Chapter #9- Inner Ear
Anterior Canal is....
at a 90 degree angle to the posterior canal. -Anterior and Posterior canals are at a right angle to the horizon.
Shearing of the Stereocilia: OHC Stereocilia in the tectorial membrane
bend back and forth.
Shearing of the Stereocilia: IHC Stereocilia will also
bend in response to fluid motion.
Auditory Nerve
enter the spiral lamina via habenula perforata
Electric Potentials: Stimulus-Related Potentials
generated in individual neurons in responses to an external stimulation. AKA: action potentials, neural potentials, neural impulses, or nerve impulses
Pathway of sound- sounds transmission from middle ear --> inner ear
inward motion stapes moves membrane or oval window-->entry point for transmitted sound to the scala vestibuli -->farther in the cochlea scala vestibule follows the spiral and connects to scala tympani through helicotrema--> scala tympani winds down to the base of the cochlea and is terminated by the membrane of the round window in the middle ear.
Summating Potential (SP)
is a direct current (DC) potential -A DC that is only during very loud intensities of acoustic simulation.
The horizontal canal....
is at a 30 degree angle off the horizontal plane.
Inner Ear
located in the bony labyrinth within the petrous portion of the temporal bone.
The semicircular canals and vestibule contain the....
organs of balance.
**Dendrites**
part of the cell that receives signals from other cells.
**Axons**
part of the cell that transmits signals to other cells
Direct Current (DC) potential
stimulus doesn't change with time, constant; i.e. battery 1. Endocochlear Potential (EP) 2. Intracellular Potential (IP) or Organ of Corti Potential 3. Summating Potential (SP)
**Habenula Perforata**
the holes that the auditory nerve enters through the spiral lamina.
Shearing of the Stereocilia: When the stereo cilia bend back and forth,
the tip-to-side links open and close the ion channels.
Shearing Motion
up-and-down motion of the scala media will cause the basilar membrane and tectorial membrane to move back and forth relative to each other in a direction that is perpendicular to the motion of the basilar membrane. -Results in a shearing force acting on the stereo cilia, which makes them bend.
Organ of Corti Support Cells
-**Deiters cells** -Inner sulcus cells (inner spiral sulcus cells) -Inner phalangeal cells (inner supporting cells) -Hensen cells -Claudius Cells -Outer spiral sulcus cells -Boettcher cells (basal turn of cochlea only)
**Vestibule**: In the membranous Labyrinth
-**Utricle** -**Saccule**
Type 2 Afferent Neurons
-5% are type 2 afferent neurons *(outer spiral fibers)* Each type 2 afferent auditory neuron sends branches to several different OHCs -Thinner and not covered with myelin.
Tuning Curves
-A curve representing cell firing as a function of frequency.
Coding of intensity (cont.)
-Because humans have an overall dynamic range around 120 dB, the ear must use a combination of strategies to code intensity. -These strategies include the area of the basilar membrane that is affected and the number and types of neurons that fire.
Semicircular Canals: **Ampulla**
-Bulge at one end of each canal contains a sense organ called *crista ampullataris* -The cristae contain hair cells that project into a gelatinous mass called a **cupola**.
Tuning Curves (cont)
-Cells can fire in response to various frequencies, but they are most effective at their CF. -The CF depends on the cell's location.
Neurotransmitter
-Chemical messenger from the base of the cell into the synapse with the afferent auditory nerve fibers. -Transmission of information to a neuron.
OHC Motility: Nonlinear Distortions
-Created by nonlinear amplifiers -Frequency components that may not exist in the input signal.
Vestibule (cont.)
-Ducts from the utricle and saccule join to form the *Endolymphatic duct*, which projects to the *endolymphatic sac.* -The saccule is connected to the cochlear duct (scala media) via the *ductus reunions.* -The utricle is connected to all three semicircular canals.
Electric Potentials: Action Potential (AP)
-Electrical activity from the 8th nerve -Can be measured from anywhere in the cochlear or in the auditory nerve.
Tuning Curves: Characteristic Frequency
-Hair cells and nerve cells are tuned to respond best to one specific frequency.
Vestibular System (cont.)
