Chapter 15: Special Senses
Eye and Vision




The eye is enclosed in the boney orbit and cushioned by fat.
Accessory structures include
 Eyebrows, which shade and protect the eye
 Eyelids, protect and lubricate the eyes by reflex blinking
 Within the eyelids are the orbicularis oculi and levator
palpebrae muscles and modified sebaceous and sweat
glands
 The conjunctiva is a mucosa that lines the eyelids and covers
the anterior eyeball surface
 Its mucus lubricates the eyeball surface
The lacrimal apparatus consists of the lacrimal gland (produces
saline solution containing mucus, lysozyme, and antibodies) the
lacrimal canaliculi, the lacrimal sac, and the nasolacrial duct.
The extrinsic eye muscles (superior, inferior, lateral and medial
rectus and superior and inferior oblique) move the eyeballs.
Structure of the eyeball

The wall of the eyeball is made up of 3 layers
 The fibrous layer (the outer most) consists of sclera, and the
cornea
 The sclera protects the eye and gives it shape
 The cornea allows light to enter the eye
 The middle, pigmented vascular layer (uvea) consists of the
choroid, the ciliary body, and the iris
 The choroid provides nutrients to the eye and prevents
light scattering within the eye
 The ciliary muscles of the ciliary body control lens
shape
 The iris controls the size of the pupil




The sensory layer, or retina, consists of an outer pigmented
layer and an inner neural layer.
 The neural layer contains the photoreceptors (rods and
cones), bipolar cells, and ganglion cells.
 Ganglion cell axons form the optic nerve which
exits the eye via the optic disk (“blind spot”)
 The outer segments of the photoreceptors
contain the light-absorbing pigment membrane –
bounded discs
The posterior segment of the eyeball, behind the lens,
contains vitreous humor, which helps support the eyeball
and keep the retina in place.
The anterior segment, anterior to the lens, is filled with
aqueous humor, formed by capillaries in the ciliary
processes and drained into the scleral venous sinus.
 Aqueous humor is a major factor in maintaining
intraocular pressure
The biconvex lens is suspended within the eye by the ciliary
zonule attached to the ciliary body. It is the only adjustable
refractory structure of the eye.
Physiology of Vision:



Light is made up of those wavelengths of the electromagnetic
spectrum that excite the photoreceptors - ROYGBIV. Red
wavelengths are the longest and have the lowest energy and
violet are the shortest and the most energetic.
Light is refracted (bent) when passing from one transparent
medium to another of a different density.
 Concave lens disperse the light, convex lens converge the
light and bring its rays to a focal point.
 The greater the lens curvature, the more light bends
As light passes through the eye, it is bent by the cornea and the
lens and focused on the retina.
 The cornea accounts for most of the refraction.
o The lens allows for active focusing for different distances
 Focusing for distance vision requires no special
movements of the eye structures.
 Focusing for close-up vision requires accommodation
(bulging of the lens), pupillary constriction, and
convergence of the eyeballs.
 Accommodation is the process that increases the
refractory power of the lens
 Pupillary constriction prevents the most divergent
light rays from entering the eye. These rays would
pass through the extreme edge of the lens causing
blurred vision
 Convergence is the medial rotation of the eyeballs
by the medial rectus muscles so that each is
directed toward the object being viewed
 All three reflexes are controlled by Cranial nerve
III
 Refractory problems include presbyopia, myopia, hyperopia and
astigmatism
 99% of refractory problems are related to the eyeball shape,
either too long of too short
 Presbyopia, (old people’s vision), the eyeball is nonaccommodating – the lens losing its elasticity. Having to hold a
book at arm’s length
 Myopia (short vision) occurs when distant objects are focused not
on, but in front of, the retina. Eyeball is too long
 Myopic people can see close objects without problems but distant
objects are blurred. Nearsighted
 Hyperiopia, (far vision) or far- sightedness. When distant objects
are focused behind the retina. These people can see far objects
well because the ciliary muscles contract continuously to increase
the light bending power of the lens, which brings the focal point
back to the retina.
 Astigmatism, unequal curvatures of different
parts of the cornea and/or the lens causes blurry
images.
Functional Anatomy of Photoreceptors
 Photoreceptors are modified neurons
 Rods respond to low intensity light and provide night and
peripheral vision
 Cones are bright light, high discrimination receptors that
provide color vision
o Anything that must be viewed precisely is focused on
the cone-rich fovea centralis
 Rods and cones have an outer segment (receptor region)
and an inner segment (connecting to the cell body)
 These cells are highly vulnerable to damage and
immediately begin to degenerate if the retina becomes
detached.
o They can also be destroyed by intense light
 Rods and cones contain unique visual pigments that absorb
different wavelengths of light.
o They also have different thresholds for activation
 Rods are very sensitive and respond to very dim
light ( a single photon) making them best suited for
night vision and peripheral vision
 They contain a single pigment so it is perceived as
tones of gray
 Cones need bright light, have 3 different pigments
that provide a vividly colorful view
 Light absorbing molecule is called Retinal which is
chemically related to Vitamin A and is made from it.
o Retinal combines with a proteins called Opsins to form
4 types of visual pigments.
 Rods visual pigment is rhodopsin
 Three types of cones all contain retinal but each
has a different type of opsin. Each responding to
different wavelengths of light – red, blue, green
 Color blindness is the absence of one of more cones.
Usually red or green, it is an X-linked trait and is more
prevalent in males than females
 Night blindness is a condition in which rod function is
severely hampered impairing the ability to drive safely at
night. Most common cause is a vitamin A deficiency which
leads to rod degeneration.
Visual Pathways to Brain:



Begins with the optic nerve fibers (ganglion cell axons) from
the retina.
To the optic chiasma (cross)
To the optic tracts to synapse with the thalamus neurons
which connect to the primary visual cortex in the occipital
lobes where seeing occurs.
Chemical Senses: Taste and Smell
 Taste and smell are primitive senses that alert us to stuff to
be savored or avoided
 Taste (gustation) and Smell (olfaction: olfact = to smell)
are chemoreceptors
o Respond to chemicals in an aqueous solution
o They complement each other and respond to different
classes of chemicals
o Taste receptors are excited by food chemicals
dissolved in the saliva




o Smell receptors, by the airborne chemicals that dissolve
in the fluids coating the nasal membranes
Olfactory epithelium is located on the roof of the nasal cavity.
o Receptor cells are ciliated (increasing surface area)
bipolar neurons.
Individual neurons show a range of responsiveness to
different chemicals
o Smell is difficult to research because any given odor
made up of hundreds of different chemicals.
o Some studies have identified 1000 smell genes that are
active only in the nose. Each gene encodes for a
specific receptor protein
o Olfactory neurons are exceptionally sensitive with some
taking a few molecules to activate them.
For us to smell a particular odor, it must be volatile, that
means it must be in a gaseous state as it enters the nasal
cavity.
o It must also dissolve in the fluid coating of the olfactory
epithelium
Pathways:
o Action potentials of olfactory nerve filaments are
transmitted to the olfactory bulb where to filaments
synapse with mitral cells. The mitral cells send
impulses via the olfactory tract to the olfactory cortex.
Fibers carrying impulses from the olfactory receptors
also project to the limbic system. (our emotional, or
affective (feelings) center in the brain)
Taste:
 Taste buds are scattered in the oral cavity and pharynx but
are most abundant on the tongue papillae
 Gustatory cells, the epithelial receptor cells of the taste buds,
have gustatory hairs (microvilli) that serve the receptor
regions.
 We have 5 basic taste qualities: Sweet, Sour, Salty, Bitter
and Umami (allows you to experience the “beef taste” of
steak, the tang of aging cheese
o Most taste buds respond to 2 or more taste qualities
o There are only slight differences in the localization of
specific taste receptors in the different regions of the
tongue
 Pathway: through the cranial nerves VII, IX, and X which
send impulses to the nucleus of the medulla. From there to
the thalamus and the taste cortex.
Hearing and Balance:
 Our hearing apparatus allows us to hear an extraordinary
range of sound.
 Our equilibrium receptors keep our brain informed of head
movements and position.
 Structurally, these are interconnected but they respond to
different stimuli and are activated independently of one
another.
Structure of the Ear:
 The ear is divided into 3 major areas: external ear, middle
ear and internal ear.
o The external and middle ear are involved in hearing
only (relatively simple structurally)
o The internal ear is involved with hearing and balance
and is extremely complex
 The external ear consists of the auricle and external
acoustic meatus. The auricle (pinna) is what most people
call the ear.
o Composed of elastic cartilage covered with thin skin
and an occasional hair
o Function is to direct sound waves to the external
auditory meatus (auditory canal)
o Sound waves entering the auditory meatus hit the
tympanic membrane (eardrum), the boundary
between the external ear and the middle ear.
 Thin translucent membrane shaped like a flattened
