The Ear Flashcards
What fluids are in each labyrinth?
Osseous - Perilymph (high Na low K) - like blood plasma
Membraneous - Endolymph (low Na high K)
What provides blood to the inner ear?
Labyrinth artery, a branch of the basilar artery
What is the vestibule and its relation to other structures?
Part of the osseous labyrinth, connects posteriorly to the three semicircular canals, and anterior to the cochlea. Also gives off a smaller bony channel called the vestibular aqueduct
What inserts on the wall of the vestibule?
The stapes, because the oval window is on the wall of the vestibule
What is the modiolus? What does it give off?
The bony central axis for the cochlea’s 2.5 turns, it gives off the osseous spiral lamina which indents the inner edge of the cochlea
What is the spiral ganglion and where does it run?
It is the ganglion inside the modiolus for the cochlear nerve, composed of bipolar cells. The cell process of these CN8 cells insert in the inner tunnels between hair cells.
What is the spiral ligament?
The outside wall of the cochlea, a layer of dense connective tissue
What are the two osseous compartments of the cochlea and what do they connect to?
Scala vestibuli - oval window
Scala tympani - round window
What is the middle compartment flanked by the two osseous compartments?
The scala media, part of the membranous labrinythe
What is the apex of the cochlea?
Helicotrema - where the scala vestibuli and scala tympani meet
What secretes the perilymph?
Blood vessels in the connective tissue of the osseous labyrinth
What is the membranous labyrinth derived from? How do the cells keep the endolymph from the perilymph?
Derived from otocysts.
Endolymph kept from perilymph via occluding zonules (tight junctions)
What is the sensory structure of the semicircular ducts and where is it found?
At the end of the ducts, connecting to the utricle in the vestibule, there is the ampulla
Sensory: Crista ampullaris
What structures detect linear acceleration?
Utricle and saccule, via the maculae.
Where does the endolymphatic duct and sac travel?
Through the vestibular aqueduct
What is the connection between the saccule and the cochlear duct?
The ductus reuniens
What is the roof of the cochlear duct?
Vestibular membrane / Reissner’s membrane - A membrane composed of two simple squamous epithelial sheets with a basal lamina in between.
What is the basilar membrane derived from and what sits upon it?
Osseous labyrinth tissue
it borders the scala tympani. Organ of Corti sits ontop of it.
What structure sits atop the organ of Corti as it transduces auditory signals?
The tectorial membrane
Where does the tectorial membrane come from?
The angle between the vestibular membrane and the organ of Corti gives off a ridge called the spiral limbus. The spiral limbus gives of the tectorial membrane
What supports the spiral limbus?
The osseous spiral lamina, which is part of the osseous membrane / modiolus
What is on the outer wall of the cochlear duct and what is its function?
Stria vascularis - one of the only vascularized epithelia you will see. It provides blood, and also produces the ENDOLYMPH
It is pseduostratified
What is beneath the stria vascularis?
The spiral ligament - connective tissue, part of the osseous labyrinth
What are the sensory cells of the ear and what do they contain?
They are hair cells, they contain stereocilia and either a kinocilium (longest hair) or basal body (in case of cochlea)
What is the kinocilium and where is it not found?
It is a cilium-like structure containing 9+2 microtubules, probably not motile. It is the longest hair, and is lost during development of cochlear hair cells
What describes the arrangement of stereocilia in hair cells?
Organ pipe
How does membrane polarization work with respect to ions?
Stereocilia are connected to adjacent stereocilia via tip links. When they tilt towards the kinocilium, K+ gates open. When they tilt away from the kinocilium, the K+ gates are shut closed harder
Thus,
Towards: depolarization
Away: hyperpolarization
How does afferent and efferent communication to hair cells work?
Afferent - depolarization causes neurotransmitter to hit the dendrites of CN8, which is relayed to sensory ganglion cells and ultimately the brain.
Efferent - hair cells can have activity decreased by negative feedback from the brain (so all of the neurotransmitter being released does not damage the cells)
What is the crista ampullaris and how is it oriented?