-Hair cells from the maculae and cristae communicate with the vestibular portion of the vestibulocochlear nerve. -The cell bodies (vestibular ganglion) are located in the internal auditory canal. -Houses cranial #8 and #7 nerves
IHC action
-IHCs release neurotransmitters into the synaptic cleft. -Neurotransmitters bind to specialized receptor sites on the auditory nerve endings. -The auditory nerve will respond to this chemical message by firing (sending an electrical signal down the nerve). -The sound is converted into a series of nerve impulses.
Coding of Intensity
-Low-intensity sounds create a fairly narrow traveling wave on the basilar membrane. -Large-intensity sounds create a broader traveling wave. -Thus, the area of the cochlea affected by a sound will vary depending on the intensity. -As intensity increases, the firing rate of a neuron will increase, up to the saturation point.
OHC Motility (cont.)
-Observed directly in the lab and indirectly by measuring Otoacoustic Emissions (OAEs) -The OHC motion causes an increase in the intensity of sound at the cochlea; acts as a **cochlear amplifier.** -However, other theories to explain possible sources of amplification int he cochlea are also under investigation. -OHC motility operates only for low to moderate intensities. -Because low-intensity sounds are amplified more than high-intensity sounds, the cochlea is a nonlinear amplifier.
Perilymph
-Same as CSF -fills the inside of the bony labyrinth: scala vestibule, scala tympani, vestibule, semicircular canals. -High in sodium and low in potassium. -imcompressible
OHC Motility: Otoacoustic Emissions (OAEs)
-Sounds that are observed in the outer ear as a result of vibrations generated in the inner ear. -The tympanic membrane creates sound waves in the ear canal.
**The Cochlea**
-The bony labyrinth of the cochlea coils to form a spiral tunnel. -The widest part (coil of the spiral is the base -The narrowest part (narrowest end of the spiral) is the apex.
Electric Potentials
-The inner ear requires a source of electricity. Electricity is created by separating positive and negative *ions* (charged particles) -Electric potentials in the ear are created by ions in perilymph, endolymph, and hair cells.
Place Theory
-The mapping of stimulus frequencies to a place on the basilar membrane is referred to as *place coding.* -The theory explaining the process of hearing based on place coding is called *place theory.* -Two sounds are perceived as different because they cause two different groups of hair cells and neurons to fire.
Dynamic Range
-The range between threshold and the saturation point. -The dynamic range of an auditory neuron is about 25 to 40 dB.
Traveling Wave Frequency
-The sound is medium frequency. -High-frequency sounds vibrate at the base. -Low-frequency sounds vibrate more at the apex.
**Cuticular Plate**
-Thickening at the top edge of the hair cell. -This is where the stereo cilia attach to the hair cell.
OHC Electromotility
-This refers to mechanic changes in the length of the OHCs -Electromotility -OHC Motility is atrributed to Prestin -This motion increases the displacement of the traveling wave.
Cochlea Cross Section: Tectorial Membrane
-attached to the spiral limbus and extends out over the organ of corgi -Makes contact with the tallest tips of the OHC stereocilia.
Within the bony labyrinth...
-cochlea -semicircular canals -vestibule
Endolymph
-fills the inside of the membranous labyrinth: scala media. -High in potassium and low in sodium. -imcompressible
Electric potentials: In order for the stored electricity of resting potentials to do work...
.....a trigger mechanism must cause the flow of charged particles from one place to another. -In the inner ear, this trigger is the shearing of the stereocilia
**3 Semicircular Canals**
1. Anterior (superior) 2. Posterior (inferior) 3. Horizontal (lateral)
Semicircular Canals: The cristae provide information about angular acceleration.
1. When the head turns in any direction, a series of signals are sent from the cristae. 2. The specific cristae that are affected depends on the direction of acceleration.
Cochlear Microphonic and CAP are ...
Alternating current (AC) potentials -The magnitude and direction of the electrical flow change periodically.
Cochlea Cross Section: **Spiral Limbus**
Attached to Reissner's membrane on its inside edge.
Nerves
Axons extending from several neurons are bundled together form a communication pathway called......
Cochlea Cross Section: **Spiral Ligament**
Basilar membrane is attached to the spiral lamina (inside edge) and spiral ligament (outside edge).