cone. Vibrations then transmit the sound waves to
the bones of the middle ear.
 The middle ear or tympanic cavity, is a small, air filled,
mucosa lined cavity in the temporal bone
o Connected by the pharyngotympanic tube to the
naso-pharynx.
o The ossicles, which help amplify the sound, span the
middle ear cavity and transmit sound vibrations from
the eardrum to the oval window (vestibular window).
 The internal ear
o Consists of the boney labyrinth (maze), within which
the membranous labyrinth (continuous series of
membranous sacs and ducts) is suspended
o Boney labyrinth chambers contain perilymph (fluid
similar to cerebral spinal fluid)
o The membranous sacs and ducts contain endolymph
(K+ rich intracellular fluid)
 These two fluids conduct the sound vibrations
involved in hearing and respond to the mechanical
forces occurring changes in body position and
acceleration
o The vestibule is the central egg shaped cavity of the
boney labyrinth.
 Contains the saccule and utricle which house
receptor regions called maculae that respond to
the pull of gravity and changes in head position.
 The semicircular canals extend posteriorly from
the vestibule in 3 planes. These contain the
semicircular ducts which respond to angular
rotational movements of the head.
o The Cochlea
 Houses the cochlea duct (scala media),
containing the spiral organ (of Corti)
 This is receptor organ for hearing
 Within the cochlear duct are hair cells
(receptors) which rest on the basilar
membrane, hairs project into the gelatinous
tectorial membrane
Physiology of Hearing:
 Sound originates from a vibrating object and travels is waves
consisting of alternating areas of compression and
rarefaction of a medium
 The distance from crest to crest on a sine wave is the
sound’s wavelength. The shorter the wavelength, the higher
the frequency (pitch) (measured in hertz)
 The amplitude of sound is the height of the peaks of the sine
wave, which reflects the sounds intensity (loudness). Sound
intensity is measured in decibels.
 Auditory processing is analytic; each tone is perceived
separately.
o Perception of pitch is related to the position of the
excited hair cells along the basilar membrane.
o Intensity perception reflects the fact that as sound
intensity increases, basilar membrane mobility is
increased and the frequency of impulse transmission to
the cortex is enhanced.
o Cues for sound localization include the intensity and
timing of sound arriving at each ear.
Homeostatic imbalances:
 Conduction deafness results from interference with the
conduction of sound vibrations to the fluids of the internal ear
 Sensorineural deafness reflects damage to the neural
structures
 Tinnitus is an early sign of sensorineural deafness; it may
also result from use of certain drugs
Equilibrium and Orientation:
 The equilibrium receptor regions of the internal ear are
called the vestibular apparatus
 Receptors for static equilibrium are the maculae of the
saccule and utricle
 The dynamic equilibrium receptor, the crista ampullaris
within each semicircular duct, responds to angular
movement in one plane.
Developmental Aspects of the Special Senses:
 Chemical senses are sharpest at birth and gradually decline
with age as replacement of the receptor cells slows
 The eye reaches adult size by age 8-9.
 With age, the lens loses elasticity and clarity, there is a
decline in the iris ability to dilate, and visual acuity decreases
 Response to sound in infants is reflexive. By 5 months an
infant can locate sound. Critical listening develops in
toddlers
 Deterioration of the spiral Organ of Corti occurs throughout
life
 Age-related loss of hearing (presbycusis) occurs in the 60’s
– 70’s