Lecture 46 Flashcards
Ear anatomy
Outer ear (made of cartilage):
- Pinna aka auricle
- funnels the sound waves into the outer ear canal aka external auditory ear canal - External auditory canal (ear canal)
- has hairs and some glands to secrete ear wax.
- ear wax and hairs together protect ear canal from dirt and small animals
Separates outer ear and middle ear:
- Tympanic membrane (eardrum)
- covered by skin on the external surface and by simple cuboid ep on the inner surface
- the core of the membrane contains fibroelastic CT
Middle Ear (is an air-filled space that ends with oval window (membranic window), which separates the middle ear from inner ear):
- Three ossicles (smallest bones in the body): Malleus, incus, stapes (aka stirrup)
- these ossicles are connected together through joints and suspended with tendons.
- the malleus picks up movement of tympanic membrane, translates it to movement of incus, then to stapes.
- uses lever forces to enhance movement of tympanic membrane, that then leads to stapes pushing against the oval window
- stapes aka stirrup is locked against the oval window and as sound waves hit our ear, the stirrup moves like piston against the oval window. This puts pressure on the inner ear. Inner ear is a bony labyrinth filled with liquid.
- how does stirrup move against oval window? The stirrup goes against oval window, which covers perilymph fluid. This fluid will be dislocated through the push from the stirrup all the way to the tip of the coil (Apex) and down to the other side. The top fluid filled space is scala vestibuli and bottom fluid filled space is Scala tympani, which are both in the cochlea (but influenced by movements in middle ear)
Inner Ear (bony labyrinth filled with liquid):
- Cochlea (has perilymph fluid, helicotrema, endolymph fluid and basilar membrane)
- with oval window, round window, cochlear nerve, endolymph-filled cochlear duct
- has cochlear nerve (afferent fibers via neurons of the spiral ganglion, inhibitory impulses via the olivocochlear efferent)
- cochlear nerve: our hearing organ is aligned all along the cochlea duct. Our cells are innervated by spiral ganglions; the 8th vestibulocochlear nerve innervates the inner ear
- scala tympani and the scala vestibuli join together at the apex of the cochlea to form the helicotrema.
- bw scala vestibuli and scala tympani, there is an endolymph-filled cochlear duct. It is filled with endolymph fluid. - Semicircular canals
Additional parts:
1. Eustachian tube aka auditory tube
- middle ear is connected to outside in order for air pressure to be same as environment.
- if pressure is diff from middle ear to environment, it causes discomfort or ruptures ear drum.
- to prevent quick changes in air pressure, we can open up eustachian tube (connects
middle ear with nasal pharynx) (allows us to crack our ear or open when we yawn).
- can only open from a certain age on, newborns cannot do this.
2. Auditory nerve, vestibular nerve (these both might be referring to cochlear nerve)
Basilar membrane in cochlea of inner ear
Oval window makes perilymph move all the way up to the apex or helicotrema at the tip of the coil, then the fluid moves back and
bulges out the round window
As fluid moves back and forth bw the 2 perilymph spaces, it makes the
membrane (called basilar membrane) on the bottom of the cochlear duct move.
Basilar membrane is stiff and narrow close to the window, but becomes wide and floppy at the end near the apex.
- These properties make the basilar membrane swing, depending on the sound frequency.
- high frequency (short wavelength) will maximally move the basilar membrane
very close to the round window.
- low frequency (long wavelength) makes
movement go to the end at apex and makes basilar membrane swing
- Overall: high-frequency sound stimulates sensory cells near the base of the cochlear spiral, whereas the low sound frequencies that are most important for speech and music perception cause maximal stimulation of hair cells near the apex of the spiral
Cochlea
Spiral ganglion (on the side) are nerves that enter the cochlea
Osseus spiral lamina (on the side)
From top to bottom of structure:
- Scala vestibuli (filled with perilymph)
- Vestibular membrane aka Reissner’s membrane (above cochlear duct)
- Cochlear duct (filled with endolymph) (in middle) (bounded by Scala media)
- scala vascularis (next to scala media or cochlear duct)
- Organ of corti (next to cochlear duct): hair cells, tectorial membrane
- internal spiral sulcus (next to organ of corti)
- basilar membrane (below coclear duct)
- spiral ligament on the side
- Scala tympani (filled with perilymph)
Organ of corti
Tectorial membrane, hair cell, supporting cells
Organ of corti has supporting cells and hair cells, all sitting on basilar membrane.
- 3 rows of external hair cells aka outer hair cells and 1 row of inner hair cells.
- hair cells are embedded in supporting cells, and are innervated by spiral ganglion cells that pick up neural signals from the hair cells, but they are also innervated by efferent fibers (from CNS to the hair cells)
Hair cells have stereocilia that is in touch with tectorial membrane (collagen-rich membrane that covers organ of corti)
Shearing forces
When basilar membrane swings, the stereocilia in the hair cells will get bent by the tectorial membrane. This is called shearing forces.
As organ of corti gets lifted up, it moves agains the tectorial membrane and this makes the stereocilia bend. This bending of stereocilia makes the neural signal in the hair cell
Hair Cells
• has Sterocilia (60-100 per cell, supported by actin containing cuticular plate)
- embedded in tectorial membrane at the top
• has Kinocilium (only 1 per cell, gets lost in auditory hair cells during development)
• Deflection of stereocilia activate mechanically activated ion channels
Hair cells are imbedded in supporting cells and innervated by afferent and efferent fibers.
- afferent fibers pick up neural signal.
- efferent nerve endings come from the brain.
Destroying stereocilia and kinocilium usually causes noise-induced hearing loss. Once they break off, they will not grow back.
Hair cell - signal transduction
Hypothetical
Stereocilia of hair cells are linked together by filaments called tip links, which mechanically open up ion channels when the stereocilia bend.
These ion channels, potassium and calcium channels, lead to depolarization of hair cells that lead to neurotransmitter release and signal in the cell
Outer hair cells
Embedded in supporting cells called phalangeal cells. Phalangeal cells make a weird process that forms a apical cuticular plate.
From basilar membrane to upwards:
- basilar membrane
- outer phalangeal cell (supporting cell)
- facet for outer hair cell
- outer hair cell
- apical cuticular plate (has a phalangeal process that bends downwards to connect back to outer phalangeal cell)
Outer hair cell motor
Outer hair cells not really engaged in hearing, but mostly to amplify the movement of basilar membrane
Through swinging of membrane, the stereocilia bends, the cell gets depolarized. When the cell is depolarized, the outer hair cell shortens due to protein motors. When they get shorter, they pull the basilar membrane up
