Anatomy & Physiology The Ear and Hearing (Test 1)

What is the ear?

An energy transducer where sound is converted from acoustic to mechanical to electrical energy

Physical processing of acoustic information occurs in 4 groups of what structures?

1. Outer

2. Middle

3. Inner

4. Cochlear Nerve (8th cranial)

What are the two main components of the outer ear?

- Pinna (auricle)

- External Auditory Canal (ear canal)

What are the characteristics of the Pinna?

- designed to enhance high frequency sounds

- first 1/3 cartilage

---- numerous hair cells and a modified sebaceous gland

- remaining 2/3 bone

What are the characteristics of the External Auditory Canal/Meatus?

- connects the outer and middle ear together

- adult size is 2.5 cm on average

- Largest part of the EAC is the opening

- The smallest part is the Isthmus

What are both the Pinna and EAC lined with?

both lined with skin and has ceruminous glands

What do the glands produce?

cerumen (ear wax)

- dead skin and oil

How many decibels of hearing would we lose without the pinna?

The outer ear is a natural hearing aid or amplifier and 60dB would be lost without it

What are the parts of the pinna?

- Helix

- Antihelix

- Concha (bowl area)

- Tragus

- Antitragus

- Lobule

How many layers does the tympanic membrane have?

3

What is the first layer of the tympanic membrane?

Epithelial layer: lining of the ear canal

What is the middle layer of the tympanic membrane?

- fibrous, providing strength and support

--- there are three layers within this portion

What is the inner layer of the tympanic membrane?

- mucosa, located behind the eardrum and completely lined with mucous membrane

What are the characteristics of tympanic membrane?

slightly coned shaped, pearl grey in color and semi-translucent

What are the two main sections of the tympanic membrane?

- Pars Flaccida

- Pars Tensa

What are the characteristics of the Pars Flaccida?

- smaller more compliant portion of the tympanic membrane

- located superiorly and contains two layers of tissue

What are the characteristics of the Pars Tensa?

- larger portion

- located inferiorly and contains 4 membranous layers and is stiffer than the pars flaccida

What is the manubrium?

the arm of the malleus

What does the umbo do?

attaches the tympanic membrane to the manubrium

What is the cone of light?

- shaped like a triangle

- should reflect light back

- signifies a healthy tympanic membrane

What are the characteristics of the middle ear?

small air-filled cavity located within the temporal bone of the skull

What are the structures of the middle ear?

- Tympanic membrane

- Ossicles (Ossicular Chain)

- Tympanic muscle

- Eustachian tube

What makes up the ossicular chain?

the malleus, incus, stapes

Mallues

- consists of a long process called the manubrium (handle) attached to the TM

- The head is attached to the body of the incus

Incus

- articulates with the head of the malleus and the stapes

Stapes

- head articulates with the incus

- footplate rests in the oval window of the inner ear

The ossicular chain is held together by what?

several ligaments

Impedance

the opposition to the flow of energy through a system

There is an impedance mismatch between what?

the air-filled ear canal and the fluid filled inner ear

The difference in the size of the TM and the oval window produces an area ration hydraulic effect providing....

25dB of pressure gain

The difference between the length of the malleus and the stapes cause a lever action adding...

2-3dB of pressure gain

The buckling of the TM adds...

4-6dB of pressure gain

What is the function of the middle ear with regards to impedance mismatch?

Middle ear transfers the vibration from the large, low impedance at TM to the smaller, high impedance at the oval window of inner ear

What is the function of the ossicular chain?

conduct sound from the air outside to the fluid of the inner ear and this is called the impedance mismatching transformer

What are the tympanic muscles?

stapedius and tensor tympani

What are the characteristics of the stapedius muscle?

- attached to the neck of the stapes

- innervated by the VII (facial) cranial nerve

--- This muscle pulls on the stapes in response to loud sounds

What are the characteristics of the tensor tympani muscle?

- originates from the eustachian tube

- attached to the manubrium of the malleus

- innervated by the motor branch of the V (trigeminal) cranial nerve

--- pulls on the malleus and runs parallel to the eustachian tube

--- contacts and pulls on the malleus

What is the acoustic reflex?

o These muscles contract and tighten the ossicular chain and provide protection against loud sounds

Both muscles pull on opposite ends of the ossicular chain protecting the inner ear from loud sounds

-- occurs in both ears

What is the Eustachian tube?

- Cartilaginous tube from the ME space to the nasopharynx

- Serves as the passageway from the nasopharynx to the anterior wall of the ME to bring oxygen to the ME space

What can happen in mastoid air cells?

o On the floor of the middle ear cavity where infection may result

What is the job of the eustachian tube?

- Normally closed but can be readily opened when we yawn, or swallow and a rush of air enters the middle ear space and provides an equalization of air pressure on both sides of the ear drum

What are the two purposes of the eustachian tube?

o Ventilation of middle ear cavity

o Facilitates drainage of fluids from the middle ear to the nasopharynx

The middle ear is kept at what by the eustachian tube?

atmospheric pressure

Infants and children are prone to what?

- prone to frequent ear infection because tube not fully developed (short and horizontal)

What is eustachian dysfunction?

- when the tube fails to open to equalize pressure (seasonal allergies, Flu, Cold, Sinus Infection, smokers)

What is within the inner ear?

houses the sensors for balance (semicircular canals) and hearing (cochlea)

What is the osseous (bony) labyrinth?

cavity that houses the membranous labyrinth

What is the membranous labyrinth?

- soft-tissue fluid-filled channels housing the inner ear structures

What does the balance portion include?

