Download The Ear - Dr Magrann

SPECIAL SENSES: Smell, taste, hearing, equilibrium (balance), vision
OLFACTORY SENSE (smell)
Olfactory receptors are CHEMORECEPTORS; a special type of neuron which senses
particular chemicals and triggers an action potential. Chemoreceptors are at the roof of
the nasal cavity. There are hundreds of thousands of types, and they can smell a wide
variety of substances. They are extremely sensitive, and can detect parts per billion, as in
the scent of natural gas…just a few molecules! The olfactory nerve goes through the
cribiform plate to the OLFACTORY BULB (one of the shortest nerves in the body) and
into the limbic system.
Scientists who are trying to find a way to make neurons divide to heal nerve injuries
often study the body’s only mitotic neurons. These neurons are the olfactory receptors.
People who experience imaginary odors have what are called “unicate fits”.
GUSTATORY SENSE (taste)
Sensed on taste buds, which are located mostly on the tongue surface, but are also on the
palate, pharynx, and a few on the lips. Taste buds have specialized cells, which increase
surface area and have chemoreceptors. They are surrounded by support cells (like glia).
They synapse on sensory neurons, which go to the facial nerve. Someone with a damaged
facial nerve can not easily taste sweet, sour, or salty substances. Taste buds are the only
parts of the nervous system that can regenerate completely. The taste information is sent
to the primary gustatory (taste) cortex, located in the insula region (under the temporal
lobe) of the brain.
How many different tastes are there? Dozens. Salt, sweet, bitter, and sour are only a
few. Where are they located on the tongue? All tastes are located all over the tongue.
The picture in the book was drawn 120 years ago by an anatomist that his drawing was
not right; he just wanted to use it as a starting point for further experimentation.
Taste appreciation is also involved in texture (a mealy apple is not as good), temperature
(cold pizza tastes different than warm), and smell (perfume or cigarette smoke clog the
senses and decrease taste). There are dozens of taste receptors, hundreds of thousands of
smell receptors, so the subtly of taste is from smell. Foods people like are in opposite
proportion to the numbers of taste receptors for that. People that love sweets have
FEWER taste receptors for sweets, so they crave more taste of sweet things. If you
dislike something, it’s because you have lots of receptors for it. Also, as you get older,
you become less tolerant of sweets and more tolerant of bitter tastes (like beer and
coffee).
FUN FACTS
The catfish has over 27.000 taste buds. (What could be so tasty on the bottom of a pond?)
Flies taste with their feet.
THE EAR
1. OUTER EAR consists of the PINNA and the EXTERNAL AUDITORY CANAL.
The pinna is the cartilage of the ear; it acts as a funnel to capture the sound. If you cup
your hands to your ears (do it now), you’ll notice the sound of my voice is louder. If you
rolled up a piece of paper like a funnel and put it to your ear, it functions like the pinna.
The transmission of sound vibrations through the outer ear occurs chiefly through AIR.
2. MIDDLE EAR is an AIR filled space with structures. The TYMPANIC
MEMBRANE (ear drum) vibrates in response to sound. Attached to it are 3 bones: The
MALLEUS (hammer), INCUS (anvil), and the STAPES (stirrup) are the smallest bones
in the body. Together, they are only one inch long. Their function is to amplify sound
vibrations. The malleus vibrates the incus, which vibrates the stapes. The middle ear is
open to the nasopharynx by way of the AUDITORY TUBE (also called eustachian
tube or nasopharyngeal tube), which is only the thickness of a pencil lead. If this tube
is closed, the ears feel plugged up. The function of the auditory tube is to equalize the
pressure of the middle ear and the outside air so the ear bones can vibrate. Tubes are put
in the tympanic membrane to drain fluids in kids with frequent ear infections.
3. INNER EAR exists within the temporal bone (petrious portion). It is a complex
structure. It is located in a bony cavity called the BONY LABYRINTH (“maze”). The
bony labyrinth is filled with a fluid called PERILYMPH, which is similar to CSF. The
bony labyrinth is the only place where perilymph is found.
Within the bony labyrinth is a snail-shaped structure, called the MEMBRANOUS
LABYRINTH, which is filled with ENDOLYMPH.
The snail-shaped structure is divided into two main components. One is the COCHLEA
(“snail shell”). This is responsible for hearing.
The other structure is responsible for balance and consists of three parts:
– Semicircular Canals
– Utricle
– Saccule
Utricle
Semicircular canals
Saccule
Inside the cochlea are special neurons called HAIR CELLS; their axons form CN VIII.
The stapes is attached to the OVAL WINDOW, and vibrations cause the endolymph to
vibrate; the hair cells transmit this vibration. Therefore, the HAIR CELLS in this region
are receptors for HEARING.
Low frequencies (like the longer strings of a piano) cause a response in the tip of the
cochlea, and high frequencies cause a response at the larger end. The hair cells are
connected to CN VIII, the VESTIBULOCOCHLEAR NERVE, which takes the signals
to the brain. Therefore, the cochlea is where the hearing receptors are located, so the
cochlea is responsible for all of the hearing of sounds. However, the ear does more than
just hear; it is also responsible for balance and equilibrium.
