Inner Ear The inner ear is divided into 3 parts: a) Pars superior: scc + utricle b) Pars inferior: saccule + cochlea c)
Endolymphatic sac + duct
Osseous labyrinth:
Located in the petrous portion of the temporal bone
Surrounds the membranous labyrinth
Contains perilymph
Connects the middle ear via fenestra vestibule & fenestra
cochleae
Divided into 3 parts: 1. Vestibule 2. Cochlea 3. Semicircular canal
Membranous labyrinth:
Located within the bony labyrinth
Contains endolymph
Divided into : 1. Cochlear duct= sacula media for hearing 2. vestibular apparatus for balance and motion sense which is made of a) Semicircular ducts b) Saccula c) Utricle
Note : The oval window and round window open into the vestibule , at the base of the cochlea
Cochlea
The cochlea is coiled around the modiolus
The Modiolus:
The spongy bone spongy bone axis around axis around which the cochlea is colied (central axis central axis of the cochlea) Contains:
1. cochlear nerve ganglia : comprised of bipolar neurones neurones that: that: 2. Spiral ganglia: peripherally innervate the hair cells a) peripherally innervate b) centrally form centrally form the cochlear nerve 3. Blood vessels
The cochlear coil extends “up” from its base (2 3/4 X) The chambers spiral and exhibit 5 cross sectional profiles. profiles. Different pitches (frequencies) (frequencies) of of sound are detected at different positions along positions along this long coiled structure. ( structure. (high high frequency at the base, base, low frequency low frequency at the tip) tip)
Cross section through 1 turn of the cochlea shows that the cochlea is dividied by Reissner’s membrane and the basilar membrane into 3 scalae(chambers): 1. Scala vestibule 2. Scala media (cochlear duct) 3. Scala tympani
N o t e :
o
The p e r i l y m p h of of The scala vestibuli and scala tympani are c o n t i n u o u s with with one another at the tip of the cochlea cochlea ( (helicotrema) The bony wall wall of the cochlea has 2 defects, each covered by a thin membrane PA( Round & oval window) Scala vestibule is connected to the oval window window
o
Scala tympani connected connected to the round window window
o
o
Cochlea
The cochlea is coiled around the modiolus
The Modiolus:
The spongy bone spongy bone axis around axis around which the cochlea is colied (central axis central axis of the cochlea) Contains:
1. cochlear nerve ganglia : comprised of bipolar neurones neurones that: that: 2. Spiral ganglia: peripherally innervate the hair cells a) peripherally innervate b) centrally form centrally form the cochlear nerve 3. Blood vessels
The cochlear coil extends “up” from its base (2 3/4 X) The chambers spiral and exhibit 5 cross sectional profiles. profiles. Different pitches (frequencies) (frequencies) of of sound are detected at different positions along positions along this long coiled structure. ( structure. (high high frequency at the base, base, low frequency low frequency at the tip) tip)
Cross section through 1 turn of the cochlea shows that the cochlea is dividied by Reissner’s membrane and the basilar membrane into 3 scalae(chambers): 1. Scala vestibule 2. Scala media (cochlear duct) 3. Scala tympani
N o t e :
o
The p e r i l y m p h of of The scala vestibuli and scala tympani are c o n t i n u o u s with with one another at the tip of the cochlea cochlea ( (helicotrema) The bony wall wall of the cochlea has 2 defects, each covered by a thin membrane PA( Round & oval window) Scala vestibule is connected to the oval window window
o
Scala tympani connected connected to the round window window
o
o
Cochlea: 3 scala: scala media
Scala media is more or less triangular:
formed by : a.
Reissner’s membrane ( above)
b.
basilar membrane (below)
c.
stria vascularis:
(laterally/outer part) base of the triangle lying against the bony wall of the cochlea
derived from the proper cochlear artery
made of basal & marginal cells
marginal cells are the one responsible 4 formation + absorption of endolymph ( hence the source of action potentials in the inner ear= endocochlear potential)
It contains endolymph ( produced by sacula vascularis)
a) The Reissner membrane : 3 layered structure Consisting of 2 cell layers separated by a basal l amina Attached medially to the modiolar edge of the spiral imbus laterally : spiral ligament at the apical edge of stria vascularis
b) The basilar membrane contains the organ of corti The side of the duct where the nerve fibers exit is the “inner” /“modiolar” side of the duct. The opposite side is the“outer” side
The spiral ligament: the prominent cells in the matrix is fibroblast type I
c) Stria Vascularis :
It extends from the attachment of Rei ssner's membrane to the spiral prominence.
it is an unusual epithelium in that it lacks a basement membrane.
