Lecture 4 - Auditory & Vestibular Systems Flashcards
What is sound?
Sequenced waves of rarified and compressed air
What are the 2 parameters of sound waves? What does each correspond to?
- Pitch: wave frequency
2. Intensity/loudness: wave amplitude
What is the human audible range?
20 to 20,000 Hz
What do we call sound with a frequency below 20 Hz?
Infrasound
What do we call sound with a frequency above 20,000 Hz?
Ultrasound
Why can the human audible range change with time?
Damage to hair cells of the ear
What decibel limit will cause permanent damage to hair cells of the ear?
Above 130 dB
What are the 3 major components of the gross anatomy of the ear? Describe each.
- Outer ear = auditory canal + pinna
- Middle ear = tympanic membrane + ossicles
- Inner ear = oval and round windows + cochlea + semicircular canals + auditory-vestibular nerve + otolith organs
Where does the auditory canal terminate?
At the tympanic membrane
What are the 3 bones of the middle ear? What are they called?
Ossicles:
- Stapes
- Incus
- Malleus
What is special about the ossicles?
Smallest bones in the human body
What can each ossicle be compared to?
- Stapes: stirrup
- Incus: anvil
- Malleus: hammer
What is another name for the tympanic membrane?
Ear drum
To what structure are the vibrations that cause undulation of the ear drum transmitted? How?
The oval window:
ear drum => malleus => incus => stapes => oval window
Explain what mechanical amplification of sound means.
- The area of the oval window is smaller than the area of the tympanic membrane, so since F=P.A, there will be a larger amount of pressure on the oval window
- The force applied on the incus is smaller than the force applied on the oval window
What parts of the ear mediates loud sounds? What is the second one innervated by?
How does each work?
TWO MUSCLES:
- Tensor tympani: origin point deeper in inner ear and target is the malleus
- Stapedius (innervated by facial nerve): from middle ear to stapes
Goal: attenuate the amount of sound being propagated
Can the tympanic membrane heal if it bursts?
YUP
What are the 3 parts of the cochlea? In which one does signal transduction occur?
- Scala vestibuli
- Scala media***
- Scala tympani
Which part of the scala media of the cochlea performs signal transduction? Explain.
Organ of Corti: converts air vibrations into electrical signals
List and describe the 7 parts of the organ of Corti.
- Basilar membrane
- Outer hair cells with stereocilia
- Inner hair cells with stereocilia
- Tectorial membrane: acellular, gelatinous (composed of collagen), and has a web-like reticular lamina
- Rods of Corti: connect basilar lamina and retucular lamina of tectorial membrane
- Modiolus: medial most aspect of organ of Corti
- Spiral ganglion: synapse with inner hair cells
What is the entire auditory signal pathway.
Vibration of tympanic membrane => malleus => incus => stapes => oval window => cochlea => organ of Corti with hair cells => synapse at spiral ganglion => auditory nerve => synapse at ventral cochlear nucleus in the medulla => one branch decussates and both synapse at the superior olives => lateral lemniscus => synapse at the inferior colliculus in the corpora quadrigemina in the tectum => synapse at medial geniculate nucleus (MGN) in the thalamus => synapse auditory cortex
What are the ionic concentrations of the fluids in the cochlea balanced by? Where is it located? Explain how it works.
Stria vascularis in scala media
It secretes K+ into the scala media and absorbs Na+
Describe the ionic concentration of the scala vestibuli and tympani.
- High Na+
- High Cl-
- Low K+
Describe the ionic concentration of the scala media. What does this mean for its potential?
- High K+
- High Cl-
- Low Na+
+80 mV potential
What ion is responsible for depolarization of the organ of Corti cells?
K+
What are the 3 types of cells in the stria vascularis? What is the function of each?
- Basal: regenerate cell portions of the stria vascularis
- Intermediate: no function
- Marginal cells: take in Na+ and pump out K+ in scala media
What are the air pressure waved converted to in the cochlea?
Fluid pressure waves
What is the auditory sensory epithelium?
Organ of Corti
On what do the hair cells of the organ of Corti synapse?
The spiral ganglion
What is the point of contact between the stapes and the cochlea?
The scala vestibuli
How are the hair cells of the organ of Corti activated?
They are mechanoreceptors and are activated by the movement of the tectorial membrane and allow K+ to flow through
What part of the organ of Corti gives rise to the auditory nerve?
Spiral ganglion
Describe the signal transduction pathway inside the cochlea.
- A vibration is transmitted and amplified to the oval window
- Reverberations are sent through fluid in the vestibular canal of the cochlea (scala vestibuli) to the apex, and then back through the tympanic canal (scala tympani)
- This increase in pressure causes the round window to bulge out
- This displacement pulls the tectorial membrane, producing a shearing force
- Hair cells are embedded in the basilar membrane, and this shearing force causes them to shift
- The stereocilia of hair cells are all joined at the top by actin (tip-links), so when one moves they all move
- This shifting will open mechanically-gated potassium channels located in stereocilia
- Because the concentration of K+ is high in the scala media, it will flood in and depolarize the cell
- Voltage-gated calcium channels will open, allowing neurotransmitter-filled vesicles to dock and release glutamate to the nearby afferent nerves in spiral ganglion
What shift will open mechanically-gated potassium channels located in stereocilia?
