Download The Special Senses

Chapter 15:
The Special Senses
J.F. Thompson, Ph.D. & J.R. Schiller, Ph.D. & G. Pitts, Ph.D.
The Five Special Senses:

Smell and taste: chemical senses
(chemical transduction)

Sight: light sensation (light
transduction)

Hearing: sound perception
(mechanical transduction)

Equilibrium: static and dynamic
balance (mechanical transduction)
Special Sensory Receptors

Distinct types of receptor cells are
confined to the head region

Located within complex and discrete
sensory organs (eyes and ears) or in
distinct epithelial structures (taste buds
and the olfactory epithelium)
The Chemical Senses:
Taste and Smell

The receptors for taste (gustation) and
smell (olfaction) are chemoreceptors
(respond to chemicals in an aqueous
solution)

Chemoreception involves chemically
gated ion channels that bind to odorant
or food molecules
Taste
Location of Taste Buds


Located mostly on
papillae of tongue
Two of the types of
papillae:


fungiform
circumvallate
Taste Buds



Each papilla contains
numerous taste buds
Each taste bud
contains many
gustatory cells
The microvilli of
gustatory cells have
chemoreceptors for
tastes
The Five Basic Tastes

Sweet: sugars, alcohols, some amino
acids, lead salts

Sour: H+ ions in acids

Salty: Na+ and other metal ions


Bitter: many substances including
quinine, nicotine, caffeine, morphine,
strychnine, aspirin
Umami: the amino acid glutamate (“beef”
taste)
Taste Transduction
 Incompletely
understood
A
direct influx of various ions (Na+,
H+) or the binding of other molecules
which leads to depolarization of the
receptor cell
 Depolarization
of the receptor cell
causes it to release neurotransmitter
that stimulates nerve impulses in the
sensory neurons of gustatory nerves
Sensory Pathways
for Taste


Afferent impulses of
taste stimulate many
reflexes which promote
digestion (increased
salivation, and
gastrointestinal motility
and secretion)
“Bad” taste sensations
can elicit gagging or
vomiting reflexes
Smell
Location of Olfactory (Odor)
Receptors
Odor Receptors



Bipolar neurons
Collectively constitute
cranial nerve I
Unusual in that they
regenerate (on a ~60
day replacement cycle)
Odors

Very complicated

Humans can distinguish thousands


More than a thousand different
odorant-binding receptor molecules
have been identified
Different combinations of specific
molecule-receptor interactions
produce different odor perceptions
Transduction of Smell


Binding of an odorant molecule to a specific
receptor activates a G-protein and then a second
messenger (cAMP)
cAMP causes gated Na+ and Ca2+ channels to
open, leading to depolarization
•
Olfactory Pathway



One path leads from the olfactory bulbs via
the olfactory tracts to the olfactory cortex
where smells are consciously interpreted and
identified
Another path leads from the olfactory bulbs
via the olfactory tracts to the thalamus and
limbic system where smells elicit emotional
responses
Smells can also trigger sympathetic nervous
system activation or stimulate digestive
processes
Vision
Surface Anatomy of the Eye



Eyebrows divert sweat from the eyes and
contribute to facial expressions
Eyelids (palpebrae) blink to protect the eye
from foreign objects and lubricate their
surface
Eyelashes detect and deter foreign objects
Conjunctiva

A mucous membrane
lining the inside of the
eyelids and the anterior
surface of the eyes



forms the conjunctival sac
between the eye and eyelid
Forms a closed space
when the eyelids are
closed
Conjunctivitis
(“pinkeye”): inflammation
of the conjunctival sac
The Lacrimal Apparatus

Lacrimal Apparatus:





lacrimal gland
lacrimal sac
nasolacrimal duct
Rinses and lubricates
the conjunctival sac
Drains to the nasal
cavity where excess
moisture is
evaporated
Extrinsic Eye Muscles

Lateral, medial, superior, and inferior rectus
muscles (recall, rectus = straight); superior and
inferior oblique muscles
Internal Anatomy of the Eye--Tunics

Fibrous tunic: sclera & cornea

Vascular tunic: choroid layer

Sensory tunic: retina
Internal Anatomy of the Eye

Anterior Segment
contains the Aqueous
Humor





Iris
Ciliary Body
Suspensory Ligament
Lens
Posterior Segment
contains the Vitreous
Humor
Autonomic Regulation of the Iris
Pupil
Constricts
Pupil
Dilates
The Two Layers of the Retina


Outer pigmented layer
has a single layer of
pigmented cells,
attached to the choroid
tunic, which absorbs
light to prevent light
scattering inside
Inner neural layer has
the photosensory cells
and various kinds of
interneurons in three
layers
Neural Organization in the Retina
 Photoreceptors:
rods
(for dim light) and
cones (3 colors: blue,
green and red, for
bright light)
 Bipolar
cells are
connecting
interneurons
 Ganglion
cells’ axons
become the Optic
Nerve
Neural Organization in the Retina


