Chapter 17
The Special Senses
Lecture Outline
Principles of Human Anatomy and Physiology, 11e
1
Chapter 17
The Special Senses
•
•
•
•
Smell, taste, vision, hearing and equilibrium
Housed in complex sensory organs
Ophthalmology is science of the eye
Otolaryngology is science of the ear
Principles of Human Anatomy and Physiology, 11e
2
Chemical Senses
• Interaction of molecules with receptor cells
• Olfaction (smell) and gustation (taste)
• Both project to cerebral cortex & limbic system
– evokes strong emotional reactions
Principles of Human Anatomy and Physiology, 11e
3
Anatomy of olfactory receptors
• The receptors for olfaction, which are bipolar neurons, are in
the nasal epithelium in the superior portion of the nasal
cavity (Figure 17.1).
• They are first-order neurons of the olfactory pathway.
• Supporting cells are epithelial cells of the mucous
membrane lining the nose.
• Basal stem cells produce new olfactory receptors.
Principles of Human Anatomy and Physiology, 11e
4
Olfactory Epithelium
• 1 square inch of membrane
holding 10-100 million receptors
• Covers superior nasal cavity
and cribriform plate
• 3 types of receptor cells
Principles of Human Anatomy and Physiology, 11e
5
Cells of the Olfactory Membrane
• Olfactory receptors
– bipolar neurons with cilia or
olfactory hairs
• Supporting cells
– columnar epithelium
• Basal cells = stem cells
– replace receptors monthly
• Olfactory glands
– produce mucus
• Both epithelium & glands innervated
cranial nerve VII.
Principles of Human Anatomy and Physiology, 11e
6
Physiology of Olfaction - Overview
• Genetic evidence suggests there are hundreds of primary
scents.
• In olfactory reception, a generator potential develops and
triggers one or more nerve impulses.
• Adaptation to odors occurs quickly, and the threshold of
smell is low: only a few molecules of certain substances
need be present in air to be smelled.
• Olfactory receptors convey nerve impulses to olfactory
nerves, olfactory bulbs, olfactory tracts, and the cerebral
cortex and limbic system.
• Hyposmia, a reduced ability to smell, affects half of those
over age 65 and 75% of those over 80. It can be caused by
neurological changes, drugs, or the effects of smoking .
Principles of Human Anatomy and Physiology, 11e
7
Olfaction: Sense of Smell
•
•
•
•
Principles of Human Anatomy and Physiology, 11e
Odorants bind to receptors
Na+ channels open
Depolarization occurs
Nerve impulse is triggered
8
Adaptation & Odor Thresholds
• Adaptation = decreasing sensitivity
• Olfactory adaptation is rapid
– 50% in 1 second
– complete in 1 minute
• Low threshold
– only a few molecules need to be present
– methyl mercaptan added to natural gas as warning
Principles of Human Anatomy and Physiology, 11e
9
Olfactory Pathway
• Axons from olfactory receptors form the olfactory nerves
(Cranial nerve I) that synapse in the olfactory bulb
– pass through 40 foramina in cribriform plate
• Second-order neurons within the olfactory bulb form the
olfactory tract that synapses on primary olfactory area of
temporal lobe
– conscious awareness of smell begins
• Other pathways lead to the frontal lobe (Brodmann area 11)
where identification of the odor occurs
Principles of Human Anatomy and Physiology, 11e
10
GUSTATORY: SENSE OF SMELL
• Taste is a chemical sense.
– To be detected, molecules must be dissolved.
– Taste stimuli classes include sour, sweet, bitter, and salty.
Principles of Human Anatomy and Physiology, 11e
11
Gustatory Sensation: Taste
• Taste requires dissolving of
substances
• Four classes of stimuli--sour, bitter,
sweet, and salty
– Other “tastes” are a combination
of the four taste sensations plus
olfaction.
• 10,000 taste buds found on tongue,
soft palate & larynx
• Found on sides of circumvallate &
fungiform papillae
• 3 cell types: supporting, receptor &
basal cells
Principles of Human Anatomy and Physiology, 11e
12
Anatomy of Taste Buds
• An oval body consisting of 50
receptor cells surrounded by
supporting cells
• A single gustatory hair projects
upward through the taste pore
• Basal cells develop into new
receptor cells every 10 days.
Principles of Human Anatomy and Physiology, 11e
13
Physiology of Taste
• Receptor potentials developed in gustatory hairs cause
the release of neurotransmitter that gives rise to nerve
impulses.
• Complete adaptation in 1 to 5 minutes
• Thresholds for tastes vary among the 4 primary tastes
– most sensitive to bitter (poisons)
– least sensitive to salty and sweet
• Mechanism
– dissolved substance contacts gustatory hairs
– receptor potential results in neurotransmitter release
– nerve impulse formed in 1st-order neuron
Principles of Human Anatomy and Physiology, 11e
14
Gustatory Pathway
• First-order gustatory fibers found in cranial nerves
–V
– VII (facial) serves anterior 2/3 of tongue
– IX (glossopharyngeal) serves posterior 1/3 of tongue
– X (vagus) serves palate & epiglottis
• Signals travel to thalamus or limbic system & hypothalamus
• Taste fibers extend from the thalamus to the primary gustatory
area on parietal lobe of the cerebral cortex
– provides conscious perception of taste
Principles of Human Anatomy and Physiology, 11e
15
VISION
• More than half the sensory receptors in the human body are
located in the eyes.
