VESTIB

   Vestibule: posterior lateral to the cochlea

o   3 semicircular canals

§  3 types

·      Horizontal semicircular canal

o   Lateral

o   Pushes on medial wall of the middle ear

o   Innervated by the superior vestibular nerve

·      Superior semicircular canal

o   Anterior

o   Innervated by the superior vestibular nerve

·      Posterior semicircular canal

o   Innervated by the inferior vestibular nerve

§  They are all 90 degrees with each other (perpendicular to each other)

§  They help with detecting angular movements/accelerations

o   Utricle

§  Closer to the semicircular canals

§  Known as an otolith by itself or otoliths when paired with saccule

§  Helps with detecting linear acceleration

·      Horizontal linear acceleration specifically

o   Saccule

§  Relatively closer to the cochlea by the oval window

§  Known as an otolith by itself or otoliths when paired with utricle

§  Helps with detecting linear acceleration

·      Vertical linear acceleration specifically

·      Mid sagittal

o   Perfectly dividing into two halves

·      Middle ear infection can have some symptoms of dizziness in a very chronic and profound stage of cholesteatoma

o   Rare but it possible

·      What is the purpose of the vestibular system?

o   To work with visual and proprioceptive systems to maintain balance

·      The vestibular system predominantly dominates in the higher frequencies and the visual system predominantly dominates in the lower frequencies????

·      Any object can move in 6 different directions (even the human head/rigid body)

o   Front and back

o   Up and down

o   Side to side

o   Rotate in 3 different planes

·      Once a movement occurs, the vestibular system will send a signal to the vestibular nucleus in brainstem and reflexes will occur at this moment.

o   Messages will then be sent to move the eyes as well as other muscles in the body to keep the person from feeling dizzy/off balance.

·      The vestibular nerve carries information to the vestibular nucleus

o   From there it can go upstream, downstream, to the cerebellum, to the cerebral cortex, or the autonomic centers/system

§  Upstream

·      Vestibular ocular: vestibular system, vestibular nerve, and goes upstream, and then goes to eye muscles and eye movement occurs

·      Controls eye movement

o   Cranial nerves III (oculomotor), IV (trochlear), and VI (abducens)

§  Downstream

·      Postural corrections

·      The muscles are innervated by cranial nerve XI (accessory)

§  Cerebellum

·      Important for postural maintenance

·      Refining of movements happens here

§  Vestibular nucleus

§  Cerebral cortex

§  Autonomic centers/system

·      Fight and flight responses

o   The two vestibular nuclei communicate with each other unlike the cochlear nuclei for the auditory system

§  The communication is very efficient

·      Vestibular apparatus

o   Located in the petrous portion of temporal

o   Bony and membranous labyrinth

o   5 sensory end organs on each side (10 in entire body due to two ears)

§  3 semi-circular canals

§  2 otolith organs

·      Crista/Cristae

o   Crista = one semicircular canal

o   Cristae = more than one semicircular canal

·      Macula/Maculae = belongs to the otoliths (utricle/saccule)

·      Macula/Macullae: sensory organ for detection of movement

o   This is within the saccule and utricle

o  

§  Hair cells

·      Type 1: flask

·      Type 2: cylindrical shape

§  Otolithic membrane: gelatinous mass

·      Otoconia particles:

o   Made of calcium carbonate crystals

o   Are within the otolithic membrane

o   BPPV: benign paroxysmal positional vertical

§  Can be dislodged and moved to the semicircular canals

·      Causes dizziness

·      Happens with age

·      Happens with very hard movement of the head

·      Most common disorder and easily treatable

§  Kinocilium: tallest stereocilia

o   Supplied by superior vestibular nerve

·      Crista ambularis: sensory organ for detection of movement

o   This is within the 3 semicircular canals

§  The canals have Ampullary ends which just means the bulging ends where the crista ambularis is

·      Meaning the crista is within the ampulla

·     

o   Pink is endolymph (high concentration of potassium)

§  At baseline the cupula and endolymph have the same density

o   Cupula: gelatinous mass

§  Has higher and richer vascular/blood supply

§  Positional alcohol nystagmus (PAN)

·      At high alcohol intake OH is higher in cupula than endolymph

·      Cupula is lighter than the endolymph in this instance and they are not the same density

§  Nystagmus: involuntary eye movement

o   Hair bundles (stereocilia)

o   Hair cells

o   Posterior semicircular canal is innervated by the inferior vestibular nerve

o   Horizontal semicircular canal is innervated by the superior vestibular nerve

o   Superior/Anterior semicircular canal is innervated by the superior vestibular nerve

·      Otolith structures

o  

o   Type 1 hair cells

§  Have a calyx nerve terminal

·      the nerve surrounds the entire structure

§  responsible for coding sudden/transient movements

§  very fast transmission of signals

·      transmission is almost 6 ms faster than auditory system due to the calyx terminal

o   Type 2 hair cells

§  Has button shaped terminal not a calyx nerve terminal

§  Responsible for coding sustained/ongoing movements

·      Example: riding in a car at a constant speed

·      The vestibular system is always firing from 80 to 100 spikes per second

o   Inhibition (hyperpolarization): firing goes from 100 to 0 spikes

§  Hair cells flowing from tallest to shortest

o   Excitation (depolarization): firing goes from 100 to 400 spikes

§  Hair cells flowing from shortest to tallest

·      Different fibers (vestibular afferents)

o   Regular fibers

§  Like type 2 hair cells

§  Code for sustained movement

o   Irregular fibers

§  Like Type 1 hair cells

§  Code sudden movement

·      Semicircular canals function together when they are in the same plane

o   RALP:

§  Right anterior pairs with left posterior

o   LARP

§  Left anterior pairs with right posterior

o   Horizontal pairs with horizontal

o   When one side (left or right ear) semicircular canal gets excited the opposite side gets inhibited

·      Ewald’s law (principles that describe how the semicircular canals respond to endolymph movement and also how that causes involuntary eye movement (nystagmus))

o   If you stimulate in the horizontal plane then the eyes will move horizontally

o   Excitation of a semicircular canal creates a greater response than inhibition

o   Ampullopetal: fluid moving toward ampulla

o   Ampullofugal: fluid moving away from ampulla

o   The tallest hair cell (Kinocilium) is towards the midline

o   Vertical canals (posterior and anterior)

·      Macula physiology

o   Saccule orientation is vertical

o   Utricle orientation is more horizontal

o   Striola: the midline structure in the utricle and saccule

§  Half of the hair cells are oriented in one direction and the other half are oriented in the opposite direction

§  This means that half of the otolith gets excited and the other side gets inhibited

·      Saccule and utricle don’t work in pairs like the semicircular canals

§  The type 1 hair cells are predominantly closer to the striola while the type 2 hair cells are further away

·      The stereocilia of type 1 hair cells are not embedded in the gelatinous mass

o   this means that they are highly sensitive to movement