Anatomy Lesson 11-Ear, Audition and Equillibiumz

Four (4) Cranial Nerves involved in hearing (directly & indirectly)

CN VIII (Vestibulocochlear Nerve)

• Transfers vibrations from the cochlea into the cochlear nuclei in the brain

• Connects the inner ear to the primary auditory cortex via the brainstem and thalamus

  CN V (Trigeminal Nerve)

• Provides motor innervation to the tensor tympani muscle in the middle ear

  CN VII (Facial Nerve)

• Innervates the stapedius muscle in the middle ear

  CN X (Vagus Nerve)

• Provides sensory innervation to the external ear and auditory canal

• This keeps our ears safe as we can sense foreign objects and injuries in it

Parts of the External Ear

-Auricle

-External Auditory Canal

-Ceruminous Glands

-Tympanic Membrane

Auricle

• The outer ear

• A funnel for sound waves to enter the external auditory canal

External Auditory Canal

• Outer ear to tympanic membrane

• Sound waves travel through this area

• Inner ⅓ of the skin in the auditory canal is glued to the bone. The outer ⅔ is glued to fat. This makes pimples inside the ear very painful

Ceruminous Glands

• Inside the auditory canal

• Secrete ear wax that helps to trap foreign objects and repel insects

Tympanic Membrane

• Has thin skin on the outside (facing the auricle) and a mucous membrane on the inside (facing inner ear). 

• When sound waves hit the tympanic membrane it vibrates

Parts of the Middle Ear (5)

-Ossicles (Malleus, Incus, Stapes)

-Tensor Tympani Muscle

-Stapedius Muscle

-Auditory Tube

-Mastoid Air Cells

Ossicles

• The malleus incus and stapes are three bones in the middle ear responsible for transmitting vibrations

• They are suspended by ligaments and connected through synovial joints which can develop arthritis leading to conduction deafness

• Malleus: Receives vibrations from the tympanic muscle and transfers to the incus

• Incus: Transfers vibrations from the malleus to the stapes

• Stapes: The vibrations cause the stapes to move and push against the oval window which moves fluid inside the inner ear

Tensor Tympani Muscle. What is it’s innervation and action?

• Attaches to the tympanic membrane

• Trigeminal Nerve

• Because it is innervated by the same CN that innervates the jaw muscles, the tensor tympani muscle dampens sounds such as your chewing or shouting. This is why you sound different when you hear yourself talk vs hear an audio of yourself talk. This is also why you can’t hear as well when you are chewing

Stapedius Muscle: Innervation and action

• This muscle pulls the neck of the stapes. When you hear a loud sound this muscle contracts and the vibrations are not as intense in the stapes and therefore it doesn’t damage your hearing as much as it could without this muscle

• Facial Nerve

Auditory Tube

• Also called the nasopharynx tube or the eustachian tube

• This tube is responsible for balancing pressure

• It opens in the nasopharynx so when pressure builds in your ear it can be released to the outside world through this tube

• If you develop an infection in your nasopharynx it could transfer to your middle ear and cause an ear infection and buildup of mucus and pressure

Mastoid Air Cells

• A spongy part of your middle ear that contains air bubbles

• Inflammation of this area can lead to meningitis

Parts of the Inner Ear (2)

• Vestibular apparatus - balance

• Cochlea – hearing

Structures of the Cochlea (5)

-Oval Window

-Round Window

-Membranous labyryinth within bony labyrinth

-Perilymph

-Endolymph

Oval Window

 The stapes hits the oval window which transmits vibrations down the perilymph of the cochlea.

Round Window

• The vibrations exit the cochlea through the round window. Without this, pressure would build in the inner ear

Membranous Labyryinth within Bony Labryinth

Within the cochlea is the cochlear duct. The cochlear duct’s endolymph vibrates due to the cochlea’s perilymph

Perilymph

Fluid found in the outer part of the cochlea. Vibrations from the middle ear are transmitted here first. The perilymph’s vibrations vibrate the endolymph.

Endolymph

 Found in the cochlear duct inside the cochlea. Vibrations on the outside perilymph vibrate the endolymph.

