Vestibular System

Question 1

Vestibular System 5 Components

Answer 1

– essential for posture, balance, and movement
o Peripheral receptor apparatus – converts mechanical energy into electrical
 Hair cells located in vestibular labyrinth innervated by vestibular nerve (CN8)
o Central vestibular nuclei – primary sensory nuclei for vestibular system
o Vestibulo-thalamo-cortical network – brainstem  thalamus  cortex (central network)
o Vestibulo-ocular network – via semi-circular canals; involved with dynamic function
o Vestibulospinal network – via efferent muscle control; involved with balance/posture

Question 2

Vestibular Organization

Answer 2

o 2 components – bony labyrinth and membranous labyrinth
o 2 vestibules each side – detect translational/linear acceleration in horizontal & vertical plane
 Utricle – horizontal plane mainly
 Saccule – vertical mainly
o 3 semicircular canals (anterior, posterior, horizontal) – detect rotational acceleration in 3-D
o 2 fluids – perilymph (CSF) and endolymph

Question 3

Hair Cell Characteristics and Innervation

Answer 3

o Hair cells are specialized sensory receptor
o Macula – internal structure of static system that contains specialized sensory hair cells
o Crista ampullaris – internal structure for dynamic system that contains specialized hair cells
o Innervated by peripheral proceses of CN 8 (vestibular division)
o Cell bodies reside in Scarpa’s ganglion
o Receive Efferent innervation from brainstem areas

Question 4

Otolithic Membrane and Hair Cells

Answer 4

– encapsulates the kinocilia & stereocilia and contains carbonate crystals that prevent non-specific movement; helps detect gravity; and adds extra weight to allow detection of movement changes; ONE kinocilium and many stereocilia
o Type I – circular; contain large afferent nerve fiber that forms a cup around the hair cell
 Increases the sensitivity and amplifies the signal
 Efferent fiber synapses on the afferent fiber
 Located close to the middle of the cupula
o Type II – columnar; contain a smaller afferent nerve terminal
 Efferent nerve fiber synapses directly onto the hair cell
 Located closer to the edges of the cupula

Question 5

Transduction Mechanism and Adaptation

Answer 5

o Tip link – “spring like” connection between kinocilium and adjacent stereocilium
 Connected to a “trapdoor” of mechanically sensitive K+ ion channel on stereocilia
 Movement of stereocilia towards kinocilium pulls open the trapdoor, K+ influx, depolarizing the hair cell
o Adaptive Mechanism – occur during hyper- and depolarization
 Sustained depolarization leads to Ca+ channels staying open in stereocilia
 As Ca+ increases it causes slippage of the motor protein and tension decreases causing the channel (“trapdoor”) to close; takes 5-10 seconds for slippage to occur
 As Ca+ decreases the motor can now retension the spring so that it is ready to detect change again

Question 6

Static Vestibular Functions

Answer 6

o Detection of linear acceleration (translational motion)
o Vestibulocolic reflexes – vestibular apparatus to motor neurons of neck muscles
o Vestibulospinal reflexes – vestibular apparatus to motor neurons in spinal cord controlling anti-gravity muscles (axial muscles of trunk)
o Maintains postural status quo (detection of head and body orientation)
 Keeps head upright on body
 Keeps trunk upright & centered over pelvis

Question 7

Sensory Apparatus for Static Vestibular Functions

Answer 7

o	Striola – indentation in the macula structure that divides the saccule/utricle into 2 polarities
	Polarity flips as you pass the striola so depolarization occurs on one side and hyperpolarization on the other
	Kinocilia located on the same side of every cell on one side of striola and on opposite side on the other side of the striola
	Turn head to left  left side discharge frequency decreases and  right side discharge frequency increase  allows brain to interpret head movement from these 2 signals
	Each side of striola is innervated separated
o	Spontaneous (constant) firing occurs even when stereocilia is not moving
o	Stereocilia move towards kinocilia  trapdoor opens and hair cells depolarize
	Mechanism: cilia movement opens mechanosensitive K+ channels  K+ enters the hair cell causing depolarization  opens voltage-gated Ca+ channels  opening of Ca+ dependent K+ channels  allows K+ to leave the cell to repolarize the hair cell
o	Stereocilia move away from kinocilia  reflexive loosening of the spring  channel is closed  hair cell hyperpolarizes

Question 8

Static Vestibular Signaling Pathway: General

Answer 8

utricle/saccule  CN 8 lateral/medial/descending nuclei of primary vestibular nucleus  Medial/Lateral vestibulospinal tracts (MVST, LVST)
 Some fibers go from CN8 to the flocculonodular cerebellum bypassing the primary vestibular nucleus
 Utricle and saccule send afferent fibers that branch to innervate lateral/medial/descending nuclei of primary vestibular nucleus
• Superior nucleus NOT innervated by the static portion of vestibular system

