#14

Question 1

. Most of the functions
of the vestibular system are related to important reflexes that

Answer 1

stabilize gaze and the axial skeleton

Question 2

The vestibular system detects head motion
Input:
Output:

Answer 2

detection of linear and angular acceleration of the head in 3D space

reflexive control of eye position and axial posture

Question 3

4 components of vestibular system

Answer 3

1. Peripheral apparatus (static and kinetic labyrinths)
2. Vestibular (Scarpa’s) ganglion and nerve (part of CN VIII)
3. Vestibular nuclei complex (MVN, LVN, SVN, IVN) in brainstem
4. Outputs to cerebellum, spinal cord and brainstem motor neurons, and cortex

Question 4

In the hair cells of the vestibular system, stereocilia are
arranged in ranks of increasing length, headed by a
single tall

Answer 4

kinocilium

Question 5

• Vestibular hair cells are X when
  stereocilia are deflected toward the kinocilium.
• Vestibular hair cells are Y when stereocilia are deflected away from the kinocilium.

Answer 5

x depolarized
y hyperpolarized

Question 6

The static labyrinth, comprised of the X, detects Y. The major component is the:

Answer 6

x utricle and saccule
y linear accelerations
experienced by the head.
Otolithic maculae

Question 7

Otolithic maculae

Answer 7

Located within the membranous vestibule
(surrounded by endolymph), these vesicles each possess a small patch of hair cells
(maculae) whose cilia are embedded in a
gelatinous matrix (otolithic membrane)
capped with calcium carbonate crystals
(otoconia, lit. “ear dust”)

Question 8

the maculae detect

Answer 8

static head position relative to gravity (gravito-inertial
acceleration) as well as changes in position (linear acceleration)

Question 9

The kinetic labyrinth detects X, comprised of Y which arise from the utricular portion of the membranous vestibule and are filled
with Z.

Answer 9

x angular acceleration
y three semicircular
ducts
z endolymph

Question 10

The kinetic labyrinth detects angular acceleration experienced by the head when it
moves. How?

Answer 10

• At the base of each semicircular duct is a raised, dilated region called the ampulla.
  The ampulla contains a patch of hair cells (crista) whose cilia project into a gelatinous
  matrix (cupula) that spans the cross-sectional area of the ampulla and therefore
  interrupts the endolymphatic space.
• When the head experiences
  an angular acceleration,
  endolymph inside the
  semicircular ducts lags
  behind due to its inertia. This deflects the cupula, causing excitation or
  inhibition of afferent
  neurons in the vestibular nerve, depending on the
  direction of the movement.
• All hair cells in a given ampulla are oriented in the same direction, making each duct
  maximally sensitive to angular acceleration experienced in its preferred plane.
• Because the semicircular ducts are mutually orthogonal and bilaterally symmetrical,
  ampullary hair cells in all six ducts can accurately detect angular accelerations about three perpendicular axes (pitch, yaw, roll).

Question 11

Excitation of afferent neurons in one semicircular duct (causing an increase in firing rate of the VIIIth cranial nerve) is matched by

Answer 11

inhibition (decrease in
firing) of its contralateral pair.
In operation, the semicircular duct system is the biological equivalent of the gyroscope used to detect attitude (angle of bank and pitch up or down) in modern airplanes.

Question 12

Hair cells are innervated by peripheral processes of the bipolar cells of the X located in the Y.

Answer 12

x vestibular (or
Scarpa’s) ganglion
y internal acoustic meatus

Question 13

The central processes of vestibular ganglion cells form the X and enter the pontomedullary junction with the cochlear and facial
nerves. These axons mainly synapse on nuclei in the y

Answer 13

x vestibular component of cranial nerve VIII
y vestibular nuclear complex.

Question 14

There are four main vestibular nuclei:

Answer 14

a) lateral vestibular nucleus
b) medial vestibular nucleus
c) superior vestibular nucleus
d) inferior vestibular nucleus

Question 15

Main outputs of the vestibular nuclei are to the spinal cord (X), oculomotor system (via Y), and to the cerebellum. Minor
outputs travel via the thalamus to the parietal cortex, where balance sense reaches
conscious perception.

Answer 15

x medial and lateral
vestibulospinal tracts
y MLF)

Question 16

Ultimately, vestibular information is used to drive reflexes associated with

Answer 16

eye position, neck position, and limb and body position

Question 17

There are three main vestibular-mediated reflexes

Answer 17

• VSR (Vestibulo-spinal reflexes): act to stabilize the limbs and body when perturbed
• VCR (Vestibulo-collic reflexes): act on the neck musculature to stabilize the head
• VOR (Vestibulo-ocular reflexes): act to maintain stable vision during head motion

Question 18

Functionally, the two most important outputs of the vestibular nuclear complex are to the

Answer 18

oculomotor system (vestibuloocular system) and to the spinal cord (vestibulospinal system)

Question 19

The medial longitudinal
fasciculus is a midline fiber bundle that connects

Answer 19

CN III, IV,
and VI nuclei with themselves
and with the vestibular nuclei inferiorly

Question 20

The MLF is an
important tract for

Answer 20

“yoking” together the nuclei that drive
movements of the eyeballs

Question 21

The VOR is a three neuron reflex arc,
involving

Answer 21

a primary afferent neuron, a vestibular nuclear neuron, and an oculomotor (III,
IV and VI) motoneuron.

Question 22

During a head turn in VOR, the superior vestibular nucleus sends inhibitory projections (via X) to oculomotor motoneurons while the medial vestibular
nucleus sends crossed excitatory fibers (again via Y).