VESTIBULAR SYSTEM: This system regulates balance.
It is also within the inner ear.
SEMI-CIRCULAR CANALS (Three of them, all in different planes) determine
movement in three planes. Within each semi-circular canal is HAIR CELLS and
ENDOLYMPH, whose axons go to the cerebellum.
When you move in one direction, like sliding across the room, the fluid sloshes like a cup
of coffee, and it makes the hair cells move. Attached to the semi-circular canals are two
joined structures called the UTRICLE and the SACCULE. These also contain HAIR
CELLS and ENDOLYMPH. Within the endolymph of the utricle and saccule are
OTOLITHS (“ear rocks”) which are calcium deposits. When you stand perfectly
upright, these otoliths fall directly down and bend the hair cells (a special type of neuron)
on the lower cells. When you tip your head to the side, they will stimulate the hairs on
that side. The otoliths stimulate the hair cells to tell you what position your head is in
and give you a sense of equilibrium. Therefore, the HAIR CELLS in the utricle and
saccule are receptors for equilibrium, and the OTOLITHS are an essential component of
this process.
EAR PROBLEMS
Inflammation of the semi-circular canals give you a sense of motion when you’re not
moving = VERTIGO (dizziness) or LABYRINTHITIS. This can be debilitating.
Sometimes only one canal is affected, so you only get dizzy if you turn your head one
way.
VISUAL VERTIGO is when your eyes get one set of information that conflicts with the
vestibular structures, such as when you are on a roller coaster or reading in a car.
Whether the vertigo is from visual or vestibular disturbances, your body interprets the
signals as a poison invasion, so it initiates a vomit reflex.
“Cauliflower Ear”, Hematoma auris or Traumatic auricular hematoma
Common in boxers and wrestlers. A blood clot or other fluid collects under the
perichondrium. This separates the cartilage from the overlying perichondrium that is its
source of nutrients, causing the cartilage to die. This leads to a formation of fibrous tissue
in the overlying skin.
Hearing Loss
• Conductive hearing loss happens when there is a problem conducting sound
waves through the outer ear, tympanic membrane (eardrum) or middle ear
(ossicles). It may be caused from excess was, damaged eardrum, or arthritis of
the ossicles.
•
Sensorineural hearing loss is a problem in the vestibulocochlear nerve (Cranial
nerve VIII), the inner ear, or central processing centers of the brain.
Weber Test: only tests unilateral problems. A tuning fork is touched to the middle of the
forehead:
– Sensorineural hearing loss: sound is heard louder in the normal ear
because the damage is to the nerve, so bone conduction of the sound is
ineffective.
– Conductive hearing loss: sound is heard louder in the problem ear
(earwax, etc) because reflected soundwaves cannot escape the ear canal,
so they penetrate deeper into the inner ear.
Rinne Test
• Performed by placing a vibrating tuning fork on the mastoid process until sound is
no longer heard, the fork is then immediately placed just outside the ear.
Normally, the sound is audible at the ear, indicating a positive Rinne test.
• If they cannot hear the sound at the ear, it is a negative Rinne test, and indicates
Sensorineural hearing loss
Cochlear Implants
• A cochlear implant is a small, complex electronic device that can help to provide
a sense of sound to a person who is profoundly deaf or severely hard-of-hearing.
The implant consists of an external portion that sits behind the ear and a second
portion that is surgically placed under the skin. An implant has the following
parts:
• A microphone, which picks up sound from the environment.
• A speech processor, which selects and arranges sounds picked up by the
microphone.
• A transmitter and receiver/stimulator, which receive signals from the speech
processor and convert them into electric impulses.
• An electrode array, which is a group of electrodes that collects the impulses from
the stimulator and sends them to different regions of the auditory nerve.
• An implant does not restore normal hearing. Instead, it can give a deaf person a
useful representation of sounds in the environment and help him or her to
understand speech.
The following is not on the exam
Otosclerosis
• The stapes becomes fixed, cannot move, and dampens sound conduction.
• Stapedotomy: A portion of the stapes is removed and replaced with a titaniumnickel prosthesis.
At what volume does sound damage your ears?
Hearing damage from headphones is more common than from loudspeakers, because
people listen at higher volumes. Even at comparable volumes, hearing damage from
headphones is higher than with loudspeakers, due to the close coupling of the transducers
to the ears.
90 dbA
8 hrs
92 dbA
6 hrs
95 dbA
4 hrs
97 dbA
3 hrs
100 dbA 2 hrs
102 dbA 1.5 hrs
105 dbA 1 hr
110 dbA 0.5 hr
115 dbA 0.25 hr or less
60 dB
Everyday conversation, ringing telephone.
70 dB
Restaurant.
80 dB
Heavy city traffic, alarm clock at 2 feet, factory noise,
vacuum cleaner, garbage disposal.
90 dB
Subway trains, motorcycle, workshop tools, lawn mower.
100 dB Chain saw, pneumatic drill.
110 dB Dance club.
120 dB Rock concert speaker sound, sandblasting, thunderclap.
130 dB Jet take off.
140 dB gunfire
Nerve damage occurs immediately
150 dB rock music peak