Anatomy
Chapter 8 – Special Senses
Special Senses:
Humans are responsive creatures.
“Irritants” are the stimuli that continually enter our nervous
system for integration, and may elicit a response.
We are usually told we have five senses that keep us in touch with
what is going on in the external world: taste, sight, smell, touch,
and hearing .
Touch is a mixture of general senses – temperature, pressure, and
pain receptors of the skin and proprioceptors of muscle and joints.
Smell, sight, taste, and hearing are called special senses. Special
sense receptors are either large, complex sensory organs (eyes,
ears) or localized clusters of receptors (taste buds and olfactory
epithelium).
The Eye and Vision
The adult eye is a sphere about 1
inch (2.5cm) in diameter.
The eye is enclosed and protected
by a cushion of fat and the bony
walls of the orbit.
Accessory organs:
Eyelids
Eyelashes
Tarsal glands
Conjunctiva
Lacrimal apparatus
Lacrimal glands
Lacrimal canal
Lacrimal sac
Nasolacrimal duct
Lysozyme
Frontal bone of orbital
70% of
sensory input
comes
through the
eyes.
Extrinsic muscles of the Eye
Lateral view of right eye
Superior view of the right eye
Six extrinsic or external, eye muscles attach to the outer surface of the eye.
These muscles produce gross movements in the eye allowing the eye to follow
objects. The muscles control the reflexive movements of convergence, the
movement of the eyes medially when we view close objects. These muscles lose
elasticity as we age (40’s).
The eye is commonly called
the eyeball.
Internal anatomy of the Eye
The wall is composed of
three tunics or coats:
Sclera – outermost tunic
Cornea
Choroid – middle tunic
Pupil
Iris
Retina – inner tunic
Photoreceptors
Optic disc
Fovea centralis
Lens focuses light on the retina. It is a flexible, biconvex crystal-like structure.
Lens divides the eye into segments or chambers – anterior aqueous humor and
posterior vitreous humor.
Pathway of light through the eye and light refraction
When light passes from one substance to
another , with different densities, its speed
changes and its rays are bent, or refracted.
Light rays are bent by the cornea, aqueous
humor, lens, vitreous humor , before they
encounter the retina. The refracting power
of the lens can vary by being more or less
convex, allowing light to be properly focused
on the retina.
The eye is set for distance vision – light
comes in a parallel waves.
Close objects tend to cause light to scatter
and diverge (spread out). This causes the
lens to bulge to make vision possible, called
accommodation.
Relative convexity of lens
focusing on objects.
Visual Fields and Visual
Pathways to the Brain
Optic chiasma – the fibers from the
medial side of each eye cross over
to the opposite side. The fiber tracts
that result are the optic tracts. The
optic tract fibers synapse with
neurons in the thalamus which runs
to the occipital lobe of the brain.
Eye Reflexes:
Photopupillary reflex
Accommodation Pupillary reflex
Convergence
The visual fields overlap considerably
(area of binocular vision).
Errors of refraction and Eye Dysfunction
Ophthalmoscope is an instrument
used to illuminate the interior of
the eye.
Nearsighted and Farsighted
Fundus of the eye, posterior wall.
Cataracts appear as a milky
structure the seems to fill the
pupil.
Night blindness – vitamin A deficiency
Color blindness – genetic, x linked
Glaucoma – increase pressure in the eye
The Ear: Hearing and Balance
Outer Ear:
Pinna – Auricle
External auditory canal
Ceruminus glands - cerumin
Tympanic membrane
Middle Ear:
Ossicles (3)
Auditory tube
Inner Ear:
Cochlea
Vestibule
Semicircular canals
The bony labyrinth of the inner ear is filled with a plasma-like fluid called perilymph.
Suspended in the bony labyrinth are membranous sacs that contain a thicker fluid
called endolymph.
Mechanisms of Equilibrium:
The equilibrium sense responds to the various
movements of our head. The equilibrium
receptors of the inner ear, the vestibular
apparatus, can be divided into two functional
groups - static equilibrium and dynamic
equilibrium.
