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Chapter 16
The Special Senses

2. 2
The Special Senses
 Chemical senses
 Taste (gustation)
 Smell (olfaction)
 Vision
 The ear
 Hearing
 Equilibrium

3. 3
re: 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

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TASTE
 Taste buds: mostly on tongue
 Two types
 Fungiform papillae (small, on entire surface of tongue)
 Circumvallate papillae (inverted “V” near back of tongue)

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 Taste buds of 50-100
epithelial cells each
 Taste receptor cells
(gustatory cells)
 Microvilli through pore,
bathed in saliva
 Disolved molecules
bind & induce receptor
cells to generate
impulses in sensory
nerve fibers

6. 6
 Types of taste
 Sweet
 Sour
 Salty
 Bitter
 Glutamate (MSG)
 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

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Olfactory epithelium in roof of nasal cavity
 Pseudostratified columnar epithelium
 Has millions of bipolar neurons = olfactory receptor cells
Only neurons undergoing replacement throughout adult life
Olfactory 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
Smell
(olfaction)

8. 8
 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
Olfactory bulb__
_______Olfactory tract
___Filaments of Olfactory nerve (CN I)
*
*

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 1000 types of smell receptors (approx.)
 Convergence of many receptor cell signals
onto one glomerulus registers a signature
pattern
 Brain recognizes the pattern: sent to
unclus (olfactory center) and limbic area
Anosmia: absence of the sense of smell
 Trauma
 Colds or allergies producing excessive mucus
 Polyps causing blockage
 1/3 are from zinc deficiency

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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

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Accessory structures of the eye
 Eyebrows
 Eyelids or palpebrae
 Upper & lower separated by palpebral fissure
 Corners: medial & lateral canthi
 Eyelashes

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 Eyelid tarsal plates give structure
 Where orbicularis oculi muscles attach (close eyes)
 Levator palpebrae superioris muscle
 Lifts upper lid voluntarily (inserts on tarsal plate)

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 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)

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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)

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Extraocular (extrinsic) eye muscles: 6 in #
“EOMs intact” means they all work right
 Four are rectus muscles (straight)
 Originate from common tendinous or anular
ring, at posterior point of orbit
 Two are oblique: superior and inferior

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Extraocular (extrinsic) eye muscles
Cranial nerve innervations:
 Lateral rectus: VI (Abducens n.) – abducts eye outward
 Medial, superior, inferior rectus & inf oblique: III (Oculomotor n.)
– able to look up and in if all work
 Superior oblique: IV (Trochlear n.) – moves eye down and out

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Innervation

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 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

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3 Layers form the external wall of the eye
1. (outer) Fibrous: dense connective tissue
 Sclera – white of the eye
 Cornea
 100s of sheets of collagen fibers between
sheets of epithelium and endothelium
 Clear because regular alignment
 Role in light bending
 Avascular but does have pain receptors
 Regenerates
2. (middle) Vascular: uvea
 Choroid – posterior, pigmented
 Ciliary body
 Iris (colored part: see next slide)
3. (inner) Sensory
 Retina and optic nerve

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1. (outer layer) Fibrous:
dense connective tissue
 Sclera – white of the eye
 Cornea
2. (middle) Vascular: uvea
 Choroid – posterior,
pigmented
 Ciliary body
 Muscles – control lens
shape
 Processes – secrete
aqueous humor
 Zonule (attaches lens)
 Iris
3. (inner layer) Sensory
 Retina and optic nerve

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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
Pigmented put incomplete: pupil lets in light
Sphincter of pupil: circularly arranged smooth muscle -
parasympathetic control
for bright light and/or close vision
Dilator of pupil: radiating smooth muscle –
sympathetic control
for dim light and/or distance vision
3. (inner) Sensory
 Retina

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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

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some pictures…

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Chambers and fluids
 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
(see previous pics)

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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

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Cataract (opaque lens)

