Transcript THE SPECIAL SENSES

THE SPECIAL SENSES
THE CHEMICAL SENSES:
TASTE AND SMELL
• The receptors for taste (gustation) and smell (olfaction) are
chemoreceptors that respond to chemicals in aqueous solution
• Taste Buds and the Sense of Taste
– Taste buds, the sensory receptor organs for taste, are located in the
oral cavity with the majority located on the tongue (few scattered on soft
palate, inner surface of cheek, pharynx, and epiglottis of larynx)
• Most found in papillae (peglike projections of the tongue mucosa that give
the tongue surface a slightly abrasive feel)
– Taste sensations can be grouped into one of five basic qualities:
sweet, sour, bitter, salty, and umami (meaty or savory taste produced by
monosodium glutamate)
– Physiology of Taste:
• For a chemical to be tasted it must be dissolved in salvia, move into
the taste pore, and contact the gustatory hairs
• Each taste sensation appears to have its own special mechanism for
transduction
THE CHEMICAL SENSES:
TASTE AND SMELL
• Taste Buds and the Sense of Taste
– Afferent fibers carrying taste information from the
tongue are found primarily in the facial nerve and
glossopharyngeal cranial nerves
– Taste impulses from the few taste buds found on
the epiglottis and the lower pharynx are covered
via the vagus nerve
– Taste is strongly influenced by smell and stimulation
of thermoreceptors, mechanoreceptors, and
nociceptors
TONGUE
TASTE
•
Sweet:
–
•
Sour:
–
•
Elicited by many organic substances including sugars, saccharin, alcohols, some amino
acids, and some lead salts (found in lead paints)
Produced by acids, specifically their hydrogen ions (H+) in solution
Salt:
–
Produced by metal ions (inorganic)
•
•
Table salt (sodium chloride) tastes the saltiest
Bitter:
–
Elicited by:
•
alkaloids: organic alkaline substances that react with acids to form salts that are used for medical
purposes
•
Some nonalkaloids:
–
–
•
Quinine, nicotine, caffeine, morphine, and strychnine
Aspirin
Umami:
–
–
–
Discovered by the Japanese?
Elicited by the amino acid glutamate
Responsible for the beef taste of steak, tang of aging cheese, and the flavor of the food
additive monosodium glutamate (sodium salt of glutamic acid)
TASTE
• Most taste buds respond to two or
more taste qualities, and many
substances produce a mixture of the
basic taste sensations
TASTE BUDS
• Taste buds are located
mainly on the:
– Tops of fungiform
papillae which are
scattered over the entire
tongue surface
– In epithelium of the side
walls of the large round
circumvallate (vallate)
papillae
• Largest and least
numerous
• Inverted V shape at back
of tongue
TASTE BUDS
• Gourd (squash) shape
• Epithelial cells of three
types:
– Supporting cells:
• Form bulk of taste bud
• Insulate the receptor cells
from each other and form the
surrounding tongue
epithelium
– Receptor cells:
• Called taste cells or gustatory
cells
– Basal cells:
• Act as stem cells, dividing
and differentiating into
supporting cells, which in turn
give rise to new gustatory
cells
TASTE BUDS
• Gustatory hairs:
– Long microvilli that
project from the tips of
both supporting and
gustatory (taste) cells
– Extend through a taste
pore to the surface of
the epithelium
– Bathed by saliva
– Receptor membranes
of gustatory cells
TASTE BUDS
• Coiling around gustatory cells
are sensory dendrites that
represent the initial part of the
gustatory pathway to the brain
– Each afferent fiber receives
signals from several receptor
cells within the taste bud
• Because of their location,
taste buds cells are
subjected to huge amounts
of friction and are routinely
burned by hot foods
– Replace every 7 to 10 days
TASTE MAPS
•
•
•
•
•
Sweet: tip of tongue
Salt: front sides
Sour: mid/rear sides
Bitter: back
Umami: pharynx
TASTE MAPS
• Maps are dubious
(uncertain)
TASTE HOMEOSTATIC VALUES
• Likes:
– Umami guides intact of protein
– Sugar and salt helps satisfy the body’s need
for carbohydrates, minerals, and some amino
acids
– Sour rich source of vitamin C (oranges,
lemons, tomatoes)
• Dislike:
– Bitter is protective since many poisons and
spoiled foods tend to be bitter
GUSTATORY CORTEX
PHYSIOLOGY OF TASTE
• For a chemical to be
tasted it must be
dissolved in salvia,
diffuse into the taste
pore, and contact the
gustatory hairs
• Different gustatory hairs
have different
thresholds for
activation:
– Bitter receptors detect
substances in very minute
amounts
– Others are less sensitive
PHYSIOLOGY OF TASTE
•
Transduction: process by which
stimulus energy is converted
into a nerve impulse:
– Each taste quality appears to
have its own mechanism
• Salt is due to Na+ influx through
sodium channels followed by Ca2+
influx
• Sour is mediated by H+ which
appears to act on a taste cell in
one of three ways
– Directing entering the cell
– Opening channels that allow
other cations (+) to enter
– Blockage of K+ channels
• Bitter, sweet, and umami are
mediated by G protein-dependent
mechanisms that act via second
messengers to promote
depolarization by increasing
intracellular levels of Ca2+
(bitter) or closing K+ channels
(sweet)
PHYSIOLOGY OF TASTE
•
Gustatory Pathway: cranial nerves
–
Tongue:
•
•
–
Epiglottis and lower pharynx:
•
•
Facial nerve (VII): chorda tympani
transmits impulses from taste receptors
in the anterior 2/3 of the tongue
Glossopharyngeal nerve (IX):
services the posterior 1/3 and the
paharynx just behind
Vagus nerve (X)
These afferent fibers synapse in the
solitary nucleus of the medulla to the
thalamus and ultimately to the
gustatory cortex in the parietal lobes
–
–
Fibers also project to the hypothalamus
and limbic system which determine our
appreciation of what we are tasting
Initiate reflex synapses with the
parasympathetic system increasing
salivary secretion in the mouth and
gastric secretion in the stomach
Influence of Other Sensations on Taste
• Taste is 80% smell
– When olfactory receptors in the nasal cavity
are blocked (congestion/pinching) food is
bland
• Mouth also contains thermoreceptors,
mechanoreceptors, and nociceptors
(pain: chili peppers)
– The temperature and texture of foods can
enhance or detract from their taste
THE CHEMICAL SENSES:
TASTE AND SMELL
• The Olfactory Epithelium and the Sense of
Smell
– The olfactory epithelium is located in the roof of
the nasal cavity and contains the olfactory
receptor cells
– To smell a particular odor it must be volatile and it
must be dissolved in the fluid coating the
olfactory epithelium
– Axons of the olfactory receptor cells synapse in the
olfactory bulbs sending impulses down the olfactory
tracts to the thalamus, the hypothalamus, amygdala,
and other members of the limbic system
OLFACTORY EPITHELIUM
• Organ of smell is a yellowtinged patch of
pseudostrtified epithelium
(olfactory epithelium)
located in the roof of the
nasal cavity
• Air entering the nasal cavity
must make a hairpin turn to
stimulate olfactory receptors
before entering the respiratory
passageway
– Poor location
– This is why sniffing, which
draws more air superiorly
across the olfactory
epithelium, intensifies the
sense of smell
OLFACTORY EPITHELIUM
•
Olfactory epithelium covers the
superior nasal concha on each
side of the nasal septum, and
contains millions of bowling pinshaped olfactory receptor cells
(surrounded and cushioned by
columnar supporting cells, which
make up the bulk of the penny-thin
epithelial membrane)
– Supporting cells contain a yellowbrown pigment similar to lipofuscin
(insoluble fatty pigment found in
aging cells that they have
ingested but not completely
digested) which gives the olfactory
epithelium its yellow hue
•
At the base of the epithelium lie
the short basal cells
OLFACTORY RECEPTORS
OLFACTORY RECEPTORS
• Olfactory receptor cells:
– Unusual bipolar neurons
– Each has a thin apical (apex)
dendrite that terminates in a
knob from which several long
cilia radiate (olfactory cilia)
• Increase the receptive
surface area
• Covered by a thin coat of
mucus produced by
supporting cells and by
olfactory glands in the
underlying connective tissue
– Mucus is a solvent that
captures and dissolves
airborne odorants
– Olfactory cilia are largely
nonmotile
OLFACTORY RECEPTORS
• Unmyelinated axons of the
olfactory receptor cells gather
into small fascicles that
collectively form the filaments
of the olfactory nerve (cranial
nerve I)
• Olfactory neurons are unique
in that they undergo noticeable
turnover throughout adult life
– Remember that taste
receptors are epithelial cells
– Their location puts them at risk
of damage
– Typical life span is 60 days
– Replaced by differentiation of
the basal cells in the olfactory
epithelium
Specificity of Olfactory Receptors
• Taste is classified into 4 to 5 categories
• Humans can distinguish approximately 10,000 odors
but research suggests that our olfactory receptors
are stimulated by different combinations of a more
limited number
– There are at least 1000 smell genes that are only active in
the nose
– Each gene encodes an odorant binding protein that responds to
several different odors and each odor binds to several different
receptor types
• However, each receptor cell has only one type of receptor protein
• Pain receptors (irritants, temperatures) send
impulses to the CNS by means of the trigeminal
nerves
Physiology of Smell
Activation
• Particular odorant must be
volatile (gaseous state)
• Must dissolve in the fluid
coating the olfactory
epithelium
• Dissolved odorants stimulate
olfactory receptors by binding
to protein receptors in the
olfactory cilium membranes
and opening specific Na+
channels
– Leads to an action potential
(impulse) that is conducted to
the olfactory bulb (distal end of
the olfactory tracts)
Physiology of Smell
Mechanism
• Transduction (biochemical
conversion: docking of specific
chemicals to receptors
resulting in production of
specific enzymes or nerve
impulses)
• Odorant chemical binding to
the G protein-associated
receptor sets the cyclic AMP
(cAMP) second messenger
system into motion causing
Na+ and Ca2+ channels to
open (note Ca2+ channels
not shown)
• Influx of Na+ causes
depolarization and impulse
transmission
OLFACTORY TRANSDUCTION
PROCESS
Olfactory Pathway
• Axons of the olfactory
receptors that constitute
the olfactory nerves that
synapse in the overlying
olfactory bulbs (distal
ends of the olfactory
tracts)
Olfactory Pathway
•
Filaments of the olfactory nerves
synapse with second-order neurons
(mitral cells) in complex structures
called glomeruli
– Axons from neurons bearing the
same kind of receptor converge
on a given type of glomerulus
– Each glomerulus receives only
one type of odor signal
– Mitral cells: Refine, amplify, relay
signal
• Olfactory bulbs also
contain granule cells that
inhibit mitral cells
contributing to olfactory
adaptation
– Subconsciously turn
off smell (smoke,
cabbage, etc.)
