Vestibular System

Question 1

What is the vestibular apparatus?

Answer 1

Specialized set of receptors in the non-auditory part of the inner ear, consisting of utricle, saccule, and three semi-circular ducts.

Question 2

What type of information does the vestibular system primarily deal with?

Answer 2

Unconscious proprioception. Vestibular nuclear complex integrates this information, along with vision and conscious proprioception, to orient the system to space.

Question 3

What are the three main functions of the motor vestibular system?

Answer 3

1. Maintain muscle tone in extensors -> fight gravity
2. Maintain equilibrium
3. Produce compensatory eye movements in response to head movements

Question 4

What are the orientation of utricle and saccule, and what are their sensory parts called? What is this covered with?

Answer 4

Utricle = sac next to semicircular ducts, detecting horizontal linear acceleration

Saccule = sac next to cochlea, detecting vertical horizontal acceleration

Sensory part called macula, covered with otoliths

Question 5

What are the three planes of the semicircular ducts?

Answer 5

Oriented 90 degrees apart from eachother, there are two perpendicular to the ground (anterior and posterior), and one horizontal to the ground (horizontal aka lateral).

Question 6

What is the orientation of the “horizontal” semicircular duct with respect to the ground, and what corresponds to depolarization of its hair cells?

Answer 6

Horizontal is 30 degrees above true horizontal, with the assumption that you will look down 30 degrees with your head to watch your feet as you walk.

Depolarization = movement of hair cells towards utricle, which occurs when the head is rotating in the opposite direction.

Question 7

What is the sensory apparatus of the semicircular ducts called?

Answer 7

Crista ampullaris, each of which is covered by a gelatinous cupula.

Question 8

The anterior duct on the right side is in the same plane as what?

Answer 8

The posterior duct on the left side

Question 9

What is the name of the vestibular ganglion? Where does the vestibular nerve enter the brainstem?

Answer 9

Scarpa’s ganglion. Enters the brainstem at the cerebellopontine angle, distributing to vestiular nuclei and cerebellum

Question 10

What is the vestibular nuclear complex and where is it located?

Answer 10

Composed of 4 nuclei, located at border of rostral medulla and caudal pons. It is the center of integration for the entire vestibular system.

Semicircular ducts empty onto superior/medial nuclei.
Maculae empty onto lateral nucleus.

Question 11

Which deep cerebellar nucleus is involved in the vestibular system and where is it located?

Answer 11

Fastigial nucleus - located most medially

Question 12

What three areas of the cerebellar cortex are involved in the vestibular system? What are they called?

Answer 12

Floccus (laterally), nodulus (part of vermis), uvula (posterior vermis). Collectively called vestibulocerebellum

Floccus + nodulus were ancestral and originally called “archicerebullum”

Question 13

What are the direction projections of the vestibular nerve?

Answer 13

Ipsilateral to vestibulocerebellum and vestribular nuclear complex.

does NOT project to fastigial nucleus

Question 14

Where does the vestibular nuclear complex project?

Answer 14

Ipsilaterally to vestibulocerebellum as well as festigial nucleus

Question 15

What are the inputs of the cerebellum back on the vestibular nuclear complex? What is one additional one?

Answer 15

1. Vestibulocerebellum - most caudal, ipsilateral input, inhibitory
2. Spinocerebellum - most rostral, ipsilateral, inhibitory
3. Fastigial nucleus - bilateral, excitatory.

Additionally:
Reticular formation sends fibers to vestibular nuclear complex

Question 16

What gives input to the fastigial nucleus?

Answer 16

1. Vestibular nuclear complex
2. Spinocerebellum (anterior lobe vermis)
3. Vestibulocerebellum

Question 17

What are the two vestibular efferents to the spinal cord? Where do they arise from?

Answer 17

1. Lateral vestibulospinal tract - LVST - arises from lateral nucleus, so is more responsible for responding to linear acceleration (falling changes)
2. Medial vestibulospinal tract - MVST - arises from medial nucleus, responsible for reflex head/neck movements in response to stimuli

Question 18

What is the path of LVST? What is its job?