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(to a lesser degree, not
shown here, the cornea also)
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
c. Lens accommodates (becomes rounder) so as to bend divergent rays
more sharply, thereby allowing convergence on the retina
Note: images are
upside down and
reversed from left to
right, like a camera
The eye is an optical
device: predominantly
the lens

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Retina: develops as part of the brain
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. (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
Remember
the 3 layers of
the external
eye?
1. Photoreceptors
2. Bipolor cells
3. Ganglion cells

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Light passes through pupil in iris, through vitreous
humor, through axons, ganglion cells and bipolar cells, to
photoreceptors next to pigmented layer

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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

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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

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One of the Ishihara charts for color blindness
Commonly X-linked recessive: 8% males and 0.4% females

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35. 35
If you want more
detail, it’s
fascinating…

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Retina through
ophthalmoscope
 Macula: at
posterior pole
 Fovea: maximal
visual acuity
(most
concentrated
cones)
 Optic disc: optic
nerve exits
 Vessels

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Visual pathways
Green is area seen by both eyes, and is
the area of stereoscopic vision
At optic chiasm, medial fibers from each eye
(which view lateral fields of vision) cross to
opposite side. Optic tracts (of crossed and
uncrossed, sensing opposite side of visual field of
both eyes) synapse with neurons in lateral
geniculate of 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.

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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
1. Optic nerve
ipsilateral (same side) blind eye
2. Chiasmatic (pituitary tumors
classically)
lateral half of both eyes gone
3. Optic tract
opposite half of visual field gone
4. & 5. Distal to geniculate
ganglion of thalamus:
homonymous superior field (4)
or homonymous inferior field (5)
defect
Visual cortex
Visual fields
1.
2.
3.
5.
4.
Location of lesion:
1.
2.
3.
4.
5.

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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

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The Ear Parts of the ear
 Outer (external)
ear
 Middle ear
(ossicles) for
hearing)
 Inner ear
(labyrinth) for
hearing &
equilibrium

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Sound in external acoustic meatus hits
tympanic membrane (eardrum) – it vibrates
Pressure is
equalized by the
pharyngotympanic
tube (AKA
eustachian or
auditory tube)

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TM causes ossicles in air filled middle ear to move:
 Malleus (hammer)
 Incus (anvil)
 Stapes (stirrup)
Ossicles articulate to
form a lever system
that amplifies and
transmits the vibratory
motion of the TM to
fluids of inner ear
cochlea via oval
window
These are 3 of the
smallest bones of the body

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Skeletal muscles of middle ear
When loud, muscles
contract, limiting
vibration and
dampening the noise

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Inner ear = bony “labyrinth” of 3 parts
1. Cochlea - hearing
2. Vestibule - equilibrium
3. Semicircular canals -
equilibrium
Filled with
perilymph and
endolymph
fluids
Cochlea_______________________
Vestibule___________
Semicircular canals____
In petrous part of
the temporal bone

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Spiral organ of Corti in cochlea contains
receptors for hearing (amplifies sound 100 times)
 Vibration of
stirrup at oval
window starts
traveling
waves
displacing
basilar
membrane
 Sensory hair
cells
stimulated
 Nerve ending
of cochlear
nerve division
of VIII
(Vestibulo-
cochlear n.)

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Auditory pathway

47. 47
Equilibrium pathway
 Via vestibular nerve branch of VIII
(Vestibulocochlear n.) to the brain
stem
 Only special sense for which most of
the information goes to lower brain
centers

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Vestibule contains
utricle and saccule
 Each contains a macula
 Senses static equilibrium
and linear acceleration of
the head
(not rotational movements)
 Tips of hairs imbedded in
otolithic membrane (calcium
carbonate “stones”)
 Vestibular nerve branches of
VIII (Vestibulocochlear n.)

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Semicircular canals
 Each of the 3 lies in one of the 3 planes of space
 Sense rotational acceleration of the head
 Duct with ampulla housing a small crest: crista ampulla
 Hairs project into jellylike cupula & basilar cells synapse
with fibers of vestibular nerve

50. 50
VIII Vestibulocochlear nerve