Olfactory Pathway
•
Impulses flow from the olfactory bulbs
via olfactory tracts (composed mainly
of mitral cell axons) to two destinations
–
1.via thalamus to the piriform lobe of
the olfactory cortex and part of the
frontal lobe just above the orbit (smells
are consciously interpreted and
identified)
•
–
Each olfactory cortical neuron receives
input from up to 50 receptors to analyze
2.via the subcortical to the
hypothalamus, amygdala, and other
regions of the limbic system
•
Emotional response to odors (trigger
sympathetic system)
–
–
•
•
Danger: gas
Attractiveness: perfume
Appetizing odors: cause increase
salivation and stimulate digestive
tract
Unpleasant odors: trigger
protective reflexes such as
sneezing and coughing/choking
THE CHEMICAL SENSES:
TASTE AND SMELL
• Homeostatic Imbalances of the Chemical
Senses
– Anosmias (loss of smell) are olfactory disorders
resulting from head injuries that tear the olfactory
nerves, nasal cavity inflammation, or aging
• 1/3 of chemical sense loss is due to zinc deficiency
– Zinc is a growth factor for the receptors of the chemical senses
– Uncinate seizures (brain disorders):
• Some temporal lobe epileptics (repetitive abnormal electrical
discharges within the brain)
• Olfactory hallucinations
• Psychological
• Irritation of olfactory pathways (surgery/trauma)
THE EYE AND VISION
• Vision is our dominant sense with 70% of
our body’s sensory receptors found in the
eye
• Nearly half of the cerebral cortex is
involved in some aspect of visual
processing
THE EYE AND VISION
• Accessory Structures of the Eye:
– Eyebrows are short, coarse hairs overlying the supraorbital
margins of the eye that shade the eyes and keep perspiration out
– Eyelids (palpebrae), eyelashes, and their associated glands
help to protect the eye from physical danger as well as from
drying out
– Conjunctiva is a transparent mucous membrane that lines the
eyelids and the whites of the eyes
• It produces a lubricating mucus that prevents the eye from
drying out
– The lacrimal apparatus consists of the lacrimal gland, which
secretes a dilute saline solution that cleanses and protects the
eye as it moistens it, and ducts that drain excess fluid into the
nasolacrimal duct
– The movement of each eyeball is controlled by six extrinsic
eye muscles that are innervated by the abducens and
trochlear nerves
EYE
EYEBROWS
• Short, coarse hairs that overlie
the supraorbital margins of the
skull
• Shade the eyes from sunlight
• Prevent perspiration trickling
down the forehead from
reaching the eyes
• Orbicularis oculi muscle
depresses the eyebrow
when contracted
• Corrugator muscle contraction
moves eyebrow medially
EYELIDS
• Called palpebrae
• Separated by palpebral
fissure (eyelid slit)
– Meet at the medial and
lateral commissures
(canthi)
• Medial canthus is the
location of the lacrimal
caruncle:
– Contains sebaceous and
sweat glands
– Produces the whitish,
oily secretion
(sandman’s eye-sand)
EYELIDS
• Thin, skin-covered folds
supported internally by
connective tissue sheets
called tarsal plates
– Anchor muscles
• Upper lid more mobile
• Eyelid muscles
activated reflexively to
cause blinking every 37 seconds:
– Prevents drying
– Spreads accessory
structure secretions (oil,
mucus, and saline solution)
across eyeball surface
EYELIDS
•
•
•
•
•
Follicles of the eyelash hairs are richly
innervated by nerve endings (hair follicle
receptors)
– Anything that touches the eyelashes (even
puff of air) triggers reflex blinking
Tarsal glands (Meibomian)
– Embedded in the tarsal plates
– Ducts open at the eyelid edge just posterior to
the eyelashes
– Modified sebaceous glands
– Produce oily secretion that lubricates the
eyelid and eye
• Prevents eyelids from sticking together
– Infection results in cyst (chalazion)
• Closed sac or pouch containing fluid
and solid material resulting from
clogged duct
Associated with the eyelash follicles are typical
sebaceous glands
Between the hair follicles are modified sweat
glands called ciliary glands
Inflammation of sebaceous or ciliary glands
results in a sty
– Purulent fluid (pus)
CONJUNCTIVA
•
•
•
•
•
•
Transparent membrane
Produces lubricating mucus that prevents the
eyes from drying
Lines the eyelids as the palpebral conjunctiva and
reflects (folds back) over the anterior surface of the
eyeball as the ocular (bulbar) conjunctiva
Ocular (bulbar) conjunctiva:
– Covers only the white part of the eye not the
cornea (clear window over iris and pupil)
– Very thin
– Blood vessels are clearly visible beneath it
• More visible in irritated “bloodshot” eyes
Conjunctival sac:
– Space between conjunctiva-covered eyeball
and eyelids
– Where contact lens lies
Inflammation called: conjunctivitis
– Red, irritated eyes
– Pinkeye: conjunctival infection caused by
bacteria or viruses
• Highly contagious
EYE
Lacrimal Apparatus
•
•
•
Consists of the lacrimal gland and the
ducts that drain excess lacrimal
secretions into the nasal cavity
Lacrimal gland lies in the orbit above the
lateral end of the eye and is visible through
the conjunctiva when the lid is everted (turn
inside out)
Releases a dilute saline solution called
lacrimal secretion (tears) into the
superior part of the conjunctival sac
through several small excretory ducts
– Blinking spreads the tears downward
and across the eyeball to the medial
commissure
– Tears enter the paired lacrimal
canaliculi (canals) via two tiny openings
called lacrimal puncta (visible as tiny
red dots on the medial margin of each
eyelid
– From the canals the tears drain into
the lacrimal sac and then into the
nasal cavity at the inferior nasal
meatus
– Can fill nasal cavity causing
congestion (sniffles)
Lacrimal Apparatus
•
•
•
•
Lacrimal fluid contains mucus,
antibodies, and lysozyme (enzyme
that destroys bacteria)
– Cleanses and protects the eye
surface as it moistens and
lubricates
Enhanced tearing serves to
wash away or dilute irritating
substances
Importance of emotionally
induced tears is poorly
understood
Nasal inflammation can
constrict the nasolacrimal duct
preventing tears from draining
from the eye surface causing
“watery” eyes
EYE MUSCLES
EYE MUSCLES
• Movement of eyeball
is controlled by six
straplike extrinsic
muscles which
originate from the
bony orbit and insert
into the outer surface
of the eyeball
• Help maintain the
shape of the eyeball
and hold it in the orbit
EYE MUSCLES
HOMEOSTATIC IMBALANCE OF THE EYE
MUSCLES
• Diplopia: movements of the external muscles of
the two eyes are not perfectly coordinated
– Two images instead of one (double vision)
– Paralysis, muscle weakness, alcohol
• Strabismus: cross eyed
– Might be due to congenital (present at birth)
weakness of the external muscles
– Condition in which the eyes rotate either medially or
laterally
– To compensate the eyes may alternate in focusing or
only the controllable eye is used (brain disregards
inputs from the deviant eye, which then becomes
functionally blind)
THE EYE AND VISION
• Structure of the Eyeball
– Three tunics form the wall of the eyeball
• The fibrous tunic is the outermost coat of the eye and is made of a dense
avascular connective tissue with two regions: the sclera and the cornea
• The vascular tunic (uvea) is the middle layer and has three regions: the
choroid, the ciliary body, and the iris
• The sensory tunic (retina) is the innermost layer made up of two layers:
the outer pigmented layer absorbs light; the inner neural layer contains
millions of photoreceptors (rods and cones) that transduce light energy
– Internal Chambers and Fluids
• Posterior segment (cavity) is filled with a clear gel called vitreous humor
that transmits light, supports the posterior surface of the lens, holds the
retina firmly against the pigmented layer, and contributes to intraocular
pressure
• Anterior segment (cavity) is filled with aqueous humor that supplies
nutrients and oxygen to the lens and cornea while carrying away wastes
– The lens is an avascular, biconcave, transparent, flexible structure that
can change shape to allow precise focusing of light on the retina
INTERNAL EYE STRUCTURES
FIBROUS TUNIC
• The fibrous tunic is the
outermost coat of the eye
and is made of a dense
avascular connective tissue
with two regions: the sclera
and the cornea
• Sclera:
– Forming the posterior portion
and the bulk of the fibrous
tunic (glistening white and
opaque)
– Protects and shapes eyeball
– Provides sturdy anchoring site
for extrinsic muscles
– Posteriorly , where the sclera
is pierced by the optic nerve, it
is continuous with the dura
mater of the brain
FIBROUS TUNIC
•
Cornea: anterior portion
– Crystal clear
– Light enters the eye
– Light bending apparatus
– Covered by epithelial sheets on both
faces
– Outer epithelial cells (merge with the
