells
• Partial or complete deafness, or gradual loss
over time
Tinnitus- ringing or clicking sound in the absence of
auditory stimuli; 1st symptom of cochlear nerve
degeneration
•
may result from inflammation of the inner or
middle ear
•
side effect from medicine such as aspirin
•
Symptoms- vertigo, nausea, hearing loss
Meniere's Syndrome- labyrinth disorder; effects both
semicircular canals and cochlea