Lab 9: Inner Ear, Auditory Pathways Flashcards
Modiolus
Central bony core around which coils of cochlea wind
Auditory part of VIII nerve
Fibers in central space of modiolus
Part of Spiral Ganglion
Cell bodies of auditory nerve located near the projections off the modiolus to the basilar membrane
Scala Vestibuli
Upper space of the BONY labyrinth that contains perilymph
Scala Tympani
lower space of the BONY labyrinth that contains perilymph
Scala Media (cochlear duct)
Middle space, consisting of the MEMBRANOUS labyrinth
Contains endolymph and the receptor cells for hearing
Vestibular Membrane
Separates the scala media and the vestibuli
Basilar membrane
Separates scala media and tympani
Spiral lamina
Shelf-like projection of the modiolus that extends to the basilar membrane
Auditory Nerve Fibers
Dark fibers in the spiral lamina
Vestibular Membrane
Delicate membrane between the scala vestibuli and cochlear duct (Scala media)
Basilar Membrane
Membranous floor of the cochlear duct
Stria Vascularis
A highly vascularized epithelium that lines the lateral wall of the scala media, that extends from the vestibular membrane to the spiral ligament
Organ of Corti
Supporting and hair cells on the basilar membrane
Inner hair cells
1 row in the image
Non-neural receptors for hearing
Outer hair cells
3 rows in the image
non-neural receptors for hearing
Tectorial Membrane
Plastic-looking, rigid membrane; receives cilia of hair cells
Petrous part of temporal bone
Contains middle and inner ear
External Auditory Meatus
- Lateral surface of temporal bone
- the path for sound waves=external auditory meatus—-middle ear—–inner ear
Internal Auditory Meatus
Channel in the petrous Temporal bone from the posterior cranial fossa; intracranial foramen for the 7th and 8th cranial nerves
Mastoid process
Lateral surface of temporal bone
Carotid canal
Basal surface of skull
Stylomastoid foramen
Basal surface of skull
Tegmen Tympani
- A thin plate of the anterior surface of the petrous temporal bone
- above the middle ear cavity
- located in the middle cranial fossa
Foramen lacerum
Visible on dried skull only
Between the apex of the petrous temporal bone and the sphenoid bone
Groove for Cartilaginous part of Auditory Tube
Visible on skull only
Between the petrous temporal bone and sphenoid bones
Laterally (boundaries of the middle ear)
Tympanic membrane
Epitympanic Recess: a small area that’s superior to the level of the tympanic membrane
Posteriorly (boundaries of the middle ear)
The mastoid air cells: the aditus leads to the antrum and the air cells of the mastoid process
-a route for the spread of infection
Anteriorly (boundaries of the middle ear)
Auditory tube: which leads to the nasopharynx.
Note the cardioid canal passes anterior to the tympanic cavity and inferior to the auditory tube
Inferiorly (boundaries of the middle ear)
Floor of the tympanic cavity
This floor is related to the superior jugular bulb
Medially (boundaries of the middle ear)
The inner ear
Superiorly (boundaries of the inner ear)
The tegmen tympani or thin roof formed by a plate of the petrous temporal bone that separates the middle ear from the middle cranial fossa
Auditory Ossicles—3 (contents of the middle ear)
Connect the tympanic membrane with the inner ear and transmit vibrations of the tympanic membrane to the inner ear. Of the ossicles, the MALLEUS and INCUS act as an angular lever and the STAPES titles to enable the footplate of the stapes to transmit the vibration produced by sound waves to fluid in the inner ear
Movements of the Malleus and Stapes
Influence by the tensor tympani muscle: innervated by CN V and is activated by sudden contact in the facial region. This reflex may stabilize the ossicles during head impact
The stapedius muslce which is innervated by CN VII and is activated by loud sound to reflexively protect the hair cells from damage due to excessive vibrations
Chorda Tympani Nerve (structure passing through the middle ear)
A branch of the facial nerve in the facial canal that carries parasympathetic and taste fibers to the oral region; it loops anteriorly between the malleus and the incus, medial to the tympanic membrane and exits the skull between the petrous and tympanic parts of the temporal bone at the petrotympanic fissure
Aditus (structures of the posterior wall of the middle ear)
Leads to antrum and mastoid air cells
Facial Canal (structures of the medial wall of the middle ear)
For the facial nerve (CN VII); trace the facial nerve through the petrous part of the temporal bone as it bends posteriorly, then turns and proceeds inferiorly to reach the stylomastoid foramen
A portion of the cochlea (structures of the medial wall of the middle ear)
Embedded in the petrous part of the temporal bone
Semicircular Canals (structures of the medial wall of the middle ear)
Embedded in the petrous part of the temporal bone
Promontory of the middle ear (structures of the medial wall of the middle ear)
Formed by the basal turn of the cochlea; has openings called the oval and round windows
-in life, the promontory is covered by mucous membrane
Tympanic plexus (structures of the medial wall of the middle ear)
Fibers that are located deep to the mucous membrane of the promontory. This plexus contains fibers of the glossopharyngela nerve (CN IX) that provide sensory supply to the mucous membrane of the middle ear, mastoid cells, and auditory tube. In addition,this plexus gives rise to the LESSER PETROSAL NERVE, which brings preganglionic parasympathetics to the otic ganglion
Pyramid (structures of the medial wall of the middle ear)
Small elevation of bone which houses the stapedius muscle
Other structures in the medial wall of the middle ear
Stapes
Footplate of stapes
Oval window
Lesser Petrosal Nerve
Derived from the tympanic plexus
This nerve is the visceral motor component of the glossopharyngeal nerve and brings preganglionic parasympathetic fibers to the optic ganglion.