ocular conjunctiva at the sclera-cornea
junction) continuously renew the
cornea
– Deep epithelial cells have active Na
pumps that maintain the clarity by
keeping the water content low
– Well supplied with nerve pain receptors
(problem with contacts)
– Capacity for regeneration and repair is
extraordinary
– Only tissue in the body that can be
transplanted with little or no possibility
of rejection (no blood vessel-beyond
the reach of the immune system)
VASCULAR TUNIC (UVEA)
• The vascular tunic (uvea) is
the middle layer and has
three regions: the choroid,
the ciliary body, and the iris
• Choroid:
– Highly vascular (blood vessels
supply nutrients to the eye)
– Dark brown (melanocytes)
membrane that forms the
posterior 4/5 of the uvea
• Helps absorb light, preventing
it from scattering and
reflecting within
VASCULAR TUNIC (UVEA)
• Ciliary body:
– Anterior thicken ring of tissue
that encircles the lens
– Consist mainly of smooth
muscle bundles called ciliary
muscles which control lens
shape
– Posterior surface of the ciliary
body forms the ciliary process
which contains the capillaries
that secrete the fluid that fills
the cavity of the anterior
segment of the eyeball
– Suspensory ligament (zonule)
• Extends from the ciliary
process to the lens
• Holds the lens in its upright
position
VASCULAR TUNIC (UVEA)
•
Iris:
– Visible colored part of the eye
– Most anterior portion of the uvea
– Lies between the cornea and lens
– Continuous with the ciliary body
posteriorly
– Its round opening, the pupil,
allows light to enter
– Different colors but only brown
pigment:
• Large amount of brown
pigment: brown
• Varying amounts: shorter
wavelengths of light are
scattered from the
unpigmented parts, and the
eyes appear blue, green, or
gray
PUPIL
PUPIL SIZE
• IRIS:
– Made of two smooth
muscle layers with bundles
of sticky elastic fibers that
congeal into a random
pattern before birth
• These muscle fibers allow
it to vary pupil size
• Pupil dilation is controlled
by sympathetic fibers
• Pupil constriction is
controlled by
parasympathetic fibers
SENSORY TUNIC (RETINA)
• The sensory tunic
(retina) is the innermost
layer made up of two
layers:
– Outer pigmented layer
absorbs light and prevents
it from scattering in the eye
• Also acts a phagocytes
and stores vitamin A
needed by the
photoreceptor cells
– Inner neural layer
contains millions of
photoreceptors (rods and
cones) that transduce light
energy
RETINA
RETINA
• Neural layer: three
types of neurons
– 1.photoreceptors: rods
and cones
– 2.bipolar cells
– 3.ganglion cells
RETINA
• Photoreceptors:
– Rods:
• Dim light and peripheral
vision receptors
• Do not provide either sharp
images or color vision
• Retinal periphery: only rods
– Cones:
• Operate in bright light
• Provide high-acuity color
vision
• Fovea centralis region (center
of the macula lutea): all cones
• Toward the retina periphery
cone density decreases
• Anything we wish to view
critically is focused on the
fovea centralis (center of the
macula lutea)
MICROSCOPIC ANATOMY OF
THE RETINA
•
•
•
•
•
•
Light (yellow arrow) passes
through the retina to excite the
photoreceptors
Flow of electrical signals occurs in
the opposite direction
Local currents are produced in
response to light and spread from
the photoreceptors to the bipolar
neurons and then to the innermost
ganglion cells, where action
potential is generated
Ganglion cell axons leave the
posterior aspect of the eye as the
thick optic nerve
Horizontal and amacrine cells play
a role in visual processing
Optic nerve exits the eye at the
optic disc (blind spot)
POSTERIOR WALL (FUNDUS) OF
RETINA
POSTERIOR WALL (FUNDUS) OF
RETINA
• Part of the posterior
wall (fundus) of the
eye as seen with an
ophthalmoscope
• Note the optic disc
from which the
retinal blood
vessels radiate
• Note: Macula which
is the area of most
acute vision
HOMEOSTATIC IMBALANCE OF
RETINA
• Retinal Detachment:
– Pigmented and nervous layers separate
and allow the jellylike vitreous humor to
seep between them
• Can cause permanent blindness because it
derives the neural retina of its nutrient source
– Head trauma or jerking of the head
(auto/bungee jumping)
– Wet/Dry variations
LENS
LENS
• Biconvex, transparent,
flexible structure that
can change shape to
allow precise focusing of
light on the retina
• Enclosed in a thin, elastic
capsule and held in place
just posterior to the iris by
the suspensory ligament
• Avascular (blood
vessels interfere with
transparency)
LENS
•
Two regions:
– Lens epithelium:
• Confined to the anterior lens
surface
– Lens fiber:
•
•
•
•
•
Formed from the lens epithelium
Bulk of lens
No nuclei
No organelles
Contain transparent protein called
crystallins
– Converts sugar into energy for
use by the lens
• Since new fibers are continually
added, the lens enlarges
throughout life, becoming
denser, more convex, and less
elastic. All of which gradually
impair its ability to focus light
properly
CATARACT
CATARACT
• Clouding of the lens
• Most result from age-related
hardening and thickening of
the lens or as a secondary
consequences of diabetes
mellitus
• Heavy smoking and frequent
exposure to intense sunlight
• Whatever the promoting
factors, the direct cause
seems to be inadequate
delivery of nutrients to the
deeper lens fibers resulting
in clumping of the crystallin
proteins
INTERNAL CHAMBERS AND
FLUIDS
• Internal Chambers and Fluids:
– Posterior segment (cavity) is filled with a clear
gel called vitreous humor that transmits light,
supports the posterior surface of the lens,
holds the retina firmly against the pigmented
layer, and contributes to intraocular pressure
– Anterior segment (cavity) is filled with
aqueous humor that supplies nutrients and
oxygen to the lens and cornea while carrying
away wastes
AQUEOUS HUMOR
•
•
Anterior segment is partially subdivided
by the iris into the anterior chamber
(between the cornea and the iris) and the
posterior chamber (between the iris and
the lens)
The entire anterior segment is filled with
aqueous humor, a clear fluid similar in
composition to blood plasma
– Forms and drains continually and is in
constant motion
– Filters from the capillaries of the ciliary
processes into the posterior chamber
and freely diffuses
– Drains into the venous blood via the
scleral venous sinus (canal of
Schlemm)
– Normally produced and drained at the
same rate, maintaining a constant
intraocular pressure of about 16 mm
Hg, which helps to support the eyeball
internally
– Supplies nutrients and oxygen to the
lens and cornea and to some cells of
the retina
– Carries away metabolic wastes
VITREOUS HUMOR
• Posterior segment
• Clear gel that binds a
tremendous amount of water
• Transmits light
• Supports the posterior surface
of the lens and holds the
neural retina firmly against the
pigmented layer
• Contributes to intraocular
pressure, helping to
counteract the pulling force of
the extrinsic eye muscles
• Forms in the embryo and
lasts a lifetime
HOMEOSTATIC IMBALANCE OF
HUMORS
• Glaucoma
– Drainage of aqueous humor blocked
– Fluid backs up
– Pressure in the eye builds up compressing
the retina and optic nerve
– Result is blindness unless the condition is
detected early
– Damage can be done before you realize it
– Late signs include seeing halos around lights
and blurred vision
WAVELENGTH AND COLOR
• Electromagnetic radiation
includes all energy waves,
from long radio (meters) to
very short gamma waves
(nanometers)
• 1 nm = 10-9 m
• Our eyes respond to the part of
the spectrum called visible
light (wavelength range of 400700 nm)
• Light is packets of energy
(photons) traveling in a
wavelike fashion at very high
speed (186,00 miles per
second; 300,000 km/s
VISIBLE SPECTRUM
• Red wavelengths are the
longest and have the lowest
energy
• Violet wavelengths are the
shortest and most energetic
• Objects have color because
they absorb some wavelengths
and reflect others
• White reflects all wavelengths
• Black absorbs all wavelengths
• Red apple reflects red
• Green grass reflects green
ELECTROMAGNETIC
SPECTRUM
REFLECTION
• Light travels in straight lines
– Easily blocked by any nontransparent object
– Like sound, light can reflect, or bounce, off a
surface
– Reflection of light by objects in our
environment accounts for most of the light
reaching our eyes
REFRACTION
• Speed of light traveling in a given medium is
constant
– Passing from one transparent medium into another
with a different density changes its speed
• Speeds up as it passes into a less dense medium
• Slows down as it passes into a denser medium
– Because of these changes in speed, bending or
refraction of a light ray occurs when it meets the
surface of a different medium at an oblique angle
rather than at a right angle (perpendicular)