It is a stratified epithelium containing primarily three cell types: 1. Marginal 2. Intermediate 3. basal cells
intraepithelial capillaries
1. marginal cells:
the primary functional units of the stria vascularis
produce the positive DC endocochlear potential
They are the only cells of the stria vascularis that directly face the endolymph
their basolateral surface is convoluted into numerous mitochondria-packed folia that +
+
are rich in Na and K -ATPase and Na-K-Cl cotransporter 2. Intermediate cells:
stellate-shaped
contain melanin
have phagocytic activity
contain carbonic anhydrase enzyme activity
3. Basal cells: The basal surface of the stria vascularis is seale d from paracellular transport of materials by several layers of tightly interdigitated basal cells. the basal cells exhibit numerous tight junctions with adjoining cells of the stria vascularis and the spiral ligament. It contains lipid inclusions and has apical processes that interdigitate with processes of the marginal and intermediate cells. 4. melanocytes Intermediate cell + intraepithelial capillaries comprise the middle layer of the stria vascularis: The capillaries are formed by nonfenestrated endothelial cells that are surrounded by a dense homogeneous basement membrane . This basement membrane may be formed by the fusion of epithelial and endothelial basement membranes during development. The capillaries are also surrounded by pericyte cells.
Organ of Corti/ spiral organ
Specialized epithelium (sensory receptor for sound) Located within the inner part of the basilar membrane
Components of the organ of Corti : a) cells : 1. hair cells: inner + outer 2. support cells (pharyngeal cells): Deiters, Hensen, Claudius
provide structural and metabolic support for the organ of Corti. The phalangeal processes of the Deiters cells form tight cell junct ions of the reticular lamina.
b) tectorial membrane c) reticular lamina
Hair cells of corti organ
1. Hair cells: “Receptor” cells ( the ones that transduce sound) It possesses stereocilia. Name of pillar cells Inner hair cells
Outer Hair cells
Site Number of rows
Modiolar side 1 row
Outer side of the tunnel of corti 3 rows
Total number
3.000
12.000
Shape of the cell
Bulbous(flask)
Columnar (cylineric)
Shape of cilia
gentle curve (From inside toward outside Each stereocilia is taller than the next
‘W’ shape (with a very shallow central notch)
Number of cilia
Few
Many
Attachment of cilia to tectorial membrane Function
Non attached
Longest stereocilia is firmly attached
principal transducer of motion from the basilar membrane to a nerve impulse (Se nd inf about hearing to the brain)
Relation to the support cells Afferent innervations
Surrounded completely by the supporting cells Type I
Cochlear amplifier: amplify motion from the basilar membrane at the point of maximum response (which increase sensivity & frequency selectivity) controlling and fine tuning mechanisms Contains actin and myosin to adjust the size of the cell Only the base
Afferent innervations % Efferent source
90%
10%
Lateral superior olivary complex
Medial superior olivary complex
Post synaptic target
Denterics of the afferent fibers
Base of the hair cells
Type II
The tips of the stereocilli are mechanically linked by tip filaments, which are directly connected to mechanically gated cation channels in the plasma membrane covering the stereocilia. When the bundle of stereocilia is deflected in the direction towards the longer stereocilia, the tension on the tip filaments is increased and the mechanically gated ions channels are opened, allowing potassium and calcium to enter the cell, which becomes depolarized.
When the bundle of stereocilia is deflected in the direction towards the shorter stereocilia, tension is reduced on the tip filaments (also referr ed to as gating springs) and the ion channels close and the cell becomes hyperpolarized. The state of polarization of the hair cell controls the release of neurotransmitter at the cell's basal end, where the cell synapses with a nerve ending. The tops of the hair cells form the reticular lamina, which isolates the hair cells’ stereocilia from Their cell bodies
The probable site of noise induced hearing loss is stereociliary rootlets of the hair cells. Tectorial membrane (non-cellular/extracellular matrix membrane) is in intimate contact with the stereocilia on the apical surface of the outer hair cells.