It leaves the petrous temporal bone via a hiatus for the lesser petrosal nerve in the floor of the middle cranial fossa.
It exits the middle cranial fossa into the infratemporal fossa through the foramen oval and synapses with postganglionic parasympathetic neurons in the otic ganglion
Postganglionic parasympathetic fibers travel with the auriculotemporal nerve to innervate the parotid gland
Greater Petrosal Nerve (branch of the petrous part of the facial nerve)
A branch that will bring parasympathetic fibers to deep structures of the head
It branches from the facial nerve at the bend, then gravels to the anterior surface of the petrous temporal bone through the hiatus for the greater petrosal nerve.
The groove for the greater petrosal nerve can be seen on a skull
The temporal bone specimen has a string-like bulge representing the greater petrous all nerve in the upper area of the medial view of the middle ear cavity traveling anteriorly from the facial nerve to the area of the carotid canal
Chorda Tympani Nerve—-Again (branch of the petrous part of the facial nerve)
A branch from the facial nerve in the facial canal
It loops anteriorly, medial to the tympanic membrane between the malleus and incus and exits the skull between the petrous and tympanic parts of the temporal bone through the petrotympanic fissure
It enters the infratemporal fossa and travels to structures of the oral cavity
Nerve to the Stapedius (branch of the petrous part of the facial nerve)
This branch leaves the facial nerve in the facial canal and immediately enters the muscle with the same name
Imagine this branch exciting from the facial nerve in the facial canal and traveling to the stapedius muscle
In the Clinic: Infection
The bony roof and floor of the middle ear cavity can be very thin. Infection can easily erode them away
If infection spreads through the roof, there can be involvement of the meninges and brain (meningitis, cerebral abscess in the temporal lobes)
If infection spreads through the floor into the jugular vein there can be jugular thrombosis
Review on Brain stem
Vestibulocochlear Nerve at cerebellopontine angle
Flocculus and facial nerve: near VIIIth cranial nerve
Inferior colliculus: nucleus for synapse in auditory pathway
Brachium of inferior colliculus: path for fibers from the inferior colliculus to the thalamus (medial geniculate nucleus or body)
Medial geniculate body: thalamic nucleus for hearing
Review certain structures on the cortex of the brain
Transverse Temporal Gyri (of Heschl): primary auditory cortex
Auditory association cortex: peripheral to the primary auditory cortex in the superior Temporal gyrus
Wernicke’s Area: usually on the left side; the planum temporale (posterior part of the superior Temporal gyrus and parts of the inferior parietal lobule)
Dorsal and Ventral Cochlear Nuclei (rostral medulla)
- Located lateral and dorsal to the inferior cerebellar peduncle
- contain cell bodies of second order neurons in the auditory pathway
- receive fibers from the cochlear nerve
- send axons into the acoustic stria
Lateral Lemniscus (caudal pons)
Passage of fibers from the cochlear nucleus across the midline into the auditory stria (dorsal acoustic stria, intermediate acoustic stria, and ventral acoustic stria)
These projections will gather into the pathway called lateral lemniscus
In the Clinic: Lesion of Pathway
- The auditory info from the cochlear nuclei will ascend through the brainstem bilaterally
- consequently, a lesion of the ascending pathways distal to the cochlear nucleus will result in virtually no loss of hearing
- the deficit that the patient will experience is an impairment of the ability to localize the direction and distance of sounds
Superior Olivary Nucleus
- Some of the ascending fibers of the central auditory pathway will synapse in the superior olivary nuclei in the ventrolateral tegmentum of the caudal pons
- This nuclear complex receives bilateral impulses from the cochlear nuclei and is involved in auditory processing related to sound localization
- the SON will send axons into the lateral lemniscus
Lateral Lemniscus (in the rostral pons)
- Lateral to the superior cerebellar peduncle
- contains fibers from the acoustic stria and superior olivary nucleus
- contains ascending acoustic fibers that are both crossed and uncrossed
Lateral Lemniscus (pons-midbrain junction)
-tract and dark rim below the inferior colliculus
Inferior colliculus (pons/midbrain junction)
Nuclei which receive terminals from lateral lemniscus fibers
Superior Colliculus (midbrain/diencephalon transition)
Reflex center for visual system
Brachium of inferior colliculus (midbrain/diencephalon transition)
Ventral to the medial geniculate Nucleus
-fibers that travel from the inferior colliculus to the medial geniculate Nucleus
Medial Geniculate nucleus (midbrain/diencephalon junction)
The thalamic nucleus for hearing
-receives fibers from the brachium of the inferior colliculus
Brachium of the superior colliculus (midbrain/diencephalon junction)
Dorsal to the medial geniculate Nucleus