• A large part of the cerebral cortex is devoted to processing
visual information.
Principles of Human Anatomy and Physiology, 11e
16
Accessory Structures of Eye - Overview
• Eyelids or palpebrae
– protect & lubricate
– epidermis, dermis, CT,
orbicularis oculi m., tarsal
plate, tarsal glands &
conjunctiva
• Tarsal glands
– oily secretions
• Conjunctiva
– palpebral & bulbar
– stops at corneal edge
Principles of Human Anatomy and Physiology, 11e
17
Eyelids
• The eyelids shade the eyes during sleep, protect the eyes
From superficial to deep, each eyelid consists of epidermis,
dermis, subcutaneous tissue, fibers of the orbicularis oculi
muscle, a tarsal plate, tarsal glands, and conjunctiva (Figure
17.4a).
– The tarsal plate gives form and support to the eyelids.
– The tarsal glands secrete a fluid to keep the eye lids from
adhering to each other.
– The conjunctiva is a thin mucous membrane that lines
the inner aspect of the eyelids and is reflected onto the
anterior surface of the eyeball.
– Eyelashes and eyebrows help protect the eyeballs from
foreign objects, perspiration, and the direct rays of the
sun.
Principles of Human Anatomy and Physiology, 11e
18
Eyelashes & Eyebrows
Eyeball = 1
inch diameter
5/6 of Eyeball
inside orbit &
protected
• Eyelashes & eyebrows help protect from foreign objects,
perspiration & sunlight
• Sebaceous glands are found at base of eyelashes (sty)
• Palpebral fissure is gap between the eyelids
Principles of Human Anatomy and Physiology, 11e
19
Lacrimal Apparatus
• About 1 ml of tears produced per day. Spread over eye by blinking. Contains
bactericidal enzyme called lysozyme.
Principles of Human Anatomy and Physiology, 11e
20
Extraocular Muscles
• Six muscles that insert on
the exterior surface of the
eyeball
• Innervated by CN III, IV or
VI.
• 4 rectus muscles -- superior,
inferior, lateral and medial
• 2 oblique muscles -- inferior
and superior
Principles of Human Anatomy and Physiology, 11e
21
Tunics (Layers) of Eyeball
• The eye is
constructed of
three layers
(Figure 17.5).
– Fibrous Tunic
(outer layer)
– Vascular
Tunic
(middle layer)
– Nervous Tunic
(inner layer)
Principles of Human Anatomy and Physiology, 11e
22
Fibrous Tunic -- Description of Cornea
• Transparent
• Helps focus light(refraction)
– astigmatism
• 3 layers
– nonkeratinized stratified squamous
– collagen fibers & fibroblasts
– simple squamous epithelium
• Transplants
– common & successful
– no blood vessels so no antibodies to cause rejection
• Nourished by tears & aqueous humor
Principles of Human Anatomy and Physiology, 11e
23
Fibrous Tunic -- Description of Sclera
• “White” of the eye
• Dense irregular connective tissue
layer -- collagen & fibroblasts
• Provides shape & support
• At the junction of the sclera and
cornea is an opening (scleral
venous sinus)
• Posteriorly pierced by Optic Nerve
(CNII)
Principles of Human Anatomy and Physiology, 11e
24
Vascular Tunic -- Choroid & Ciliary Body
• Choroid
– pigmented epithilial cells
(melanocytes) & blood vessels
– provides nutrients to retina
– black pigment in melanocytes
absorb scattered light
• Ciliary body
– ciliary processes
• folds on ciliary body
• secrete aqueous humor
– ciliary muscle
• smooth muscle that alters
shape of lens
Principles of Human Anatomy and Physiology, 11e
25
Vascular Tunic -- Iris & Pupil
• Colored portion of eye
• Shape of flat donut suspended
between cornea & lens
• Hole in center is pupil
• Function is to regulate amount of
light entering eye
• Autonomic reflexes
– circular muscle fibers contract in
bright light to shrink pupil
– radial muscle fibers contract in
dim light to enlarge pupil
Principles of Human Anatomy and Physiology, 11e
26
Vascular Tunic -- Muscles of the Iris
• Constrictor pupillae (circular) are innervated by parasympathetic
fibers while Dilator pupillae (radial) are innervated by
sympathetic fibers.
• Response varies with different levels of light
Principles of Human Anatomy and Physiology, 11e
27
Vascular Tunic -- Description of lens
• Avascular
• Crystallin proteins arranged
like layers in onion
• Clear capsule & perfectly
transparent
• Lens held in place by
suspensory ligaments
• Focuses light on fovea
Principles of Human Anatomy and Physiology, 11e
28
Vascular Tunic -- Suspensory ligament
• Suspensory ligaments attach lens to ciliary process
• Ciliary muscle controls tension on ligaments & lens
Principles of Human Anatomy and Physiology, 11e
29
Nervous Tunic -- Retina
• Posterior 3/4 of eyeball
• Optic disc
– optic nerve exiting back of
eyeball
• Central retina BV
– fan out to supply
nourishment to retina
– visible for inspection
• hypertension &
diabetes
• Detached retina
– trauma (boxing)
• fluid between layers
• distortion or blindness
View with Ophthalmoscope
Principles of Human Anatomy and Physiology, 11e
30
Photoreceptors
• shapes of their outer segments differ
• Rods
– specialized for black-and-white vision in dim light
– allow us to discriminate between different shades of dark
and light
– permit us to see shapes and movement.