2. Support cells: Support hair cells Such as the Deiter,Claudius and Hensen. Notice that : The Deiter’s cells support the Outer hair cells at their base so the outer hair cell walls o are surrounded by fluid. The inner hair cell is compleately surrounded by support cells o
ACOUSTIC NERVE
auditory nerve has approximately 30,000 fibers
it is made of: 1. afferent Fibers 2. efferent Fibers
1. Afferent fibers
Carries information: from the hair cells in the inner ear to the Ipsilateral cochlear nucleus in the brainstem Bipolar neurons Their cell body reside in the spiral ganglion in the modioludus
Types of afferent neurons: Type I neurons
Type II neurons
Innervate
Inner hair cells
Outer hair cell
% of afferent
90%
10%
Mylination of the central & peripheral process Other name
Mylinated
unmylinated
Radial fibers
Outer spiral fibers
Differences in afferent nerve (typeI, typeII)innervations of (IHC,OHC) pattern:
A. convergent manner of Type I innervating IHC: Each IHC has its own “private” set of fibers ( each inner hair cell is connected to 20 afferent fibers that does not share with surrounding cells) Type I afferent innervates the IHC via a peripheral process
that terminates on the hair-cell base as a bulb. B. Divergent manner of Type II innervating OHC :
peripheral processes enter the organ of Corti through the same route as that of the type I
traverse the inner hair cell and inner pillar cell rows crossing the floor of the tunnel of Corti as basilar fibres
They then travel towards the cochlear base for varying distances as outer spiral fibres
branching to innervate up to ten outer hair cells each
Note: The peripheral process of type 1 neurons becomes unmyelinated in the osseus spiral lamina just before it enters the organ of Corti through a hole (the foramen nervosum) in the upper border of the spiral lamina, the habenula perforata, to approach the inner hair cell
Most, if not all, recordings from auditory nerve fibers are from the larger type I fibers in contact with inner hair cells.
low frequency fibers occupy the centre of the auditory nerve while the high frequency occupy the peripheral part this tonotopic organization is continous throughout the central auditory complex
the 1st synapse is at dorsal & ventral cochlear nuclei (so the 1st obligatory relay center for CN VIII afferent fibers is the cochlear nucleus) then fibers goes to the ipsilateral/contralateral superior olive
majority ascend in the contralateral lateral leminiscus to inferior colliculus ; medial genigulate ganglion: auditory cortex ( note it relays=synapse in all of them) The primary auditory centers are located in pons: 1. Inferior colliculus ( inferior corpora quadrigemina ) 2. Medial geniculate ganglion the following mnemonics outline the neural pathway: E COLI: (Eighth nerve, Cochlear nucleus, superior Olivary nuclei, Lateral lemniscus, Inferior colliculus) S-L-I-M: (sup olivary n,lateral lemniscus,inferior colliculus,medial geniculate)
The primary auditory cortex is: Brodmann's area 41 in the superficial area of temporal lobe The blood supply of the auditory cortex is the middle cerebral artery
Type 1 fibers: sparsely granulated
Patterns of efferent Fibers (olivochochlear fibers):
carries inf from auditory cortex into SOC then into hair cells highly granulated
monopolar
Note: both IHCs and OHCs receives bilateral innervations The majority of innervations into the outer hair cells Efferent fibers to OHC are mylinated while IHC unmylinated
Medial efferent system
Lateral efferent system
Origin
medial superior olivary complex
lateral
Crossed vs uncrossed
Majority cross midline
Do not cross
Type of innervated cells
outer hair cells directly
Inner hair cells indirectly
Mylination
Mylinated
unmylinated
Function
suppress outer hair cell motility to make the cells less sensitive, providing protection from very loud sounds
1-inducing either slow increases or decreases in the magnitude of the response of auditory nerve fibers. 2-Since these fibers originate in the lateral superior olive, which is involved in sound localization, they may be useful in maintaining accurate binaural comparisons during slow changes in interaural sensitivity
Implicated in the production of the otoacoustic emission + tinnitus
Note that efferent fibers terminate in the basal turn of the cochlea Related to otoacoustic emission & tinnitus
The action of the outer hair cell in more details The motile response of outer hair cells:
consist of a length change in the outer hair cell body.
part of the mechanism by which the outer hair cel ls amplify the travelling wave to increase the cochlea's sensitivity and frequency selectivity
this response is nonlinear
The motile mechanism resides in the membrane, and is a property of the protein prestin which is situated in the membrane prestin changing its configuration as the electric potential across the membrane varies: -
Cl ions occupy a position in the intracellular domain of the membrane-bound prestin protein
Cl move along an inward electrical gradient away from the extracellular domain during
depolarization.