Static Equilibrium:
Maculae
Otolithic membrane
Otoliths
Vestibular nerve
When the head is tipped, the
maculae are stimulated by
movement of otoliths in the
otolithic membrane with
gravitational pull, which creates
pull on the hair cells
Dynamic Equilibrium: receptors respond to angular or rotatory movements of
the head. The semicircular canals are oriented in the three planes of space, so
regardless of which plane one moves in, there will receptors to detect the
movement.
Semicircular canals
Sight and proprioceptors of the
Crista ampullaris
muscles and tendons provide
Cupula
information used to control balance
Vestibular nerve
to the cerebellum.
Three spatial planes.
Ampulla contains a
crista ampillaris.
Head position changes.
Mechanisms of Hearing:
Within the cochlear duct is the organ of Corti, which contains the hearing
receptors or hair cells. The chambers above and below the cochlear duct
contain perilymph. Sound waves reach the cochlea by way of the eardrum,
ossicles, and oval window. The transfer of waves sets the cochlear fluids in
motion. The receptor cells on the basilar membrane, in the organ of Corti, are
stimulated when their “hairs” are bent by the movement of the gel-like
tectorial membrane.
Once stimulated, the
hair cells transmit
impulses along the
cochlear nerve to the
auditory cortex in the
temporal lobe. The
sound is interpreted
and hearing occurs.
Sound reaches both
ears at different times
giving us “stereo”.
Cross-sectional view of
the cochlea showing
the organ of Corti in the
cochlear duct.
Detailed structure of the
organ of Corti.
Hearing and equilibrium deficits:
Otitis media – ear infection
Fungus in the ear
Normal ear drum
Perforated ear drum
Swimmers ear
Tubes in the ear
Deafness – hearing loss of any degree; slight to total inability to hear sound.
Conduction deafness - temporary or permanent; something interferes with
conduction of sound vibrations in the inner ear – earwax, fusion of ossicles,
ruptured eardrum, otitis media.
Misshapen stapes
Sensorineural deafness – degeneration or damage to the receptor cells of the
organ of Corti, cochlear nerve, neurons or auditory cortex; listening to loud or
extended sounds.
Meniere’s disease – may be caused by arteriosclerosis, degeneration of cranial
nerve VIII, increased pressure of inner ear fluids - progressive deafness.
Vertigo – sensation of spinning; nausea.
Chemical Senses: Taste and Smell
The receptors for taste and smell are classified as chemoreceptors. These
receptors respond to chemicals. There are 5 identified taste receptors. The
olfactory receptors are sensitive to a much wider range of chemicals. The sense
of smell and taste complement each other and will respond to some of the same
stimuli.
There are 1000’s of olfactory receptors for the sense of smell that occupy a
postage stamp-sized area in the roof of each nasal cavity. Our sense of smell is far
less acute than other animals.
Olfactory receptor cells are
neurons equipped with
olfactory hairs. The hairs are
stimulated by chemicals
dissolved in the mucus. The
hairs transmit impulses along
the olfactory filaments making
up the olfactory nerve.
Olfactory impressions can be long-lasting and form permanent memories and
emotions. Grandma’s house, a vacation.
Taste buds ad the sense of taste:
We have 10,000 taste buds, most are of which are found on the tongue.
Papillae – small peg-like projections on the surface of the tongue
Filiform – sharp
Fungiform - rounded
The cells that respond to the
Circumvallate
chemicals dissolved in the
saliva are epithelial cells called
gustatory cells.
There are 5 basic tastes:
Sweet –
Salt – metal ions (found on the tip)
Sour –
Bitter – alkaloids (posterior);
protection – gag reflex
Umami – protein, glutamate (beef
steak taste)
Influences on taste: taste and smell influence each other
Smell – olfactory receptors
Temperature –
Texture Developmental aspects of senses:
Senses form early, the eye by the forth week. The eye functions at birth but is
the last to completely develop – learning involved.
Strabismus – congenital eye disorder; “cross-eyed”, can be corrected.
Infants have poor visual acuity – lack complete color vision until 5; depth
perception is present.
Aging –
Presbyopia – lens elasticity is less, difficult to focus on close objects (40 yrs)
Atherosclerosis – hardening of arteries in the eye; decrease oxygen and
nutrients
Decrease in olfactory and gustatory receptors – taste and smell diminishes: 80
years and older; cannot smell and taste, highly season food; lose appetite.