• The greater the incident angle, the greater the amount of
bending
REFRACTION OF LIGHT
REFRACTION
• Image demonstrates
the consequence of
light refraction when
a spoon is placed in a
half-full glass of water
• Spoon appears to
break at the air-water
interface
LENS
• Transparent object curved on one or both
surfaces
• Light hitting the curve at an angle is
refracted
CONVEX LENS
• Lens that is thickest in the
center (convex)
• Light rays are bent so that they
converge (come together) or
intersect at a single point
called the focal point
• The thicker (more convex) the
lens, the more the light is bent
and the shorter the focal
distance (distance between the
lens and focal point)
• Image formed by a convex
lens is called a real image
– Upside down and reversed
from left to right
CONCAVE LENS
• Thicker at the edges than at the center
• Magnifying glasses
• Diverge the light (bend it outward) so that
the light rays move away from each other
• Prevents light from focusing and
extends the focal distance
Focusing of Light on the Retina
•
•
As light passes from air into the
eye, it moves sequentially
through the cornea, aqueous
humor, lens, and vitreous
humor, and then passes
through the entire thickness of
the neural layer of the retina to
excite the photoreceptors that
abut (border) the pigmented
layer
Light is bent three times: as it
enters the cornea and on entering
and leaving the lens
– The refractory power of the
humors and cornea is constant
– The lens is highly elastic, and its
curvature and light-bending power
can be actively changed to allow
fine focusing of the image
Focusing for Distant Vision
• Aim both eyes so that they
are both fixated on the same
spot
• Our eyes are best adapted
for distant vision
• The far point of vision is that
distance beyond which no
change in lens shape is
required for the normal
(emmetropic) eye
– About 6m or 20 ft
• Ciliary muscles are
completely relaxed, and the
lens (stretched flat by
tension in the suspensory
ligaments) is as thin as it
gets
Focusing for Distant Vision
• (a) Light from distant objects (over 6m away)
approaches as parallel rays and, in the normal
eye, need not be adjusted for proper focusing on
the retina. Image is a real image (inverted and
reversed from left to right)
Focusing for Close Vision
• Light from objects less
than 6m away diverges
as it approaches the eye
and it comes to a focal
point farther from the lens
• Focusing for close vision
demands that the eye
make three adjustments:
accommodation of the
lens, constriction of the
pupils, and convergence
of the eyeballs
Focusing for Close Vision
• (b) Light from close objects (less than 6m away)
tends to diverge and lens convexity must be
increased (accommodation) for proper focusing.
Image is a real image (inverted and reversed
from left to right)
FOCUSING
Focusing for Close Vision
• Lights from objects less than
6m away diverges as it
approaches the eyes and it
comes to a focal point farther
from the lens
• Close vision demands that the
eye make adjustments
• To restore focus, three
processes– accommodation of
the lenses, constriction of the
pupils, and convergence of the
eyeballs– must occur
simultaneously
Focusing for Close Vision
Accommodation of the lenses
• Is a process that
increases the
refractory power of
the lens
• Near point is 10 cm (4
inches) from the eye
• The gradual loss of
accommodation
with age reflects the
lens’s decreasing
elasticity
Focusing for Close Vision
Constriction of the Pupils
• Circular (constrictor)
muscles of the iris reduce
the size of the pupil
• Accommodation papillary
reflex, mediated by
parasympathetic fibers of the
oculomotor nerves, prevents
the most divergent light rays
from entering the eye
– Such rays would pass through
the extreme edge of the lens
and would not be focused
properly (blurred vision)
Focusing for Close Vision
Convergence of the Eyeballs
• The visual goal is always to
keep the object being viewed
focused on the retinal fovea
• When we fixate on a close
object our eyes converge
• Controlled by somatic motor
fibers of the oculomotor nerves
• Closer the object, the greater
the degree of convergence
required
– When you focus on the tip
of your nose, you go crosseyed
Focusing for Close Vision
• Reading or other close work requires
almost continuous accommodation,
papillary constriction, and convergence
• Prolonged periods of reading tire the
eye muscles and can result in eyestrain
– Helpful to look up and stare into the
distance occasionally to relax the intrinsic
muscles
Homeostatic Imbalances of Refraction
• Visual problems related to refraction can
result from a hyperrefractive
(overconverging) or hyporefractive
(underconverging) lens or from structural
abnormalities of the eyeball
Problems of Refraction
• (a) In the emmetropic (normal) eye, light from
both near and distant objects is focused properly
on the retina
Problems of Refraction
• (b) In a myopic eye, light from a distant object
comes to a focal point before reaching the retina
and then diverges again
Homeostatic Imbalances of Refraction
•
•
•
Myopia, or nearsightedness,
occurs when objects focus in front
of the retina and results in seeing
close objects without a problem
but distance objects are blurred
Eyeball too long
Correction has traditionally
involved use of concave lenses
that diverge the light before it
enters the eye, but procedures to
flatten the cornea slightly—a
painless 10-minute surgery called
radial keratotomy, or PRK and
LASIK procedures using a laser—
have offered other treatment
options
Problems of Refraction
• (c) In the hyperopic eye, light from a near
object comes to a focal point behind (past) the
retina. Refractory effect of the cornea is ignored
Homeostatic Imbalances of Refraction
• Hyperopia or farsightedness
occurs when objects are
focused behind the retina and
results in seeing distance
objects clearly but close
objects are blurred
• Eyeball is too short or a lens
with poor refractory power
(lazy lens)
• Convex corrective lenses
are needed to converge the
light more strongly for close
vision
Homeostatic Imbalances of Refraction
• Astigmatism:
– Unequal curvatures in different parts of the
lens (or cornea) lead to blurry images
– Special cylindrically ground lenses and laser
procedures are used to correct this problem
PROBLEMS OF REFRACTION
THE EYE AND VISION
• Physiology of Vision:
– Photoreception is the process by which the eye detects
light energy
• Photoreceptors are modified neurons that structurally resemble tall
epithelial cells
• Rods are highly sensitive and are best suited to night vision
• Cones are less sensitive to light and are best adapted to bright
light and colored vision
• Photoreceptors contain a light-absorbing molecule called
retinal
– Stimulation of the Photoreceptors
• The visual pigment of rods is rhodopsin and is formed and
broken down within the rods
• The breakdown and regeneration of the visual pigments of the
cones is essentially the same as for rhodopsin
Functional Anatomy of the Photoreceptors
• Photoreceptors are
modified neurons
• Structurally they
resemble tall epithelial
cells turned upside down
with their tips immersed
in the pigmented layer of
the retina
• Named according to the
shape of the outer
segment: rod/cone
shape
Functional Anatomy of the Photoreceptors
•
Outer segments contain an
elaborate array of visual
pigments (photopigments) that
change shape as they absorb
light:
– Pigments embedded in areas of
the plasma membrane that forms
discs
• In rods, discs are discontinuous—
stacked like a row of pennies in a
coin wrapper
• In cones, disc membranes are
continuous with the plasma
membrane; thus, the interiors of
the cone discs are continuous with
the extracellular space
– Tips of the outer segments of
rods and cones are removed
(phagocytized by cells of the
pigmented layer) and renewed
daily
Functional Anatomy of the Photoreceptors
• Rods:
– Very sensitive (respond to
very dim light)
– Best suited for night vision
and peripheral vision
– Absorb all wavelengths of
visible light, but their inputs
are perceived only in gray
tones
• Cones:
– Need bright light for activation
(have low sensitivity)
– Have pigments that furnish
a vividly colored view of the
world
PHOTORECEPTORS
Rods and Cones
• Wired differently to other
retinal neurons
• Rods participate in
converging pathways
– Effects are summated and
considered collectively
– Vision fuzzy and indistinct
• Cones have a straightthrough pathway via their
own personal bipolar cell to
a ganglion
– Has its own line to the higher
visual centers
– Accounts for the detailed,
high-resolution
Rods and Cones
• Because rods are absent from the
foveae and cones do not respond to
low-intensity light, we see dimly lit
objects best when we do not look at