This charge movement produces, in turn, a conformational change in the protein that reduces its surface area and shortens the cell;
-
the opposite action occurs during hyperpolarization, when Cl ions move toward the extracellular space within the domain of the protein (although the anion is never translocated to the extracellular space), causing an increase in surface area and lengthening of the cel l
depolarization: contraction
hyperpolarization: increase the length
so the process is dependent on prestin changing its configuration voltage generated by intracellular electrolyte movement
Fluid filled Spaces of the inner ear
1. The perilymphatic space:
in which the membranous labyrinth is suspended
located within located scala tympani + scala vestibuli
contains the perilymph a) similar to extracellular fluid+ CSF ( rich in Na= 150/ k=7) b) formed by infiltrate of blood c)
similar to the blood but different from CSF
d) Connected to the CSF via cochlear aqueduct 2. The endolymphatic spaces:
Located within the membranous labyrinth (scala media)
Separated from the perilymphatic space by reissner membrane & basilar membrane
Contains endolymph: a.
the only extracellular fluid that resemble intracellular fluid component (rich in K=150/Na=16)
b. produced by perilymph infiltrate ( produced by sacula vascularis)
endocochlear potential in the sacla media is generated by sacula vascularis =+ve 80 mEqv
Note: there is a negative intracellular potential, which is:
-45 m V for inner hair cells and -70 m V for outer hair cells 3. The cortilymphatic space:
lying within the organ of Corti
separated from the endolymphatic space by the reticular membrane
contains corticolyph: a) similar to perilyph b) derived from scala tympani via foramina in the osseous spiral lamina
Location Source
Electrolyte component
PH
Perilymph
Endolymph
corticolymph
Sacla tympani & vestibule (Surrounding the membranous labrynthin) Blood infilterate
Sacal media
Corti organ
Cochlea: marginal cells of the stria vascularis Vestibular system except saccule: Dark cells Rich in K (similar to intracellular)
derived from scala tympani via foramina in the osseous spiral lamina
1-Rich in Na 2-Low in amino acids & glycerin in comparasion to blood & high in comparasion to the CSF 3-Does not contain CL 7.2
Rich in Na
Rich in chlorid 7.5
Note:
Perilymph:
The site of perilymph production is still controversial.
It is unclear whether perilymph is derived as an ultrafiltrate of blood, from cerebrospinal fluid (CSF) or from both.
CSF can reach the vestibule by means of: 1. vestibular aqueduct 2. perivascular channels 3. perineural channels.
There is more convincing evidence in favor of perilymph originating as an ultrafiltrate from blood. 1.
In terms of chemical composition, the amino acid content of perilymph, particularly gl ycine and alanine, is low compared with blood but much higher than CSF.
2.
changes in blood composition are reflected much more rapidly in perilymph than in CSF.
Kellerhals has proposed a dual origin of perilymph. He found that the majority was derived from blood and the remainder from CSF
Perilymph leaves the ear by drainage through:
1. venules 2. through the middle ear mucosa
Endolymph:
dark cells (of the cristae and maculae ) & marginal cells of the stria vascularis have: 1. Ultrastructural morphologic properties of secretory cells:
deeply invaginated nuclei (indicating a cell actively engaged in production and secretion);
many microvilli on their apical surface
large number of free ribosomes and many vesicles (again indicating active production and packaging of secretory products);
deep infoldings on their basal surface containing many mitochondria(Presumably, the mitochondria provide the fuel for the energy-consuming process that pumps the endolymph out of the cell)
long, thin cytoplasmic extensions.
2. biochemical properties of secretory cells,they have a high concentration of enzymes associated with active ion pumps and fluid transport:
+
+
Na =K =ATPase
adenylate cyclase
carbonic anhydrase
the "dark cells" of the cristae and maculae,which are separated from the neuroepithelium by a
transitional zone
absorption of endolymph :
site: endolymphatic sac
Morphologically: the columnar cells located here are specialized for absorption. Like intestinal cells, they have long microvilli on the luminal surface and contain many pinocytotic vesicles and
vacuoles
Energy transduction: Transform the mechanical energy (sound wave pressure) into electrochemical Energy ( nerve
impulses) The process of transduction occurs in the structures within scala media, sitting on the basilar membrane (organ of Corti).
I.
Summary of the Pathway for sound Sound waves in the air enter the outer ear canal. A.
B. C. D. E. F.
G. H. I. J.
Vibration of the tympanic membrane is transmitted through the chain of three (3) ossicles (which amplify the sound 20 X) to the oval window. Vibration of the oval window induces pressure waves in the perilymph of the scala vestibuli. Vibration of the vestibular (Reissner's) membrane transfers pressure waves from perilymph to the endolymph of the scala media (cochlear duct). Pressure waves of the endolymph induce the basilar membrane to vibrate. Vibration of the basilar membrane (which is the critical step in the transduction process) induces a shearing of the apical ends of the hair cells against the tectorial membrane. This will bends the bundle of stereocilia on each hair cell resulting in the bending of the bundle of stereocilia and the opening of mechanically gated ion channels in the membrane of the stereocilia + Influx of K ions, and depolarization of the hair cell depolarizing the hair cell, which releases neurotransmitters. This results in an action potential being propagated along nerve fibers of sensory neurons in the spiral ganglion.
