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Division of Ear Anatomy into 4 Areas
A, Outer Ear
B. Middle Ear
C. Inner Ear
D. Higher Auditory Pathways (HAP)
Introduction to the ear
From outer ear to HAP
A. Outer ear : auricle or pinna, the outermost part.
B. Ear canal: is external auditory meatus.
C. Middle ear is an air-filled space in the temporal bone.
D. The auditory tube extends down from it.
E. The cochlea and semi circular canal are called the inner ear.
F. The 8th nerve connects to the brain stem and ascends up to the cortex.
**The cochlea & semi-circular canal may be called the sensory part of the inner ear, and the higher auditory pathways as the neural inner ear.
Auricle AKA Pinna
2 PARTS OF OUTER EAR: AURICLE AND EAR CANAL
The auricle, or pinna (people refer to it as the ear) is visible on the outside. The ear canal is the other part of the outer ear.
A. Made up of cartilaginous core with coetaneous lining.
1) Except lobule which is made of fat
B. Cartilaginous core and coetaneous lining extend down into the temporal bone and constitute part of the ear canal.
C. Not highly innervated (not many nerves)
D. Not highly vasculated (does not have a dense vascular system)
1. Susceptible to cold
2. Vascular arteries are fairly superficial they bleed profusely
E. Not very important acoustically
F. Helps slightly in our ability to localize sound from front to back of our head. (not so good side to side)
G. Completely unique. Not even your own two ears are alike.
H. Not everyone has darwin's Tubercle
***NEED TO BE ABLE TO IDENTIFY Identify helix, anti helix, crus of helix, scaphoid fossa, triangular fossa, concha cymba, concha cabum, tragus, anti tragus, lobe, intertragal inscisure, Darwin's tubercle.
External Auditory Canal
Lateral end connects to Auricle.
Medial end connects to Drum Membrane
A. About an inch long. (25 to 35 mm) 0 oval opening about 6 - 8mm in diameter
B. External part of canal is lower than the middle of the ear canal.
C. Cartilage along lower lateral ear canal connects directly to bone.
D. Towards the ear drum the canal drops down again.
E. Lateral Part (1/3 to 1/2 of the canal) is cartilage and medial (wall) of the canal is bone
F. Isthmus = (Narrowing in the canal)separation between cartilage and Bone
G. Ear canal is "S" shaped
1. Pars Externa - Outer third
a. moves medially towards center of the head, anteriorly towards the nose and superior towards the top
2. Pars Media - middle third, level off moving Medially towards center of the head, then turn posteriorly towers back of the head and then towards eardrum
3. Pars Interna - medially towards center of the head, turn towards from of the face, then drop inferiorly towards tympanic membrane
Ear Canal Surface Hair
A. Near lateral opening are surface hair:
B. More predominant in some males than others
C. Easy to see in some impossible to see in others
D. Below the coetaneous lining of the lateral part are cerumen gland which produce cerumen which is ear wax.
E. Ear wax is normal and particularly helpful in many cases with the physiology of the auditory mechanism.
F. Has a noxious taste and sticky.
G. These elements discourage small organisms from entering the ear canal.
Function of the EAM (External Auditory Meatus)
A. The primary function of the EAM is to direct sound energy to the tympanic membrane (TM) or ear drum
B. Because the essential anatomy of the hearing mechanism is recessed into the boney structures of the skull, it is thought by some that the EAM serves as a protective mechanism.
Resonant Characteristics of the EAM
A. Acts like a closed tube resonator
B. Resonates incoming Sounds from about 2500hz to 5000hz
C. The intensity of these resonances are in the neighborhood of about 17db
D. The ear canal by itself has a resonant frequency (peak resonance) somewhere near 2500hz
E. The Concha by itself has a resonant frequency of about 5000hz
F. The combination of all the structures of the ear have a resonance frequency of about 3800hz
The Tympanic Membrane
A. AKA ear drum
B. Located at the termination of the EAM
C. Forms the lateral wall of the middle ear cavity
1.Smooth - has kind of a sheen
2. Pearl gray - sets it off from the surrounding tissue
3. Translucent - can see some light through also some structures behind it
4. Concave (cone shaped) - from the external auditory meatus side
5. Small mass = 14 milligrams
6. The cone shape and small mass is conducive to receiving and reacting to sound waves because of the minute nature of the stimulus (cannot have a large mass or flat surface).
7. Situated in such a way that at the TM's superior margin there is a 140 degree angle
E. The TM is held in place by an annulus, an annular fibrocartilagenous ring that surrounds the TM, and is open at the top. It's thicker at the bottom, narrower at the top sides, and open at the top which is called the Notch of Rivinus.
F. Tympanic Sulcus (depression)--Along the floor of ear canal toward middle ear there is a sulcusor depression there that houses the annular ring.