Answer 22

x the medial longitudinal fasciculus
y MLF

Question 23

for a left lateral head turn, input from the left lateral
semicircular duct will excite the X lateral rectus and Y rectus, and inhibit the Y lateral rectus and X medial rectus

Answer 23

x contralateral
y ipsilateral medial

Question 24

The flocculus of the cerebellum connects to the superior vestibular nucleus and inhibits the vestibuloocular reflex, permitting

Answer 24

smooth pursuit: eyes remain fixated on
a moving visual target, even though the head remains still.

Question 25

doll’s head maneuver

Answer 25

Presence or absence of vestibulo-ocular reflex can be used in comatose patients to determine whether the brainstem is intact. If the head of a comatose patient is moved from side to side, the eyes should move conjugately in the opposite direction if the circuit is intact. This is the so-called oculocephalic or doll’s head maneuver

Question 26

medial vestibulospinal tract

Answer 26

The medial vestibulospinal tract arises mostly from the medial vestibular nucleus on both sides (i.e., each tract contains crossed and uncrossed fibers). * Fibers descend bilaterally in the midline, supplying medial (axial) lower motor neurons in the ventral horn of the cervical and upper thoracic spinal cord. * Main function is to adjust position of the head in response to changes in posture (head stabilization via VCR)

Question 27

lateral vestibulospinal tract

Answer 27

The lateral vestibulospinal tract arises mostly from the lateral vestibular nucleus on the same side (hence this is an uncrossed tract). * Descends the full length of the spinal cord, innervating lower motor neurons in both medial (majority) and lateral (small number) groups in the ventral horn. * Activation of this pathway facilitates axial extensor muscles and inhibits axial and appendicular flexors. * Primary function is to maintain upright and balanced posture in response to inputs from the static and kinetic labyrinths (via VSR).

Question 28

Outputs of vestibular nuclei to cerebellum

Answer 28

The strong connections between the vestibular nuclei and the cerebellum do not constitute a separate output system, rather they provide fine control over postural adjustments and eye movements. Note that damage to the anterior cerebellar lobe (spinocerebellum) and flocculonodular lobe (vestibulocerebellum) lead to clinical signs similar to those found after damage to the vestibular system. We will look at vestibular inputs to the cerebellum in more detail when we discuss the motor system.

Question 29

Outputs of medial vestibular nucleus to the thalamus

Answer 29

Fibers from the medial vestibular nucleus project bilaterally alongside MLF (or possibly in the lateral lemniscus –unclear) to the ventral posterior intermediate nucleus (VPi) of the thalamus. VPi projects diffusely to parietal and temporal cortex. This allows for the conscious experience of “balance sense.” Given the diffuse arrangement of cortical inputs, the vestibular sense is weak compared to other sensory systems, and we’re generally only aware of it when we feel out-of-balance. One of the targets of VPi projections is an area of the parietal lobe where the intraparietal sulcus meets the postcentral gyrus, which is close to the somatic sensory representation of the head. This probably explains why we experience dizziness “in our head.”

Question 30

Clinical signs associated with damage to vestibular system: VANN

Answer 30

Vertigo: a sensation of motion when one is stationary; a type of “dizziness.” Ataxia: truncal ataxia (body position is difficult to maintain). Nystagmus: Rapid, oscillating conjugate eye movements, generally horizontal. Nausea and vomiting. Other autonomic signs may include sweating, excessive salivation, et al.

Question 31

Benign paroxysmal positional vertigo (BPPV)

Answer 31

* Benign – not life-threatening * Paroxysmal – comes in sudden, brief spells * Positional – triggered by certain head positions or movements specific to the directional sensitivity of the effected canal * Vertigo – false sense of rotational movement

Question 32

There are two forms of BPPV

Answer 32

Canalithiasis: * Most common * Occurs when otoconia are moving within the semicircular duct * Vertigo and nystagmus resolves within 60 sec. Cupulolithiasis: * Less common * Occurs when otoconia adhere to the cupula * Vertigo and nystagmus persist for longer periods

Question 33

Canalith repositioning

Answer 33

An effective physical therapy intervention for BPPV involves a series of specifically patterned head and trunk movements while observing the effect on eye position, termed “canalith repositioning,” with the goal being to flush the displaced otoconia back into the utricle (where it won’t cause vertigo), where the stones typically re-adhere to the otolithic membrane.

Question 34

The ear structures are supplied by the X, typically a small branch of Y.

Answer 34

x labyrinthine artery y AICA

Question 35

Effects of Alcohol

Answer 35

Alcohol makes blood less dense (due to its lower specific gravity). Recall that alcohol has high lipid solubility and easily diffuses into brain tissue. Blood alcohol diffuses more rapidly into semicircular duct cupulae (via small capillaries) than into the endolymph (via stria vascularis). As a result, during intoxication the cupula becomes less dense than the surrounding endolymph. The lower-density cupula now floats relative to the endolymph, producing hair cell deflections and subsequent dizziness (“the spins”). Assuming one stops consuming alcohol, eventually the density of the endolymph and cupulae normalize (the “honeymoon period” between intoxication and hangover), but then as blood alcohol levels fall during liver metabolism, alcohol is eliminated faster from the blood (supplying the cupula) than the endolymph, so the “heavier” cupulae now sink relative to the endolymph (inducing “hangover spins” experienced in the opposite direction). Incidentally, horizontal gaze nystagmus (HGN) can be evoked in most individuals with Blood Alcohol Levels between 0.08% and 0.10%, a major reason HGN tests remain popular for evaluating field sobriety in law enforcement, and one of the reasons .08% blood alcohol concentration was established as the legal limit for driving under the influence in the first place. Before breathalyzers came along, it was the best test going