Shift towards the highest stereocilia = the kinocilium
What is the change in potential of the hair cell proportional to? What does this mean exactly?
The degree of displacement of the cilia
This means that the further hair cells are displaced, the longer the channels will stay open, the greater the influx of potassium, the greater the depolarization will be and the longer it will last
What can be said of the phases/frequencies of the sound pressure and the hair cell receptor potential?
They are timed together = phase locked
What is the difference between inner and outer hair cells of the organ of Corti? Which ones mediate the majority of your ability to hear?
- ***Inner hair cells are located more medially, between the modiolus and rods of corti, and are arranged in a single row
- Outer hair cells are located more laterally and are arranged in 3 rows => they modulate the sensitivity of inner hair cells, representing a regulatory function
What % of spiral ganglion synapse with inner hair cells of the organ of Corti?
95%
What are the 3 codes of the auditory system?
- Frequency (pitch)
- Amplitude
- Source location
Describe the pitch code of the auditory system. What is this called?
TONOTOPY
If you unwind the cochlea, the basilar membrane is narrow and stiff at the base, and wide and floppy at the apex and different hair cells respond to different frequencies according to their location along the basilar membrane:
- Hair cells closer to the base of basilar membrane are more sensitive to higher frequency sounds
- Hair cells closer to the apex have greater sensitivity to lower frequencies
What joins the scala vestibuli and scala tympani? Where is it located in the cochlea? Purpose?
The helicotrema: so the fluid in them remains contiguous
Located at the apex of the cochlea
Is the pitch code preserved in the central auditory system? Explain.
Yes, the organization is preserved:
- Different auditory nerves transmit different frequencies based upon their location along the basilar membrane to cochlear nuclei in the medulla
- The Primary Auditory Cortex has isofrequency bands: different bands correspond to different frequencies
What is labeled line coding of hair cells?
Each hair cell has a characteristic frequency of vibration that is optimal for its response
What is the organization of the tonotopy in the primary auditory cortex?
Increases from rostral to caudal
Describe the amplitude/intensity code of the auditory system.
An increase in intensity is similar to a step in voltage or injection of current in
the sense that higher intensity leads higher frequency action potential firing.
Describe the source location coding of the auditory system.
- Interaural Time Delay: this represents a mechanism for locating sounds within the horizontal plane => a sound coming from the left will reach your left ear before it reaches your right ear
- Interaural Intensity Difference: this represents a mechanism for locating sounds within the horizontal plane => a sound coming from the left will sound more intense to your left ear than your right ear
- Location of sounds within the vertical plane involves the pinna and the reflection of sounds: both direct and reflected sound bounce off structures within the pinna creating different vibrations at different times
At what point of the auditory pathway does the afferent have two branches?
At the medulla
Where is the auditory cortex located? 2 names
Transverse temporal lobe
= Heschl’s gyrus
What is another name for the scala tympani?
Tympanic duct
What is the word scala synonymous with?
Duct
What is the role of the vestibular labyrinth?
Provides sense of balance and spatial orientation
Describe the orientation of the semicircular canals.
Each on a different plane: x, y, z (3D)
What does each semicircular connect to?
Scarpa’s ganglion
What does the vestibular labyrinth consist of? What does each convey?
- 3 semicircular canals: rotational accelerations of the head
- 2 otolith organs: horizontal and linear accelerations of the head
- cochlea
What are the 2 otolith organs?
- Saccule (vertical acceleration)
2. Utricle (horizontal acceleration)
How do the otolith organs work? What to note?
During movement, the gelatinous membrane is responsible for the shearing force between the otolithic membrane and macula needed to bend the stereocilia in the direction opposite from that of the head and causing increased firing of the hair cells
What is the clinical significance of binaural cells in the central auditory system?
If you damage one ear, you will notice changes in both cerebral hemispheres
What physical motions doe the vestibular system use to produce electrical signals?
- The movement of your head
2. Gravity
What is the vetibular sensory epithelium? Where is it? How is it oriented?
In the otolith organs: the macula in each
Oriented differently in the saccule and utricle: horizontal and vertical
What is found within the otolith organs? Where? Purpose?
Within the membranous sacs of the otolith organs is the fluid endolymph = fluid medium that can flow in either direction in the semicircular canals to register movement
Describe the composition of both otolith organs.
Hair cells are found in the macula of both organs and contain stereocilia, which project into a gelatinous cap above the hair cells.
On top of the gelatinous cap is the otolith layer called the otoconia, which is a layer of calcium carbonate crystals.
What is the firing of the hair cells of the otolith organs at rest due to?
Small influx of potassium into the cells
Describe excitation and inhibition of hair cells.
- Inhibition of hair cells: stereocilia move away from kinocilium = hyperpolarization
- Excitation of hair cells stereocilia move toward kinocilium = depolarization
What are stereocilia vs kinocilium?