Question 9

Static Vestibular Signaling Pathway: MVST and LVST

Answer 9

o Lateral Vestibulospinal Tracts (LVST) – descending AND lateral nuclei of primary vestibular nuclei sends signal via Lateral VST to innervate only the ipsilateral side motor neurons
 Innervate alpha motor neurons; responsible for body posture
o Medial Vestibulospinal Tracts (MVST) – medial nuclei of primary vestibular nucleus sends signal via Medial VST to innervate both the ipsilateral and contralateral sides
 Innervate interneurons and gamma motor neurons; keeps head upright

Question 10

Dynamic Vestibular Functions

Answer 10

o Detection of angular acceleration – roll, pitch, and yaw
o Detects CHANGE
o Uses semicirucular canals for detection
o Vestibulo-ocular reflex (VOR) – help controlling eye movement
 Acts on extrinsic eye muscles through outputs to CN 3,4,6
 Causes eye movements that are compensatory to head movements
 Reflexively stabilizes the visual field on the retina during movement of the head

Question 11

Vestibulo-ocular Reflex Pathway

Answer 11

o Detects in semicircular canals  vestibular nuclei  nuclei of CN 3,4,6
o Axons travel in ascending medial longitudinal fasciculus (MLF)
o Innervate extraocular eye muscles

Question 12

Semicircular Canals Function

Answer 12

o Detect angular acceleration in any spatial dimension (X, Y, Z)
 Vector analysis of activity on the 6 canals computes the position of the head in space
 Maximum response in canals which are in parallel planes
• Right anterior canal and left posterior canal
 Head must be tilted down at 30 degrees to orient the horizontal canal parallel to ground
• Evolutionarily Derived – look slightly down so we can see where we are walking
o Cristae ampullaris – found within the ampulla of each canal
• Area where the specialized sensory hair cells are embedded
 Cupula – gelatinous structure within the ampulla; encapsulates the kino- and stereocilia
• Synonymous with macula in utricle and saccule
• Fluid stays in same position; cupula actually moves when we move

Question 13

Dynamic Vestibular Signaling Pathway: General

Answer 13

o Semicircular canal (cristae ampullaris hair cell)  CN 8  superior/medial and descending vestibular nuclei  ascending medial longitudinal fasciculus (MLF)  nuclei of CN 3/4/6
 Superior medial nuclei – involved when maintaining eye fixture during head movments
 Some axons bypass the primary vestibular nucleus and send signal directly to flocculonodular cerebellum
 NO INFORMATION BEING SENT TO SPINAL CORD
 Afferent fibers from horizontal & anterior canals  synapse on the superior and medial nuclei of the primary vestibular nucleus
 Afferent fibers from posterior canal branch to innervate superior/medial/descending nuclei of the primary vestibular nucleus
o Complex connections and integrations exist between dynamic nuclei of the 2 hemispheres of the brain; also involves connections from other areas of brain such as cerebellum and spinal cord
o Vestibular Thalamo-cortical Pathway

Question 14

Dynamic Vestibular Signaling Pathway: Thalamo-cortical

Answer 14

 From superior/lateral/descending vestibular nuclei  ventral posterolateral nucleus in thalamus  area 3a and 2v of postcentral gyrus (relatively small area) in cortex
• Helps us recognize why certain motions are occurring and allows us to have a bit of control as to how we feel at times (ex: car sickness)

Question 15

Vestibular Deficits (Trauma, schwannoma, cellular composition, vestibular dysfunction)

Answer 15

o Damage to petrous portion of temporal bone may result in vestibular deficits
o Vestibular schwannoma – vestibulocochlear deficits and deficits indicative of potential damage to facial nerve
o Disturbance in the ionic content of endolymph
o Nystagmus – indiciator of vestibular system dysfunction

Question 16

Vestibular Deficits: Meniere, Dizziness, Vertigo, Alcohol)

Answer 16

o Meniere disease – results from changes in volume of endolymph
 Symptoms: vertigo, nausea, nystagmus, vomiting, tinnitus, balance problems
o Dizziness – perception of spatial disorientation that may be accompanied by nausea and ataxia
o Vertigo – vestibular neuritis may result from acute viral infection
 Symptoms: nausea and vomiting; no hearing loss
 Perception of body motion (spinning) experienced even when no real motion is occuring
o Alcohol changes the specific gravity of endolymph and makes it difficult to walk in a straight line