Neuroanatomy Lecture #30: The Vestibular System

Front 1

Nystagmus

Back 1

Alternating rhythmic eye movements, usually in combinations of slow eye movements driven by reflex circuitry followed by fast saccades in the opposite direction

Front 2

Vertigo

Back 2

Sensation of rotational self or the environment

Front 3

Tinnitus
• What are some ways it can be experienced?

Back 3

The perception of sound under silent conditions. Can be experienced as buzzing, ringing or whistling

Front 4

Sensorineuronal Hearing Loss

Back 4

Hearing loss associated w/ pathology of the cochlea or auditory nerve

Front 5

Hair cells:
• main fxn
• The apex of the hair cell
- is where this process takes place
- the structure it contains to facilitate the above process
- the type of fluid that surrounds the hair cell and the main ion of this fluid

• Base of hair cells is where these two structures communicate w/ each other
- it contains these three components for synaptic transmission
- synaptic transmission stimulates these structures

Back 5

Hair Cells
• transduce mechanical stimuli in the inner ear

• Apex of hair cell
- signal transduction (mechanical stimulus to cellular response)
- cilia (stereocilia)
- endolymph, mainly potassium

• Base of hair cells is where the hair cell and afferent fibers communicate w/ each other
- synaptic terminal w/ vesicles, containing the excitatory NTs
- synaptic transmission stimulates the afferent fibers of CNVIII

Front 6

Exact numbers aren't necessary but know the difference btwn these two compartments and how they compare with the intracellular compartments of nerve and muscle cells.

Back 6

Front 7

Depolarization of hair cells:
• Process is due to the activity of these channels
• The channels cause these types of potentials
• The ion and direction of its current that lead to depolarization
• Briefly explain the "tip link"

Back 7

Depolarization of hair cells:
• Mechanically-gated cation channels, specifically TRPA1
• cause graded potentials that can range from depolarization to hyperpolarization
• K+, into the cell
• "Tip link"
- Neighboring cilia produce a mechanical force on one another causing the TRPA channels to open or stay closed.
- when the deflection is in the direction of the tallest cilium, the TRPA channel opens and the hair cell depolarizes
- A deflection in the direction of the smallest cilia lead to the channel staying closed and hyperpolarization

Front 8

The effects of hair cell depolarization:
• Once the cells are depolarized, these channels open leading to this event
• this leads to the activation of afferent fibers
• The location of afferent fibers
• Afferent fibers produce these kinds of potentials
• Explain the relationship btwn the actions of stereocilia and the APs generated by afferent fibers

Back 8

The effects of hair cell depolarization:
• Voltage-gated Ca2+ channels open leading to an inward current of intracellular Ca2+
• Intracellular Ca2+ rise, leading to a release of excitatory NTs at the base of hair cells which excite the afferent fibers

• they produce APs
• Relationship of stereocilia and afferent fiber activity
- When stereo cilia are deflected towards the tallest cilium, the hair cell is depolarized and the afferent fiber produces APs at a very high frequency.
When stereo cilia are deflected towards the smallest cilium, the hair cell is hyperpolarized and the afferent fiber produces APs at a very low frequency.

Front 9

Otolith organs:
• Two components
• filled with this type of fluid
• substance at the tips of their stereocilia
• What causes the depolarization/hyperpolarization of their hair cells

Back 9

Otolith organs:
• Utricle and saccule

• endolymph

• at the tips are calcium carbonate crystals, glued together with a jelly-like otolith membrane

• gravitational forces or linear acceleration bending the cilia

Front 10

Semicircular canals:
• filled with this type of fluid
• the cells the canals contain, along with any special structures
• What causes the depolarization/hyperpolarization of hair cells

Back 10

Semicircular canals:
• endolymph

• contain crista (patch of sensory hair cells), that have their cilia inserted into a jelly-like structure called a cupula

• angular rotation of the head that leads to the bending of the cilia in the cupula

Front 11

Characteristics of Otolith organs-

• What is the striola?
• Structural difference btwn the saccule and the utricle.
• The significance of the arrangement of hair cells around the striola

Back 11

The striola is the center of each otolith organs that divides the organ into halves. There are sensory hair cells on each side, and each hair cell has a increasing sizes of kinocilium.

The utricle has their kinocilium oriented towards the striola while the saccule has theirs oriented away from it.

The arrangement allows a single stimulus to concurrently activate one group of hair cells and inhibit an adjacent group.

Pictured above is the utricle. Notice the orientation of kinocilium.
- Hair cells with kinocilium oriented away from the striola are inhibited when bent while those who have kinocilium oriented towards the striola are excited

Front 12

Briefly explain how the hair cells in the semicircular canals detect angular acceleration

• What type of fluid are the canals filled with and how does it respond to movement?
• Where are the hair cell cilia inserted?
• How does the fluid move w/ respect to the direction of head rotation?

Back 12

• The semicircular canals are filled with endolymph.

• When the head turns, the endolymph, due to inertia, initially delays moving through the canal

• As the endolymph moves, it bends the cupula which contains the cilia, in the direction opposite to movement.

