Auditory System Flashcards
What are the two components of the External Ear?
- Auricle
- External Auditory Meatus
- (Tympanic Membrane)
What is the structure and function of the Auricle?
S: Irregularly shaped plate of elastic cartilage
F: Focuses sound into external auditory meatus
What is the structure and function of the External Auditory Meatus?
S: Canal in the tympanic part of the temporal bone
F: Conducts sound from the auricle to the tympanic membrane.
What is the tympanic membrane?
Thin, oval, semitransparent membrane
-Forms partition between external and middle ear.
F: moves in response to alternating increases and decreases in air pressure.
What are the components of the middle ear?
- Auditory Ossicles
- Muscles
- Pharyngotympanic Tube
What are the auditory ossicles in the middle ear?
mobile chain of 3 small bones extending from the tympanic membrane to the oval window
What are the 3 small bones that make up the auditory ossicles?
- Malleus
- Incus
- Stapes
Where is the malleus located?
Handle is embedded in the tympanic membrane and head articulates with the incus
Where is the incus located?
Between the malleus and stapes
Where is the stapes located?
Head articulates with the incus
Base fits into the oval window
What is the function of the auditory ossicles?
- Match the relatively low impedance airborne sounds to the higher-impedance fluid in the inner ear.
- Reduce the magnitude of the movements of the tympanic membrane, while increasing their force at the oval window.
What does impedence matching depend on?
- The ratio of the surface area of the tympanic membrane to that of the oval window
- The mechanical advantage of the lever system formed by the ossicle chain
What is the function of the muscles in the middle ear?
Dampen movements of the auditory ossicles, decreasing the sensitivity of the auditory apparatus.
-This action is thought to protect the auditory apparatus against damaging sounds that are anticipated.
What is the structure and function of the Pharyngotympanic Tube?
S: Connects the middle ear to the nasopharynx
F: Equalizes pressure in the middle ear with the atmospheric pressure, allowing free movement of the tympanic membrane.
What would a lesion in the middle or external ear results in?
Ipsilateral conduction deafness
What is the inner ear made up of?
- Bony Labyrinth- Cochlea
2. Membranous Labyrinth
What is the cochlea made up of?
3 fluid filled tubes, wound helically around a conical bony core called the modiolus
What is the scala vestibuli?
Upper most chamber, filled with perilymph. Begins at oval window (Base) and is continuous with scala tympani (apex)
What is the Scala Tympani?
Lower most chamber, filled with perilymph. Continuous with Scala vestibuli at apex, and ends at round window (base)
Where is the round window located?
Vestibule
What is the scala media?
Central chamber filled with endolymph
AKA: Membranous Labyrinth or Cochlear Duct
Describe the action of sound propagation through the External–>Inner Ear.
Action of Stapes at oval window produces pressure changes. The pressure changes propagate throughout the fluid of Scala Vestibuli, which moves the Scala Media, causing a wave to travel along it. In turn increases the pressure in the Scala Tympani. Enhanced pressure causes outbowing of round window.
Where is the membranous labyrinth and what happens here?
Suspended within the bony labyrinth. Bound by elastic partitions.
-Sound transduction
What are the two membranes that are part of the membranous labyrinth?
- Vestibular Membrane
2. Basilar Membrane
What does the vestibular membrane do?
Separates the Scala Media from Scala Vestibuli
What does the Basilar Membrane do?
Separates the Scala Meda from the Scala Tympani
-Location of the Organ of Corti
What is the Organ of Corti?
Receptor for auditory stimuli, where auditory transduction occurs
What unique about the structure of the basilar membrane?
- Shape and mechanical properties vary along its length, allowing it to recognize different frequencies.
- Narrow and stiffest at its base, and wider and more flexible towards apex
What is sound?
- sequence of waves of pressure that propagate through compressible media such as air or fluid.
What are the relevant properties of sound?
- Amplitude
2. Frequency
What is amplitude?
- Peak to peak change in sound pressure
- Related to perception of loudness
What is frequency?
- Number of cycles per second
- Related to perception of pitch
What are pure tones?
Sounds with a sinusoidal waveshape, characterized by one frequency
What are complex sounds?
Mixtures of pure tones (what we hear in conversation)
What are the components of the Organ of Corti?
- Hair Cells
2. Tectorial Membrane
What is the structure of the hair cells in the organ of corti?
- Apical surface which contains stereocilia
- Stereocilia are arranged in curving rows, graded by height.
- Tip Links (filamentous structures) extend from the tip of each stereocilium to the next tallest neighbor.
What are the two types of hair cells and what do they do?
- Inner- Sensory receptors, form a single row of cells
2. Outer- Sound amplifiers, form 3 rows of cells
What are the two types of hair cells separated by?
Tunnel of Corti
What is the tectorial membrane?
- Gelatinous shelf extending outward from the modiolus over the organ of corti
- lower surface in contact with tips of tallest stereocillia
What are the steps to mechanoelectrical transduction of sound waves?
- Sound waves produce vertical movement of basilar membrane, creating shear force that deflects the stereocilia.
- Hair bundle is deflected toward tallest stereocilium, ion channels near the tips open, allowing K+ to flow into the cell down its electrochemical gradient.
- Results in depolarization, opening voltage gated Ca++ channels in the base of the cell.
- Synaptic vesicles fuse with presynaptic membrane and release neurotransmitters.
What is the place theory of cochlear tuning?
The basilar membrane and hair cells act as a frequency filter to separate and analyze individual frequencies from complex sounds.
What is the job of the basilar membrane in cochlear tuning?
- Moves maximally at different positions as a function of stimulus frequency
- Points responding to high frequencies= base
- Points responding to low frequencies= apex
What is the job of the hair cells in cochlear tuning?
