Chapter 15: Sense Organs
SENSE OF SMELL
Olfactory receptors
Olfactory sense organs consist of epithelial
support cells and olfactory sensory neurons
(Figure 15-6)
Olfactory cilia: line the upper surface of the nasal cavity
Olfactory cells: chemoreceptors; gas molecules or
chemicals dissolved in the mucus covering the nasal
epithelium stimulate the olfactory cells
Olfactory epithelium: located in most superior portion of
the nasal cavity
Olfactory receptors: extremely sensitive and easily
fatigued; rapidly adapt with continual stimuli
SENSE OF SMELL (cont.)
Olfactory pathway: when the level of odorproducing chemicals reaches a threshold
level, the following occur (Figure 15-7):
Receptor potential and then action potential are
generated and passed to the olfactory nerves in
the olfactory bulb
The impulse then passes through the olfactory
tract and into the thalamic and olfactory centers
of the brain for interpretation, integration, and
memory storage
SENSE OF TASTE
 Taste buds: sense organs that respond to gustatory
(taste) stimuli; associated with papillae
Chemoreceptors stimulated by chemicals dissolved in the
saliva
Gustatory cells: sensory cells in taste buds
Sense of taste depends on the creation of a receptor
potential in gustatory cells because of taste-producing
chemicals in the saliva
Taste buds are similar structurally; functionally, each taste
bud responds most effectively to one of five primary taste
sensations: sour, sweet, bitter, umami, and salty (and
perhaps others, such as metallic) (Figures 15-8 and 15-9)
Adaptation and sensitivity thresholds differ for each primary
taste sensation
SENSE OF TASTE (cont.)
Neural pathway for taste
Taste sensation begins with a receptor potential in the
gustatory cells of a taste bud; an action potential is
generated and activated, which then transmits the
sensory input to the brain
Nerve impulses from the anterior two thirds of the tongue
travel over the facial nerve; those from the posterior third
of the tongue travel over the glossopharyngeal nerve
Nerve impulses are carried to the medulla oblongata,
relayed into the thalamus, and then relayed into the
gustatory area of the cerebral cortex in the parietal lobe
of the brain
SENSES OF HEARING AND BALANCE:
THE EAR
External ear: two divisions (Figures 15-10
and 15-11)
Auricle, or pinna: the visible portion of the ear
External acoustic meatus: tube leading from the
auricle into the temporal bone and ending at the
tympanic membrane
SENSES OF HEARING AND BALANCE: THE EAR (cont.)
Middle ear (Figure 15-11)
Tiny, epithelium-lined cavity hollowed out of the
temporal bone
Contains three auditory ossicles
Malleus (hammer): attached to the inner surface of the
tympanic membrane
Incus (anvil): attached to the malleus and stapes
Stapes (stirrup): attached to the incus
Openings into the middle ear cavity
Opening from the external acoustic meatus covered with
tympanic membrane
Oval window: opening into inner ear; stapes fits here
Round window: opening into inner ear; covered by a
membrane
Opening into the auditory (eustachian) tube
SENSES OF HEARING AND BALANCE:
THE EAR (cont.)
Inner ear (Figure 15-12)
Structure of the inner ear
 Bony labyrinth: composed of the vestibule, cochlea, and
semicircular canals
 Membranous labyrinth: composed of utricle and saccule inside the
vestibule, cochlear duct inside the cochlea, and membranous
semicircular ducts inside the bony semicircular canals
 Vestibule and semicircular canal organs are involved with balance
 Cochlea: involved with hearing
 Endolymph: clear, potassium-rich fluid filling the membranous
labyrinth
 Perilymph: similar to cerebrospinal fluid; surrounds the
membranous labyrinth, filling the space between the membranous
tunnel and its contents and the bony walls that surround it
SENSES OF HEARING AND BALANCE:
THE EAR (cont.)
Cochlea and cochlear duct
Cochlea: bony labyrinth
Cochlear duct
 Lies inside the cochlea; only part of the internal ear
concerned with hearing; contains endolymph
 Shaped like a triangular tube
 Divides the cochlea into the scala vestibuli, the upper
section, and the scala tympani, the lower section; both
sections filled with perilymph
 Vestibular membrane: the roof of the cochlear duct
 Basilar (spiral) membrane: floor of the cochlear duct
 Organ of Corti: rests on the basilar membrane; consists of
supporting cells and hair cells; also called spiral organ
 Axons of the neurons that begin around the organ of Corti
and extend in the cochlear nerve to the brain to produce the
sensation of hearing
SENSES OF HEARING AND BALANCE: THE EAR (cont.)
 Sense of hearing
Sound is created by vibrations
Ability to hear sound waves depends on volume, pitch, and
other acoustic properties
Sound waves must be of sufficient amplitude to move the
tympanic membrane and have a frequency capable of
stimulating the hair cells in the organ of Corti (spiral organ)
(Figure 15-13)
Basilar membrane width and thickness varies throughout its
length
 High-frequency sound waves vibrate the narrow portion near the
oval window
 Low frequencies vibrate the wider, thicker portion near the apex
of the cochlea
 Each frequency stimulates different hair cells and facilitates
perception of different pitches
 Perception of loudness is determined by movement amplitude;
the greater the movement, the louder the perceived sound
 Hearing results from stimulation of the auditory area of the
cerebral cortex
SENSES OF HEARING AND BALANCE:
THE EAR (cont.)