Kinocilium is the tallest stereocilia
How do the semicircular canals work?
The endolymph from the otolith organs moves in them and disturbs the cupula in which there are hair cells (surrounded by support cells). These hair cells synapse with afferents of the vestibular nerve
What is the ampula?
The part of the semicircular canals that hosts the cupula
What are the 3 rotational accelerations of the head sensed by the semicircular canals? Explain each and note the axis it corresponds to.
- Roll (head tilt left and right): y axis
- Pitch (head up and down): z axis
- Yaw (head turn left and right): x axis
What does the fusion of the vestibular and auditory nerves
give rise to?
CN VIII
To where do the vestibular end organs send information?
Medulla oblongata:
- Otolith organs: lateral vestibular nucleus
- Semicircular canal: medial vestibular nuclei
To what 4 other NS structures does the medulla send information after receiving it from the vestibular end organs? For what purpose?
- Cerebellum: motor coordination to offset any type of imbalance
- VP nucleus of thalamus: major sensory relay station
- Extraocular motor neurons (3, 4, 6)
- Spinal cord: limb/neck motor neurons to help with balance
What is the vestibulo-ocular reflex (VOR)?
Central projection of the vestibular system allows us to maintain our gaze while rotating our head:
- Head moving the left = eyes moving to the right: left MEDIAL vestibular nucleus of medulla (receicing input from semicircular canals) inhibits the left abducens cranial nerve and stimulates the right one => left abducens nerve inhibits lateral rectus muscle of left eye and medial rectus muscle of right eye + right abducens nerve stimulates lateral rectus muscle of right eye and medial rectus muscle of left eye
- Head moving the right = eyes moving to the left: vice versa
What is the vestibulospinal reflex? Purpose? 2 types?
Extensor activity is induced on the side to which the head is inclined and flexor activity is induced on the opposite side of the body => balance
- Lateral vestibulospinal tract: limbs and trunk
- Medial vestibulospinal tract: upper back and neck
What is dizziness?
Impairment in spatial perception and stability
What is vertigo?
Perception of spinning motion
What are 3 vestibular disturbances?
- Dizziness
- Vertigo
- Abnormal vestibular reflexes
Why is the function of the middleear paradoxical?
Because it amplifies (ossicles) and softens (tensor tympani, stapedius muscles) sound
Do the auditory and vestibular nerves ever come together or totally separate entities?
Packaged together
What is a Rinne tuning fork test?
- Put tuning fork on mastoid behind ear to cause bone conduction directly to test sensorineural hearing (if he cannot hear than you know it’s a sensory-neural issue)
- Move tuning fork to outside the ear to cause air conduction: tests both conductive and sensorineural hearing: if he can no longer hear then you know it’s a conductive hearing loss
What is conductive hearing loss?
Any hearing loss between stapes and outer ear = external and middle ears
What is sensory-neural hearing loss?
Any hearing loss in inner ear and CN VIII
What are the 2 types of conduction? Which is greater?
- Air conduction (2x greater)
2. Bone conduction
What does it mean if the bone conduction is greater than the air conduction?
Patient has conductive hearing loss
What does it mean if the air conduction is greater than the bone conduction?
- Patient is normal
2. Patient has sensoryneural hearing loss
What is a positive Rinne test?
Air conduction is greater than the bone conduction
What is a negative Rinne test?
Bone conduction is greater than the air conduction
What is a Weber test?
Put tuning fork on top of head so vibrations go straight to both cochleas (testing sensory neural hearing):
- Normal: both ears will hear (no lateralization)
- Sensoryneural hearing loss: will not hear in one ear (lateralization to good ear)
- Conductive hearing loss: he will hear better on the side with the conductive hearing loss because there is no noise interfering (lateralization to bad ear)
What could cause a false negative Rinne test? Why?
SEVERE cochlear damage because it would result in sensorineuronal hearing loss, which the Rinne test does not test for
Purpose of Weber and Rinne tuning fork tests?
Allows to differentiate between:
- Conductive hearing loss
- Sensory-neural hearing loss
- Mixed hearing loss
What is the absolute bone conduction test?
Comparing patient’s bone conduction to examiner’s bone conduction (tragus must be occluded)
- If the patient has conductive hearing loss he will hear the sound for same duration
- If the patient has sensorineural hearing loss he will hear the sound for a shorter duration
What is the Schwabach test?
ABC test but without occlusion of the ear:
- If the patient has sensoryneural hearing loss he will hear the sound for a shorter duration
- If the patient has conductive hearing loss he will hear the sound for a longer duration
What does an unsteady gait indicated?
Vestibular issue
What is a caloric test?
Tests for dizziness/vertigo and whether it’s caused by an issue with the vestibular portion of the inner ear
What are 4 causes of dizziness/vertigo?
- Inner ear dysfunction
- Disorders of the brain
- Low BP
- Anxiety
What are binaural neurons? At what point in the auditory pathway are these seen?
Receiving input from both ears
All neurons after the cochlear nuclei neurons (starting with superior olive neurons)