Tympanic Membrane - 3 layers of tissue
1.Cutaneous-continuous with lining of the E.A.M. (skin lining of the EAM)
2.Mucous-continuous with lining of the Middle Ear
3.Fibrous layer sandwiched (situated) between the Cutaneous and mucous layers-gives resilience to T.M. It is made up of 2 sublayers:
a) Radial fibers - laterally
b) Circular fibers - medially
4. Radial and circular fibers together make something like a spider web -built for resiliency and strength.
2 parts of the eardrum
If you could see the entire eardrum you would see that it had two distinct anatomical parts
a) pars flaccida-more floppy, flacid
b) pars tensa, more tense, firm.
- You can tell a right or left ear by the handle of the malleus. 11:00 angle for left ear, 1:00 angle for right.
Tympanic Cavity (middle ear cavity)
A. Middle ear cavity - Air filled
B. Air on both sides of the tympanic membrane
C. Air enters cavity through auditory tube.
D. Middle ear space is in the petrous portion of the temporal bone, one of the most dense areas of bone in the body.
E. The middle ear cavity has somewhat of an hourglass shape.
F. The superior to Inferior (top to bottom) of middle ear is about 15 mm, and from anterior to posterior (front to back) is about the same.
G. Across the superior portion of middle ear is about 6 mm, narrowing to 2mmat the middle and 4mmat the bottom.
H. The Epitympanic Recess is attic of tympanic cavity.
I. The TegnmanTympani is a thin piece of bone at the superior margin of the middle ear (epitympanic recess) The cortex of the brain sits along there.
J. The Mastoid Antrum (cavity) is made of quite porous bone. The opening from the mastoid antrumand the typmaniccavity is called the Aditus to Antrum.
Walls of the middle ear cavity
(say you're in the right ear)
1) Superior (top) - Tegmental Wall (Sits below the epitympanic recess)
2) Inferior (bottom) - jugular Wall
- Jugular Fossa (prominence formed by the jugular vein)
3) Posterior (back) - mastoid wall
- Aditus to Atrum, Fossa Incudis & Pyramidal Eminence
4) Anterior (front) - carotid wall
- carotid canal moves down through the body very near this wall and you can see the fossa for the carotid canal makes an eminence into the middle ear cavity onto the anterior wall
- opening for the auditory tube (exits the middle ear cavity
5) Lateral (outside, right) - membranous
- Tympanic membrane, malleus head and manibrium, incus body, annulus, pars tensa & chord tympani
6) Medial (inside, left) - labyrinthian wall
- named because of its proximity to the inner ear labyrinth
- upper right corner a partial canal for the tensor tympani muscle to get in there
- upper left corner = two prominences into the middle ear
- first caused by the lateral semicircular canal which comes by and protrudes into the middle ear
- 2nd is where the vii nerve (facial nerve) comes by (it has a canal/fossa) which is a protrudent into the middle ear space
- Promontory: large protrudence caused by the basil turn of the cochlea.
- Fenestra Vestibuli: (oval window)
- FenestraRotunda: (round window)
The Chorda tympani
branch of VII nerve that goes under the incus body and on top of manubrium, carries information from the anterior portion of the tongue to the central nervous system.
TM CAN BE DIVIDED INTO 4 QUADRANTS:
2 muscles to middle ear:
stapedial (comes in through the posterior wall in the pyramidal emminence) & tensor tympani (comes in through the Cochlear form process)
Cochlear form process canal
opening into the middle ear, lets tensor tympani muscle enter the middle ear.
Two prominences into middle ear
one by lateral semicircular canal
one by canal for facial nerve VII.
A chain of three small bones that bridge the gap of the middle ear space from the tympanic membrane to the oval window
- Malleus (hammer)
- Incus (anvil)
- Stapes (stirrup)
***Examine the nature of not only each ossicle but also the ossicular chain including it's position in the middle ear
Largest of the ossicles
The malleus has a
• 3 processes
- Lateral process
- Anterior process
- manubrium (handle of the malleus)
The head occupies ½ of the epitympanic recess
Manubrium attached to the middle connective tissue of the T.M. (Umbo - middle connective tissue of T.M.)
This accounts for the concave nature of the T.M.
Posterior surface of the head contains an articular facet for attachment to the incus
Where the manubrium joins the neck, there is a small boney projection for attachment of the tensor tympani tendon (one of two muscles)
A body and 2 processes
The body occupies ½ of the epitympanic recess
2 processes arise from the body at nearly right angles
- The short process (5 mm) runs horizontally backward (posteriorly) and occupies the fossa incudis
- The long process(7 mm) runs vertically, parallel to the manubrium
- The inferior end turns sharply medially towards the center of the head and terminates at the lenticular process
Smallest i the ossicular chain and the smallest bone in the body.