It is the sensory portion of the semicircular ducts, runs perpendicular to the duct axis (thus picking up on fluid flow)
What epithelium lines the semicircular ducts and how does it change at crista ampullaris?
It’s just simple epithelium that changes into stratified epithelium. Ultimately, this lining will become tall columnar supporting cells for the sensory hair cells
What is the cupula?
The membrane which embeds the stereocilia and kinocilia of the crista ampullaris and acts like a sail. It will move all the hair cells
What are sensory maculae?
The sensory organs of the utricle and macule, have supporting and sensory cells just like the crista ampullaris
How is the maculae different from the crista ampullaris?
Rather than the cupula, the sensory and supporting cells are embedded in the otolithic membrane. It is thin and gelatinous, and has crystalline inclusinos called otoliths / otoconia
What is the purpose of the otoliths?
They increase the mass on the otolithic membrane to add more torque to the accelerating forces on the membrane. This depolarizes the membrane more by adding a lag to a fast depolarization effect.
How do hair cells at the base of the organ of Corti differ from those at the apex?
Base - high frequencies
Apex - low frequencies
This is the “tonotopic” arrangement of hair cells
What are the two layers of hair cells in the organ of Corti?
Inner layer: 1 row - most important for hearing
Outer layer: 3-5 rows - less imporatnt for hearing
Separated by a space called the inner tunnel
Where do hair cells of the organ of Corti rest?
stereocilia are embedded in tectorial membrane, but their bases are actually attached to phalangeal cells. These cells rest on the basement membrane
What is the function of phalangeal cells?
They are cells which hold the hair cells in the organ of Corti and anchor to the basilar membrane. They send processes up which hold the reticular lamina and keeps the apical portions of hair cells steady
What is the reticular lamina?
Also called the cuticular plate, it is held onto by the phalangeal cells and steadies the apical surface of the hair cells. It is also the separation between endolymph and perilymph in the organ of Corti
What reinforces the reticular lamina?
It is locked together by adherent junctions and tight junctions from the phalangeal cells.
What filaments to phalangeal cells + pillar cells have?
They contain many microtubules and intermediate filaments
What are pillar cells? What is the function of the cavity they line?
Cells lining the inner tunnel, which carries the CN8 dendritic process of cells from the spiral ganglion to the hair cells
How does the tectorial membrane function in the organ of Corti?
It is rigid with keratin-like protein. Outer hair cells stereocilia are definitely embedded in it. Inner cells is less certain.
What is the basilar membrane? How does it respond to sound?
It is a dense but but supple mat of collagenous fibers which act like a guitar string. It moves up and down when the oval window stimulates surrounding perilymph to make the basilar membrane to move.
How does the brain discern pitch and loudness?
How often the signals travel through the cochlea from the round window = frequency
How high the basilar membrane is depolarized via shearing forces in that segment = amplitude
Both the organ of Corti and basilar membrane move. The cells with the maximum shearing force in a given pitch range is the frequency
What causes vertigo?
Disturbances in vestibular function (dizziness)
What causes Meniere’s disease?
Swelling of membranous labyrinth leads to dizziness and ringing of ears and low frequency hearing loss. Makes people feel motion sickness
What is conductive vs sensorineural hearing loss?
Conductive - something wrong mechanically (i.e. otosclerosis or tympanic membrane disruption
->surgical treatment
Sensorineural - loss of hair cells of CN8 fibers -> cochlear implant treatment or just lip reading + sign language
What is presbyacusis?
Form of sensorineural hearing loss, poor high frequency perception due to loss of hair cells at base of cochlea. Most common in aging
What can hypothyroidism do?
Cause hearing loss, need to replace hormones to alleviate the hearing loss
What is Pendred syndrome?
Genetic disorder causing early hearing loss in children, related to thyroid disorder and can cause problems with balance. Thyroid hormone replacement later in life will not reverse hearing loss.