• Cones
– specialized for color vision and sharpness of vision (high
visual acuity) in bright light
– most densely concentrated in the central fovea, a small
depression in the center of the macula lutea.
Principles of Human Anatomy and Physiology, 11e
31
Photoreceptors
• The macula lutea is in the exact center of the posterior
portion of the retina, corresponding to the visual axis of the
eye.
– The fovea is the area of sharpest vision because of the
high concentration of cones.
– Rods are absent from the fovea and macula and
increase in density toward the periphery of the retina.
Principles of Human Anatomy and Physiology, 11e
32
Layers of Retina
• Pigmented epithelium
– nonvisual portion
– absorbs stray light &
helps keep image clear
• 3 layers of neurons
(outgrowth of brain)
– photoreceptor layer
– bipolar neuron layer
– ganglion neuron layer
• 2 other cell types (modify the
signal)
– horizontal cells
– amacrine cells
Principles of Human Anatomy and Physiology, 11e
33
Rods & Cones--Photoreceptors
• Rods----rod shaped
– shades of gray in dim light
– 120 million rod cells
– shapes & movements
– distributed along periphery
• Cones----cone shaped
– sharp, color vision
– 6 million
– fovea of macula lutea
• densely packed region
• at exact visual axis of eye
• 2nd cells do not cover
cones
• sharpest resolution (acuity)
Principles of Human Anatomy and Physiology, 11e
34
Pathway of Nerve Signal in Retina
• Light penetrates retina
• Rods & cones transduce light
into action potentials
• Rods & cones excite bipolar cells
• Bipolars excite ganglion cells
• Axons of ganglion cells form
optic nerve leaving the eyeball
(blind spot)
• To thalamus & then the primary
visual cortex
Principles of Human Anatomy and Physiology, 11e
35
Lens
• The eyeball contains the nonvascular lens, just behind the
pupil and iris.
• The lens fine tunes the focusing of light rays for clear vision.
– With aging the lens loses elasticity and its ability to
accommodate resulting in a condition known as
presbyopia.
Principles of Human Anatomy and Physiology, 11e
36
Cavities of the Interior of Eyeball
• Anterior cavity (anterior to lens)
– filled with aqueous humor
• produced by ciliary body
• continually drained
• replaced every 90 minutes
– 2 chambers
• anterior chamber between cornea and iris
• posterior chamber between iris and lens
• Posterior cavity (posterior to lens)
– filled with vitreous body (jellylike)
– formed once during embryonic life
– floaters are debris in vitreous of older individuals
Principles of Human Anatomy and Physiology, 11e
37
Eye Anatomy
• The pressure in the eye, called intraocular pressure, is
produced mainly by the aqueous humor.
• The intraocular pressure, along with the vitreous body,
maintains the shape of the eyeball and keeps the retina
smoothly applied to the choroid so the retina will form clear
images.
• Glaucoma
– increased intraocular pressure
– problem with drainage of aqueous humor
– may produce degeneration of the retina and blindness
Principles of Human Anatomy and Physiology, 11e
38
Aqueous Humor
• Continuously produced
by ciliary body
• Flows from posterior chamber
into anterior through the pupil
• Scleral venous sinus
– canal of Schlemm
– opening in white of eye
at junction of cornea & sclera
– drainage of aqueous humor
from eye to bloodstream
Principles of Human Anatomy and Physiology, 11e
39
Major Processes of Image Formation
• Refraction of light
– by cornea & lens
– light rays must fall upon the retina
• Accommodation of the lens
– changing shape of lens so that light is focused
• Constriction of the pupil
– less light enters the eye
Principles of Human Anatomy and Physiology, 11e
40
Definition of Refraction
• Bending of light as it passes from one substance (air) into a 2nd
substance with a different density(cornea)
• In the eye, light is refracted by the anterior & posterior surfaces of the
cornea and the lens
Principles of Human Anatomy and Physiology, 11e
41
Refraction by the Cornea & Lens
• Image focused on retina is inverted &
reversed from left to right
• Brain learns to work with that
information
• 75% of Refraction is done by
cornea -- rest is done by the lens
• Light rays from > 20’ are nearly parallel
and only need to be bent enough to
focus on retina
• Light rays from < 6’ are more divergent
& need more refraction
– extra process needed to get
additional bending of light is called
accommodation
Principles of Human Anatomy and Physiology, 11e
42
Accommodation & the Lens
• Accommodation is an increase in the curvature of the lens, initiated by
ciliary muscle contraction, which allows the lens to focus on near objects
(figure 17.10c).
• Convex lens refract light rays towards each other
– Lens of eye is convex on both surfaces
• Viewing a distant object
– lens is nearly flat by pulling of suspensory ligaments
• View a close object
– ciliary muscle is contracted & decreases the pull of the suspensory
ligaments on the lens
– elastic lens thickens as the tension is removed from it
– increase in curvature of lens is called accommodation
– The near point of vision is the minimum distance from the eye that an
object can be clearly focused with maximum effort.
Principles of Human Anatomy and Physiology, 11e
43
Near Point of Vision and Presbyopia
• Near point is the closest distance from the eye an object
can be & still be in clear focus
– 4 inches in a young adult
– 8 inches in a 40 year old
• lens has become less elastic
– 31 inches in a 60 to 80 year old
• Reading glasses may be needed by age 40
– presbyopia
– glasses replace refraction previously provided by
increased curvature of the relaxed, youthful lens
Principles of Human Anatomy and Physiology, 11e
44
Refraction Abnormalities
• Myopia is nearsightedness (Figure 17.11).