them directly, and recognize them best
when they move
– Moonlit evening
Chemistry of Visual Pigments
• Light-absorbing molecule called retinal
combines with proteins called opsins to form
four types of visual pigments:
– Depending on the type of opsin to which it is
bound, retinal preferentially absorbs different
wavelengths of the visible spectrum
– Retinal is chemically related to vitamin A and is
made from it
– Liver stores vitamin A and releases it as needed by
the photoreceptors to make visual pigments
– Cells of the pigmented layer of the retina absorb
vitamin A from the blood and serve as the local
vitamin A depot for the rods and cones
RETINAL ISOMERS IN
PHOTORECEPTION
Chemistry of Visual Pigments
•
•
Retinal can assume a variety of
distinct three-dimensional
forms (isomers)
Bound to opsin, retinal has a
bent shape called the 11-cis
isomer
– When the pigment is struck by
light and absorbs photons, retinal
twists and snaps into a new
configuration (all-trans
isomer/pigment is bleached)
which causes retinal to detach
from opsin
– This is the only light-dependent
stage, and this simple
photochemical event initiates a
whole chain of chemical and
electrical reactions in rods and
cones causing electrical impulses
to be transmitted along the optic
nerve
Stimulation of the Photoreceptors
• Excitation of rods:
– The visual pigment of
rods is a deep purple
pigment called rhodopsin
– Rhodopsin molecules are
arranged in a single layer
in the membranes of each
of the thousands of discs in
the rods’ outer segments
– Although rhodopsin
absorbs light throughout
the entire visible
spectrum, it maximally
absorbs green light
Stimulation of the Photoreceptors
•
•
•
•
•
(b): shows a small segment of the
membrane of a visual pigmentcontaining disc of the outer
segment of a rod cell
The visual pigments consist of a
light-absorbing molecule called
retinal bound to an opsin protein
Each type of photoreceptor has a
characteristic kind of opsin
protein, which affects the
absorption spectrum of the retinal
In rods, the pigment-opsin
complex is called rhodopsin
Notice that the light-absorbing
retinal occupies the core of the
rhodopsin molecule
Stimulation of the Photoreceptors
•
Excitation of Rods:
– Rhodopsin forms and
accumulates in the dark
– Vitamin A is oxidized to the 11-cis
retinal form and then combined
with opsin to form rhodopsin
– Rhodopsin absorbs light and
changes to all-trans retinal isomer
– Retinal-opsin combination breaks
down
• Retinal and opsin separate
(bleaching)
– All-trans retinal reconverted with
enzymes (aid of light) to 11-cis
retinal isomer
– Rhodospin is regenerated when
11-cis retinal is rejoined to
opsin
RHODOPSIN
Stimulation of the Photoreceptors
• Excitation of Cones
– Essentially the same as for rhodopsin but the threshold for
cone activation is much higher than that for rods because
cones respond only to high-intensity (bright) light
– Visual pigments of the three types of cones (like rods) are a
combination of retinal and opsins
• However, the cone opsins differ from rods and from one another
• The naming of cones reflects the colors (wavelengths) they
absorb (e.g.: blue cones, red cones, etc)
– Absorption spectra overlaps with perception of intermediates
hues (orange, yellow, purple, etc)
» Yellow light stimulates both red and green cones
» If red is stimulated more than green, we see orange
» When all cones are stimulated equally, we see white
HOMEOSTATIC IMBALANCE
• Color blindness is due to a congenital
lack of one or more of the cone types
– Sex linked condition
– Most common red-green color blindness
PHOTOTRANSDUCTION
Light Transduction in Photoreceptors
• Light and dark Adaptation
– Rhodopsin is amazingly sensitive to light (even
starlight causes some of the molecules to be
bleached)
– As long as the light is low intensity, little rhodopsin is
bleached and the retina responds to light stimuli
– In high intensity light there is bleaching of the
pigment and rhodopsin breaks down as rapidly as
it is made
• Rods become nonfunctional and cones begin to respond
THE EYE AND VISION
• Physiology of Vision
– Light adaptation occurs when we move
from darkness into bright light
• Retinal sensitivity decreases dramatically and the
retinal neurons switch from the rod to the cone
system
– Dark adaptation occurs when we go from a
well-lit area into a dark one
• The cones stop functioning and the rhodopsin
starts to accumulate in the rods increasing retinal
sensitivity
HOMEOSTATIC IMBALANCE
• Night blindness (nyctalopia)
– Condition in which rod function is
seriously hampered
– Most common cause is Vitamin A
deficiency which leads to rod degeneration
THE EYE AND VISION
– Visual processing occurs when the action of
light on photoreceptors hyperpolarizes them,
which causes the bipolar neurons from both
the rods and cones to ultimately send signals
to their ganglion cells
VISUAL FIELDS
• Physiology of Vision
– Visual Pathway to the
Brain
• The retinal ganglion
cells merge in the back
of the eyeball to
become the optic nerve
VISUAL FIELDS
• At the X-shaped optic
chiasma, fibers from
the medial aspect of
each eye cross over to
the opposite side and
continue on via the
optic tracts:
– Thus, each optic tract:
• Contains fibers from the
lateral (temporal) aspect
of the eye on the same
side and fibers from the
medial (nasal) aspect of
the opposite eye
• Carries all the information
from the same half of the
visual field
VISUAL FIELDS
• Also notice that, the lens
system of each eye reverses
all images:
– The medial half of each retina
receives light rays from the
temporal (lateral-most) part of
the visual field (that is, from
the far left or far right rather
than from straight ahead)
– The lateral half of each retina
receives an image of the nasal
(central) part of the visual field
– Consequently, the left optic
tract carries (and sends on) a
complete representation of the
right half of the visual field,
and the opposite is true for the
right optic tract
VISUAL FIELDS
•
•
The paired optic tracts send their
axons to neurons within the lateral
geniculate body of the thalamus,
which maintains the fiber
separation established at the
chiasma, but balances and
combines the retinal input for
delivery to the visual cortex
Axons from the thalamus project
through the internal capsule to
form the optic radiation of fibers in
the cerebral white matter
– These fibers project to the primary
visual cortex in the occipital lobes,
where conscious perception of
visual images (seeing) occurs
VISUAL FIELDS
• Notice that although
both eyes are set
anteriorly and look in
approximately the same
direction, their visual
fields, each about 170
degrees, overlap to a
considerable extent,
and each eye sees a
slightly different view
• Crossing over is not
complete
VISUAL FIELDS
• Cortical “fusion” of the slightly different images delivered
by the two eyes provides us with depth perception, an
accurate means of locating objects in space (threedimensional vision)
• Many animals (pigeons, rabbits, etc.) have
panoramic vision. There eyes are placed more laterally
on the head, so that the visual fields overlap verylittle,
and crossover of the optic nerve fibers is almost
complete
– Consequently, each visual cortex receives input principally
from a single eye and a totally different visual field
HOMEOSTATIC IMBALANCE
• Loss of left eye:
– Nothing would be
seen in the visual
field area colored
yellow
– Loss of true depth
perception
– Peripheral vision is
lost on the side of
damage
HOMEOSTATIC IMBALANCE
• If neural destruction
occurs beyond the
optic chiasma—in
an optic tract, the
thalamus, or visual
cortex — then part
or all of the opposite
half of the visual
field is lost
HOMEOSTATIC IMBALANCE
• A stroke affecting the
left visual cortex leads
to blindness in the right
half of the visual field
– But, since the right
(undamaged) visual
cortex still receives
inputs from both eyes,
depth perception in the
remaining half of the
visual field is retained
OPTIC NERVE
VISUAL FIELDS
RESPONSES OF RETINAL
GANGLION
THE EAR:
HEARING AND BALANCE
Outer (External) Ear
• The outer (external)
ear consists of the
auricle (pinna):
– Composed of elastic
cartilage covered with
thin skin and an
occasional hair
– Function is to direct
sound waves into the
external auditory canal
Outer (External) Ear
• The external auditory canal
(meatus) extends from the
auricle to the eardrum
• Passes through the temporal
bone
• Lined with skin bearing
hairs, sebaceous glands,
and modified sweat glands
called ceruminous glands:
– Secret yellow-brown waxy
cerumen (earwax) which
provides a sticky trap for
foreign bodies and repels
insects
Outer (External) Ear
• Tympanic Membrane
(eardrum):
– Boundary between the outer
and middle ears