- Has a head which is slightly concave to receive articular facet for reception of the lenticular process of the incus
- Small boney spine on the head-neck for attachment of the stapedial tendon (2nd of two muscles)
2 Crura (Kroo'rah) (Post. & Ant.)
- Originate near the inferior margin of the footplate
- ANTERIOR CRUS = slenderer, shorter, and less curved than posterior Crus
- Partially osseous and partially cartilaginous
- Occupies the oval window
- Held in place by an annular ring
- OBTURATOR FORAMEN - (one that closes) = the opening in the stapes (were the cowboy boot goes)
The Auditory Tube aka Eustachian tube
Name after Eustachius a 16th century anatomist
Definition: The canal which establishes communication
between the middle ear and the nasopharynx
In adults = 35 - 38 mm in length
Directed = downward (45 degrees)- forward (35 degrees) and medially
The auditory tube is opened by a combination of contractions of the tensor palatini and the levator palatini muscles
The above action tends to pull the lateral cart. away from the medial cart.
Opens approx. 1000 times per day
Study text and figures in Zemlin - 4th Ed.
At the pharyngeal ostium (entrance into a hollow organ or cavity), the cartilage and coverings form a prominent elevation.
This prominence is the Torus Tubaris
The Osseous portion of the Auditory Tube
- 12 - 14 mm = in length
- Begins at the carotid wall just beneath the septum canals musculotubarii
- Lumen (opening of the tube) = 3 - 6 mm in dia.
- Lumen = normally open
- Lumen = 3 mm above floor of Md. ear
Function of the Auditory Tube
Equalize middle ear air pressure with outside air pressure
Drainage of normal and diseased secretions from the middle ear
--the ligament that runs from the head of the malleus to Tegmentympani (ceiling or roof of middle ear)
--ligament that runs from the neck of the malleus to the boney wall near the notch of Rivinus
--Attaches from the anterior process and runs over and attaches to the carotid wall.
4.POSTERIOR LIGAMENT OF THE INCUS
--Runs from the tip of the short process of the incus to the Fossaincudis.
5.ANNULAR LIGAMENT OF THE STAPES
-Runs the periphery of the Footplate of the Stapes and secures the footplate into the oval window.
THE TYMPANIC MUSCLES
A. There are two TYMPANIC MUSCLES:
1.-The Tensor Tympani Muscle
• These are the smallest striated muscles in the body
• Penate in nature (feather-like) they have central tendon and muscle fibers coming off it in leaf-like fashion
• The muscles are encased in bone -it is only the tendon that enters the middle ear
B. The Stapes and tensor tympani muscles exert a force in opposite directions (opposing each other) and perpendicular to the rotational axis of the ossicular chain.
C. Tympanic muscle's action is:
-Elicited by sound energy
-Reflexive in nature, reflexive action to intense sound
-Elicited by either ipsilateralor contralateralstimulation
-can put sound in one ear and get contractions in both sides
D. Ipsilateral: pertaining to the same side
E. Contralateral: associated with a part on the other side
F. At the level of the superior olive there is a mediation of the reflexive action from one superior olive to the opposite superior olive.
G. At the level of the superior olivary complex, there is a motor response initiated at that level , and the motor response descends to the facial nerve, into and back to the middle ear, and the middle ear muscles, the stapedial muscle.
H. Of the 2 muscles, the stapedial is the major player in obtaining an acoustic reflex.
THE TENSOR TYMPANI
1.Described by Eustachius in 1564
-- Length = 25 mm
-- Cross sectional area = 5.85 mm2
2. Origin: the cartilaginous portion of the auditory tube just above the boney portion of the auditory tube. The septum canalis tuberi separates this capsule from the auditory tube.
3. Insertion: inserts into the handle of the malleus
4. Nerve supply: trigeminal V
5. Location: Lies superior and parallel to the auditory tube
6. Contraction of the tensor tympani :
-Draws the malleus medially and anteriorly, toward middle and front of head
-Applies a force at a right angle to the rotation of the ossicles
-Increases the tension of the T.M
***Figure A: the tensor tympani muscle is running into the middle ear. The central tendon comes from muscle and enters the middle ear at the cochleoform process, goes over and connects to middle of malleus. Only the tendon enters.