• Hyperopia is farsightedness (Figure 17.11).
• Astigmatism is a refraction abnormality due to an irregular
curvature of either the cornea or lens.
Principles of Human Anatomy and Physiology, 11e
45
Correction for Refraction Problems
• Emmetropic eye (normal)
– can refract light from 20 ft away
• Myopia (nearsighted)
– eyeball is too long from front to back
– glasses concave
• Hypermetropic (farsighted)
– eyeball is too short
– glasses convex (coke-bottle)
• Astigmatism
– corneal surface wavy
– parts of image out of focus
Principles of Human Anatomy and Physiology, 11e
46
Constriction of the Pupil
• Constrictor pupillae muscle contracts
• Narrows beam of light that enters the eye
– Prevents light rays from entering the eye through the
edge of the lens
– Sharpens vision by preventing blurry edges
– Protects retina very excessively bright light
Principles of Human Anatomy and Physiology, 11e
47
Convergence of the Eyes
• Binocular vision in humans has both eyes looking at the
same object
• As you look at an object close to your face, both eyeballs
must turn inward.
– In convergence, the eyeballs move medially so they
are both directed toward an object being viewed.
– required so that light rays from the object will strike
both retinas at the same relative point
– extrinsic eye muscles must coordinate this action
Principles of Human Anatomy and Physiology, 11e
48
Physiology of Vision
• The first step in vision transduction is the absorption of light
by photopigments (visual pigments) in rods and cones
(photoreceptors) (Figure 17.12).
Principles of Human Anatomy and Physiology, 11e
49
Photoreceptors
• Named for shape of outer
segment
• Receptors transduce light energy
into a receptor potential in outer
segment
• Photopigment is integral
membrane protein of outer
segment membrane
– photopigment membrane is
folded into “discs” & replaced
at a very rapid rate
• Photopigments
– opsin (protein) + retinal
(derivative of vitamin A)
Principles of Human Anatomy and Physiology, 11e
50
Physiology of Vision
• Photopigments are undergo structural changes upon light
absorption.
• Retinal is the light absorbing part of all visual
photopigments.
• All photopigments involved in vision contain a glycoprotein
called opsin and a derivative of vitamin A called retinal.
– There are four different opsins
• A cone contains one of three different kinds of
photopigments so there are three types of cones.
– permit the absorption of 3 different wavelengths
(colors) of light
• Rods contain a single type of photopigment
(rhodopsin)
Principles of Human Anatomy and Physiology, 11e
51
Physiology of Vision
• Figure 16.14 shows how photopigments are activated and
restored.
• Bleaching and regeneration of the photopigments
accounts for much but not all of the sensitivity change
during light and dark adaptation.
Principles of Human Anatomy and Physiology, 11e
52
Photopigments
• Isomerization
– light cause cis-retinal to
straighten & become transretinal shape
• Bleaching
– enzymes separate the transretinal from the opsin
– colorless final products
• Regeneration
– in darkness, an enzyme
converts trans-retinal back to
cis-retinal (resynthesis of a
photopigment)
Principles of Human Anatomy and Physiology, 11e
53
Application: Color Blindness & Night Blindness
• Most forms of colorblindness (inability to distinguish certain
colors) result from an inherited absence of or deficiency in
one of the three cone photopigments and are more common
in males. A deficiency in rhodopsin may cause night
blindness (nyctalopia)
• Color blindness
– inability to distinguish between certain colors
– absence of certain cone photopigments
– red-green color blind person can not tell red from green
• Night blindness (nyctalopia)
– difficulty seeing in low light
– inability to make normal amount of rhodopsin
– possibly due to deficiency of vitamin A
Principles of Human Anatomy and Physiology, 11e
54
Regeneration of Bleached Photopigments
• Pigment epithelium near the photoreceptors contains
large amounts of vitamin A and helps the regeneration
process.
– After complete bleaching, it takes 5 minutes to
regenerate 1/2 of the rhodopsin
• Full regeneration of bleached rhodopsin takes 30
to 40 minutes
• Rods contribute little to daylight vision, since they
are bleached as fast as they regenerate.
– Only 90 seconds are required to regenerate the cone
photopigments
Principles of Human Anatomy and Physiology, 11e
55
Light and Dark Adaptation
• Light adaptation
– adjustments when emerge from the dark into the light
• Dark adaptation
– adjustments when enter the dark from a bright situation
– light sensitivity increases as photopigments regenerate
• during first 8 minutes of dark adaptation, only cone
pigments are regenerated, so threshold burst of light is
seen as color
• after sufficient time, sensitivity will increase so that a
flash of a single photon of light will be seen as graywhite
Principles of Human Anatomy and Physiology, 11e
56
Details: Formation of Receptor Potentials
• In darkness
– Na+ channels are held open and photoreceptor is always
partially depolarized (-30mV)
– continuous release of inhibitory neurotransmitter onto
bipolar cells suppresses their activity
• In light
– enzymes cause the closing of Na+ channels producing a
hyperpolarized receptor potential (-70mV)
– release of inhibitory neurotransmitter is stopped
– bipolar cells become excited and a nerve impulse will
travel towards the brain
Principles of Human Anatomy and Physiology, 11e
57
Release of Neurotransmitters
Principles of Human Anatomy and Physiology, 11e
58
Visual Pathway
• Horizontal cells transmit inhibitory signals to bipolar cells
• bipolar or amacrine cells transmit excitatory signals to
ganglion cells
• ganglion cells which depolarize and initiate nerve impulses
(Figure 17.8).