– Thin, translucent,
connective tissue
membrane, covered by skin
on its external face and by a
mucosa internally
– Shapped like a flattened cone,
with its apex protruding
medially into the middle ear
– Sound waves make the
membrane vibrate:
• The eardrum, in turn,
transfers the sound energy
to the tiny bones of the
middle ear and sets them
into vibration
Middle Ear
•
•
The middle ear, or tympanic
cavity, is a small, air-filled,
mucosa-lined cavity in the petrous
portion of the temporal bone
Flanked laterally by the eardrum
and medially by a bony wall
with two openings:
– The superior oval (vestibular)
window
– The inferior round (cochlear)
window
•
Mastoid antrum (canal in the
posterior wall of the middle ear)
located in the roof (epitympanic
recess) of the tympanic cavity
allows it to communicate with
mastoid air cells housed in the
mastoid process
Middle Ear
•
Pharyngotympanic (auditory)
tube:
– Eustachian tube
– Links middle ear with the
nasopharynx
– The mucosa of the middle ear is
continuous with the lining of the
pharynx
– Normally flattened and closed:
• Swallowing or yawning opens it
briefly to equalize pressure in
the middle ear cavity with
external air pressure
• Important because the eardrum
vibrates freely only if the
pressure on both of its surfaces
is the same
– Otherwise, sounds are
distorted
• Ear-popping sensation of the
pressure equalizing is familiar
to anyone
Middle Ear
•
•
It is spanned by the auditory
ossicles (three smallest bones in
the body)
Named for their shape:
–
–
–
•
•
•
•
Malleus: hammer
Incus: anvil
Stapes: stirrup
Handle of the malleus is secured to
the eardrum, and the base of the
stapes fits into the oval window
Tiny ligaments suspend the
ossicles, and mini-synovial joints
link them together
Incus articulates with the malleus
laterally and the stapes medially
Ossicles transmit the vibratory
motion of the eardrum to the oval
window, which in turn sets the
fluids of the inner ear into motion,
eventually exciting the hearing
receptors
Middle Ear
• Tiny skeletal
muscles attached to
the ossicles prevent
damage due to loud
sounds by
restricting their
movements thus
limiting the movement
of the stapes in the
oval window
HOMEOSTATIC IMBALANCE
• Otitis media
–
–
–
–
Middle ear inflammation
Eardrum bulges and becomes inflamed and red
Most cases treated with antibiotics
If large amounts of pus or fluids accumulate a
myringotomy (lancing of the eardrum) may be
required to equalize the pressure
• Tiny tube implanted in the eardrum permits pus to drain into
the external ear
• Tube falls out into the external ear in time
– Non-infectious causes: food allergies (typically
milk or wheat)
Inner (Internal) Ear
• Lies deep in the
temporal bone behind
the eye socket
• The inner (internal)
ear has two major
divisions: the bony
labyrinth and the
membranous
labyrinth
Inner (Internal) Ear
• Bony (osseous)
labyrinth:
– System of twisting
channels through the
bone
– Three regions:
• Vestibule
• Cochlea
• Semicircular canals
Inner (Internal) Ear
• Bony labyrinth is
filled with perilymph
– Fluid similar to
cerebrospinal fluid and
continuous with it
Inner (Internal) Ear
•
•
•
•
Membranous labyrinth is a
continuous series of
membranous sacs and ducts
contained within the bony
labyrinth following its contour
Surrounded by and floats in the
perilymph
Interior contains endolymph which
is chemically similar to K+ rich
intracellular fluid
These two fluids (perilymph and
endolymph) conduct the sound
vibrations involved in hearing
and respond to the mechanical
forces occurring during
changes in body position and
acceleration
Inner (Internal) Ear
• Bony labyrinth:
– The vestibule is the central
cavity of the bony labyrinth
with two membranous sacs
suspended in the perilymph,
the saccule and the utricle
– The semicircular canals
project from the posterior
aspect of the vestibule, each
containing an equilibrium
receptor region called a crista
ampullaris
– The spiral, snail-shaped
cochlea extends from the
anterior part of the vestibule
and contains the cochlear
duct, which houses the spiral
organ of Corti, the receptors
for hearing
Inner (Internal) Ear
• Vestibule:
– The vestibule is the
central egg-shaped
cavity of the bony
labyrinth
– In its lateral wall is the
oval window
Inner (Internal) Ear
•
Vestibule:
– Suspended in its perilymph and
united by a small duct are two
membranous labyrinth sacs:
• Saccule:
– Small sac
– Continuous with the
membranous labyrinth
extending anteriorly into
the cochlea (cochlear
duct)
• Utricle: continuous with the
semicircular ducts extending
into the semicircular canals
posteriorly
– The saccule and utricle house
equilibrium receptor regions
called maculae that respond to
the pull of gravity and report on
changes of head position
Inner (Internal) Ear
• Semicircular Canals:
– The semicircular canals lie
posterior and lateral to the
vestibule and each of these
canals defines about 2/3 of
a circle
– Cavities of the bony
semicircular canals project
from the posterior aspect of
the vestibule, each oriented
in one of the three planes
of space
– There is an anterior,
posterior, and lateral
semicircular canal in each
inner ear
Inner (Internal) Ear
•
Semicircular Canals:
– The anterior and posterior canals
are oriented at right angles to
each other in the vertical plane,
whereas the lateral canal lies
horizontally
– Snaking through each
semicircular canal is a
corresponding membranous
semicircular duct, which
communicates with the utricle
anteriorly
• Each of these ducts has an
enlarged swelling at one end
called an ampulla, which houses
an equilibrium receptor region
called a crista ampullaris
– These receptors respond to
angular (rotational)
movements of the head
Inner (Internal) Ear
• Cochlea:
– Latin for snail
– Spiral, conical, bony
chamber about the
size of a pea
Inner (Internal) Ear
• Cochlea:
– Running through its
center like a wedgeshaped worm is the
membranous cochlear
duct, which ends
blindly at the cochlear
apex
– Cochlear duct
houses the spiral
organ of Corti, the
receptor for hearing
Inner (Internal) Ear
• Cochlea:
– Cochlear duct houses the
spiral organ of Corti, the
receptor for hearing
– Divided into 3 chambers
(scalas
• Scala vestibuli:
– Contained perilymph
• Scala media (cochlear
duct):
– Contained endolymph
• Scala tympani:
– Contained perilymph
Inner (Internal) Ear
• Cochlea:
– Floor of the cochlear duct
(Scala media) is composed
of the bony spiral lamina
and the flexible fibrous
basilar membrane, which
supports the organ of Corti
– Cochlear nerve, division of
the vestibulocochlear
(Cranial Nerve VIII) nerve,
runs from the organ of Corti
through the modiolus on its
way to the brain
Inner (Internal) Ear
• (c): detailed structure of the spiral organ of Corti
COCHLEA
• (d): Electron
micrograph of
cochlear hair cells
THE EAR:
HEARING AND BALANCE
• Physiology of Hearing
– Properties of Sound:
• Sound is a pressure disturbance produced by a vibrating
object and propagated by the molecules of the medium
• Frequency is the number of waves that pass a given point in
a given time
• Amplitude, or height, of the wave reveals a sound’s intensity
(loudness)
– Airborne sound entering the external auditory canal
strikes the tympanic membrane and sets it vibrating
– The resonance of the basilar membrane processes
sound signals mechanically before they ever reach
the receptors
HUMAN HEARING
• Sounds set up vibrations in air that
beat against the eardrum that pushes a
chain of tiny bones that press fluid in
the inner ear against membranes that
set up shearing forces on tiny hair cells
that stimulate nearby neurons that give
rise to impulses that travel to the brain,
which interprets them—and you hear
SOUND
• Light can be transmitted through a vacuum (outer
space)
• Sound depends on an elastic medium for
transmission
• Sound travels much more slowly than light:
– Sound travels in dry air: 0.2 miles/second (331 m/s)
– Light travels 186,000 miles/second (300,000 km/s
• A lightning flash is almost instantly visible, but the sound it
creates (thunder) reaches our ears much more slowly
– For each second between the lightning bolt and the roll of thunder, the
storm is 1/5 mile farther away
– Speed of sound is constant in a given medium:
• It is greatest in solids and lowest in gases, including air
SOUND
•
•
•
•
•
Sound is a pressure disturbance—
alternating areas of high and low
pressure—produced by a vibrating
object and propagated by the
molecules of the medium
Tuning fork struck on left
Prong will first move to the right,
creating an area of high pressure by
compressing the air molecules there
Then, as the prongs rebound to the
left, the air on the left will be
compressed, and the region on the
right will be rarefied, or low-pressure,