Electric Potentials: Endocochlear Potential (EP)
Bekesy discovered EP by putting the electrode (batter tester- 2 wires) in the scala media and discovered a +100 mV potential with respect to a neutral point on the body. -Tasaki discovered EP was due to the Stria Vasualris and that it was related to metabolic activity.
Electric potentials: Resting Potentials
Bioelectric potentials that exist normally in the human body even without stimulation.
-**Spiral Lamina**
Bony shelf that projects from the modiolus
**Efferent Neurons**
Carry information from the central nervous system to the peripheral nervous system. (information to move the muscles in our leg that allow us to walk) -Make up the *olivocochlear bundle (OCB)* -The OCB begins in the *superior olivary complex (SOC) *in the brainstem. 1.*Medial olivocochlear neurons* begin in the medial portion of the SOC -2.*Lateral olivocochlear neurons* begin in the lateral portion of the SOC These two make up the olivocochlear bundle.
**Afferent Neurons**
Carry sensory information from the peripheral nervous system to the central nervous system.
**Spiral Ganglia**
Cell bodies from auditory neurons that are located in the **Rosenthal's canal** in the modiolus
-**Modiolus**
Cochlea's bony core
Electric Potentials: Bioelectric Potentials
Electric potentials in the human body.
Coding of intensity: Lieberman (1978) identified three auditory neurons based on spontaneous activity:
High, Medium, Low
Vestibular System
Human ability to maintain balance depends on the ability of the central nervous system to coordinate sensory input from 3 systems: 1. Vestibular 2. Visual 3. Somatosensory -Also includes the Vestibule and Semicircular Canals
Periodicity (temporal) Theory: Phase Locked Phase locked not on study guide.
Individual neurons are phase locked to the stimulus, meaning the firing pattern is synchronized with the phase of the signal. -Individual neurons cannot fire more than 1000 times per second, but a group of neurons can work together to follow the phase of the wave (Wever, 1949). -Phase locking has been measure in mammals up to 4000 to 5000 Hz (For frequencies higher than about 5000 Hz, place cues are probably the primary coding strategy.)
OHC Motility: Electromotility (or just motility)
Is a change in the length of the OHC in response to changes in the electrical potential of the cell.
Theories of Hearing (cont.)
It appears that both place coding and temporal coding are used together by the auditory system to code the frequency of sound.
Anions
Negatively charged particles
**Space of Nuel**
OHC is not firmly supported but is surrounded by perilymph-filled space.
Contralateral
Opposite Side
Theories of Hearing
Place theory and periodicity theory are the two possible explanations for how the inner ear creates a coding scheme for frequency, intensity, and phase information.
Saturation Point
Point at which the firing rate wil stay the same reguardless of an increase in intensity.
Cations
Positively charged particles
**Phalangeal Processes**
Project around the OHC and extend up to the surface of the Organ of Corti
OHC Motility: Prestin
Protein in the OHC wall
Vestibule: Macula
Sense organ in both the utricle and saccule which provides information about: -Head position relative to the earth. -Linear acceleration of the body. -Maculae contain hair cells -The stereo cilia project into an *otolithic membrane.* -The otolithic membrane contains *otoconia*, which makes it responsd to gravity and lag behind the motion of the head (inertia)
Periodicity (temporal) Theory
States that frequency is coded based on the period of the waveform. -Rate of frequency of neural impulses is the mechanism for coding frequency. -Nerve fibers fire spontaneously at a resting firing rate. -The firing rate increases when IHCs are excited and decreases when IHCs are inhibited.
Perilymph vs. Endolymph
The chemical difference between these two fluids is important for the function of the inner ear.
Traveling Wave: Tonotopic Organization
The correspondence between stimulation frequency and place along the cochlea -First described by Georg Von Bekesy (1960).
Electric Potentials: Electric Potential
The force between 2 opposing charges.
**Label and Define**
**Label and Define**
**Soma**
Cell Body
**Scala Vestibuli**
Filled with perilymph -contiguous with the vestibule at the base of the cochlea. -connects to the scala tympani at the helicotrema located at the apex of the cochlea.
