Vestibular System

Question 1

Where are the organs of balance located?

Answer 1

The organs of balance are located in the inner ear
- Recall that the inner ear consists of
interconnecting fluid-filled tunnels and chambers in the temporal bone.

Question 2

What are the 2 structures of the vestibular labyrinth?
Functions?

Answer 2

1. the otolith organs (utricle & saccule)
   which detect the force of gravity and tilts of the head
2. the semicircular canals which are sensitive to head rotation

Question 3

Describe the inner membranous labyrinth?

Answer 3

Filled with a special extracellular fluid (ECF) called endolymph
- that bathes the apical cellular surfaces
- Endolymph is rich in K+ but relatively poor in Na+ and Ca2+

Question 4

Describe the outer bone labyrinth?

Answer 4

• Is a layer of laminar bone that surrounds the membranous labyrinth,
  separating it from the spongy bone of the skull
• normal ECF called perilymph is found between the bony and membranous labyrinth

Question 5

What is static equilibrium?

Answer 5

• Maintenance of the body position (head) relative to gravity
• Stimulated by tilting of the head, linear acceleration/deceleration
  e.g. Elevator, car speeding up/slowing down

Question 6

What is dynamic equilibrium?

Answer 6

1. Maintenance of the body position relative to rotational acceleration/deceleration
2. which way our head is moving in three-dimensional space

Question 7

What are the 5 receptor organs housed in each of the 2 vestibular labyrinths?

Answer 7

1. hair cells in the utricle static equilibrium
2. hair cells in the saccule
   - static equilibrium
3. hair cells in the anterior vertical semicircular canal dynamic
4. hair cells in the horizontal semicircular canal
5. hair cells in the posterior vertical semicircular canal
   - dynamic equilibrium
   Note: The displacement of hair cells due to the forces of gravity and inertia transduces mechanical stimuli into receptor potentials

Question 8

What is the macula?
Where is it?

Answer 8

Each otolith organ contains a sensory epithelium called a macula,
- vertically oriented within the saccule
- horizontally oriented within the utricle
> when the head is upright

Question 9

Histology of the macula?

Answer 9

1. hair cells, which lie among a bed of supporting cells with their cilia projecting into a gelatinous cap
   - 30,000 in the utricle, 6,000 in the saccule
2. Otoconia (calcium carbonate crystals) encrust the surface of the macula’s gelatinous cap
   - the key to the tilt sensitivity of the macula.
   - have a higher density than the endolymph that surrounds them

Question 10

How do the otolith organs work?

Answer 10

1. When you tilt your head, the angle between your otolith organs and the direction of the force of gravity changes
2. During linear acceleration, the membranous labyrinth moves as well because it is fixed to the skull
3. Due to its inertia, the free-to-move otoconial mass lags behind the movement of the head
4. Movement of the otoconia is transferred to the otolithic membrane, which then shifts with respect to the underlying epithelium
5. This bends the hair cells and initiates action potentials in 1st order sensory neurons

Question 11

Describe the vestibular receptor organs in the semicircular canals?

Answer 11

• The hair cells of the semicircular canals are clustered within a sheet of cells, the crista, located within a bulge at the base of the canal called the ampulla
• The cilia project into the gelatinous cupula, which spans the lumen of the canal within the ampulla.
• All the hair cells in an ampulla have their kinocilia oriented in the same direction
  > they all get excited or inhibited together

Question 12

How do the vestibular receptor organs of the semicircular canal work?

Answer 12

1. Rotation of the canal along its axis bows the cupula
2. This bends the cilia, which (depending on the direction of the rotation) either excites or inhibits the release of neurotransmitter from the hair cells onto the vestibular nerve axons

Question 13

Why do we get dizzy and stagger when we stop spinning?

Answer 13

• During prolonged head rotation, the friction of endolymph with the canal walls eventually makes the two move together, thereby reducing and then eliminating the bending of the cupula after 15–30 seconds
• When rotation is stopped, the inertia of the endolymph causes the cupula to bend in the other direction, generating an opposite response from the hair cells and a temporary sensation of counter- rotation.
  Note: Dancers use spotting: staring at a fixed point while the body is turning to prevent dizziness and loss of balance

Question 14

How do the visual and vestibular systems work together?

Answer 14

Visual and vestibular systems involve keeping the head in a constant position that is upright and on an even horizontal plane
When the head tilts too much to one side, the visual system senses the horizontal shift, and the vestibular system signals the brain to force the body to right itself
Simply turning the head while spotting eliminates any tilting motions.
Waiting to quickly rotate the head back to the focal point until the last possible moment will also ensure that there is as little movement as possible.
By adding these two concepts to the idea of staring at a focal point, dancers can improve their turning technique immensely

Question 15

Describe the vestibular signal transduction - in depolarization of the cell?

Answer 15

1. A stimulus (acceleration or rotation) that displaces the stereocilia towards the kinocilium (longest stereocilium) stretches the tip link
2. This pulls the mechanically-gated ion channel open, allowing an influx of K+, depolarizing the hair cell
3. Depolarization activates voltage-gated calcium channels
4. The entry of Ca2+ triggers the release of the neurotransmitter glutamate, which activates the Scarpa`s ganglion fibers lying postsynaptic to the hair cell

Question 16

Describe the vestibular signal transduction - is hyperpolarization of the cell?

