Chapter 15 notes
Special Senses
Eye
Eyebrows – provide shade and keeps sweat from dripping into the eye.
Palpebrae – eyelids
Palpebral fissure – eyelid slit
Medial and lateral commissures – corners of eye
Lacrimal caruncle – small fleshy spot at medial commissure where “sandman’s sand” collects
Conjunctiva – thin layer of tissue that covers
the underside of palpebrae and sclera.
Conjunctivitis – infection of the conjunctiva
Viral or bacterial that causes inflammation
(pink eye)
Sty – inflammation of the sebaceous glands
by the eyelashes.
Lacrimal apparatus “ tear” – Lacrimal gland
secretes saline solution “tears”. Blinking
spreads tears across the eye medially to the
lacrimal caruncle. Tears travel down the
lacrimal canaliculus into the lacrimal sac and
nasolacrimal duct. Lacrimal fluid contains
mucus, antibodies and lysozymes.
Nasal inflammation causes constriction of the
ducts and prevents tears from draining.
“watery eyes”
Diplopia – double vision
Strabismus – “cross-eyed” Also “lazy eye” one eye has muscles that are weak, so the brain learns to
rely off the one eye that is working properly and ignores impulses from the weak eye. Treatment –
eyepatch or special glasses that force the brain to use the signals from the weak eye. Once it has been
trained with increased strength, it can re-join the good eye and work together.
Fibrous layer – sclera (white part of eye) and cornea (clear bulge over the iris and pupil)
Vascular layer – choroid (under the sclera), ciliary body(holds the lens and secretes aqueous humor), iris
(contains only brown pigment. Other colors are seen because there is less brown pigment, and the light
is scattered over the surface of the iris. The reflected wavelengths result in seeing blue, green or grey. )
Inner layer – retina
Retina has 2 layers. The pigmented layer is closest to the choroid , and the neural layer is the layer that
has all the neurons. (photoreceptors, bipolar cells, ganglion cells.)
Optic disc – “blind spot” this is where all the neurons converge and enter the optic nerve to transmit
info to the brain.
Photoreceptors
Cones = color vision. Require bright light We have cones for red, green and blue.
Rods = shape vision- Sensitive to low light. This helps us make out shapes at night.
Eye is generally spherical, so it has an anterior and posterior pole. Anterior pole goes through the
cornea and pupil. Posterior pole goes through the macula lutea “yellow spot”. This area of the retina is
mostly cones.
Fovea Centralis – center of macula lutea and contains only cones. This is where the focused light coming
into the eyes is projected for clear, crisp vision.
Glaucoma – blindness due to pressure on the optic nerve. This is usually caused by blocked drainage of
aqueous humor.
Lens – focuses light on the macula lutea. The lens continues to grow throughout life making it less
flexible and sometimes cloudy. Cataracts are a condition where the lens becomes cloudy and distorts
vision.
More terms:
Emmetropic – normal eye. Normal vision
Presbyopia – old person’s vision. Can not focus on close up items.
Myopia – nearsightedness – short vision. Can not see well far away. Cause – eyeball is too long.
Hyperopia – farsightedness – long vision. Can not see well up close. Cause – eyeball is too short.
Astigmatism – “not a point” unequal curvatures of the cornea or lens. Results in blurry images.
Color blindness – lack of one or more of the cone types. This is a recessive x linked trait. Men are more
likely to be color blind than women. Most common is red/green colorblindness.
Transmission = Rods/cones to Optic Nerve to optic chiasma to optic tracts to primary visual cortex
(occipital lobes)
Chemical senses – taste and smell.
Both taste and smell have chemoreceptors. Chemicals are dissolved in saliva or in mucus in the nasal
membranes. These two senses compliment each other.
Smell – olfactory receptors are located in the roof of the nasal cavity. This is not the best location for
adequate stimulation. Sniffing increases the opportunity for the smell to pass over the olfactory
receptors and increases the ability to smell. Olfactory receptors have cilia that lay flat and are covered
in mucus. They increase the surface area for the chemicals to dissolve in mucus and then affect the
receptors.
The olfactory receptor cells are gathered into fascicles and then make up the olfactory nerve. Olfactory
neurons do not last a lifetime. They are replaced every 30-60 days. Pain receptors are also present and
help to detect chemicals like ammonia, the hotness of chili peppers, and the chill of menthol.