– Impulses are conveyed through the retina to the optic
nerve, the optic chiasma, the optic tract, the thalamus,
and the occipital lobes of the cortex (Figure 17.15).
Principles of Human Anatomy and Physiology, 11e
59
Retinal Processing of Visual Information
• Convergence
– one cone cell synapses onto one bipolar
cell produces best visual acuity
– 600 rod cells synapse on single bipolar
cell increasing light sensitivity although
slightly blurry image results
– 126 million photoreceptors converge on 1
million ganglion cells
• Horizontal and amacrine cells
– horizontal cells enhance contrasts in
visual scene because laterally inhibit
bipolar cells in the area
– amacrine cells excite bipolar cells if levels
of illumination change
Principles of Human Anatomy and Physiology, 11e
60
Brain
Pathways of
Vision
Principles of Human Anatomy and Physiology, 11e
61
Processing of Image Data in the Brain
• Visual information in optic nerve travels to
– hypothalamus to establish sleep patterns based upon
circadian rhythms of light and darkness
– midbrain for controlling pupil size & coordination of head
and eye movements
– occipital lobe for vision
Principles of Human Anatomy and Physiology, 11e
62
Visual fields
• Fibers from nasal 1/2 of
each retina cross in optic
chiasm
• Left occipital lobe
receives visual images
from right side of an
object through impulses
from nasal 1/2 of the right
eye and temporal 1/2 of
the left eye
• Left occipital lobe sees
right 1/2 of the world and
Right occipital lobe sees
left 1/2 of the world.
Principles of Human Anatomy and Physiology, 11e
63
Anatomy of the Ear Region
Principles of Human Anatomy and Physiology, 11e
64
HEARING AND EQUILIBRIUM - Overview
• The external (outer) ear collects sound waves.
• The middle ear (tympanic cavity) is a small, air-filled cavity
in the temporal bone that contains auditory ossicles (middle
ear bones, the malleus, incus, and stapes), the oval window,
and the round window (Figure 17.17).
• The internal (inner) ear is also called the labyrinth because
of its complicated series of canals (Figure 17.18).
Principles of Human Anatomy and Physiology, 11e
65
Anatomy of the Ear Region
Principles of Human Anatomy and Physiology, 11e
66
External Ear
• The external (outer) ear collects
sound waves and passes them
inwards (Figure 17.16)
• Structures
– auricle or pinna
• elastic cartilage covered with skin
– external auditory canal
• curved 1” tube of cartilage & bone leading into temporal
bone
• ceruminous glands produce cerumen = ear wax
– tympanic membrane or eardrum
• epidermis, collagen & elastic fibers, simple cuboidal epith.
• Perforated eardrum (hole is present)
– at time of injury (pain, ringing, hearing loss, dizziness)
– caused by explosion, scuba diving, or ear infection
Principles of Human Anatomy and Physiology, 11e
67
Middle Ear Cavity
Principles of Human Anatomy and Physiology, 11e
68
Middle Ear Cavity
• Air filled cavity in the temporal bone
• Separated from external ear by
eardrum and from internal ear by
oval & round window
• 3 ear ossicles connected by synovial joints
– malleus attached to eardrum, incus & stapes attached by
foot plate to membrane of oval window
– stapedius and tensor tympani muscles attach to ossicles
• Auditory tube leads to nasopharynx
– helps to equalize pressure on both sides of eardrum
• Connection to mastoid bone =mastoiditis
Principles of Human Anatomy and Physiology, 11e
69
Muscles of the Ear
• Stapedius m. inserts onto stapes
– prevents very large vibrations of stapes from loud noises
• Tensor tympani attaches to malleus
– limits movements of malleus & stiffens eardrum to prevent damage
Principles of Human Anatomy and Physiology, 11e
70
Bony Labyrinth
• The bony labyrinth is a series of cavities in the petrous
portion of the temporal bone.
• It can be divided into three areas named on the basis of
shape: the semicircular canals and vestibule, both of which
contain receptors for equilibrium, and the cochlea, which
contains receptors for hearing.
• The bony labyrinth is lined with periosteum and contains a
fluid called perilymph. This fluid, chemically similar to
cerebrospinal fluid, surrounds the membranous labyrinth.
Principles of Human Anatomy and Physiology, 11e
71
Inner Ear---Bony Labyrinth
• Bony labyrinth = set of tubelike cavities in temporal bone
– semicircular canals, vestibule & cochlea lined with periosteum & filled
with perilymph
– surrounds & protects Membranous Labyrinth
Principles of Human Anatomy and Physiology, 11e
72
Inner Ear---Membranous Labyrinth
• Membranous labyrinth = set of membranous tubes containing sensory
receptors for hearing & balance
– utricle, saccule, ampulla, 3 semicircular ducts & cochlea
Principles of Human Anatomy and Physiology, 11e
73
Membranous Labyrinth
• The membranous labyrinth is a series of sacs and tubes
lying inside and having the same general form as the bony
labyrinth.