Horizontal Cells
enhance contrast
(light versus dark
boundaries) and
help differentiate
colors
Amacrine cells
detect changes in
the level of
illumination
The Optic Disc



Axons of ganglion
cells exit to form the
optic nerve
Blood vessels enter
to serve the retina by
running on top of the
neural layer
The location of the
“blind spot” in our
vision
Micrograph of the Retina

Light must cross
through the
capillaries and
the two layers of
interneurons to
reach the
photoreceptors,
the rods and
cones
Light
Opthalmoscope Image of the Retina



The Macula Lutea (“yellow
spot”) is the center of the
visual image
The Fovea Centralis is a
central depression where
light falls more directly on
cones providing for the
sharpest image
discrimination
Light bouncing off RBCs’
hemoglobin causes “red
eye” in flash photos
Circulation of the Aqueous Humor



Ciliary process at the base
of the iris produces
aqueous humor
Scleral venous sinus
returns aqueous humor to
the blood stream
Glaucoma – any
disturbance that increases
aqueous humor volume
and pressure which causes
pain – ultimately the
vitreous humor crushes
the retina causing
blindness
Hearing
External Ear


Pinna (auricle):
focuses sound
waves on the
tympanic
membrane
Ceruminous glands
guard the external
auditory canal
Middle Ear & Auditory Tube


Three auditory
ossicles (bones) serve
as a lever system to
transmit sound to the
inner ear
Pharyngotympanic
(auditory tube):
connects to pharynx,
allowing air pressure
to equalize on both
side of the tympanic
membrane
Middle Ear Ossicles — (median view)


Malleus (hammer), incus
(anvil) and stapes
(stirrup) act to increase
the vibratory force on the
oval window
Tensor tympani and
stapedius muscles control
the tension of this lever
system to prevent
damage to the delicate
tympanic and round
window membranes
The Membranous Labyrinth




A series of tiny fluid-filled chambers in the temporal
bone
Cochlea tranduces sound waves
Semicircular canals and their ampullae transduce
balance and equilibrium
The vestibule connects the two portions
The Cochlea – Two Coiled Tubes


Larger outer tube is folded but continuous (like a
coiled letter “U”) – the scala vestibuli and scala
tympani –contains perilymph fluid
Smaller inner tube is the scala media (cochlear duct)
contains endolymph fluid
The Spiral Organ of Corti


Between the scala tympani and the scala
media/cochlear duct is the complex receptor system:
the spiral organ of Corti
Sensory Hair Cells stand on the basilar membrane and
their processes are attached to the Tectorial
Membrane
Wave Pulses in the Cochlea


Stapes moving at the
oval window creates
pulses of vibration in
the perilymph of the
scala vestibuli and
scala tympani
Harmonic vibrations
are created at right
angles in the
endolymph of the scala
media which move the
basilar membrane
Transduction of Sound Waves


Movement against the
tectorial membrane
stimulates the hair
cells to send impulses
to the auditory cortex
Round window moves
to accommodate the
vibrations initiated by
the stapes
Apex
Base 
Wave Pulses in the Cochlea


Stapes moving at the oval window creates
pulses of vibration in the perilymph of the
scala vestibuli and scala tympani
Harmonic vibrations are created at right
angles in the endolymph of the scala media
which move the basilar membrane
Transduction of Sound Waves
Resonance of Basilar Membrane



High notes are
detected at the base
of the cochlea
Low notes are
detected at the apex
Due to differences in
the width and
flexibility of the
basilar membrane
Apex
Base 
Auditory Pathway

Afferent impulses for
sounds are routed:



Vestibulocochlear Nerve
VIII (cochlear branch)
Nuclei in the medulla
oblongata where motor
responses can turn the
head to focus on sound
sources
Primary Auditory Cortex
in the temporal lobe for
conscious interpretation
Balance and Coordination
Macula in the Saccule
& Utricle



Chambers near the
oval window filled
with perilymph
CaCO3 otoliths (“ear
stones”) slide over
the surface lining cells
in response to gravity
Static equilibrium tells
the CNS “which way
is up”
Macular Transduction


Hair cells’ stereocilia move in response to
the sliding otoliths
To send impulses to the CNS for
interpretation
Semicircular Canals



Three endolymph-filled
tubes in the bony
labyrinth
Each C-shaped loop is in
a plane at right angles to
the other two
Each has an expanded
ampulla containing a
sensory structure, the
cupula
Ampullar Transduction



Movement in the plane of one of the canals causes
endolymph to flow and bends the cupola
Hair cells’ stereocilia move in response to the
movement
Dynamic
equilibrium tells
the CNS “which
way is the head
or body is
moving”
Pathways of Balance and Orientation

Integration of
sensory modalities:





Sight
Proprioception
Static equilibrium
Dynamic equilibrium
Output to skeletal
muscles to position:



Eyes
Head and neck
Trunk
Take a Tour of the Virtual Ear at:
http://www.augie.edu/perry/ear/hearmech.htm
End Chapter 15