- Exhibit the strongest response at the point of greatest displacement of the basilar membrane
- Every hair cell is most sensitive to stimulation at a specific frequency
- Gives rise to a topographical mapping of frequency
What steps are involved with the processing of auditory information?
- Innervation of hair cells
2. cochlear ganglion cells encode stimulus attributes
What type of innervation occurs at the hair cells, and which cells are involved?
- Afferent- Cochlear Ganglion Cells
2. Efferent- Olivocochlear Cells
What is the location of the cell bodies of the Cochlear Ganglion cells?
Within the modiolus
What is the location of the cell bodies of the Olivocochlear Cells?
Superior olivary nucleus of the brainstem
What is the distribution of the afferent cells on the hair cells?
- 90% terminate on the inner hair cells
- 10% terminate on the outer hair cells
What is the distribution of the efferent cells on the hair cells?
Majority terminate directly on outer hair cells
What is the job of the efferent fibers in processing auditory information?
- Helps create specificity that exists along the basilar membrane
- Causes outer hair cells to change length, amplifying basilar membrane oscillations, increasing mechanical stimulation of inner hair cells.
- This enhances cochlear sensitivity and frequency selectivity.
How is amplitude coded by the cochlear ganglion cells?
-Both by discharge rate of the fibers and by the recruitment of additional fibers as stimulus intensity increases.
How are low frequencies coded by the cochlear ganglion cells?
coded by the tendency of the afferent fibers to discharge in phase with the stimulus (phase locking)–> fibers fire at the same rate
How are high frequencies coded by the cochlear ganglion cells?
coed by the position of the afferent fibers along the tonotopically organized basilar membrane (place theory)
What is characteristic frequency?
The frequency at which the fiber has the lowest threshold
How are pathways in the auditory system different from the somatosensory and visual systems?
In the auditory system, once the information reaches the brainstem, it doesn’t cros contra laterally, it crosses everywhere.
-Lesions are not as localized, which means deficits are not as localized.
What is the central projection of the cochlear ganglion cell?
Cochlear Nerve (cochlear division of the vestibulocochlear nerve)
Where do axons of the cochlear ganglion cells convey input from?
inner hair cells
What is the structure of the cochlear nerve?
Extends from the inner ear through the internal auditory meatus in the brainstem
What is the function of the cochlear nerve?
Each fiber gives off an ascending and descending branch to the ipsilateral cochelar nuclear complex
What would happen if there was a lesion of the cochlea or cochlear nerve?
Result in ipsilateral sensorinerual deafness.
What are the subcortical regions of the pathways of the auditory system?
- Cochlear Nuclei
- Relay Nuclei of the Brainstem
- Inferior Colliculus
- Medial Geniculate Nucleus
What is the structure of the cochlear nuclei?
Two collections of neuron cell bodies (ventral and dorsal) at the pontomedullary junction.
What is the function of the cochlear nuclei?
Different cell types analyze and encode different sound features (i.e. timing, intensity) while preserving frequency information
Where do the axons of the cochlear nuclei project?
Project to more rostral levels of brainstem
What is the job of the relay nuclei in the auditory system?
Process auditory information in parallel pathways, each of which is dedicated to the analysis of a particular feature of sound
What are the two components of the Relay Nuclei in the brainstem?
- Superior Olivary Complex
2. Nuclei of Lateral Lemniscus
Why is the Superior Olivary complex important?
First site in the brainstem where information from both ears converges (binaural input)
What is the structure of the Superior Olivary Complex?
-3 collectsions of neuron cell bodies in the caudal pons (medial superior olive, lateral superior olive, and medial nucleus)
What is the function of the medial superior olive?
- Sound localization based on interaural time differences
- Ex. one ear will detect sound before the other
What is the function of the lateral superior olive and medial nucleus?
- Sound localization based on interaural intensity differences
- Ex. Sound will sound louder in one ear compared to the other
What is the structure of the nuclei of the lateral lemniscus?
A collection of neuron cell bodies at the pons-midbrain junction, inferior to the inferior colliculus
-Receives input from the contralateral cochlear nuclei, constituting monaural input
What is the structure of the inferior colliculus?
- Alternating gray and white layers in the tectal portion of the midbrain
- Receives fibers via the lateral lemniscus
What is the function of the inferior colliculus?
- Sound localization based on interaural time AND intensity differences.
- Processes progressively more complex aspects of sounds
What is the structure of the medial geniculate nucleus?
Layers of neuron cell bodies located in the dorsal thalamus
What is the function of the medial geniculate nuclesu?
Processes progressively more complex aspects of sounds
What are the target areas of the auditory system?
- Primary Auditory Cortex
2. Secondary Auditory Cortex
What is the structure of the Primary Auditory Cortex?
Location: Transverse Gyrus
Composition: Six layers of cell bodies
How is the primary auditory cortex organized?
Subdivided into alternating zones of neurons excited by stimulation of either ear (EE cells) and neurons excited by stimulation of the contralateral ear and inhibited by the ipsilateral ear (EI cells)
What are the neurons in the primary auditory cortex sensitive to?
combinations of simple sounds
What would happen if there was electrical stimulation to the primary auditory cortex?
-Evokes a variety of elements of sounds, but never complicated (words) ones.
What would happen if there was a central unilateral lesion in the primary auditory cortex?
-Infrequently causes unilateral deafness, but could interfere with sound localization or discrimination of tones
What would happen if there was a bilateral lesion in the primary auditory cortex?
Ability to localize sound and understand speech is reduced
What area makes up the secondary auditory cortex and what does it do?
- Superior Temporal Gyrus (wernicke’s area)
- Instrumental in the interpretation of sound
What would happen if you had a left side lesion of Wernicke’s Area?
Auditory Aphasia- impairment of the comprehension of speech sounds