Answer 16

1. A stimulus (acceleration or rotation) that displaces the stereocilia away from the kinocilium
2. This reduces the stretch on the tip link, keeping the mechanically-gated ion channel closed, preventing an influx of K+, hyperpolarizing the hair cell

Question 17

What are the 1st order sensory neurons of the vestibular pathways?

Answer 17

• Cell bodies located in the vestibular ganglion
• About 20,000 axons join to form vestibular nerve, which joins cochlear nerve to form vestibulocochlear nerve
• Vestibular nerve portion projects to the ipsilateral complex of four major vestibular nuclei in the dorsal part of the pons and medulla

Question 18

What are the 2n order sensory neurons of the vestibular pathway?

Answer 18

Integrate signals from vestibular organs with those from
1. spinal cord
2. cerebellum
3. visual system

Question 19

What are the 3rd order sensory neurons of the vestibular pathway?

Answer 19

Located in:
1. the ventral nuclei of the thalamus
2. oculomotor nuclei
3. reticular centers occupied with skeletal movement
4. spinal centers occupied with skeletal movement
5. cerebellum

Question 20

What is the vestibulo-ocular reflex?

Answer 20

The VOR works by sensing rotations of the head and immediately commanding a compensatory movement of the eyes in the opposite direction.
The movement helps keep your line of sight tightly fixed on a visual target.
Because the VOR is a reflex triggered by vestibular input rather than visual input, it works amazingly well even in the dark or when your eyes are closed

Question 21

What happens when the head turns to the left and the VOR induces both eyes to turn right?

Answer 21

1. Axons from the left horizontal canal innervate the left vestibular nucleus, which sends excitatory axons to the contralateral (right) cranial nerve VI nucleus (abducens nucleus).
2. Motor axons from the abducens nucleus in turn excite the lateral rectus muscle of the right eye
3. Another excitatory projection from the abducens crosses the midline, back to the left side, and ascends (via the medial longitudinal fasciculus) to excite the left cranial nerve III nucleus (oculomotor nucleus), which excites the medial rectus muscle of the left eye.
4. To further ensure speedy operation, the left medial rectus muscle also gets excited via a projection from the vestibular nucleus directly to the left oculomotor nucleus
5. Speed is also maximized by activating inhibitory connections to the muscles that oppose this movement
   the left eye’s lateral rectus and right eye’s medial rectus, in this case

Question 22

What is Acoustic neuroma/ vestibula Schwannoma?

Answer 22

• noncancerous, usually slow growing tumours that form along the branches of the vestibulocochlear nerve
• Arise from Schwann cells

Question 23

What is Barotrauma?

Answer 23

• Occurs with a blocked or inflamed Eustachian tube
• Air pressure builds up in the space behind thr ear drum, causing the pressure in your middle ear to become unbalanced.
• The air pressure difference can make your ears hurt

Question 24

What is Benign paroxysmal positional vertigo (BPPV)?

Answer 24

1. Definition
   - caused by crystals dislodged from the utricle of the inner ear
2. Causes
   - head injury, infection, advanced age
3. Symptoms
   - dizziness and vertigo is triggered by particular positions

Question 25

What is Labyrinthitis or vestibular neuritis?

Answer 25

Infection that causes inflammation of the labyrinth/vestibule

Question 26

What is Mal de débarquement aka Disembarkment or Debarkation Syndrome?

Answer 26

• Rare disorder that leaves patients feeling as if they are rocking and swaying
• Triggered by exposure to passive motion (i.e. boat, airplane, automobile, train, etc.
• symptoms tend to be exacerbated when a patient is not moving

Question 27

What is Ménière’s disease?

Answer 27

Causes sudden attacks of vertigo, dizziness, tinnitus, pressure in the ear, hearing loss

Question 28

What is Migraine-associated dizziness/ vestibular migraine?

Answer 28

a type of migraine where people experience a combination of vertigo, dizziness or balance problems with other migraine symptoms

Question 29

What is orthostatic hypotension?

Answer 29

• a form of low blood pressure that happens when moving from a laying down (supine) position to a standing (upright) position
• can cause light-headedness, dizziness , fainting

Question 30

What is ototoxicity?

Answer 30

Ear poisoning from medicine e.g. quinine, streptomycin

Question 31

What is perilymph fistula?

Answer 31

1. Cause
   • a tear or defect in one or both of the small, thin membranes (the oval window and the round window) that separate the air-filled middle ear and the fluid-filled perilymphatic space of the inner ear.
2. Consequences
   • perilymph in the inner ear escapes, driven by the hydrostatic pressure of the CSF, and is replaced by CSF
   • This can also result in lower than normal levels of CSF fluid around the brain and spinal cord, which may result in symptoms such as mild headache

Question 32

What is Superior canal dehiscence syndrome?

Answer 32

• caused by an abnormal thinness or incomplete closure of one of the bony canals in the inner ear
• Congenital, trauma, infection
• Sound vibrations escape the normally closed system of the inner ear and activate the hair cells that sense rotation in the superior canal
  > A loud noise e.g. cough, sneeze can cause a sensation of vertigo
• Autophony (“hearing one’s self”): patients may hear sound from inside the body e.g. breathing, heartbeat and blood flow
• When patients speak, they may hear their voice simultaneously from inside and outside of the body, which can be overwhelmingly loud