Path of Transmission of Vibrations

1. Sound waves are collected at the auricle (external ear) and transferred down the external auditory meatus to the tympanic membrane

2. Sound waves vibrate the tympanic membrane

3. Ossicles transmit vibrations down the middle ear (tympanic membrane >> malleus >> incus >> stapes )

4. Stapes transmits vibrations to oval window

5. Vibrations transfer through the perilymph of the scala vestibuli (first part of cochlea) of the cochlea (perilymph made my endothelial cells in the cochlea)

6. Vibrations then travel down the scala tympani and out the round window (scala tympani is bottom part of cochlea)

7. Vibrations in the perilymph set off vibrations in the endolymph

8. Vibrations in the endolymph cause the basilar membrane to vibrate which moves hairs and opens mechanoreceptors, causing nerve impulses to go down CN VIII

3 Membranes of the Cochlear Duct

-Vestibular Membrane

-Tectorial Membrane

-Bassilar Membrane

Vestibular Membrane

Separates the cochlear duct from the scala vestibuli and separates perilymph from endolymph.

Tectorial Membrane

 Lays on top of the organ of corti. As the basilar membrane moves, the organ of corti’s hair cells bend against the tectorial membrane to activate mechanoreceptors

Basilar Membrane

 Separates the cochlear duct from the scala tympani. The basilar membrane is what vibrates and causes hair cells to move

Importance of Collagen

 Provides strength yet flexibility. This is important for the vestibular and tectorial membrane as they need to be thin but strong. It also provides a gradient of stiffness in the basilar membrane which helps detect different sound frequencies.

Organ of Corti

• Hair Cells

• Support cells

How are Regions of the Cochlea Tuned to Different Hz?

At the base of the cochlea, the collagen fibers are long and stiff. They detect high pitched noises (like long strings on a guitar). At the end of the cochlea (near the inside of the spiral) the collagen fibers are short and flexible allowing them to detect bass tones.

6 Structures in the Brain’s Auditory Pathway in Order

• Cochlear Nuclei: In the medulla oblongata. Information from CN VIII decussates and travels through the brain bilaterally at this point. 

• Superior Olivary Nuclei: In the pons. Receives info from both cochlear nuclei, but obviously it will receive info fastest from the cochlear nuclei closest to it. This helps with knowing which direction a sound came from. 

• Lateral Lemniscus: A white matter tract part of the RAS

• Inferior Colliculus: Found in the dorsal midbrain. Automatic reflexes to sound.

• Medial Geniculate Nucleus: In the thalamus. Filters out unnecessary sounds

• Primary Auditory Cortex: In the temporal lobe. Conscious perception of sound.

“Confidence Supports Lasting Inner Mental Progress“

Conduction vs Sensory Deafness

• Conduction: A problem in the outer or middle ear such as wax buildup, tympanic membrane damage, or ossicle arthritis. Conduction deafness results in a loss of amplitude, as something is causing fewer vibrations to enter the inner ear. You might hear the whooshing of your blood instead.

• Sensory: A problem in the inner ear such as hair loss, damage to CN VIII, trauma, etc:. This results in a total loss of sound as something is either preventing the endolymph/perilymph to vibrate or preventing the transfer of vibrations to impulses. Can cause tinnitus which is a result of lack of stimulation.

Structures of the Vestibular Apparatus

-Utricle

-Saccule

-Semicircular Ducts

Utricle

-Detects linear acceleration on the horizontal plane. It contains a macula which has hair cells and supporting cells. The supporting cells form an otolithic membrane which is kind of like a gel. The otolithic membrane contains otoliths which are calcium carbonate crystals. When you accelerate, gravity acts on these stones and they drag the gel with them which stimulates the hair cells. 

Saccule

Detects linear acceleration on the vertical plane. Works same way as utricle

SemiCircular Ducts/Canals

• At the base of the semicircular canals lies an ampule (dilation). Inside the ampule is some endolymph and a cupula. When you have rotational acceleration or deceleration, the endolymph moves which causes the cupula to move. This moves the crista (contains hair and supporting cells) and sends nerve impulses. 

• Anterior canal detects rotation along the sagittal plane (nodding yes)

• Posterior canal detects rotation in the coronal plane (head tilt side to side)

• Lateral canal detects rotation in the horizontal plane (shaking head no)

Vestibular Pathway

Info travels from hair bundles >> CN VIII >> Vestibular nuclei in the medulla and pons >> Thalamus >> Sensory Cortex

Where is info shared?

• Info is shared with the following cranial nerves: Eye nerves (CN III, IV and VI), accessory nerve (XI) for head/neck control. 

• Info is also shared with the vestibulospinal tract and cerebellum.