Pathway of sound waves
Enter external auditory canal
Strike tympanic membrane, causing vibrations
Tympanic vibrations move the malleus, which moves the
incus and then the stapes
The stapes moves against the oval window, which
begins the fluid conduction of sound waves
The perilymph in the scala vestibuli of the cochlea
begins a “ripple” that is transmitted through the vestibular
membrane to the endolymph inside the duct, to the
basilar membrane, and then to the organ of Corti
From the basilar membrane, the ripple is transmitted
through the perilymph in the scala tympani and then
expends itself against the round window
SENSES OF HEARING AND BALANCE:
THE EAR (cont.)
Neural pathway of hearing
A movement of hair cells against the tectorial
membrane stimulates the dendrites that terminate
around the base of the hair cells and initiates impulse
conduction by the cochlear nerve to the brainstem
Impulses pass through “relay stations” in the nuclei in
the medulla, pons, midbrain, and thalamus before
reaching the auditory area of the temporal lobe
SENSES OF HEARING AND BALANCE:
THE EAR (cont.)
Vestibule and semicircular canals (Figure 1512)
Vestibule: the central section of the bony labyrinth;
the utricle and saccule are the membranous
structures within the vestibule
Three semicircular canals
Each canal is at a right angle to the other
Membranous semicircular ducts within the canals; each
contains endolymph and connects with the utricle
Each canal enlarges into an ampulla near junction with
utricle
SENSES OF HEARING AND BALANCE: THE EAR (cont.)
 Sense of balance
Static equilibrium: ability to sense head position relative to
gravity or acceleration/deceleration (Figure 15-14)
 Movements of the maculae, located in both the utricle and
saccule, provide information related to head position or
acceleration
 Otoliths are located within the matrix of the macula
 Changing head position produces a change of pressure on the
otolith-weighted matrix, stimulating the hair cells that stimulate the
receptors of the vestibular nerve
 Vestibular nerve fibers conduct impulses to the brain and sense
head position and a change in the pull of gravity
 Righting reflexes: muscular responses to restore the body and its
parts to their normal position when displaced; caused by stimuli of
the macula and impulses from proprioceptors and from the eyes
SENSES OF HEARING AND BALANCE:
THE EAR (cont.)
 Dynamic equilibrium: needed to maintain balance when the head
or body is rotated or suddenly moved; able to detect changes in
direction and rate at which movement occurs (Figure 15-15)
 Depends on the functioning of the cristae ampullaris,
located in the ampulla of each semicircular duct
 Cupula: gelatinous cap where the hair cells of cristae are
embedded
• Does not respond to gravity
• Moves with the flow of endolymph in the semicircular ducts
 Semicircular ducts are arranged at nearly right angles to
each other to detect movement in all directions
 Hair cells bend as cupula moves, producing a receptor
potential followed by an action potential
• Action potential passes through the vestibular portion of the
eighth cranial nerve to the medulla oblongata
• Sent next to other areas of the brain and spinal cord for
interpretation, integration, and response
VISION: THE EYE
Structure of the eye (Figures 15-16 to 1525)
Accessory structures (Figures 15-16 to 15-20)
Eyebrows and eyelashes give some protection
against foreign objects entering the eye; cosmetic
purposes
Eyelids consist of voluntary muscle and skin
• Lined with conjunctiva, a mucous membrane
• Palpebral fissure: opening between the eyelids
• Angle or canthus: where the upper and lower eyelids join
VISION: THE EYE (cont.)
Lacrimal apparatus: structures that
secrete tears and drain them from the
surface of the eyeball (Figure 15-19)
• Lacrimal glands: size and shape of a small almond
 Located at the upper, outer margin of each
orbit
 Approximately 12 small ducts lead from
each gland
 Drain tears onto the conjunctiva
• Lacrimal canals: small channels that empty into lacrimal
sacs
• Lacrimal sacs: located in a groove in the lacrimal bone
• Nasolacrimal ducts: small tubes that extend from the
lacrimal sac into the inferior meatus of the nose
VISION: THE EYE (cont.)
Muscles of the eye
Extrinsic eye muscles: skeletal muscles that attach to
the outside of the eyeball and bones of the orbit
• Named according to their position on the eyeball
• Include the superior, inferior, medial, and lateral rectus muscles
and superior and inferior oblique muscles
Intrinsic eye muscles: smooth muscles located within
the eye
• Iris: regulates size of pupil
• Ciliary muscle: controls shape of lens
VISION: THE EYE (cont.)
Layers of the eyeball: three coats of tissues
make up the eyeball (Figure 15-21)
Fibrous layer: outer coat
• Sclera: tough, white, fibrous tissue
• Cornea: the transparent anterior portion that lies over the iris;
no blood vessels found in the cornea or in the lens
• Scleral venous sinus (canal of Schlemm): ring-shaped venous
sinus found deep within the anterior portion of the sclera at its
junction with the cornea
VISION: THE EYE (cont.)