- Semicircular canals

- Utricle

- Saccule

Semicircular canals

sense organs for movement of the body in space

vestibule

entry way to the cochlea

Cochlear Labyrinth

sensory organ for hearing

What is the bony labyrinth filled with?

perilymph and endolymph

Characteristics of the membranous labyrinth?

o Sac filled with endolymph that rests in the cavity of the bony labyrinth

o Structure parallels the osseous labyrinth

o Houses the vestibular organs and cochlear duct

What are the characteristics of the cochlea?

- most medial part of the labyrinth

- 35 mm coil around the modiolus (core made of finely perforated bone)

What is the modiolus?

- Central core of the cochlea where sensory cells of the cochlea come together and form the auditory portion of the VIII cranial nerve

What two membranes divide the labyrinth lengthwise into 3 parallel chambers?

- Reisner's membrane

- Basilar membrane

What are the three parallel chambers?

- Scala Vestibuli

- Scala tympani

- Scala media

Which chambers contain perilymph?

scala vestibuli and scala tympani

Which chambers contain endolymph?

scala media

What are the three prominent openings?

- round window

- oval window

- cochlear aqueduct

Round window

- Communicates between the scala tympani and the middle ear

Oval window

- Permits communication between the scala vestibuli and the middle ear

Cochlear Aqueduct

- Opening between the scala tympani near the round window and the subarachnoid space

- Allows perilymph to pass through

What are the characteristics of the cochlear duct?

o Resides in the scala media

o Reissner's membrane separates the perilymph from endolymph

o Basilar membrane forms the floor of the scala media where the organ of Corti (organ of hearing) is found

Where is the organ of corti located?

- rides the basilar membrane; contains sensory cells of hearing

The organ of corti has how many rows of hair cells?

4

What are the two types of hair cells?

- outer hair cells

- inner hair cells

What are the characteristics of outer hair cells?

- Form three rows toward outer edge of cochlea; connected to tectorial membrane

- Elongated, small cilia, innervated by efferent (motor) fibers of the nervous system

- Each shares a single VIII nerve fiber innervation with ten other OHC

· More outer hair cells than inner hair cells

What are the characteristics of inner hair cells?

- Single row

- Elongated with an array of cilia on top

- Innervated by afferent (Sensory) fibers of the nervous system

- Stimulated by the motion of fluid movement

Each innervates as many as 10 VIII nerve fibers

Orientation of the canals help the brain differentiate between what?

left and right

Superior canal

- Responsive to tilting left or right

Posterior canal

- Responsive to forward/backward and tilting left or right

Horizontal Canal (Lateral)

- Responsive to rotation of the head left and right

What is the purpose of vestibular structures?

o Job is to sense head movement in three dimensions, there is a gelatinous substance that cilia hairs protrude into where movement can be sensed

Ampula

houses crista ampularis (receptor organ for movement) sense angular acceleration and deceleration

Steriocilia

embedded in the cupula inside the crista ampularis; senses movement in fluid, stimulates SCC (semi-circular canal)

What is the Utricle and Saccule?

lie within the vestibule; house the otolithic organs (crystals)

What is the purpose of the crystals?

- stimulate the utricle and saccule to help the body maintain its orientation in gravity

- important because if dislodged can cause a specific inner ear condition called benign paroxysmal positional vertigo (BPPV)

What treatment can be used to help with vertigo?

o moving head in certain positions to attempt to move crystals in correct position

What causes the traveling wave?

- Vibration of the stapes in and out of the oval window creates fluid, wavelike motion in the cochlea

What does the traveling wave do?

- As different sounds occur, the traveling wave proceeds down the cochlear partition until it reaches its point of maximum displacement

High frequencies

base, close to the oval window

Low frequencies

apex

Inner hair cells are stimulated sending neural impulses to what?

the cochlear branch of the VIII never

Outer hair cells

codes intensity of sounds

Inner hair cells

codes the frequency of sounds

VIII Cranial Nerve

Vestibulocochlear Nerve

Where does the VIII cranial nerve exit?

exits through the petrous part of the temporal bone through the internal auditory canal

- leads to the auditory and vestibular nervous system pathways

VIII connected to what?

pons affecting hearing, taste, and balance

Information is functionally crossed

right ear --> left auditory

Auditory Nervous System Pathways

- Afferent neurons transmit neural signals from the cochlea to the brainstem

- Efferent neurons regulate the outer hair cells and general inhibitory action throughout the central auditory nervous system

- Information is functionally crossed

Vestibular Nervous System Pathways

o Many afferent and a few efferent neurons communicate with hair cells of vestibular organs

o Afferent neurons carry sensory information up the central pathway to the vestibular nuclei on brainstem

o Helps control eye muscles (oculo-motor reflexes) and skeletal muscles

Central Auditory Pathways encompass what?

o Cochlear nucleus (CN)

o Reticular Formation (RF)

o Superior Olivary Complex (SOC)

o Inferior Colliculus (IF)

o Medial Geniculate Body (MGB)

Cerebral Cortex (CC)

Cochlear Nucleus

o Receives and modifies the neural responses from the VIII nerve

Reticular Formation

o Communicates with the brain

o Responsible for alertness, reflexes, habituation, selective attention and our ability to hear in noisy surroundings

Superior Olivary Complex

o Relay station

o Aids in binaural listening and sound localization

Inferior Colliculus

o Receives input from both SOCs

o Core portion of all auditory fibers

Medial Geniculate Body

o Direct connection to the auditory cortex

o Responsible for auditory attention

Cerebral (Auditory) Cortex

o Areas of auditory reception located in the temporal lobes on both sides

o Tonotopic

o Neurons use the ipsi and contra temporal and spectral information for the identification of the features of speech

o Aids in fine discrimination and speech understanding