Chapter 17
Sense Organs
 General senses
Taste
Smell
Hearing
Equilibrium
Vision
The general senses
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Refers to the senses that are relatively simple in
structure and physiology (touch, pressure, etc).
Their receptors tend to have a structure that is
specialized to detect a specific stimulus.
They may so simple as to be simple nerve dendrites.
Unencapsulated receptors are sensory dendrites that
lack a connective tissue wrapping (e.g. free nerve
endings, Merkels discs and hair receptors).
Encapsulated receptors are dendrites with a glial cell
wrap or connective tissue covering (e.g. Meissners
corpuscles, Pacinian corpuscles muscle spindles).
Receptor Classification based on origin
of stimulus
1.
2.
3.
Exteroceptors- receptors sensitive to changes
outside of body. Are found on or near the body
surface: pain, pressure, touch, temperature and
special sense organs (eye, ear, nose, mouth).
Interoceptors- visceroceptors receive stimuli from
viscera; stretch, temperature, chemical, taste.
Proprioceptors- located in skeletal muscles, joints,
tendons, and ligaments; perceive stretch in these
organs
Proprioceptors
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Encapsulated
receptors that monitor
stretch in muscles and
tendons.
Ex. Muscle spindles
Extrafusal fibers
Intrafusal
Classified by Stimulus Modality
1.
2.
3.
4.
5.
Mechanoreceptors- respond to mechanical forces:
touch, pressure, stretch, vibration, and itch.
Thermoreceptors- respond to temperature changes
Chemoreceptors- respond to chemicals in solution
and blood chemistry.
Photoreceptors- respond to changes in light-eye
Nociceptors- respond to pain and harmful stimuli
leading to pain.
Classification by Structure
1.
Free nerve endings- are found in all tissues of body;
especially abundant in epithelial and connective
tissue.
- respond to pain, temperature, itch and light pressure
Ex.- Merckels discs, hair follicle receptors
2.
Encapsulated nerve endings- enclosed in a capsule
of connective tissue; vary in shape and distribution.
Ex.- Meissner’s, Pacinian and Ruffini’s corpuscles
Taste “Gustation”
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Taste receptors are in taste buds- chemoreceptors
Occur on tongue, palate, cheeks, pharynx and
epiglottis as papillae.
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Tongue has 4 types of papillae: a). Filoform; b). Foliate;
c). Fungiform and d). Circumvallate.
In adults last 2 types function in taste on surface of tongue:
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Fungiform papillae widely distributed on tongue
Circumvallate papillae form a “V” at back of tongue
Filoform papillae do not sense taste, but the texture of foods.
Taste bud = 50-100 epithelial cells consisting of
supporting, basal and gustatory cells
Taste buds
Tastes
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There are five tastes that we can discriminate when
presented to the tongue and mouth
- Sweet
- Sour
- Bitter
- Salty
- Umami
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Taste is perceived by taste receptors than relayed to
brain via: CN VII (Facial), IX (Glossopharyngeal) and
X (Vagus) nerves.
Taste is highly dependent on smell for full perception
of the odor, without smell we lose much of our taste.
Taste pathways to CNS
via cranial nerves
VII = ant 2/3 tongue
IX = post 1/3 tongue
and pharynx
X = pharynx and
epiglottis
Smell “Olfaction”
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Chemoreceptors located in superior nasal concha and septum
in the olfactory epithelium (nasal mucosa). ~ 5 sq. cm.
Olfactory epithelium is pseudostratified columnar
Odors must be in liquid state to be perceived, thus they mix
with mucus prior to be sensed by receptors.
~ 10,000 different odor molecules and ~ 1,000 different
receptor types. Average human can detect 2,000 - 4,000 odors.
Olfactory receptor cells are bipolar and in cribriform plate of
ethmoid bone.
Olfactory nerve dendrites have olfactory cilia that bind the
odors to them. The odor is relayed to the olfactory nerve axon
which synapses with the mitral cell dendrites of the olfactory
bulb → brain.
Smell is perceived in the limbic system, hypothalamus and
olfactory cortex (areas 28 & 34) of the piriform lobe.
Olfactory pathway
Hearing and Equilibrium
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Is mediated through the structures of the inner ear.
These structures are all embedded in the temporal
bones of the cranium
Hearing involves the perception of sound waves
entering the external auditory canal which is received
by the middle ear and relayed to the inner ear where it
is then registered in the cochlea.
Equilibrium is maintained and perceived by the
structures in the semi-circular canals of the inner ear.
