Chapter 15
Sense Organs
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Slide 1
Sensory Receptors

Sensory receptors make it possible for the body to respond to
stimuli caused by changes occurring in the internal or external
environment
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Receptor response

General function—responds to stimuli by converting them
to nerve impulses
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Different types of receptors respond to different stimuli

Receptor potential
• Develops when an adequate stimulus acts on a receptor; is a graded response
• When a threshold is reached, an action potential in the sensory neuron’s axon is triggered
• Impulses travel over sensory pathways to the brain and spinal cord, where either they are
interpreted as a particular sensation or they initiate a reflex action

Adaptation—a functional characteristic of receptors; receptor potential
decreases over time in response to a continuous stimulus, which leads
to a decreased rate of impulse conduction and a decreased intensity of
sensation
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Slide 2
Sensory Receptors

Distribution of receptors
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Receptors for special senses of smell, taste, vision,
hearing, and equilibrium are grouped into localized
areas or into complex organs

General sense organs of somatic senses are
microscopic receptors widely distributed
throughout the body in skin, mucosa, connective
tissue, muscles, tendons, joints, and viscera
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Slide 3
Sensory Receptors

Classification of receptors by the following:

Location
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Type of stimulus that causes response
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Sensory Receptors

Classification by location
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Exteroceptors
• On or near body surface
• Often called cutaneous receptors; examples are pressure, touch, pain,
and temperature
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Visceroceptors (interoceptors)
• Located internally—often within body organs, or viscera
• Provide the body with information about internal environment; examples
are pressure, stretch, chemical changes, and hunger and thirst

Proprioceptors: specialized type of visceroceptor
• Location limited to skeletal muscle, joint capsules, and tendons
• Provide information on body movement, orientation in space, and
muscle stretch
• Two types—tonic and phasic receptors provide positional information
on the body or body parts while at rest or during movement
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Slide 5
Sensory Receptors

Classification by stimulus detected

Mechanoreceptors—activated when “deformed” to generate
receptor potential
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Chemoreceptors—activated by amount or changing concentration
of certain chemicals; e.g., taste and smell

Thermoreceptors—activated by changes in temperature

Nociceptors—activated by intense stimuli that may damage tissue;
the sensation produced is pain

Photoreceptors—found only in the eye; respond to light stimuli if
the intensity is great enough to generate a receptor potential

Osmoreceptors—concentrated in the hypothalamus; activated by
changes in concentration of electrolytes (osmolarity) in
extracellular fluids
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Slide 6
Sensory Receptors

Classification by structure (Figure 15-2)—divides sensory
receptors into either those with free nerve endings or those
with encapsulated nerve endings
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Free nerve endings
•
•
•
•
Most widely distributed type of sensory receptor
Include both exteroceptors and visceroceptors
Called nociceptors—are primary receptors for pain
Other sensations mediated include itching, tickling, touch, movement, and
mechanical stretching
• Primary receptors for heat and cold
• Two types of nerve fibers carry pain impulses from nociceptors to the brain:

Acute (A) fibers—mediate sharp, intense, localized pain

Chronic (B) fibers—mediate less intense, but more persistent, dull or aching pain
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Slide 7
Sensory Receptors

Classification by structure (cont.)
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Other free nerve ending receptors
• Root hair plexuses

Weblike arrangements of free nerve endings around
hair follicles
• Merkel discs

Mediate sensations of discriminative touch
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Slide 8
Sensory Receptors

Stretch receptors—two types; muscle spindles and Golgi tendon
receptors operate to provide body with information concerning
muscle length and strength of muscle contraction

Muscle spindle—composed of 5 to 10 intrafusal fibers lying between
and parallel to regular (extrafusal) muscle fibers
– Large diameter and rapid conducting, type Ia, and smaller diameter and
slower conducting, type II, afferent fibers carry messages to brain
concerning changes in muscle length
– If length of a muscle exceeds a certain limit, a stretch reflex is initiated to
shorten the muscle, thus helping to maintain posture

Golgi tendon organs—located at junction between muscle tissue and
tendon (Figure 15-2)
– Type Ib sensory neurons are stimulated by excessive contraction—when
stimulated, they cause muscle to relax
– Golgi tendon reflex protects muscle from tearing internally as a result of
excessive contractile force
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Slide 9
Special Senses

Characterized by receptors grouped closely
together or grouped in specialized organs;
senses of smell, taste, hearing, equilibrium,
and vision
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Slide 10
Sense of Smell