– lined with epithelium.
– contains a fluid called endolymph, chemically similar to
intracellular fluid.
– The vestibule constitutes the oval central portion of the
bony labyrinth. The membranous labyrinth in the
vestibule consists of two sacs called the utricle and
saccule.
• Anterior to the vestibule is the cochlea, which consists of a
bony spiral canal that makes almost three turns around a
central bony core called the modiolus (Figure 17.19a).
Principles of Human Anatomy and Physiology, 11e
74
Semicircular Canals
• Projecting upward and posteriorly from the vestibule are the
three bony semicircular canals.
– arranged at approximately right angles (X-Y-Z axis)
– The anterior and posterior semicircular canals are
oriented vertically; the lateral semicircular canal is
oriented horizontally.
– Two parts
• One end of each canal enlarges into a swelling called
the ampulla.
• The portions of the membranous labyrinth that lie
inside the semicircular canals are called the
semicircular ducts (membranous semicircular canals).
Principles of Human Anatomy and Physiology, 11e
75
Cranial nerves of the Ear Region
Principles of Human Anatomy and Physiology, 11e
76
Nerve
• Vestibulocochlear nerve = CN VIII
– The vestibular branch of the vestibulocochlear nerve
consists of 3 parts
• ampullary, utricular, and saccular nerves
– cochlear branch has spiral ganglion in bony modiolus
Principles of Human Anatomy and Physiology, 11e
77
Overview of Physiology of Hearing
Principles of Human Anatomy and Physiology, 11e
78
Physiology of Hearing - Overview
• Auricle collects sound waves
• Eardrum vibrates
– slow vibration in response to low-pitched sounds
– rapid vibration in response to high-pitched sounds
• Ossicles vibrate since malleus is attached to the eardrum
• Stapes pushes on oval window producing fluid pressure
waves in scala vestibuli & tympani
– oval window vibration is 20X more vigorous than
eardrum (but the frequency of vibration is unchanged)
• Pressure fluctuations inside cochlear duct move the hair
cells against the tectorial membrane
• Microvilli are bent producing receptor potentials
Principles of Human Anatomy and Physiology, 11e
79
Tubular Structures of the Cochlea
•
•
•
•
Stapes pushes on fluid of scala vestibuli at oval window
At helicotrema, vibration moves into scala tympani
Fluid vibration dissipated at round window which bulges
The central structure is vibrated (cochlear duct)
Principles of Human Anatomy and Physiology, 11e
80
Cochlea
• Cross sections through the cochlea show that it is divided into three
channels by partitions that together have the shape of the letter Y
(Figure 17.19 a-c).
– The channel above the bony partition is the scala vestibuli, which
ends at the oval window.
– The channel below is the scala tympani, which ends at the round
window.
– The scala vestibuli and scala tympani both contain perilymph and
are completely separated except at an opening at the apex of the
cochlea called the helicotrema.
– The third channel (between the wings of the Y) is the cochlear duct
(scala media).
– The vestibular membrane separates the cochlear duct from the scala
vestibuli, and the basilar membrane separates the cochlear duct
from the scala tympani.
Principles of Human Anatomy and Physiology, 11e
81
Cochlear
Anatomy –
Zoom In
Section thru one turn of Cochlea
• Partitions that separate the channels are Y shaped
– bony shelf of central modiolus
– vestibular membrane above & basilar membrane below form
the central fluid filled chamber (cochlear duct)
• Fluid vibrations affect hair cells in cochlear duct
Principles of Human Anatomy and Physiology, 11e
82
Cochlear Anatomy – Zoom Out
• 3 fluid filled channels found within the cochlea
– scala vestibuli, scala tympani and cochlear duct
• Vibration of the stapes upon the oval window sends
vibrations into the fluid of the scala vestibuli
Principles of Human Anatomy and Physiology, 11e
83
Anatomy
• Resting on the basilar membrane is the spiral organ (organ
of Corti), the organ of hearing (Figure 17.19, c,d).
• Projecting over and in contact with the hair cells of the spiral
organ is the tectorial membrane, a delicate and flexible
gelatinous membrane.
Principles of Human Anatomy and Physiology, 11e
84
Anatomy of the Organ of Corti
• 16,000 hair cells have 30-100 stereocilia(microvilli )
• Microvilli make contact with tectorial membrane (gelatinous
membrane that overlaps the spiral organ of Corti)
• Basal sides of inner hair cells synapse with 1st order sensory
neurons whose cell body is in spiral ganglion
Principles of Human Anatomy and Physiology, 11e
85
Sound Waves
• Sound waves result from the alternate compression and
decompression of air molecules.
– The sounds heard best by human ears are at
frequencies between 1000 and 4000 Hertz (Hz; cycles
per minute), but many people perceive a range of 20 to
20,000 Hz
– speech is 100 to 3000 Hz
• Frequency of a sound vibration is percieved as pitch
– higher frequency is higher pitch
• The volume of a sound is its intensity (the greater the size
of the vibration, the louder the sound, measured in
decibels, dB).
– Conversation is 60 dB; pain above 140dB
– OSA requires ear protection above 90dB
Principles of Human Anatomy and Physiology, 11e
86
Deafness
• Nerve deafness
– possibly nerve damage (CN VIII), but usually
damage to hair cells from antibiotics, high
pitched sounds, anticancer drugs, etc.
