UNIT 6 - The ear = hearing & equilibrium Flashcards
structure of the outer ear
- auricle (pinna)
- External acoustic meatus
- Tympanic membrane
Auricle / Pinna
The visible fleshy structure on the side of head
External Acoustic Meatus
▪ Passageway that leads to the tympanic membrane
▪ The skin lining the passageway contains ceruminous glands which secrete cerumen, or earwax
- Earwax helps trap foreign debris and repel insects
Tympanic membrane
▪ Thin connective tissue membrane covered by epithelium that separates the outer & middle ear.
▪ Vibrates when sound waves strike it.
Structure of the middle ear
- Located whitin the tympanic cavity of the temporal bone
- Houses the ossicles
Ear Ossicles - three tiny bones in the middle ear cavity
Which ossicle is in contact with the tympanic membrane?
malleus
Which ossicle inserts into the oval window?
stapes
Ear Ossicles - Function - middle ear
together these transmit sound vibrations from the tympanic to the inner ear
Muscles of middle ear - Tensor & stapedius - middle ear
Function:
To reduce vibrations of the ossicles when they become excessive to minimize damage to hearing receptors
Oval window - middle ear
Membranous area that connects the stapes to a portion of the inner ear called the scala vestibuli
Round window - middle ear
▪ A flexible, membranous area between the middle ear cavity and the scala tympani of the cochlea.
▪ We will refer back to this in a few minutes to discuss its function.
Pharyngotympanic (Auditory) Tube - middle ear
- Passageway between the middle ear and the nasopharynx.
- Usually is flattened and closed but opens briefly with swallowing or yawning to equalize pressure between the atmosphere and the middle ear cavity.
Structure of the Inner ear
General structure:
▪ The inner ear is comprised of a bony labyrinth and a membranous labyrinth.
Bony Labyrinth - inner ear
▪ Series of connected chambers within the temporal bone which houses the inner ear structures and gives them their distinct shapes.
▪ Perilymph – fluid located in bony labyrinth and surrounding the membranous labyrinth
Membranous Labyrinth - inner ear
▪ Series of membranous tubes that generally follow the contours of the bony labyrinth and form the inner portions of the vestibule, semicircular canals, and cochlea.
▪ Endolymph – fluid located in membranous labyrinth.
Functional parts of inner ear
The functional parts of the inner ear include:
◦ vestibule
◦ semicircular canals
- Vestibular apparatus ( involved with equilibrium)
◦ cochlea
Structural features of the cochlea - inner ear
▪ Scala vestibuli – open space forming the upper 1/3 of each coil in the cochlea.
- Contains perilymph
Structural features of the cochlea - inner ear
▪ Vestibular membrane – separates the scala vestibuli from the cochlear duct.
Structural features of the cochlea - inner ear
▪ Cochlear duct – central, membranous portion of the cochlea; it houses the spiral organ (organ of Corti).
- Contains endolymph
Structural features of the cochlea
Spiral organ (organ of Corti) – located within the cochlear duct; sits on the basilar membrane.
- It is the sensory receptor for sound
Structural festures of the cochlea - inner ear
▪ Hair cells – cells within the spiral organ that convert sound waves into nerve impulses.
- Have “hairs” that are embedded in the tectorial membrane.
- “Hairs” are actually microvilli.
Structural features of the cochlea - inner ear
▪ Tectorial membrane – gel-like projection sitting on top of the hair cells.
- Hairs” are embedded in this membrane.
Structural features of the cochlea - inner ear
▪ Basilar membrane – supports the spiral organ and separates the cochlear duct from the scala tympani
Structural features of the cochlea - inner ear
▪ Scala tympani – open space forming the lower 1/3 of each coil of the cochlea.
- It ends at the round window.
- contains perilymph
Structural features of the cochlea - inner ear
Helicotrema – the tip of the cochlea where the scala vestibuli is continuous with the scala tympani.
Role of the round window - Inner ear
membranous area that acts as a pressure valve to dissipate waves in the fluid at the end of the scala tympani.
Physiology of hearing
Steps in Hearing
- Sound vibrations enter the external acoustic meatus.
- Tympanic membrane vibrates.
- Ossicles (malleus, incus, stapes) vibrate.
Steps in hearings pt2
- Stapes vibrates the oval window.
- This produces pressure waves in the perilymph of the scala vestibuli.
- Perilymph bounces off the vestibular membrane, causing it to vibrate.
- This causes pressure waves in the endolymph within the cochlear duct.
Steps in hearing pt3
- Endolymph bounces off the basilar membrane and causes it to vibrate.
- The embedded “hairs” of the hair cells bend in response to movement of the basilar membrane.
- Bending of hair cells triggers a nerve impulse.
Steps in hearing pt 4
- The impulse is sent through the cochlear nerve to the vestibulocochlear nerve (cranial nerve VIII)
- The impulse travels through several structures including the medulla oblongata, inferior colliculi, and thalamus before being routed to and interpreted in the temporal lobe.
Characteristics of sound - pitch
▪ Sounds waves of different pitches (frequencies) “hit” the vestibular membrane (and thus the basilar membrane) at different regions.
▪ Perception of pitch is determined by the location of the hair cells that send the nerve impulse.
Characteristics of sound loudness - pt1
▪ A louder noise causes stronger vibrations, thus causing more hair cells to bend.
▪ The more hair cells that are stimulated, the louder the sound that is perceived.
Characteristics of sound - loudness pt2
▪ A louder noise causes stronger vibrations, thus causing more hair cells to bend.
▪ The more hair cells that are stimulated, the louder the sound that is perceived.
Characteristics of sound - location
Where a sound is coming from is determined by the brain based on:
◦ the loudness in each ear ◦ the time when the sound reaches each ear
Deafness - two types
sensorineural deafness & conduction deafness
Sensorineural deafness
due to damage to hair cells, nerves, or even cells of cerebral cortex
often difficult to correct
Conduction deafness
due to something that hinders sound waves from reaching fluids of inner ear (ex. earwax buildup, middle ear infections, or rupture of tympanic membrane)
usually can be corrected by various measures
Static equilibium
▪ The receptors to sense static equilibrium are the maculae.
- Housed in the vestibule.
static equilibium pt 2
Being the receptors for static equilibrium means the maculae monitor:
▪ the position of the head
with respect to gravity
▪ linear (straight-line)
acceleration
Static equilibium pt3
▪ Each macula is composed of a patch of epithelium that contains hair cells.
▪ These hair cells have “hairs” that are embedded in a gel-like otolithic membrane
Static equilibium pt4
▪ Movement of the head causes the otolithic membrane to shift which bends the hair cells.
p. 287
- Bending of the hair cells triggers
a nerve impulse
Dynamic equilibrium pt 1
▪ The receptor to sense dynamic equilibrium is the crista ampullaris.
- Housed in the semicircular canals
Dynamic equilibrium pt 2
▪ Being the receptors for dynamic equilibrium means these cristae ampullares* monitor rotational movement.
Dynamic equilibrium pt3
▪ Crista ampullaris contains supporting cells and hair cells.
- The hair cells, again, have “hairs” that are
embedded in a dome- shaped, gel-like
mass called the cupula.
Dynamic equilibrium pt4
▪ Rotational movements cause movement of endolymph in the semicircular canals.
▪ The movement of endolymph shifts the cupula which bends the hair cells.
▪ Bending of the hair cells triggers a nerve impluse
Neural pathway
●Nerve impulses for both static and dynamic equilibrium travel to the vestibular branch of the vestibulocochlear nerve (VIII).
●Equilibrium is perceived in the insula and parietal lobe.
End
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