Olfactory receptors

Olfactory sense organs consist of epithelial support
cells and specialized olfactory receptor neurons
(Figure 15-4)
• Olfactory cilia—located on olfactory receptor neurons that touch
the olfactory epithelium lining the upper surface of nasal cavity
• Olfactory cells—chemoreceptors; gas molecules or chemicals
dissolved in mucus covering the nasal epithelium stimulate
olfactory cells
• Olfactory epithelium—located in most superior portion
of nasal cavity
• Olfactory receptors—extremely sensitive and easily fatigued
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Slide 11
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Slide 12
Sense of Smell

Olfactory pathway—when level of odorproducing chemicals reaches a threshold
level, the following occurs (Figure 15-5):

Receptor potential, and then action potential, is
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
brain for interpretation, integration, and memory
storage
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Slide 13
Sense of Taste

Taste buds—sense organs that respond to gustatory,
or taste, stimuli; associated with papillae





Chemoreceptors that are stimulated by chemicals dissolved
in the saliva
Gustatory cells—specialized cells in taste buds; gustatory hairs
extend from each gustatory cell into the taste pore
Sense of taste depends on the creation of a receptor potential in
gustatory cells as a result of taste-producing chemicals in the saliva
Taste buds are similar structurally; functionally, each taste bud
responds most effectively to one of four primary taste sensations:
sour, sweet, bitter, and salty (and perhaps metallic and umami)
(Figure 15-6)
Adaptation and sensitivity thresholds are different for each of the
primary taste sensations
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Slide 14
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Slide 15
Sense of Taste

Neuronal pathway for taste

Taste sensation begins with a receptor potential in gustatory
cells of a taste bud; generation and propagation of an action
potential then transmits sensory input to the brain

Nerve impulses from anterior two thirds of the tongue travel
over the facial nerve; those from posterior one third of the
tongue travel over the glossopharyngeal nerve; vagus nerve
plays a minor role in taste

Nerve impulses are carried to the medulla oblongata,
relayed into the thalamus, and then into the gustatory area of
the cerebral cortex in the parietal lobe of the brain
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Slide 16
Sense of Hearing and Balance:
The Ear

External ear—two divisions
(Figures 15-7 and 15-8):

Auricle, or pinna—visible portion of the ear

External auditory meatus—tube leading from
auricle into the temporal bone and ending at the
tympanic membrane
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Slide 19
Sense of Hearing and Balance:
The Ear

Middle ear (Figure 15-8)

Tiny, epithelium-lined cavity hollowed out of the temporal bone

Contains three auditory ossicles
• Malleus (hammer)—attached to inner surface of tympanic
membrane

• Incus (anvil)—attached to malleus and stapes
• Stapes (stirrup)—attached to incus
Openings into middle ear cavity
• Opening from external auditory 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
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Slide 21
Sense of Hearing and Balance:
The Ear

Inner ear (Figure 15-9, A)

Structure of the inner ear
• Bony labyrinth—made up of the vestibule, the cochlea, and
semicircular canals
• Membranous labyrinth—made up of utricle and saccule inside the
vestibule, cochlear duct inside the cochlea, and the membranous
semicircular canals inside the bony ones
• Vestibule and semicircular canals 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 space between the membranous tunnel and its
contents and the bony walls that surround it
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Slide 22
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Slide 23
Sense of Hearing and Balance:
The Ear

Inner ear (cont.)

Cochlea and cochlear duct (Figure 15-9, B)
• Cochlea—bony labyrinth
• Modiolus—cone-shaped core of the bone that houses the spiral
ganglion, which consists of cell bodies of the first sensory neurons
in the auditory relay
• Cochlear duct







Lies inside the cochlea; only part of the internal ear concerned with
hearing; contains endolymph
Shaped like a triangular tube
Divides cochlea into the scala vestibuli, the upper section, and the scala
tympani, the lower section; both sections filled with perilymph
Vestibular membrane—roof of cochlear duct
Basilar membrane—floor of cochlear duct
Organ of Corti—rests on basilar membrane; consists of supporting cells
and hair cells
Axons of the neurons that begin around the organ of Corti, extend in the
cochlear nerve to the brain to produce the sensation of hearing
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Slide 24
Sense of Hearing and Balance:
The Ear

