The Special Senses
Chapter 16
Special Senses
• Special senses in the head?
– taste, smell, hearing, equilibrium and sight
• These special senses have NO dendritic endings of
sensory neurons.
• “Neuron-like” epithelial cells which transfer information to
true neurons in the afferent pathway.
Dominant sense.
Up to 40% of cerebral cortex dedicated to processing
visual stimuli.
Orbit provides considerable protection as well as fat layer.
• Eye lids (palpebrae) w/ lashes
– hair follicles highly innervated which aids in
blinking reflex. Protects from foreign objects
and excessive light.
• Lacrimal apparatus
– keeps the eye surface
moistened
– removes debris
• Lacrimal glands
– produces lacrimal fluid
(mucous, lysozyme,
antibodies)
• Fibrous tunic (sclera +
cornea)
–
–
–
sclera
outer layer
protection
avascular dense CT
• Sclera
–
–
–
–
posterior outer tunic
“white” of the eye
continuous w/ dura mater
acts as insertion site for
muscle
sclera
• Fibrous tunic (sclera +
cornea)
–
–
–
protection
avascular dense CT
outer layer
sclera
cornea
• Cornea
– thick transparent layer of
collagen fiber (CT)
– many pain receptors
– reflexive blinking is
triggered by nerve
endings in cornea
– helps focus objects on
retina by bending light
– avascular
sclera
• Vascular tunic (choroid +
ciliary body + iris)
–
–
–
sclera
choroid
cornea
“middle coat”
dark, vascular, innervated
continuous w/ arachnoid
and pia mater
• Choroid
– highly vascularized &
darkly pigmented
membrane
– melanocytes give dark
color and help trap light
– provides nutrients to other
tunics
choroid
sclera
• Vascular tunic (choroid +
ciliary body + iris)
ciliary zonule
sclera
choroid
– “middle coat”
cornea
– dark, vascular, innervated
– continuous w/ arachnoid
and pia mater
• Ciliary body
– continuous with choroid
– thickened ring of smooth
muscle that surrounds
lens
– suspensory ligaments
(zonual fibers) hold lens
– ciliary muscle changes
shape of lens
– secretes aqueous humor
(nourishment for cornea)
ciliary body
choroid
sclera
• Vascular tunic (choroid +
ciliary body + iris)
–
–
–
ciliary zonule
sclera
choroid
cornea
“middle coat”
dark, vascular, innervated
continuous w/ arachnoid
and pia mater
• Iris
– eye color
iris
– smooth muscle diaphragm
that regulates entrance of
light
– under ANS control
• parasympathetic
constricts pupil and
sympathetic dilates pupil
ciliary body
choroid
sclera
Eye color is determined by degree of brown pigment only on
the posterior surface of iris.
Caucasian children can change from blue to brown as they
age and melanocytes begin to function.
• Sensory tunic (retina)
– inner layer
– receptors for vision
ciliary zonule
sclera
choroid
cornea
• Retina (2 layers)
retina
– outer pigment layer
• single layer of
pigmented epithelial
iris
cells.
• helps trap stray light
– inner neural layer
• thick layer of nervous
tissue containing
photoreceptor cells.
• beginning of visual
pathway
• highly vascular
ciliary body
choroid
sclera
Photoreceptor Cells
(Rods & Cones)
Photoreceptor Cells
(Rods & Cones)
• Rods
–
–
–
–
highly sensitive (100 million cells)
Night vision (grays)
Low resolution
impulses from many rods will be transmitted on a
single nerve fiber (summation)
• Cones
–
–
–
–
low sensitivity (6 million cells)
1 cone:1 nerve fiber (no summation)
color discrimination and high acuity
3 cone types (red, blue, green)
Regional Specialization of the Retina
• Fovea centralis – posterior part of retina highly
packed with cone cell = point of visual acuity.
Regional Specialization of the Retina
• Optic disc – circular elevation located a few mm
medial to fovea centralis.
– axons of ganglion cells converge as the optic nerve
– “blind spot”, no light sensitive cells
Eye Humors
• Anterior segment w/ aqueous humor
– anterior chamber & posterior chamber
– nourishment for lens
Eye Humors
• Posterior segment w/ vitreous humor
– gel like substance which holds retina in place
and transmits light
The Ear
• Mechanoreceptor
responsible for hearing
and equilibrium.
• Contains specialized
receptors that sense
sound waves and fluid
motion.
• Outer ear collects sound waves and transfers to the external auditory canal
and ends at tympanic membrane (eardrum).
• External acoustic meatus functions to keep foreign objects out of ear.
Lined with modified sweat glands called ceruminous glands that secrete wax
(cerumen).
• Tympanic membrane is epithelium w/ CT. Its function is to transmit and
amplify sound waves.
• Middle ear (tympanic cavity) – air filled chamber between tympanum and
bone of inner ear.
• Function of middle ear:
– transmit vibrations of the tympanum to third portion of ear (inner ear)
– amplify the mechanical vibrations of the tympanic membrane (22X) via
the ossicles.
• Ossicles – amplify and transfer mechanical vibrations to the oval window.
• Attached via ligaments and synovial joints.
• Pharyngotympanic tube (auditory, eustachian) – helps in equalizing
pressure on both sides of the tympanic membrane. Typical route of ear
infections.
• Inner ear (labyrinth) – Maze-like ear is filled w/ fluid and houses organs.
• 2 main divisions of ear
– 1. Outer bony labyrinth filled w/ perilymph
• semicircular canal (function: equilibrium)
• vestibule (function: equilibrium)
• cochlea (function: hearing)
– 2. Inner membranous labyrinth filled with endolymph
• semicircular ducts, utricle and saccule in vestibule, cochlear duct
• Cochlea – a long tube w/ 2 membranes
– 1. Vestibular membrane in upper chamber (Scala vestibuli), that is
continuous w/ the scala tympani at the cochlear apex.
– 2. Basilar membrane in the lower chamber (Scala tympani), which
terminates at the round window.
• Cochlea – a long tube w/ 2 membranes
– 1. Vestibular membrane in upper chamber (Scala vestibuli), that is
continuous w/ the scala tympani at the cochlear apex.
– 2. Basilar membrane in the lower chamber (Scala tympani), which
terminates at the round window.
• Cochlear duct – houses the organ of Corti, a receptor of epithelium cells
that transmit vibrations and convert them into neuronal impulses.
Volume and Tone
• Volume discrimination
is based on displacement
of the basilar membrane
– bending of hairs is in
direct proportion to
volume.
• Tone discrimination is
based on the region in
which the vibrations reach.