***Figure B: You see the stapedialmuscle, with tendon entering the middle ear through the Pyramidal emininenceand attaches to the neck of the stapes
THE STAPEDIAL MUSCLE
1. Is the smallest striated muscle in the body.
2. Length = 6 mm & Cross sectional area = 5.0 mm2
3. Originates in a boney canal parallel to the facial nerve canal in the posterior wall of the middle ear
4. Insertion = The tendon inserts into the neck of the stapes
5. The muscle is almost vertical and the tendon is horizontal, 90 degree angle on the tendon as it attaches.
6. The nerve supply is the facial nerve (Cranial nerve VII)
7. Contraction of the muscle draws the head of the stapes posteriorly. Exerts a force at right angle with the vibration of the
THE ACOUSTIC REFLEX ARC
- SUPERIOR OLIVARY COMPLEX
- COCHLEAR NUCLEI
- AUDITORY NERVE (VIII NERVE)
- INNER EAR
- MIDDLE EAR
- OUTER EAR
Middle Ear Vibration
- Diagram A: demonstrates mode of tympanic membrane / eardrum vibration. This was demonstrated in the 60s-70s by George von Beckesy--the tympanic membrane does not vibrate as a piston but rather as a disc pivoting on a axis.
- Diagram B: the malleus comes down and attaches to the tympanic membrane. The pivot point is at the base of the malleus. The point of maximum vibration is at the umboor tip, where it connects to the eardrum. Ex: Your feet jumping down on a trampoline is like the umboon the eardrum. The point of greatest movement is under your feet -the umbo.
- Diagram C: Equal amplitude contours -the numbers show the relative magnitude of the tympanic membrane excursion. As the tympanic membrane vibrates, the greatest displacement is at the point of umbo, then as you move to the periphery of the membrane, there is gradually less displacement. At the umboit is 15 times great than at the outside.
Mode of Ossicular Vibration
- Diagram A: the ossicular chain rotates and vibrates around an imaginary axis as if it came right through the short process of the malleus. The arrow illustrates the rotation around it. That will cause the umbo to press and release on the tympanic membrane.
- Diagram B1: hereisthenormalvibratory pattern of the stapes. The axis of vibration comes through the posterior cruz. The stapesthenvibratesaroundthataxis. Thisis a motion similar to Diagram B2.
- Diagram B2: motion is like a door with hinges. The door vibrates greater away from the hinged area, or the point of rotation / point of axis.
- DigramB3: as the intensity increases, it begins to vibrate as if the axis went through both cruz (cura) of the stapes. It takes on a fluttering motion from superior to inferior. That drives the inner ear with less force than in B1, so it may be a safety mechanism.
TRANSFORMER ACTION OF MIDDLE EAR
- One of the problems with the transfer of energy is the difficulty or impedance of different mediums.
- Impedance = to inhibit, impede, interfere with.
- As we move from the gaseous air around us to the fluid in the inner ear, it presents problems of impedence., because the impedence of air is relatively less than the impedence of the fluids of the inner ear. The fluids tend to reject that sound energy coming from the gaseous air.
Diagram Ex. Fisherman on bank, fish in water. Note the gas, fluid and solid mediums. If the fisherman shouts, his voice will reach the water surface and be reflected off, because the impedence of the water is greater than air. It's an impedance mismatch. The solid's impedence is more closely matched with the fluid. So if the fisherman stomps on the ground by the lake, that energy has a better chance of penetrating the fluid/ water.
Bottom of diagram: The Middle Ear acts as middleman/ transformer between the air outside and the fluid inside the inner ear. The middle ear is an impedence matching transformer.
AREA RATIOS AND LEVER RATIOS
We're going to use area ratios and lever ratios to do this transformer thing.
Area ratios: The tympanic membrane's vibrating area is about 55 square mm. The footplate of the stapes' vibrating area is about 3.2 square mm. If we apply a force on the tympanic membrane, that force is going to be amplified as we measure the force per area at the footplate of the stapes.
Ex: Arrow on cone. Force pressing down is f1. Force at bottom of cone, f2, has a tremondous increase in force, so it is greater than the first. So if we take the TM's 55 and divide by stapes' 3.2, we get a 17, or a 17:1 ratio of the area. So it's a 17:1 area ratio.
Lever ratios: Look at the teeter-totter like diagrams. The fulcrum (balance point) is different on each one. In the left one, if you press down on d2, the lift is the same ford1---sod1=d2. Inthe one on the right, if you press down on d2, then you're lift in d1 is going to be less than d2. So if you're in a situation like the one on the right, and you want to lift a rock or heavy object, you want to place your instrumenation so the rock is on d2, and you're on d1. So you will get an enhancement of force on d2 to lift the rock.
Example: malleus is rotating around that black dot, the fulcrum. The distance from that pivot point/ fulcrum down to the end of the long process of the malleus is measured, and then the same thing is done for the incus distance. The malleus distance is larger than that of the incus, in a 1.3:1 ratio. The malleus has a 1.3:1 lever ratio.
So now we take the 17 from the area ratio and the 1.3 from the lever ratio and we multiply them to get a 22:1 ratio. A 22:1 sound pressure ratio corresponds to 27 db. So this is how the middle ear acts as an impedence transformer/transducer from the ear canal to the fluids of the inner ear.
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