Vascular layer: middle coat
• Contains many blood vessels and a large amount of pigment
• Choroid: pigmented membrane lining more than two thirds of the
posterior fibrous outer coat
• Anterior portion has three different structures
 Ciliary body: thickening of choroid; fits between
anterior margin of retina and posterior margin of iris;
 Suspensory ligament: attached to the ciliary
processes and blends with the elastic capsule of the
lens to hold it in place
 Iris: colored part of the eye; consists of circular and
radial smooth muscle fibers that form a doughnutshaped structure; attaches to the ciliary body
VISION: THE EYE (cont.)
Inner layer: incomplete innermost coat of the eyeball
• Retina: composed of an outer layer of pigmented epithelium
(pigmented retina) and an inner layer of nervous tissue
(sensory retina)
• Three layers of neurons make up the sensory retina
1. Photoreceptor cells: visual receptors,
sensitive to light rays
Rods: absent from the fovea and
macula; increased in density toward
the periphery of the retina
Cones: less numerous than rods; most
densely concentrated in the fovea
centralis in the macula lutea
2. Bipolar cells
VISION: THE EYE (cont.)
• Three layers of neurons make up the sensory retina (cont.)
3. Ganglionic cells: all axons of these neurons
extend back to the optic disk; part of the
sclera, which contains perforations through
which the fibers emerge from the eyeball as
the optic nerve
 Horizontal and amacrine cells allow lateral
connections within the sensory retina
• Optic nerve: second cranial nerve extends from the eyeball to
the brain
• Retinal blood vessels: critical to normal visual function (Figure
15-24)
VISION: THE EYE (cont.)
Cavities and humors
Cavities: eyeball has a large interior space divided
into two cavities
• Anterior cavity lies in front of the lens; has two subdivisions
 Anterior chamber: space anterior to the iris
and posterior to the cornea
 Posterior chamber: small space posterior to
the iris and anterior to the lens
• Posterior cavity is larger than the anterior cavity; occupies all
the space posterior to the lens, suspensory ligament, and ciliary
body
VISION: THE EYE (cont.)
Humors
• Aqueous humor: fills both chambers of the
anterior cavity; clear, watery fluid that often
leaks out when the eye is injured; formed from
blood in capillaries located in the ciliary body
(Figure 15-25)
• Vitreous humor: fills the posterior cavity;
semisolid material; helps maintain sufficient
intraocular pressure, with aqueous humor, to
give the eyeball its shape
VISION: THE EYE (cont.)
The process of seeing
Formation of retinal image
Refraction of light rays: deflection, or bending, of light
rays produced when they pass obliquely from one
transparent medium to another of different optical
density; cornea, aqueous humor, lens, and vitreous
humor are the refracting media of the eye
Accommodation of lens: increase in curvature of the
lens to achieve the greater refraction needed for near
vision (Figure 15-26)
VISION: THE EYE (cont.)
Formation of retinal image (cont.)
Constriction of pupil: muscles of iris are
important to formation of a clear retinal
image
• Pupil constriction prevents divergent rays from
object from entering eye through periphery of
the cornea and lens
• Near reflex: constriction of pupil that occurs
with accommodation of the lens in near vision
• Photopupil reflex: pupil constricts in bright light
VISION: THE EYE (cont.)
Formation of retinal image (cont.)
Convergence of eyes: movement of the two eyeballs
inward so that their visual axes come together at the
object viewed
• The closer the object, the greater the degree of convergence
necessary to maintain single vision
• For convergence to occur, a functional balance between
antagonistic extrinsic muscles must exist
• Strabismus is abnormal convergence (Figure 15-27)
VISION: THE EYE (cont.)
The role of photopigments: light-sensitive
pigmented compounds undergo structural
changes that result in generation of nerve
impulses interpreted by the brain as sight
Rods: photopigment in rods is rhodopsin
• Highly light sensitive
• Breaks down into opsin and retinal
• Separation of opsin and retinal in the presence of light causes
an action potential in rod cells
• Energy is needed to re-form rhodopsin (Figure 15-28)
VISION: THE EYE (cont.)
Cones: three types of cones in the retina, each with a
different variation of the rhodopsin photopigment:
blue, green, and red
• Perception of a large variety colors results from the
combination of signals from different cone types
• Cone pigments are less light sensitive than rhodopsin and need
brighter light to break down (Figure 15-29)
Ganglion cells: relay information from rods and cones
(by way of bipolar cells) but also relay nonimage light
information to the body’s biological clock;
photopigment is melanopsin
VISION: THE EYE (cont.)
Neural pathway of vision (Figure 15-30)
Fibers that conduct impulses from the rods and
cones reach the visual cortex in the occipital lobes by
way of the optic nerves, optic chiasma, optic tracts,
and optic radiations
Optic nerve contains fibers from only one retina, but
optic chiasma contains fibers from the nasal portion
of both retinas; these anatomical facts explain
peculiar visual abnormalities that sometimes occur