Inner 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
Basilar membrane is not the same width and thickness throughout its
length; high-frequency sound waves vibrate the narrow portion near
the oval window, whereas low frequencies vibrate the wider, thicker
portion near the apex of the cochlea; this fact allows different hair cells
to be stimulated and different pitches of sound to be perceived
Perception of loudness is determined by the amplitude of the
movement of basilar membrane; the greater the movement, the louder
the perceived sound
Hearing—results from stimulation of auditory area of cerebral cortex
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Slide 25
Sense of Hearing and Balance:
The Ear

Sense of hearing (cont.)
• Pathway of sound waves (Figure 15-10)

Enter external auditory canal

Strike tympanic membrane, causing vibrations

Tympanic vibrations move malleus, which in turn moves incus and
then stapes

Stapes moves against oval window, which begins fluid conduction
of sound waves

The perilymph in the scala vestibuli of cochlea begins “ripple” that
is transmitted through vestibular membrane to endolymph inside
duct, to basilar membrane, then to organ of Corti

From basilar membrane, ripple is transmitted through perilymph in
scala tympani and then expends itself against round window
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Slide 26
Sense of Hearing and Balance:
The Ear

Sense of hearing (cont.)
• Neuronal pathway of hearing

A movement of the hair cells against tectorial membrane
stimulates dendrites that terminate around base of hair
cells and initiates impulse conduction by the cochlear
nerve to the brainstem

Impulses pass through “relay stations” in the nuclei in
medulla, pons, midbrain, and thalamus before reaching
auditory area of temporal lobe
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Slide 27
Sense of Hearing and Balance:
The Ear

Vestibule and semicircular canals (Figure 15-9, A)
• Vestibule—the central section of the bony labyrinth; the utricle
and saccule are the membranous structures within the
vestibule
• Semicircular canals—three, each at right angles to the others,
are found in each temporal bone; within the bony semicircular
canals are the membranous semicircular canals, each
containing endolymph and connecting with the utricle; near
this junction, each canal enlarges into an ampulla
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Slide 28
Sense of Hearing and Balance:
The Ear

Vestibule and semicircular canals (cont.)
• Sense of balance
 Static
equilibrium—ability to sense the position of the head relative to
gravity or to sense acceleration or deceleration (Figure 15-11)
– Movements of the macula, located in both the utricle and saccule almost
at right angles to each other, provide information related to head position
or acceleration
– Otoliths are located within matrix of macula
– Changing head position produces a change of pressure on the otolithweighted matrix, which stimulates hair cells that, in turn, stimulate
receptors of vestibular nerve
– Vestibular nerve fibers conduct impulses to the brain and produce a
sensation of the position of the head and also a sensation of a change in
the pull of gravity
– Righting reflexes—muscular responses to restore the body and its parts
to their normal position when they have been displaced; caused by stimuli
of macula and impulses from proprioceptors and from eyes
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Slide 29
Sense of Hearing and Balance:
The Ear
• Sense of balance (cont.)

Dynamic equilibrium—needed to maintain balance when head or
body is rotated or suddenly moved; able to detect changes both in
direction and rate at which movement occurs (Figure 15-12)
– Depends on functioning of cristae ampullaris, which are located in
ampulla of each semicircular canal
– Cupula—gelatinous cap in which hair cells of each crista are
embedded; does not respond to gravity; moves with flow of
endolymph in semicircular canals
– Semicircular canals are placed at almost right angles to each other to
detect movement in all directions
– When the cupula moves, hair cells are bent, producing a receptor
potential followed by an action potential; action potential passes
through vestibular portion of eighth cranial nerve to medulla
oblongata, where it is sent to other areas of brain and spinal cord for
interpretation, integration, and response
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Slide 30
Vision: The Eye

Structure of the eye (Figures 15-13 and 15-14)

Coats of the eyeball—three layers of tissues compose the eyeball:
• Sclera—outer coat

Tough, white, fibrous tissue

Cornea—transparent anterior portion that lies over iris; no blood vessels found in
cornea or in lens

Canal of Schlemm—ring-shaped venous sinus found deep within anterior portion of
the sclera at its junction with the cornea
• Choroid—middle coat

Contains many blood vessels and a large amount of pigment

Anterior portion has three different structures (Figure 15-14):
– Ciliary body—thickening of choroid, fits between anterior margin of retina and
posterior margin of iris; ciliary muscle lies in anterior part of ciliary body; ciliary
processes—fold in ciliary body
– Suspensory ligament—attached to ciliary processes and blends with elastic
capsule of the lens, to hold it in place
– Iris—colored part of eye; consists of circular and radial smooth muscle fibers
that form a doughnut-shaped structure; attaches to ciliary body
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Vision: The Eye