Chemicals must be volatile for us to smell them (chemicals must be airborne).
Chemicals are picked up by receptors and impulses are transmitted toward the brain. Olfactory cortex
and association determine what is being smelled. The limbic system provides danger labels to certain
smells like smoke etc.
Taste –
Taste buds are the receptors for taste, and are primarily located on the papillae of the tongue. Taste
bud cells are replaced every 7-10 days because of their location and amounts of friction and being
bathed in damaging chemicals that we eat. Taste is divided into 5 categories.
Sweet – sugars, saccharin, alcohols, some amino acids, and some lead salts (think lead paint).
Sour – acids, caused by their H+ ions in solution.
Salty – metal ions like sodium chloride
Bitter – alkaloids such as caffeine and nonalkaloids such as aspirin.
Unami – delicious – amino acids glutamate and aspartate “beef taste” tang in cheese, and MSG
(monosodium glutamate)
For a chemical to be tasted, it must be dissolved in saliva, enter the taste pore and contact the gustatory
hairs.
Taste is 80% smell. Most disorders involve a head injury that tears the olfactory nerves. Having no
sense of smell is more common than no sense of flavor/taste.
The Ear - Hearing and Balance –
External
Auricle – cartilaginous part of the external ear that you can see.
External acoustic meatus – short, curved tube that extends from the auricle to the eardrum. The canal is
lined with skin that has hairs, sebaceous glands and ceruminous glands that produce ear wax.
Sound waves that enter the external acoustic meatus eventually contact the tympanic membrane
(eardrum) and make it vibrate. This vibration is transferred to the tiny bones of the middle ear.
Middle ear
Houses the auditory ossicles of the Malleus (hammer), Incus (anvil) and stapes (stirrup) and the drainage
tube called the Pharyngotympanic tube (formerly the Eustachian tube). The bones transmit the
vibrations to the oval widow and the inner ear.
Otitis Media is a middle ear infection – inflammation of the lining of the middle ear results in pus/fluid
that cannot drain properly through the pharyngotympanic tube.
Inner ear- contains a labyrinth of bony canals containing fluid (endo and perilymph). The sound
vibrations are conducted through the fluid to the cochlea and picked up by the vestibulochchlear nerve
that is attached to hair cells. The vibrations picked up by the movement of the hairs is what is
transmitted via nerves to the brain. This fluid also detects forces on the body, so it helps with balance.
Sound frequencies are detected as pitch. The sound wave intensity is detected as loudness.
Loudness is measured in decibels.
Deafness can be caused by many things.
Conduction deafness can be caused by something impeding the sound conduction in the inner ear. Ear
wax, perforated eardrum, middle ear infections (otitis media and otosclerosis – bony fusing of the
ossicles.)
Sensorineural deafness is the result of damage to the neural structures (cochlear hair cells all the way to
the auditory cells of the cortex) New advances in cochlear implants has allowed even children born with
deafness to be able to hear.
Tinnitus – ringing or clicking sound in the ears in the absence of auditory stimuli. It is actually a
symptom that is the first sign of cochlear nerve degeneration. It can also result from middle or inner ear
inflammation, and is a side effect of some medications.
Meniere’s syndrome –
Inner ear condition that results in repeated bouts of vertigo nausea and vomiting. Balance is affected so
much that standing upright is nearly impossible. Treatments include diuretics to reduce fluid volume and
anti-motion drugs. Severe cases may result in draining of the fluids or if hearing loss is complete,
removal of the labyrinth structures.
Equilibrium and orientation –
Vestibular apparatus – semicircular canals and vestibule that contain maculae “spots” for static
equilibrium, and the crista ampullaris for dynamic equilibrium.
Maculae have receptors for determining where you head is in space. They play a key role in posture.
They deal in straight line changes, but not rotation.
The Crista Ampularis detects dynamic equilibrium ( angular rotation). Here are one of these in each ear.
Information from the equilibrium sensors go straight to the reflex centers in the brain stem, otherwise
we would never catch ourselves before we fall.
Nystagmus – a complex of rather strange eye movements that occurs during and immediately after
rotation. Usually accompanied by vertigo.
Motion sickness is thought to be due to a sensory mismatch. The input from the eyes does not match
the input from the ears (balance and orientation).