Outer ear anatomy
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Pinna – auricle covered with skin and made of elastic
cartilage; helix and lobule
External auditory meatus- ear canal lined with skin
and ceruminous glands passes through temporal bone
Tympanic membrane – “ear drum” separates outer ear
from middle ear and is innervated with sensory fibers
from trigeminal and vagus nerves. Responds to air
vibrations or sound waves and transfers energy to
middle ear ossicles.
Hearing and Equilibrium
Middle ear anatomy
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Three middle ear ossicles joined together:
Malleus – looks like a hammer attaches to tympanic
membrane and connected to the incus.
Incus - Looks like an anvil and connected to the stapes.
Stapes- looks like a stirrup – footplate attaches to oval window
of inner ear where it creates ossicilations in the scala vestibuli
Middle ear muscles: stapedius inserts on stapes; tensor
tympani inserts on malleus. Contract during loud noises and
relieve tension on ossicles to protect inner ear.
Eustachian tube connects middle ear to nasopharynx.
Middle ear anatomy
Inner ear anatomy
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Housed in a maze of temporal bone passages called
the bony labyrinth.
Bony labyrinth is filled with perilymph and is lined
with periosteum; perilymph is continuous with CSF
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Bony labyrinth consists of:
-a). vestibule, b). cochlea and c). semicircular canals
Membranous labyrinth lines inside of bony labyrinth.
It is lined with epithelium and is filled with
endolymph (high in Na+, K+).
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Consists of semicircular ducts, utricle and saccule and the
cochlear duct.
Inner ear anatomy
Structures of hearing
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Cochlea –spiraling chamber of bony labyrinth. Looks
like a snails shell, and spirals around modiolus.
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Lined with a membranous labyrinth with 3 separate
chambers:
Scala vestibuli –filled with perilymph abuts oval window.
 Cochlear duct –filled with endolymph and contains organ of
Corti
 Scala tympani –filled with perilymph abuts round window.
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Scala vestibuli an scala tympani are joined by heliocotrema at the
apex of cochlea.
Organ of Corti
Structures of hearing
Innervation from CN-VIII
Cochlear portion of VIII
pathway to CNS for
hearing.
Structures of equilibrium
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Vestibule houses the saccule and utricle which each
contain a macculae that are receptor cells for head
position when the head is still = static equilibrium.
They also monitor head movements in a straight line
= linear acceleration.
Semicircular canals are lined with a membranous
semicircular duct.
Ampulla is bulge at each end of canal and inside is the
membranous ampulla which contains a crista ampullaris.
 These receptors detect head rotation or angular rotation.
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Macculae structure
In utricle and
saccule. Respond to:
- Static equilibrium
- Linear acceleration
Structures of equilibrium
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Semicircular canals are lined with a membranous
semicircular duct.
Ampullae is bulge at each end of semicircular canals.
 Membranous ampullae contains a crista ampullaris.
 These receptors detect head rotation or angular rotation
and dynamic equilibrium.
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Semicircular ducts
with ampullae and Crista
Ampullaris.
Respond to:
-Detect angular rotation
-Dynamic equilibrium
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The Eye and Vision
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Visual receptors account for ~ 70% of all sensory
input in the body. 40% of the cerebral cortex in
involved in processing visual perception.
Eye balls are the sense organs for vision
Eye is a sphere ~ 1” in diameter and is located in the
orbital cavity of the skull
Accessory structures of eye
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Eyebrows
Eyelids with tarsal and ciliary glands.
Eyelashes
Lateral and medial canthus (angles at corner of eye
Conjunctiva
Lacrimal apparatus
Extrinsic eye muscles (6)
Accessory structures of eye
Accessory structures of eye
Eye accessory structures
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Eyelids “palpebrae” protect eyes from foreign objects
and bright light.
- Two lids separated by palpebral fissure
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Angle of eyes is canthus- medial and lateral canthi
Lacrimal caruncle is at median canthus
Levator palpebrae muscle raises and lowers eye lids
Tarsal glands in eyelids secrete oily substance that keeps
eyes moist. Chalazion
Eyelashes on the margin of each lid help to keep
substances from entering eye; ciliary glands open into
lash follicle. Sty
Eye accessory structures
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Conjunctiva - transparent mucous membrane covering
inner surface of eyelids (palpebral conjunctiva) and anterior
surface of eye (bulbar conjunctiva), but not the cornea.
-made up of stratified columnar epithelium with numerous goblet cells
that secrete mucous and keep eyeball and lids moist.