Answer 18

Entirely ipsilateral from vestibular nuclear complex, acts on medial parts of ventral horn, with alpha motor neurons to extensors of axial muscles and proximal extremities, as well as gamma motor neurons to regulate tone.

Question 19

What is the path of MVST? What is its job?

Answer 19

Ascends from one nuclear complex bilaterally through the medial longitudinal fasciculi (MLF) on both sides. Becomes ipsilateral in caudal medulla, distributes to cervical / upper thoracic segments (T1/T2). Important for head and neck movements of cervical region. Can be found in white matter of anterior funiculus.

Question 20

What nuclei does the vestibular nuclear complex project to for the eyes? Are these ipsilateral, contralateral, or bilateral?

Answer 20

oculomotor (bilateral), trochlear (bilateral), and abducens (contralateral) nerves. This is for compensatory eye movement for head turning.

Mainly projects from medial and superior nuclei of the complex (semicircular canals, for head movement), thru MLF

Question 21

How does compensatory eye movement work when the head is turning to the right?

Answer 21

1. Horizontal duct on right side will be excited, as fluid moves forwards when head moves backwards.
2. Vestibular nucleus will be excited on right side from stimulation via vestibular nerve
3. Vestibular nucleus excites contralateral abducens nerve (lateral rectus), left eye turns to the left.
4. Vestibular nucleus excites ipsilateral oculomotor nerve, inhibits contralateral oculomotor nerve, Right eye turns to the left (medial rectus)

Question 22

What are the minor contributions of the vestibular nuclear complex to efferent control besides eyes and spinal cord?

Answer 22

1. Reticular formation (reticulospinal tract)
2. Vestibular periphery - excitatory neurons to cristae and maculae
3. Thalamus - to VPL, signalling vestibular irritation to the somatosensory cortex eventually (cause of vertigo)

Question 23

What causes benign paroxysmal positional vertigo?

Answer 23

Under age 50, it’s head injury with concussive force that displaces the otoliths.

Question 24

What causes labyrinthitis or vestibular neuritis?

Answer 24

Viral infection

Question 25

What causes Meniere’s disease?

Answer 25

Increased fluid pressure in semicircular ducts, causing spontaneous episodes of vertigo and fluctuating hearing loss with tinnitus.

Question 26

How is nystagmus clinically defined?

Answer 26

In the movements of the eye, there is a slow deviation away from center, than a fast reset back to midline. The nystagmus is defined by the direction of the fast reset. If it occurs spontaneously, there is a disease process due to imbalance of vestibular sources.

Question 27

What is a rotation test?

Answer 27

Put a person in a chair, tilt head forward 30 degrees. Spin 10-12 times. During the spin, in a healthy person, the nystagmus will be in the same direction as the spin (eye will compensate for rotation, then snap back in the same direction as the spin)

Question 28

What is post-rotary nystagmus?

Answer 28

If you abruptly stop a rotation test, the nystagmus will occur in the opposite direction due to increased resistance of endolymph movement and feeling of acceleration in opposite direction.

Question 29

How does the caloric or thermal test work? How do you remember this?

Answer 29

Tilt the head back 60 degrees to bring the horizontal semicircular duct in the vertical plane. Then, warm or cold water will be put into one ear to induce an endolymphatic flow because of convection.

Warm = fluid rises, exciting the vestibular nerve fibers on that side. Thus, the eyes will move away slowly, and snap back to midline towards the side with water put in it. Thus, nystagmus = same side.

Remember: Cold Opposite, Warm Same = COWS

Question 30

What is the difference in nystagmus between a vestibular nuclear complex lesion that is irritative vs destructive?

Answer 30

Irritative - overstimulation of horizontal duct -> nystagmus is to same side
Destructive - understimulation of understimulation of horizontal duct -> nystagmus is to opposite side.