Vibration in the perilymph are transmitted to the base o f the cochlea where it displace the round window
INTENSITY RESOLUTION Low Intensity sounds
When the organ of Corti moves in response to sounds of low intensity the cilia of the inner hair cells do not contact the tectorial membrane.
The brain receives impulses from the afferent fibres of the outer hair cells therefore, the brain will be aware that there is a sound but without perception of sound as there are no impulses from the inner hair cells.
Signals are sent along the efferent nerve fibres to the outer hair cells, causing them to shorten in length.
This pulls the tectorial membrane closer to the basilar m embrane, allowing the cilia of the inner hair cells to impact the tectorial membrane.
Deflection of the cilia leads to depolarisation and action potentials from the inner hair cells and perception of sound.
High-intensity sounds
High intensity sound lead to more movement of the organ of Corti.
In order to protect the cilia, efferent impulses cause the outer hair cells to lengthen and push the tectorial membrane away from the basilar membrane
The vestibular system: main functions of the vestibular system: 1. It is the primary organ of equili brium and thus plays a major role in the subjective sensation of motion and spatial orientation. 2. Vestibular input to areas of the nervous system involved in motor control elicits adjustments of muscle activity and body position to allow for upright posture. 3. Vestibular input to regions of the nervous system controlling eye movements helps stabilize the eyes in space during head movements. This reduces the movement of the image of a f ixed object on the retina.
Reflexes used to Counter head motion with to maitain stable vision & posture are: a) vestibulo-occular reflex: stabilize gaze despite head movement b) vestibulocollic reflex: sabilize head despite body movement c) vestibulo-spinal reflex: stabilize posture and fascilate gait
Basic elements needed for conversion of acceleration into useful signals for the nervo us system are: 1. inertial mass 2. one or more sensory hair cells 3. the nerve fibers connected to the hair cells through synaptic junctions
Inertial mass:
For saccule + utricle: otolith
For semicircular canals: endolymph cupula
Components of the vestibular system
This comprises the 3 semicircular canals and the saccule and utricle= membranous labrynithin
except the cochlear duct.
The semicircular canals are concerned with the rotation and the saccule and utricle with
gravitation Semicircular Canals (SSC): • senses rotational (angular) acceleration
Note; constant-velocity rotation will not stimulate SCC
Orientation of the semicircular canals
horizontal canals and the utricle:
lie parallel to the line between the EUC and the outer canthus of the eye, which is inclined 30 degrees above the horizontal axial plane (anterodorsally relative to the naso-occipital plane )
so in the anatomical position the plane of the lateral semicircular canal is at angle of 30 to the horizontal tilting downward Posteriorly
This orientation causes the plane of the
horizontal canal and utricle to be parallel with the earth horizontal and perpendicular to gravity.
Caloric tests: Test the function of the lateral SCC
Fitzgerald-Hallpike test: the pt lies supine with head tilted 30 forward so the lateral SSC assumes a vertical position ( the most sensitive to a thermal gradient)
Kobrak test: the pt sits with head tilted backward 60 degree
The anterior and posterior semicircular canals and the saccule are arranged vertically in the head, orthogonal to the horizontal semicircular canal and utricle
Vertical/Superior canals:
When looking down at the top of the head, the anterior canal is oriented at approximately 45 degrees off midsagittal and 45 degrees anterior to the intraaural line.
Located at right angles to: a) the horizontal canals b) to each other.
Posterior canal:
aligned roughly 45 degrees behind the intraaural line
The two vertical canals in each ear are positioned orthogonal to each other
whereas the plane of the anterior canal on one side of the head is coplanar with the plane of the contralateral posterior canal
This orthogonal pattern creates functional pairs in that increased output from one canal results in decreased output from its paired canal thus creating a “ push-pull” mechanism that enhances the sensitivity of the system
Each end of the canals opens into the utricle but there are only 5 openings as the anterior and posterior canals unite Posteriorly .
Ampulla: pear-shaped expansion of the membranous labyrinth located at one end of each SSC near the vestibular opening (where the semicircular canal meets the utricle
The location of the ampullated end of the semicircular canals
1. lateral semicircular canal : at the anterior end of the canal 2.
superior semicircular canal is also anterior
3.
posterior semicircular canal is lateral.