Front 13

The Essence of Vestibular Pathways:
• Vestibular nuclei
- location
- its input pathways (3)
- its output pathways (3)

Back 13

The Essence of Vestibular Pathways:
• Vestibular nuclei
- floor of the 4th ventricle, in the upper portions of the medulla and lower pons

- input pathways: Vestibular receptor organs, vestibular ganglia, vestibular portion of CN VIII

- output pathways: cerebellum, extraocular motor neuronsand motor neurons of the skeleto-motor via descending fiber tracts system

Front 14

Vestibulo-Ocular reflex

• The goal of the reflex
• Three general steps
• The visual input needed

Back 14

• Goal: To move the eyes in such a way that images are stabilized on the retina during head movement - conjugate eye movements in the direction opposite to head movement

• Three general steps:

1. Excitation & Inhibition of labyrinthine receptors through head rotation

2. The excited labyrinthine fibers form the afferent limb to iNN the vestibular nuclei

3. The fibers of the vestibular nuclei activate the efferent limb of the reflex to move the eyes relative to the head so images are steady on the retina

• The reflex needs NO visual input and can be carried out in darkness

Front 15

VOR - Excitation/Inhibition of Labyrinthine receptor organs:

• The direction of endolymph in horizontal semicircular canals when the head is rotated to the left

• the direction the cupula moves due to endolymph
- what direction is kinocilium moving?
- is this excitatory or inhibitory?

Back 15

VOR - Excitation/Inhibition of Labyrinthine receptor organs:

• The direction of endolymph iis to the right in both canals

• the direction the cupula movement
- left canal: the cupula and kinocilium move towards the back of the head. This is excitatory.

- right canal: the cupula and kinocilium move towards the anterior of the head. This is inhibitory.

Front 16

VOR - The Excited Labyrinth fibers

• Enter the brain stem at this landmark and synapse at this nucleus

• Fibers from the above nucleus travel and synapse at this 2nd nucleus

• Name and explain the final pathway that results in the movement of the eyes

Back 16

• Ponto-medullary jxn, synapsing in the Vestibular nucleus

• Fibers from the Vestibular nucleus cross the midline, ascend a short distance and synapse on the contralateral Abducens nucleus

• From the Abducens nucleus, The Common Final Pathway for horizontal eye movements is followed
- A 1st set of fibers from the abducens nucleus iNN the lateral rectus

- A 2nd set of fibers immediately crosses the midline and ascends up the contralateral MLF to synapse at the oculomotor nucleus. Fibers from this nucleus iNN the medial rectus

Front 17

Two types of nystagmus and their etiology

Back 17

Physiological nystagmus: can be induced in a healthy indivisual

Pathological nystagmus: caused by a defect in the mechanisms or structures controlling eye movements

Front 18

Vestibulo-ocular nystagmus
• type of nystagmus
• How the slow and fast phases move with the head and when the mechanism switches btwn phases

Back 18

Vestibulo-ocular nystagmus
• physiological
• How the slow and fast phases move with the head
- slow phase will move in the direction opposite to the head
- the fast phase is in the direction of head movement
- the mechanism resets when the eyes have reached their maximum deviation from straight forward gaze

Front 19

Two neurological examinations of brainstem fxn

Back 19

Oculocephalic Maneuver
Caloric testing of the Vestibulo-Ocular reflex

Front 20

Oculocephalic Maneuver
• Another term
• Is used to determine if the VOR pathway btwn these two structures is in tact
• One sentence describing the basic procedure

Back 20

Oculocephalic Maneuver
• Doll eyes maneuver
• medulla and midbrain
• The examiner rotates the head of a comatose patient in one plane to note whether the ocular excursions in the opposite direction occur

Front 21

Caloric Testing of the VOR:

• What is being tested?

• Briefly explain the mechanism of testing and the difference behind the results of warm water and cold water testing

Back 21

• Conjugate deviation of the eyes as driven by the VOR

• The outer ear canal of a patient is irrigated with cold (or warm) water

- For warm water, endolymph in the ipsilateral horizontal canal rises, causing an increased rate of firing in the vestibular afferent nerve.
- This situation mimics a head turn to the ipsilateral side.
- Result: Both eyes will turn toward the contralateral ear, with horizontal nystagmus to the ipsilateral ear.

• If the water is cold, the endolymph falls within the semicircular canal, decreasing the rate of vestibular afferent firing.
- Result: The eyes then turn toward the ipsilateral ear, with horizontal nystagmus (quick horizontal eye movements) to the contralateral ear.

Front 22

Meniere's Disease
• Etiology
• Systems affected
• Clinical presentation (3)

Back 22

Meniere's Disease
• Abnormalities of endolymph circulation
• Vestibular and auditory system due to dilation of endolymph compartments and degeneration of hair cells

• Sudden and recurrent attacks of vertigo, tinnitus and sensorineural hearing loss

Front 23

Motion Sickness:
• Another term
• Occurs when there is a discrepancy btwn these inputs
• This apparatus is affected

Back 23

Motion Sickness:
• Kinetosis
• Discrepancy btwn vestibular and visual inputs
• Vestibular apparatus is affected

Front 24

Why alcohol intoxication produces vestibular symptoms

Back 24

Alcohol from the blood interacts with the endolymph, causing a convection endolymph flow within the canal and thus perceived movement of the head (Bed spinning).

Front 25

How antibiotics damage the vestibular system

one example

Back 25

Abs accumulate in the endolymph, damaging the system

ex. Streptomycin

Front 26

Pathological vestibular nystagmus:
• Possible etiology
• Where the slow and fast components are directed towards
• Possible sites of the lesion (2)

Back 26

Pathological vestibular nystagmus:

• Damage to the vestibular system on one side

• Slow - towards the site of the damaged labyrinth; Fast - towards the side of the fxnal labyrinth

• Semi-circular canal receptor organs or vestibular portion of CN VIII