area since most of its air molecules
have been pushed farther to the right)
As the fork vibrates alternating from
right to left, it produces a series of
compressions and rarefactions,
collectively called a sound wave, which
moves outward in all directions (b)
SOUND
•
•
The individual air molecules
just vibrate back and forth for
short distances as they bump
other molecules and rebound
The outward-moving molecules
give up kinetic energy to the
molecules they bump, energy is
always transferred in the direction
the sound wave is traveling
– Thus, with time and distance, the
energuy of the wavw declines,
and the sound dies a natural
death
•
We can illustrate a sound wave as
an S-shaped curve, or sine wave,
in which the compressed areas
are crests and the rarefied areas
are troughs ©
SOUND
FREQUENCY
• Number of waves that
pass a given point in a
given time (waves/time)
• Sine wave of a pure tone
is periodic
– Crests and troughs repeat
at definite distances
– Distance between two
consecutive crests (or
troughs) is called the
wavelength
• Shorter the wavelength,
the higher the
frequency
FREQUENCY
•
Hertz: unit of frequency
–
–
–
•
Frequency range of human hearing is:
–
•
20 to 20,000 hertz (waves/sec)
We perceive different sound
frequencies as differences in pitch:
–
–
•
Cycle/sec
Waves/sec
Hz
The higher the frequency, the higher
the pitch
The lower the frequency, the lower the
pitch
Most sounds are mixtures of
several frequencies:
–
–
This characteristic of sound, called
quality enables us to distinguish
between the same musical note from
different sources (high C from singer,
piano, clarinet, etc)
Provides the richness and complexity
of sounds (music) we hear
AMPLITUDE
•
•
Height of the sine wave reveals a sound’s
intensity, which is related to it energy, or
the pressure differences between its
compressed and rarefied areas
Loudness refers to our subjective
interpretation of sound intensity
–
•
Pin drop to a steam whistle
Measures in logarithmic units called decibels
(dB)
–
–
Each 10-dB increase represents a tenfold
increase in sound intensity
A sound of 20 dB and 0 dB has 100 times
more energy but a 2-fold increase in loudness
•
•
•
Most people would say that a 20 dB sound
seems about twice as loud as a 10 dB sound
Threshold of pain is 130 dB
Severe hearing loss occurs with frequent
or prolonged exposure to sounds with
intensities greater than 90 dB
–
–
–
Normal conversation: 50 dB
Noisy restaurant: 70 dB
Amplified music: 120 dB or more
FREQUENCY/AMPLITUDE
Transmission of Sound to the Inner Ear
•
•
Hearing occurs when the auditory
area of the temporal lobe cortex is
stimulated but sound must be
propagated through air,
membranes, bones, and fluids to
reach and stimulate receptor cells
in the organ of Corti
Airborne sound entering the external
auditory canal strikes the tympanic
membrane and sets it vibrating at the
same frequency
–
•
Greater the frequency, the farther
the membrane is displaced in its
vibratory motion
Motion of the tympanic membrane
is amplified and transferred to the
oval window by the ossicle lever
system
–
Acts much like a hydraulic press or
piston to transfer the same total
force hitting the eardrum to the oval
window
Transmission of Sound to the Inner Ear
• Since the tympanic
membrane is 17 to 20 times
larger than the oval window,
the pressure (force per unit
area) actually exerted on the
oval window is about 20
times that on the tympanic
membrane
– Compared heel of a man’s
shoe and spiked heel of a
women’s shoe
• This increased pressure
overcomes the impedance of
cochlear fluid and sets it into
wave motion
SOUND WAVES
Resonance of the Basilar Membrane
• As the stapes rocks back and forth against the oval window, it sets
the perilymph in the scala vestibuli into a similar back-and-forth
motion, and a pressure wave travels through the perilymph from the
basal end toward the helicotrema (opening at the tip of the cochlear
canal where the scala tympani and scala vestibuli unite), much as a
piece of rope held horizontally can be set into wave motion by
movement initiated at one end
Resonance of the Basilar Membrane
• Sounds of very low
frequency (below 20 Hz)
create pressure waves
that take the complete
route through the
cochlea—up the scala
vestibuli, around the
helicotrema, and back
toward the round oval
window through the scala
tympani
– Such sounds do not
activate the organ of
Corti (below the
threshold of hearing)
Resonance of the Basilar Membrane
• Sounds of higher frequency (shorter
wavelengths) create pressure waves that take a
“shortcut” and are transmitted through the
cochlear duct into the perilymph of the scala
tympani
Resonance of the Basilar Membrane
• Fluids are
incompressible
• Water bed: sit on one
side, the other side
bulges
• Each time the fluid
adjacent to the oval
window is forced
medially by the stapes,
the membrane of the
round window bulges
laterally into the middle
ear cavity and acts as a
pressure valve
Resonance of the Basilar Membrane
• As a pressure wave descends through the
flexible cochlear duct, it sets the entire basilar
membrane into vibrations
Resonance of the Basilar Membrane
•
•
•
•
(b): Maximal displacement of the
membrane occurs where the fibers of
the basilar membranes are tuned to a
particular sound frequency
– This characteristic of many natural
substances is called resonance
Fibers near the oval window
(cochlear base) are short and stiff,
and they resonate in response to
high-frequency pressure waves
Longer, more floppy basilar
membrane fibers near the cochlear
apex resonate in time with lowerfrequency pressure waves
Thus, sound signals are
mechanically processed by the
resonance of the basilar membrane,
before ever reaching the receptors
Excitation of Hair Cells in the Organ of Corti
• The organ of Corti, which rests atop the basilar membrane, is
composed of supporting cells and hearing receptor cells called
cochlear hair cells
• Hair cells are arranged functionally:
– One row of inner hair cells and three rows of outer hair cells sandwiched
between the tectorial and basilar membranes
– Afferent fibers of the cochlear nerve (a division of the vestibulocochlear
nerve VIII) are coiled about the bases of the hair cells
Excitation of Hair Cells in the Organ of Corti
• Hairs (stereocilia) of the hair cells are stiffened by
actin filaments and linked together by fine fibers called
tip-links (arrow)
• They protrude into the K+ rich endolymph, and the
longest of them enmeshed in the overlying, gel-like
tectorial membrane
Excitation of Hair Cells in the Organ of Corti
• Physiology of Hearing
– Transduction of sound stimuli occurs after the
trapped stereocilia of the hair cells are deflected
by localized movements of the basilar membrane
• Bending the cilia towards the tallest cilium puts tension
on the tip-links which in turn opens cation (+ ion)
channels in the adjacent shorter stereocilia
– Results in an inward K+ and Ca2+ current and a graded
depolarization (receptor potential)
– Increases the release of neurotransmitters
– Impulses to the brain for auditory interpretation
– Bending away from the tallest cilium relaxes the tip-links, closes
the mechanically gated ion channels, and allows reploarization
Auditory Pathway to the Brain
• Impulses generated in the
cochlea pass through the
spiral ganglia, along the
afferent fibers of the cochlear
nerve to the cochlear nuclei of
the medulla, to the superior
olivary nucleus, to the inferior
colliculus, and finally to the
auditory cortex
• The auditory pathway is
unusual in that not all of the
fibers from each ear
decussate (cross over)
– Therefore, each auditory
cortex receives impulses
from both ears
Auditory Processing
• Auditory processing involves perception of
pitch, detection of loudness, and
localization of sound
• Auditory cortex can distinguish the
separate parts of auditory signals
– Whenever the difference between sound
wavelengths is sufficient for discrimination,
you hear two separate and distinct tones
Perception of Pitch
• Hair cells in different parts of the organ of
Corti are activated by sound waves of
different frequencies and impulses from
specific hair cells are interpreted as specific
pitches
• When the sound is composed of tones of many
frequencies, several populations of cochlear hair
cells and cortical cells are activated
simultaneously, resulting in the perception of
multiple tones
Detection of Loudness
• Perception of loudness suggests that certain
cochlear cells have higher thresholds than
others for responding to a tone of the same
frequency
– Example: some of the receptors for a tone with a
frequency of 540 Hz might be stimulated by a sound
wave of very low intensity
– As the intensity of the sound increases, the basilar
membrane vibrates more vigorously