Location of nonampullated end of semicircular canals: 1. lateral semicircular canal: enters the vestibule posterolaterally. 2. The nonampullated ends of the posterior and superior semicircular canals join to form the common crus and enter the vestibule posteromedially
Within each ampulla there is the sensory organ : the crista ampullaris.
Crista ampullaris is a saddle shaped neuroepithelium end organ
Crista ampullaris has 2 types of hair cells : 1. Type I hair cells :
Flask shaped (rounded base and narrow neck)
The body is entirely engulfed by one afferent terminal.
Efferent innervation is indirect , as the efferent nerve has its synapse on the afferent nerve ending
2. Type II hair cells :
Cylindrical in shape have one or more afferent nerve endings on the body of the cell.
Type II hair cells can also be directly or indirectly innervated by vestibular efferent terminals
Both types of hair cells apex contain: 1- stereocilia 2- 2- kinocilium
Kinocilium : A true (but non-motile) cilium, taller and larger in X-S than the stereocilia, having an array of 9 + 2 microtubules in its core
Stereocilia are arranged by height relative to kinocilium in a step-like manner (Tallest clostest, shortest farthest, from kinocilium)
there are both afferent and efferent nerve fibres to each hair cell.
the hair cells and their sup porting cells lie embedded in a saddle-shaped neuroepithelial ridge, the
crista , which extends across the base of the ampulla
Type I hair cells are concentrated in central regions of the crista, Type II hair cells are concentrated in peripheral areas.
Arising from the crista and completely enveloping the stereocilia of the hair cells is a gelatinous structure, the cupula.
The cupula attaches to the roof and walls of the ampulla, forming a fluid-tight partition that has the same specific density as that of endolymph.
Afferent fibers of the vestibular system:
The efferent fibers to the vestibular end organs arises from:
a small group of about 200 neurons
lateral to the abducens nucleus and the genu of the facial nerve , the so-called group e
These neurons project both ipsilaterally and contralaterally
The contralateral pathway crosses the midline at the level of the facial genu and joins the
ipsilateral pathway
both pass ventral to the vestibular nucleus.
At this point, they are joined by cochlear efferents originating from the superior olivary complex (the olivocochlear bundle).
All of the efferents then enter the vestibular nerve, coursing through the middle of the nerve in a small distinct bundle.
At the end organs, these relatively few fibers branch profusely to innervate the entire sensory epithelium.
the ipsilaterally projecting efferents supply the central regions of the crista, while the
contralaterally projecting efferents supply the peripheral zone.
The efferent fibers terminate as highly vesiculated boutons, making synaptic contacts with hair cells and afferent fibers
Note that the vestibular nucleus in the brain is made of: 1. Superior vesibular nucleus 2. Descending V.N 3. LATERAL There is similarity between hair cell innervations in the auditory system and the vestibular system Auditory system: inner & outer hair cell Vestibular system: Type I & Type II hair cells
Inner hair cell/type 1
Outer hair cell/type II
Shape
Flask shape
slender
Afferent
Completely surround the hair cell
Efferent
Contact the hair cell indirectly through the synapse with afferent/its extension Note the # of afferent vestibular fibers:18000 # of efferent vestibular fibers: 200
Contact the hair cell directly
There is postganglionic sympathetic efferent innervations of the vestibular end organ of unknown function (originates from the superior cervical ganglion)
Physiology:
Any change in rotation speed of the head results in mo tion in the endolymph.
This causes the cupola to move and brings about a shearing movement of the cilia.
Direction of bending of the stereocilia, relative to kinocilium, is significant:
bending away from kinocilia hyperpolarization of receptor cell (inhibition)
bending toward kinocilia depolarization of receptor cell & thus generation of an action potential (impulse) in the vestibular branch.
Because of the orientation of the semicircular canals rotation will produce an increase in firing rate in the vestibular nerve on one side of the head and a reduction on the other side.
• Horizontal (Lateral) SSC:
30° from horizontal
Kinocilia is located on the utricle side
So displacement of the steriocilia to ward the kinocilium by ampullopetal (toward vestibule) flow of endolymph increases vestibular neuron firing rate
• Vertical (Superior and Posterior ) SSC:
both canals share a nonampullated common crus,
kinocilia is located on the semicircular canal side
ampullofugal (away from vestibule) flow of endolymph increase vestibular neuron firing rate
When the head is stationary (no angular acceleration), the endolymph and the cupula remain still, and the afferents from the two horizontal semi circular canals fire at the same (resting) rate 90.