• the louder the sound the quicker the loss
of hearing
– person may fail to notice loss until they have
difficulty hearing frequencies of speech
• Conduction deafness
– perforated eardrum
– otosclerosis
– vibrations are not “conducted” to hair cells
Principles of Human Anatomy and Physiology, 11e
87
Physiology of Hearing
• The events involved in hearing are seen in Figure 17.20.
• The auricle directs sound waves into the external auditory
canal.
• Sound waves strike the tympanic membrane, causing it to
vibrate back and forth.
• The vibration conducts from the tympanic membrane
through the ossicles (through the malleus to the incus and
then to the stapes).
• The stapes moves back and forth, pushing the membrane of
the oval window in and out.
Principles of Human Anatomy and Physiology, 11e
88
Physiology of Hearing - Review
• The movement of the oval window sets up fluid pressure
waves in the perilymph of the cochlea (scala vestibuli).
• Pressure waves in the scala vestibuli are transmitted to the
scala tympani and eventually to the round window, causing
it to bulge outward into the middle ear.
• As the pressure waves deform the walls of the scala
vestibuli and scala tympani, they push the vestibular
membrane back and forth and increase and decrease the
pressure of the endolymph inside the cochlear duct.
Principles of Human Anatomy and Physiology, 11e
89
Physiology of Hearing - Review
• The pressure fluctuations of the endolymph move the
basilar membrane slightly, moving the hair cells of the spiral
organ against the tectorial membrane; the bending of the
hairs produces receptor potentials that lead to the
generation of nerve impulses in cochlear nerve fibers.
• Pressure changes in the scala tympani cause the round
window to bulge outward into the middle ear.
Principles of Human Anatomy and Physiology, 11e
90
Hair Cell Physiology - Review
• Hair cells convert mechanical deformation into electrical
signals
• As microvilli are bent, mechanically-gated channels in the
membrane let in K+ ions
• This depolarization spreads & causes voltage-gated Ca+2
channels at the base of the cell to open
• Triggering the release of neurotransmitter onto the first
order neuron
– more neurotransmitter means more nerve impulses
Principles of Human Anatomy and Physiology, 11e
91
More on Pitch and Volume
• Differences in pitch are related to differences in the width and stiffness of
the basilar membrane and sound waves of various frequencies that
cause a “standing wave.”
– High-frequency (high-pitch) tone causes the basilar membrane to
vibrate near the base of the cochlea (where it is stiff and narrow.)
– Low-frequency (low-pitch) tone causes the basilar membrane to
vibrate near the apex of the cochlea (where it is flexible and wide.)
– Hair cells beneath the vibrating region of the basilar membrane
convert the mechanical force (stimulus) into an electrical signal
(receptor potential)
• Sounds of the same pitch vibrate the same region of the membrane, and
thus stimulate the same cells, but a louder sound causes a greater
vibration amplitude -- which our brain interprets as “louder.”
Principles of Human Anatomy and Physiology, 11e
92
Auditory Pathway
• Cochlear branch of CN VIII sends signals to cochlear
and superior olivary nuclei (of both sides) within
medulla oblongata
– differences in the arrival of impulses from the
ears, allows us to locate the source of a sound
along the horizon (right vs. left)
• Fibers ascend to the
– medulla, most impulses then cross to the opposite
side and then travel to the
– midbrain (inferior colliculus)
– to the thalamus
– to the auditory area of the temporal lobe
• primary auditory cortex (areas 41 & 42)
Principles of Human Anatomy and Physiology, 11e
93
Otoacoustic Emissions
• The cochlea can produce sounds called otoacoustic
emissions.
– caused by vibrations of the outer hair cells that occur in
response to sound waves and to signals from motor
neurons.
– vibration travels backwards toward the eardrum
– can be recorded by sensitive microphone next to the
eardrum
• Purpose
– as outer hair cells shorten, they stiffen the tectorial
membrane
– amplifies the responses of the inner hair cells
– increasing our auditory sensitivity
Principles of Human Anatomy and Physiology, 11e
94
Cochlear Implants
• If deafness is due to destruction of hair cells
• Microphone, microprocessor & electrodes translate
sounds into electric stimulation of the
vestibulocochlear nerve
– artificially induced nerve signals follow normal
pathways to brain
• Provides only a crude representation of sounds
Principles of Human Anatomy and Physiology, 11e
95
Applications
• Otosclerosis
– a condition is which there is an overgrowth of spongy
bone over the oval window that immobilizes the stapes.
– prevents the transmission of sound waves to the inner
ear and leads to conductive hearing loss
Principles of Human Anatomy and Physiology, 11e
96
Vestibular Apparatus
• Notice: semicircular ducts with ampulla, utricle & saccule
Principles of Human Anatomy and Physiology, 11e
97
Physiology of Equilibrium (Balance)
• Static equilibrium
– maintain the position of the body (head) relative to the
force of gravity
– macula receptors within saccule & utricle
• Dynamic equilibrium
– maintain body position (head) during sudden movement
of any type--rotation, deceleration or acceleration
– crista receptors within ampulla of semicircular ducts
Principles of Human Anatomy and Physiology, 11e
98
Otolithic Organs: Saccule and Utricle
• The maculae of the utricle and saccule are the sense
organs of static equilibrium.
• They also contribute to some aspects of dynamic
equilibrium (Figure 17.21).