**Stereocilia**
Form a U shape on the IHC and a V or W shape on the OHC. -Rows of stereo cilia on each hair cell increase in height like a staircase (tallest away from the modiolus) -The tallest tips of the OHC stereocilia project into the tectorial membrane.
**Ganglion**
Group of cell bodies in the peripheral nervous system.
Nucleus
Group of cell bodies. -except at the surface of the brain, where it is called at **cortex** or **gray matter.**
Electric Potentials: IHCs
Have a resting potential of -40 mV compared to the perilymph. -Resting potential of the inner hair cells is lower than the endolymph by 120 mV.
Electric Potentials: OHCs
Have a resting potential of -70mV compared to the perilymph. -Resting potential of the outer hair cells is lower than than the endolymph by 150 mV
Cochlea Cross Section: **Organ of Corti**
Hearing Organ inside the cochlea. -Main structure responsible for converting mechanic vibrations that enter the cochlea into neural impulses. -Hair cells and support cells
**Neurons**
Individual cells from a nerve that transmit electrochemical info within nervous system of the body. -Afferent Neurons -Efferent Neurons ex: organ of corti communicate with the brain.
Electric Potentials: **Compound Action Potential (CAP)**
When many action potentials occur at the same time.
Type 1 Afferent Neurons
-95% are type 1 afferent neurons *(inner radial fibers)* -Type 1 fibers are larger and covered with myelin (myelin is a fatty covering that speeds the conduction of nerve impulses). -Many type 1 afferent auditory neurons connect to each IHC
Vestibulocochlear Nerve
-Connects the inner ear to the central nervous system. (systems of hearing and balance to the brain). -This nerve has a vestibular portion (begins at sensory cells in the vestibule and semicircular canals) and a cochlear portion (brainstem) -The cochlear portion is also called auditory nerve. -8th cranial nerve
Tunnel of Corti
-Inbetween the IHC and OHC. -Triangular-shaped support structure created by the inner and outer **rods (pillars) of corti**
Inner Ear Physiology
-Inner ear is a transducer that changes the mechanical energy from the middle ear into neural impulses. -The way the cochlea breaks down sound and translates the frequency, intensity, and timing characteristics into a series of nerve impulses is quite complex.
**Inner sulcus cells (inner spiral sulcus cells)**
-Located medially to the IHC -Extend from spiral limbus to inside edge of the IHCs
**Boettcher Cells**
-Named after Arthur Boettcher -Found in the basal turn of the cochlea under the Claudius cells.
Claudius Cells
-Named after Matthias Claudis -support cells adjacent to the Hensen cells
**Hensen Cells**
-Named after Viktor Hensen -Support Cells located next to the OHCs
The Reaction of the hair cells: In response to the influx of K+
-OHCs expand and contract -IHCs release neurotransmitters
Organ of Corti Hair Cells
-One row of **inner hair cells (IHCs - flask shape)** -Three rows of **outer hair cells** (OHCs- cylindrical shape)- If you lose these then you have a 70 dB hearing loss
Shearing of the Stereocilia
-Recall that the basilar membrane moves up and down in response to vibrations from the stapes. -When this happens, the hair cells on the basilar membrane will also move up and down. -The attachment point of the basilar membrane is farther from the modiolus than the attachment point of the tectorial membrane. -The basilar membrane and tectorial membrane move back and forth relative to each other, perpendicular to the motion of the basilar membrane. -Shearing motion makes the stereocilia bend.
**Hair Cells**
-Sensory cells that change mechanical motion into electrochemical impulses -Contain Stereocilla (tiny hairs) -The stereo cilia are attached to the hair cell at the **cuticular plate.**
**Outer Spiral Sulcus cells**
-Support cells which begin at the Claudius cells and continue along the Spiral ligament
Traveling Wave
-The stapes pushes on the oval window, displacing the scala media. -This causes a bulging of the round window. -When the stapes pulls on the oval window, the process is repeated in the opposite direction. -The in-and-out motion of the stapes creates a traveling wave. -A traveling wave (transverse wave) is a wave motion that is perpendicular to the direction of the moving wave. -When the oval window is stimulated at a specific frequency, the traveling wave begins at the base of the cochlea and gradually increases in amplitude to its maximum and then abruptly decays. -The basilar membrane varies in stiffness and mass, so the place of maximum displacement depends on the frequency of the motion. (high frequency = base of cochlea; low frequency = apex of cochlea) -If a sound has many components, many different places on the basilar membrane will vibrate. -This vibration is necessary for the sounds to be transmitted into electrochemical impulses.