Coats of the eyeball (cont.)
• Retina—incomplete innermost coat of eyeball

Three layers of neurons make up the sensory retina (Figure 15-16):
– Photoreceptor neurons—visual receptors, highly specialized for
stimulation by light rays
– Rods—absent from fovea and macula; increased in density toward
periphery of retina
– Cones—less numerous than rods; most densely concentrated in fovea
centralis in macula lutea
– Bipolar neurons
– Ganglionic neurons—all axons of these neurons extend back to the
optic disc; part of sclera, which contains perforations through which the
fibers emerge from the eyeball as the optic nerve
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Slide 32
Vision: The Eye

Cavities and humors
• Cavities—eyeball has a large interior space divided into
two cavities:

Anterior cavity—lies in front of lens; has two subdivisions
– Anterior chamber—space anterior to iris and posterior to cornea
– Posterior chamber—small space posterior to iris and anterior to lens

Posterior cavity—larger than anterior cavity; occupies all the
space posterior to lens, suspensory ligament, and ciliary body
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Slide 33
Vision: The Eye

Cavities and humors (cont.)
• Humors

Aqueous humor—fills both chambers of anterior cavity;
clear, watery fluid that often leaks out when eye is injured;
formed from blood in capillaries located in ciliary body
(Figure 15-17)

Vitreous humor—fills posterior cavity; semisolid material;
helps to maintain sufficient intraocular pressure, with
aqueous humor, to give the eyeball its shape
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Slide 34
Vision: The Eye

Muscles—two types of eye muscles:
• Extrinsic eye muscles (Figure 15-18)—skeletal muscles that
attach to the outside of the eyeball and to the bones of the orbit;
named according to their position on eyeball; the muscles are
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
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Slide 35
Vision: The Eye

Accessory structures (Figure 15-19)
• Eyebrows and eyelashes—give some protection against foreign
objects entering eye; cosmetic purposes
• Eyelids—consist of voluntary muscle and skin with a tarsal plate; lined
with conjunctiva, a mucous membrane; palpebral fissure—opening
between the eyelids; canthus—where upper and lower eyelids join
• Lacrimal apparatus—structures that secrete tears and drain them
from surface of eyeball (Figure 15-21)

Lacrimal glands—size and shape of a small almond; located at upper,
outer margin of each orbit; approximately a dozen small ducts lead from
each gland; drain tears onto conjunctiva

Lacrimal canals—small channels that empty into lacrimal sacs

Lacrimal sacs—located in a groove in lacrimal bone

Nasolacrimal ducts—small tubes that extend from lacrimal sac into
inferior meatus of nose
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Slide 36
Vision: The Eye

The process of seeing

Formation of retinal image
• Refraction of light rays—deflection, or bending, of light rays produced
by light rays passing obliquely from one transparent medium into
another of different optical density; cornea, aqueous humor, lens, and
vitreous humor are the refracting media of the eye
• Accommodation of the lens—increase in curvature of lens to achieve
the greater refraction needed for near vision (Figure 15-22)
• Constriction of the 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 lens in
near vision; photopupil reflex—pupil constricts in bright light
• Convergence of the 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
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Slide 37
Vision: The Eye

The process of seeing (cont.)

Role of photopigments—light-sensitive pigmented
compounds undergo structural changes that result in
generation of nerve impulses, which are 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 reform rhodopsin (Figure 15-24)
• Cones—three types of cones are present in retina, with each
having a different photopigment; cone pigments are less lightsensitive than rhodopsin and need brighter light to break down
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Slide 38
Vision: The Eye

The process of seeing (cont.)

Neuronal pathway of vision (Figure 15-25)
• Fibers that conduct impulses from rods and cones reach the
visual cortex in occipital lobes via 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
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Slide 39
Cycle of Life: Sense Organs

Sensory information is acquired through
depolarization of sensory nerve endings


Age, disease, structural defects, or lack of
maturation affect ability to identify and respond
Structure and function response capabilities
are related to developmental factors
associated with age
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Slide 40
Cycle of Life: Sense Organs

Senses become more acute with maturation

Late adulthood—loss of sensory capability

Structural change in receptor cells or other sense
organ structures
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