When the head turns to the right or left, the horizontal semicircular ducts turn with it, but the endolymph lags owing to inertial forces and the viscous drag between the fluid and the duct wall. The lagging endolymph deflects the cupula, which in turn deflects the stereocilia of the hair cells.
leftward turn of the head causes the stereocili a in the left horizontal canal ampulla to be deflected toward their kinocilia, resulting in an increase in the discharge rate of the eighth nerve afferents on the left side.
Simultaneously, the hair cells in the right horizontal canal ampulla are hyperpolarized , so their afferents show a decreased rate of fir ing.
Scarpa’s Ganglion : bipolar vestibular nerve cell bodies located in internal acoustic meatus
Plane of semicircular canal stimulation: 1. Horizontal scc: head turning in horizontal plane 45 degree off the midsagittal plane
2. Posterior SCC: nose is pitched upward & turning 45 degree off the midsagittal plane 3. Superior SCC: nose is pitched down & 45 degree off the midsagittal plane
Clinical application:
each canal has a resting basal discharge rate,and by modulating the nonezero baseline firing of vestibular afferent fibers,the semicircular canal encode rotation of the head so a lesion of the eighth nerve, such as that produced by a glomus tumor or acoustic neuroma may reduce the frequency of impulses in the ipsilateral afferent fibers or block their impulse transmission entirely. The comparator units of the vestibular nuclei will then consistently receive a higher impulse frequency from the intact side, which will be interpreted as a head turn away from the side of the lesion
The semicircular ducts are particularly concerned with reflex control of visual movements through the vestibulo-ocular reflex at the brainstem level to allow the individual to maintain optic fixation in the presence of movement(the head moves in one direction the eyes are moved in the opposite direction so they can remain focused on the same point)
The jerky eye movements caused by rotation of the head (rotatory nystagmus) are dependent on the defections of the cupulae. The slow component of the nystagmus is always in the direction of the cupula deflection
Nystagmus:
The nystagmus is named after the fast component of the nystagmus
Nystagmus components: 1. Slow component: vestibular in origin toward the defective lesion 2. Fast component: neural in origin
In caloric test:
COWS:
1. Cold: toward the opposite ear 2. warm: toward the same ear, it causes ampullopetal/uriticopetal flow
cold water makes the labyrinthine hypoactive=labrynthiectomy
vestibular neuritis has the same effect of warm water on labyrinthin
THE UTRICLE AND SACCULE:
The utricle and saccule both contain endolymph. Saccule is smaller than the utricle,& lies in a depression below & infront of the utricle Within each structure is a gravitational sense organ: the macula (otolith organ). The hair cell stereocilia of otolith organs extend into a g elatinous coating called the otolith membrane, which is covered by calcium carbonate crystals called otoconia (“ear stones”). Otoconia: 3x times as dense as the surrounding endolymph not displaced by normal endolymph movements. Instead, changes in head position relative to gravity, or linear accelerations (forward-backward, upward-downward) produce displacements of the otoconia, resulting in bending of the underlying hair cell stereocilia Any linear motion displaces this membrane in the opposite direction. This again produces deflection of the cilia and depolarization of the hair cells.
The macula is situated in: Utricle: on the floor: respond to horizontal, side-to-side motion-axial plane-inter aural saccule : on to the medial wall: responds to movement in the vertical plane.-sup inferior The hair cells within these structures are orientated with reference to a central plane ( the striola). In the utricle the cilia are arranged so that the kinocillium is towards the striola whereas in the saccule the kinocilia are arranged away from the striola.see p 100 fig4-4 There is again both an afferent and an efferent nerve supply to the hair cells of the maculae.