-fibers from the optic tract en route to the superior colliculus and pretectal area
Medial geniculate Nucleus (thalamic view)
Located medial to the lateral geniculate nucleus
- the thalamic Nucleus for hearing
- receives fibers for hearing from the inferior colliculus
- sends axons to the primary auditory cortex
- it sends auditory radiations to the primary auditory cortex (transverse temporal or Heschl’s gyri) via fibers that pass BELOW the caudal part of the lenticular nucleus
- in this part of the radiations, they’re called “sublenticular fibers” or the “sublenticular part of the internal capsule”
Location of the Brainstem auditory nuclei and fibers
-should know that they’re located in the ventral (caudal pons) and then lateral (rostral pons) reticular formation until they reach the inferior colliculi (roof of midbrain)
Paths and nuclei from the inferior colliculi to the auditory cortex are difficult to locate in most clinical scans
Markers for location on clinical scans (lateral aperture of the IV ventricle)
A marker for the general position of the termination of cochlear nerve fibers
Markers for location on clinical scans (inferior cerebellar peduncle)
General position of the cochlear nuclei are along the lateral surface of the inferior cerebellar peduncle
Caudal Pons Clinical Scan Location
-general position of auditory nuclei such as the superior olivary nuclei along the ventral edge of the tegmentum (along the horizontal reference line between the tegmentum and the base)
Nerve Deafness
Due to damage of the cochlea, cochlear receptors, or the cochlear nerve
- damage to the cochlear nerve cannot usually be corrected
- however, cochlear implants have been successful in restoring some ability to detect sounds in patients with cochlear nerve deafness
Conduction Deafness
Due to damage of middle ear structures such as the tympanic membrane or ear ossicles
-can often be corrected with surgery or hearing aids
Air-bone conduction test of Rinne
- compares the efficiency of the conduction of sound vibrates through bone vs. air
1. Places a faintly vibrating tuning fork on the “patient’s” mastoid process. Just after the sound disappear as, move the fork over the patient’s external auditory canal. He/she should be able to hear the sound again, because normally air conduction is more efficient than bone
2. If you press your fingertip in your ear while holding the tuning fork besides the external auditory canal, the sound will be blocked, but press your finger hard into your hear to occlude the canal. Then hold the tuning fork on your mastoid process. The sound gets louder. This test shows that a mechanical impediment in the auditory canal can cause an apparent increase in bone conduction
3. If anything impedes the conduction of sound vibrations through the external auditory canal or ossicles, the PT has a CONDUCTION hearing loss. Contrarily, a reduction of hearing by a lesion of the organ of Corti or the auditory nerve is called a NUEROSENSORY hearing loss. Conduction loss refers to loss of mechanical conduction of sound vibrations through the external and middle ear, not a loss of conduction of nerve impulses through the 8th nerve. Application of the vibrating tuning fork to the mastoid process causes bone conduction to the inner ear to excite auditory impulses, bypassing the air conductance channels of the external and middle ear. Bone conduction tests the integrity of the nerve even though air conduction of sound vibration through the external and middle ear is blocked. Characteristically, neurosensory losses cause a decrease in both bone conduction and air conduction. In other words, nerve lesions block hearing by either channel (air or bone), whereas mechanical lesions block only the sounds transmitted through the ear
Weber Test
A. Consists of placing a vibrating tuning fork on the middle of the forehead or the vertex of the skull. Try this test. The sound seems to come from the center and is equal in both ears if you’re normal.
B. With the vibrating fork in place on the vertex of your head, press your fingertip in one ear and then the other. The sound lateralizes to the occluded side. If a mechanical impediment blocks sound conduction in one ear, the vertex sound localized to the same side as the block
If the pt has an auditory nerve lesion on one side, the vertex vibration sounds loudest on the OPPOSITE SIDE OF THE LESION
-only a consistent lateralization to one side is considered significant
In the Clinic: after hearing tests
If your screening suggests loss of hearing, refer the patient for electronic tests. These may include pure tone audiometry, speech discrimination batteries, and a loudness discrimination test. Brainstem auditory evoked responses (BAER) do not require conscious responses and objectively tests for the integrity of the auditory pathways in conscious or unconscious patients. The commonest causes of delayed speech are mental retardation and deafness. Always investigate every infant or young kid thoroughly with bedside and electronics tests if a child doesn’t appear to hear or has delayed speech