-when eye is closed a slit-like space forms= conjunctival sac.
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Lacrimal apparatus keeps surface of eye moist with tears from
lacrimal glands. From upper lateral position, tears washover
eyes when you blink eyes and move to medial canthus to drain
into lacrimal puncta → → canaliculi → lacrimal sac and →
nasolacrimal duct → nasal cavity.
=- Lacrimal fluid “tears” contain mucus, antibodies and
lysozymes that destroy bacteria.
Eye accessory structures
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Extrinsic eye muscles control all movements of
eyeball in the socket.
Six muscles:
Superior rectus: CN III- elevates eye
 Inferior rectus: CN III- depresses eye
 Medail rectus: CN III- moves eye medially
 Lateral rectus: CN VI- moves eye laterally
 Superior oblique: CN IV- depresses eye and turns it
laterally
 Inferior oblique: CN III- elevates eye and turns it laterally
III- occulomotor; IV- trochlear; VI- abducens
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Extrinsic eye muscles
Eye anatomy
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The eye is a sphere with a bulge (cornea) at the front
and a stem at the back (optic nerve). The outside is
covered by a tough outer covering called fibrous tunic.
Consists of 3 tunics (fibrous, vascular and sensory)
and 2 chambers (anterior and posterior) separated by a
lens and iris.
The anterior chamber is filled with aqueous humor
 Posterior chamber is filled with vitreous body
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The visual receptor field (retina) occupies the major
portion of the posterior wall of the eye and light
reaching the retina is regulated by the iris.
Fibrous tunic
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The fibrous tunic consists of the cornea and sclera.
Cornea is avascular and transparent and allows light to pass
through. It is made of 100’s of sheets of collagen fibers
sandwiched between two layers of epithelium. Highly
innervated and sensitive to touch or particles on it.
 Sclera is the white of the eye which protects, shapes and
serves as the anchor site for the extrinsic eye muscles
 Limbus is junction between sclera and cornea- stem cells
here between cornea and conjunctiva allow for continual
renewal of cornea.
 Scleral venous sinus allows for drainage of aqueous humor
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Vascular tunic
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Consists of three parts: choroid, ciliary body and iris.
Choroid highly vascular dark brown pigmented layer that
covers 5/6 of posterior chamber.
-Melanocytes produce melanin that accounts for dark
brown color of choroid.
 Ciliary body anterior to choroid consists of ciliary muscle
(sm. m.), and the ciliary process. Radiating off of ciliary
process are fine fibrils that attach to the iris and control its
thickness.
 Iris is the colored portion of the eye and constricts and
dilates to regulate the amount of light entering into the eye.
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Sensory tunic “Retina”
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Consists of two layers; thin pigmented layer and a
thick neural layer
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outer layer – pigmented layer
- beginning of visual pathway to brain
inner layer – neural layer contains rods and cones
-layer where light rays are deciphered and
converted into an impulse to be relayed to brain.
- ganglion cells- synapse with bipolar cells and make a 90
degree turn on retinal surface to go into optic nerve.
- bipolar cells- synapse with rod and cone receptors to synapse with
ganglion cells.
Sensory tunic
- Rods= Blk/Whte
- Cones = Color
Sensory tunic
Sensory tunic “Retina”
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optic disc (blind spot) – site where optic nerve exits
eye ball.
macula lutea containing the fovea centralis
-small flat yellowish spot in exact center of posterior
eye
-contains only CONES, and no bipolar or ganglion
cells to scatter light
- Macula lutea is the area of highest visual acuity
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Optic disc and Macula lutea
Optic disc
Fovea centralis
Macula lutea
The LENS
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Defined: composed of proteins called crystallins
arranged in layers much like an onion. It is
completely transparent and lacks any blood vessels.
Attachments: Lens is enclosed in a clear connective
tissue capsule and is held in place by encircling
zonular fibers which attach it to the ciliary processes.
Lens divides the eye into two cavities:
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anterior chamber – lies between the cornea and iris
- is filled with aqueous humor a watery fluid that nourishes the
lens and cornea.
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posterior chamber – lies between the iris and in front of
the zonular fibers and lens.
The Photoreceptors
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RODS- receptor for black and white light
 Location – (~ 250 million) in the pigmented layer
of the retina
 Functions – low light and peripheral vision
receptors
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CONES- receptors for colored light
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Location – (~6 million) in the pigmented layer
of retina and macula lutea
Functions – operate in bright light and are high
acuity color receptors.
The Photoreceptors
Visual
pathway