**Scala Media**
Filled with endolymph. AKA Cochlear duct or cochlear partition.
**Scala Tympani**
Filled with perilymph -Ends at the middle ear wall where the round window is located.
Rod (pillars of Corti)
Inner rod sits on the spiral lamina and outer rod sits on the basilar membrane.
Synapse
Junction at which allow one nerve to communicate with other nerves.
Medial Olivocochlear Neruons
Large myelinated neurons that send branches to many OHCs, primarily to the contralateral side. -Notice the connection of the efferent neurons directly to the OHC (medial olivocochlear neurons)
Shearing of the Stereocilia: Shearing Motion
Makes the stereo cilia bend.
Electric Potentials: Electric Current
Movement of ions from one place to another
Lateral Olivocochlear Neurons
Smaller unmyelinated neurons that send branches to the type 1 afferent neurons just below the IHC, primarily to the ipsilateral side.
Shearing of the Stereocilia: When open __________ ions rush from the _____________ into the _________ ______
Potassium (K+); Endolymph; Hair Cell
Stimulus Independent
Potentials always present with and without acoustical stimuli (sound) -EP Endocohlear -IP Intracellular Potential
Stimulus Dependent
Potentials only present if there is an acoustical sound present (sound) -SP Summating Potential -CM Cochlear Microphonic -AP Action Potential
**Helicotrema**
Prior to reaching the apex, the scala media channel ends and the scala tympani and scala vestibuli are connected together by a narrow passageway.
**Inner phalangeal cells (inner supporting cells)**
Provide support for the IHCs
Electric Potentials: Intracellular Potential (IP)
Recorded -50 mV inside cells of The Organ of Corti
Electric Potentials: **Cochlear Microphonic (CM)**
Reproduces frequency and waveform of a sinusoid perfectly. -Fig. 6-115 (pg. 492) -Generated from OHC
Ipsilateral
Same side
3 channels of the cochlea
Scala Vestibuli Scala Tympani Scala Media
Cochlea Cross Section: **Basilar Membrane**
Separates the scala tympani from the scala media. -Thinnest and stiffest at the base of the cochlea (low mass high stiffness) and thickest and most flaccid at the apex. (large mass low stiffness) -This is important for the function of the basilar membrane. -stretch from the outside edge (bony labyrinth) to the inside edge (modiolus). -Thicker than Reissner's because of organ of corti -up and down motions of this membrane important in converting mechanical energy into neural impulses.
Cochlea Cross Section: **Reissner's Membrane**
Separates the scala vest from the scala media -stretch from the outside edge (bony labyrinth) to the inside edge (modiolus).
**Cross-links**
Stereocilia are connected by cross-links -Side to side -Row to Row -Tip to side/ tip link: top of smaller cilia and side to larger cilia.
Stimulus-related potentials in the inner ear
acoustic stimulus excites the cochlea, these 3 things can be measured within the cochlea. Summating potential Cochlear microphonic Compound action potential (CAP)
**Deiter Cells**
after Otto Freidrich Deiter -Cup-shaped depression that holds bottom of OHC and long phalangeal processes.
Alternating Current (AC) potential
always changing over time, looks like a sine wave. 1. Cochlear Microphonic (CM) 2. Action Potential (AP)
Cochlea Cross Section: **Stria Vascularis**
located on the outside edge of the scala media.
Membranous Labyrinth
located within the bony labyrinth and follows all the curves and coils. -Look at slide #6 for picture.
Ion Channels
tip-to-side cross-links control ion channels -allow potassium ions to pass from the endolymph into the hair cells. -when tip to side cross-links are pulled, they open the gates of the attached ion channels, and when the link return to rest position the gates close. Stereocilia are pushed from the modiolus. This leads to the process of sound perception.
Cochlea Cross Section: **Reticular Lamina**
tops of hair cells and supporting cells forms the upper surface of the Organ of Corti. -Keeps the endolymph from getting into the organ of corti