Otolith organs alone are unable to distinguish linear acceleration from vertical acceleration
So the brain uses other clues such: 1. Rotational information from SCC 2. Visual input
Summary of the Sensory Receptors of the Membranous Labyrinth: 6 regions of sensory receptors project from the wall of the membranous labyrinth: a) 3 cristae ampullaris within the ampullae of the semicircular ducts sensitive to angular acceleration (turning of the head) b) 2 maculae (macula utriculi & macula sacculus) sensitive to position relative to vertical (gravity); linear acceleration c)
the spiral organ (of Corti) of the cochlear duct sensitive to sound receptor
All of these sensory receptors share similar structural specializations and characteristics, including hair cells that:
are non-neuronal, mechanoereceptor, epithelial cells
possess numerous stereocilia (modified microvilli) called “ sensory hairs”
are associated with both afferent and efferent nerve endings
are all transducers, i.e., they convert mechanical energy (bending of stereocilia) into electrical energy transmitted by the vestibulocochlear nerve to the brain
the means by which bending or flexing occurs varies for different receptors
stretching of the plasma membrane caused by bending ge nerates transmembrane potential changes in the receptor cell that are conveyed to the afferent nerve endings associated with the cell
Summary of the plane of stimulation The max sensitivity to motion for the vestibular end organs occur along an axis oriented 1. Perpendicular to plane of the SCC 2. Parallel to the plate of otolith organ
Movements that stimulate semicircular canal: Angular rotation 1. Horizontal scc: head turning in horizontal plane 45 degree off the midsagittal plane
2. Posterior SCC: nose is pitched upward & turning 45 degree off the midsagittal plane 3. Superior SCC: nose is pitched down & 45 degree off the midsagittal plane
Movements that stimulate the otolith organ: Linear acceleration: 1. Utricle: horizontal ( ant-pos, Rt-Lt) 2. Saccule: vertical ( change in head position in relation to gravity) tilting the head
Vestibular nuclui
The vestibular nuclei is divided into 4 subdivisions
:
1. Superior vestibular nuclei 2. Medial vestibular nuclei 3. Lateral vestibular nuclei:subdivided into dorsal & ventr al 4. inferior vestibular nuclei The superior vestibular nucleus :
located dorsally and rostrally in the vestibular complex.
involved in VOR pathways.
Neurons in this nucleus fire in relation to eye movements as well as head movements
a prominent efferent projection of this nucleus is to the oculomotor nucleus via the medial longitudinal fasciculus.
The lateral vestibular nucleus: subdivided into two subnuclei: a) dorsal lateral vestibular nucleus (or Deiters' nucleus) b) ventral lateral vestibular nucleus.
dorsal lateral vestibular give rise to :
lateral vestibulospinal tract
do not give efferent fibers to the cerebellum & brainstem
ventral lateral vestibular nucleus give rise to:
vestibuloocular pathways
medial vestibulospinal tract
vestibulothalamic pathways.
The medial vestibular nucleus : extends rostrocaudally in the brainstem from the leve l of the abducens nucleus to the hypoglossal nucleus. It is bounded medially by a functionally related nucleus and the nucleus prepositus hypoglossi, and laterally by the inferior vestibular nucleus. Recent studies indicate that the medial vestibular nucleus can be subdivided into a rostral magnocellular region (MVmc) and a parvocellular region (MVpc) and a caudal region (MVc) with small- to medium-sized neurons.
[23][33]
The rostral medial vestibular
nucleus, like the superior nucleus, contains many neurons that project to the oculomotor nuclei and
whose firing behavior is related to eye movements.
[60][68][69]
Little is known about the functions of the
MVc, although many of the cells in this reg ion appear to project to the cerebellum.
[24]
The medial
vestibular nucleus also gives rise to the medial vestibulospinal tract, which descends in the medial longitudinal fasciculus to terminate on interneurons in the cervical spinal cord and ascends to terminate in the eye motor nuclei. This tract is particularly important for cervicovestibuloocular reflexes.
The inferior (or descending) vestibular nucleus
one of the primary recipients of vestibular afferents that innervate the otolithic organs.
major projections from this area are to t he cerebellum and reticular formation.
Efferent fibers from the vestibular nucleus
Reflexes are: 1. vestibulo-occular reflex:
Rotates the eye in the same speed but in the opposite direction of the head rotation so
it stabilize the image on the fovea despite head movement
vestibular nerve: Ipsilateral medial vestibular nuclui (medulla) : stimulatory impulse into
contra-lateral abducent nerve(pons): 1.lateral rectus 2.indirectly via medial longtidudinal fasciculus into the Ipsilateral oculomotor nucleus in mid brain
loss of vestibuloccular reflex in comotoze patient post head trauma indicates brain stem injury
2. vestibulo-spinal reflex:
stabilize posture and fascilate gait
Head position control Semicircular canals
Body position control Otolith organs
Vestibular nuculeus
Medial
Dorso-Lateral
Tract
Medial longtidunal fascicular tract Medial vestibulospinal tract Medial part of the ventral horn
Lateral vestibulospinal tract
Neck & axial muscles
Ipsilateral proximal limb muscles
Stimulation
Horn
Muscles
Ant horn
Classes of eye movement:
The aim of eye movement is to bring the image into the fovea:
If the head is steady & object steady:
visual fixation :
stationary object
vergene angle:
nearer stationary object
If the head is steady & the object is moving
smooth pursuit:
slowly moving object
saccule
rapidly moving object
If the object is steady & the head is moving:
vestibular : rapid,brief,high frequency head rotation
optokinatic:
slow,low frequency head rotation