• More hair cells would begin to respond and more impulses
would reach the auditory cortex and be recognized as a
louder sound of the same pitch
Homeostatic Imbalances of Hearing
• Deafness is any hearing loss, no matter how slight
• Conduction deafness occurs when something hampers sound
conduction to the fluids of the inner ear
–
–
–
–
Impacted wax
Perforated eardrum
Middle ear inflammation
Otosclerosis (hardening) of the ossicles
• Sensorineural deafness results from damage to neural structures
– Age (gradual)
– Noise related damage
• Tearing of cilia or membranes
– Tumors
– Cerebral infarcts (region of dead tissue due to lack of blood supply)
Homeostatic Imbalances of Hearing
• Tinnitus is a ringing or clicking sound in the ears in the absence of
auditory stimuli
– Symptom of pathological disease
•
•
•
•
Cochlear degeneration
Inflammation of inner and middle ear
Side effect of some medications (e.g. aspirin)
Phantom sound: destruction of some neurons and growth of nearby neurons
whose signals are interpreted as sound
• Meniere’s syndrome is a labyrinth disorder (affects both the
semicircular canals and the cochlear) that causes a person to suffer
repeated attacks of vertigo, nausea, and vomiting
– Balance is so disturbed that standing erect is nearly impossible
– Howling tinnitus is common
– Might result from distortion of membranous labyrinth
• Mixing of the perilymph and endolymph
BALANCE
• Mechanisms of Equilibrium and Orientation
– The equilibrium sense responds to various head movements and
depends on input from the inner ear, vision, and information from
stretch receptors of muscles and tendons
– Under normal conditions the equilibrium receptors in the
semicircular canals and vestibule, collectively called the
vestibular apparatus, send signals to the brain that initiate
reflexes needed to make the simplest changes in position
– The equilibrium receptors of the inner ear can be divided into two
functional sections
• Receptors in the:
– Vestibule: monitors static equilibrium
– Semicircular canals: monitor dynamic equilibrium
Static Equilibrium
• The sensory receptors for static
equilibrium are the maculae
– One in each saccule wall and one in each
utricle
– Monitor position of head in space
– Key role in maintaining normal head posture
with respect to gravity
– Respond to linear acceleration forces, that is
straight-line changes in speed and direction,
but not to rotation
Maculae
•
•
•
•
Flat epithelial patch containing supporting
cells and scattered receptor cells called hair
cells
– Hair cells have numerous stereocilia
and a single kinocilium protruding from
their apices
• Embedded in the overlying
otolithic membrane: jellylike mass
studded with tiny stones (calcium
carbonate crystals) called otoliths
In the utricle, the macula are horizontal, and
the hairs are vertical
– Responds best to acceleration in the
horizontal plane and tilting head side to
side
– Vertical moving (up-down) movements
do not displace their horizontal otolithic
membrane
In the saccule, the macula is nearly
vertical, and the hairs protrude
horizontally into the otolithic membrane
– Responds best to vertical
movements
Vestibular nerve (division of the
vestibulocochlear nerve VIII)
Activating Maculae Receptors
•
Causing the otolithic membrane to slide backward or forward like a greased
plate over the hair cells bends the hairs (e.g. your head movement in a car
stopping and starting)
– Bent toward the kinocilium: depolarize with increase neurotransmitter release
– Bent in opposite direction: hyperpolarize with declining neurotransmitter release
– In either case, the brain is informed of the changing position of the head in space
•
Fibers of the receptor cells release neurotransmitter because of
movement of the hairs
Activating Maculae Receptors
• When movement of the otolithic membrane (direction indicated by
the arrow) bends the hair cells in the direction of the kinocilium, the
vestibular nerve fibers depolarize and generate action potential
more rapidly
• When the hairs are bent in the direction away from the kinocilium,
the hair cells become hyperpolarized, and the nerve fibers send
impulses at a reduced rate i.e. below the resting rate of discharge)
Dynamic Equilibrium
•
•
•
•
The receptor for dynamic
equilibrium is the crista ampullaris
(crista), is a minute elevation in
the ampulla of the semicircular
canals and activated by head
movement
Like the maculae, the cristae are
excited by head movement
(acceleration and deceleration),
but in this case the major stimuli
are rotatory (angular) movements
Gyroscope-like receptors
Since the semicircular canals
are located in all three planes of
space, all rotatory movements
of the head disturb one or
another pair of cristae (one in
each ear)
Crista Ampullaris
• Composed of supporting
cells and hair cells
• Hair cells, like those of
the maculae, have
stereocilia plus one
kinocilium that project into
a gel-like mass (cupula)
which resembles a
pointed cap
• Dendrites of vestibular
nerve fibers encircle the
base of the hair cells
Activating Crista Ampullaris Receptors
• Cristae respond to changes in
the velocity of rotatory
movements of the head
• Because of inertia, the
endolymph in the semicircular
ducts moves in the direction
opposite the body’s rotation,
deforming the crista in the duct
• As hairs are bent, the hair cells
depolarize and impulses reach
the brain faster
• Bending the cilia in opposite
direction causes
hyperpolarization and reduces
impulse generation
Activating Crista Ampullaris Receptors
• (d): view of the
horizontal ducts from
above shows how
paired semicircular
canals work together
to provide bilateral
information on
rotatory head
movement
BALANCE
• Key point to remember when
considering both types of equilibrium
receptors is that the rigid bony
labyrinth moves with the body, while
the fluids (and gels) within the
membranous labyrinth are free to move
at various rates, depending on the
forces (gravity, acceleration, and so on)
acting on them
Equilibrium Pathway to the Brain
• Information from the
balance receptors goes
directly to reflex centers
in the brain stem
(preventing us from
falling), rather that to the
cerebral cortex
• Impulses travel initially
to one of two
destinations:
– The vestibular nuclear
complex in the brain
stem
– cerebellum
Equilibrium Pathway to the Brain
• Vestibular nuclei:
– Major integrative center for
balance (brain stem: medulla
oblongata)
– Also, receives inputs from the
visual and somatic
(proprioceptors) receptors in
the neck muscles
– Integrates this information and
then sends commands to
brain stem motor centers that
control eye muscles and reflex
movements of the neck, limb,
and trunk muscles
• Allows us to remain focused
on the visual field and to
quickly adjust our body
position to maintain or regain
balance
Equilibrium Pathway to the Brain
• Cerebellum:
– Also integrates inputs
from the eyes and
somatic receptors
– Coordinates skeletal
muscle activity
Pathways of Balance
• After vestibular
nuclear processing,
impulses are sent to
one of two major
regions:
– Areas controlling eye
movements
(oculomotor control)
– Areas controlling the
skeletal muscles of the
neck (spinal motor
control)
HOMEOSTATIC IMBALANCE
• Nausea, dizziness, and loss of balance are
common and there may be nystagmus
(involuntary movement of eyes) in the absence
of rotational stimuli
• Motion sickness:
– Due to sensory input mismatch
• Body is fixed with reference to a stationary environment
(cabin on ship)
• But as the ship is tossed by rough seas, your vestibular
apparatus detects movement and sends impulses that
disagree with the visual information
• This confusion somehow leads to motion sickness
DEVELOPMENTAL ASPECTS
OF
THE SPECIAL SENSES
• Embryonic and Fetal Development of the Senses
– Smell and taste are fully functional at birth
– The eye begins to develop by the fourth week of embryonic
development; vision is the only special sense not fully functional at birth
• German measles during this critical time: blindness or cataracts
– Development of the ear begins in the fourth week of fetal development;
at birth the newborn is able to hear but most responses to sound are
reflexive
• German measles during this critical time: deafness
• Effects of Aging on the Senses
– Around age 40 the sense of smell and taste diminishes due to a gradual
loss of receptors
– Also around age 40 presbyopia begins to set in and with age the lens
loses its clarity and discolors
– By age 60 a noticeable deterioration of the organ of Corti has occurred;
the ability to hear high-pitches sounds is the first loss
MACULA
EFFECT OF GRAVITATIONAL
PULL ON A MACULA RECEPTOR
CRISTA AMPULLARIS
PATHWAYS OF BALANCE AND
ORIENTATION SYSTEM