Principles of Human Anatomy and Physiology, 11e
99
Otolithic Organs: Saccule & Utricle
• Cell types in the macula region
– hair cells with stereocilia (microvilli) & one cilia (kinocilium)
– supporting cells that secrete gelatinous layer
• Gelatinous otolithic membrane contains calcium carbonate
crystals called otoliths that move when you tip your head
Principles of Human Anatomy and Physiology, 11e
100
Detection of Position of Head
• Movement of stereocilia or kinocilium results in the release
of neurotransmitter onto the vestibular branches of the
vestibulocochler nerve
Principles of Human Anatomy and Physiology, 11e
101
Membranous Semicircular Ducts
• The three semicircular ducts, along with the saccule and
utricle maintain dynamic equilibrium (Figure 17.22).
– anterior, posterior & horizontal ducts detect different
movements (combined 3-D sensitivity)
• The cristae in the semicircular ducts are the primary sense
organs of dynamic equilibrium.
Principles of Human Anatomy and Physiology, 11e
102
Crista: Ampulla of Semicircular Ducts
• Small elevation within each of three semicircular ducts
• Hair cells are covered with cupula (gelatinous material)
• When you move, fluid in canal tends to stay in place, thus
bending the cupula and bending the hair cells - and altering the
release of neurotransmitter
Principles of Human Anatomy and Physiology, 11e
103
Detection of Rotational Movement
• Nerve signals to the brain are generated indicating which direction
the head has been rotated
Principles of Human Anatomy and Physiology, 11e
104
Equilibrium Pathways in the CNS
Most vestibular branch fibers of the vestibulocochlear nerve
(CN VIII) enter the brain stem and terminate in the
medulla; the remaining fibers enter the cerebellum.
Fibers from these areas connect to:
• cranial nerves that control eye and head and neck
movements (III,IV,VI & XI)
• vestibulospinal tract that adjusts postural skeletal muscle
contractions in response to head movements
The cerebellum receives constant updated sensory
information which it sends to the motor areas of the
cerebral cortex
• motor cortex can then adjust its signals to maintain
balance
Principles of Human Anatomy and Physiology, 11e
105
DEVELOPMENT OF THE EYES AND EARS
Principles of Human Anatomy and Physiology, 11e
106
Eyes
• Eyes begin to develop when the ectoderm of the lateral
walls of the prosencephalon bulges to form a pair of optic
grooves (Figure 17.23a)
• As the neural tube closes the optic grooves enlarge and
move toward the surface of the ectoderm and are known as
optic vesicles (Figure 17.23b)
• When the optic vesicles reach the surface, the surface
ectoderm thickens to form the lens placodes and the distal
portions of the optic vesicles invaginate to form the optic
cups (Figure 17.23c).
• The optic cups remain attached to the prosencephalon by
the optic stalks (Figure 17.23d).
Principles of Human Anatomy and Physiology, 11e
107
Principles of Human Anatomy and Physiology, 11e
108
Ears
• Inner ear develops from a thickening of surface ectoderm
called the otic placode (Figure 17.24a).
• Otic placodes invaginate to form otic pits (Figure 17.24 a
and b)
• Optic pits pinch off from the surface ectoderm to form otic
vesicles (Figure 17.24d)
• Otic vesicles will form structures associated with the
membranous labyrinth of the inner ear.
• Middle ear develops from the first pharyngeal (branchial)
pouch.
• The external ear develops from the first pharyngeal cleft
(Figure 17.24).
Principles of Human Anatomy and Physiology, 11e
109
Principles of Human Anatomy and Physiology, 11e
110
AGING AND THE SPECIAL SENSES
• Age related changes in the eyes
– Presbyopia
– Cataracts
– Weakening of the muscles that regulate the size of the
pupil
– Diseases such as age related macular disease, detached
retina, and glaucoma
– Decrease in tear production
– Sharpness of vision as well as depth and color
perception are reduced.
Principles of Human Anatomy and Physiology, 11e
111
AGING AND THE SPECIAL SENSES
• After age 50 some individuals experience loss of olfactory
and gustatory receptors.
• Age related changes in the ears
– Presbycusis – hearing loss due to damaged or loss of
hair cells in the organ of Corti
– Tinnitus (ringing in the ears) becomes more common
Principles of Human Anatomy and Physiology, 11e
112
DISORDERS: HOMEOSTATIC IMBALANCES
• A cataract is a loss of transparency of the lens that can lead
to blindness.
• Glaucoma is abnormally high intraocular pressure, due to a
buildup of aqueous humor inside the eyeball, which destroys
neurons of the retina. It is the second most common cause
of blindness (after cataracts), especially in the elderly.
Principles of Human Anatomy and Physiology, 11e
113
DISORDERS: HOMEOSTATIC IMBALANCES
• Deafness is significant or total hearing loss. It is classified
as sensorineural (caused by impairment of the cochlear or
cochlear branch of the vestibulocochlear nerve) or
conduction (caused by impairment of the external and
middle ear mechanisms for transmitting sounds to the
cochlea).
• Meniere’s syndrome is a malfunction of the inner ear that
may cause deafness and loss of equilibrium.
• Otitis media is an acute infection of the middle ear, primarily
by bacteria. It is characterized by pain, malaise, fever, and
reddening and outward bulging of the eardrum, which may
rupture unless prompt treatment is given. Children are more
susceptible than adults.
Principles of Human Anatomy and Physiology, 11e
114
end
Principles of Human Anatomy and Physiology, 11e
115