Week 8 Auditory and Vestibular Systems

Question 1

Give an overview of the central auditory pathway.

Answer 1

• sound waves creates a wave the travels down scala vestibule. Displaces basement membrane and activates hair cell in organ of corgi. Hair cell movements generate action potentials in CN VIII
• signal from cochlea to the CNS
• auditory system analyzes frequency, volume, location of sound
• neuron chain is formed to the auditory cortex in temporal lobe

Question 2

What is neuron 1 of the auditory pathway?

Answer 2

• Cell bodies of bipolar neurons of cochlear ganglion in inner ear
• dendrite contact hair cells in organ of corti, axons form cochlear part of CN VIII
• CN VIII travels through petrous part of temporal bone, enters through internal acoustic meatus
• to brain stem at pontomedullary junction at cerebellopontine angle lateral to CN VII

Question 3

Where are the nuclei of neuron 2 of the auditory pathway?

Answer 3

• in dorsal and ventral cochlear nuclei in the medulla at the pontomedullary junction
• nuclei are wrapped around ICP

Question 4

What is the path of axons of neuron 2 in the auditory pathway?

Answer 4

• axons decussate in lower pons to contralateral side at trapezoid body
• ascent in lateral lemniscus (LL) to inferior colliculus in caudal midbrain
• many nuclei along pathway that contributes to LL axons
• multisynaptic and bilateral system
• LL synapses in inferior colliculus and ascends to medial geniculate nuc. of thalamus (MGN) bilaterally

Question 5

After axons synapses at MGN of thalamus, what pathway does it take in the auditory pathway?

Answer 5

• pass through sublentiform part of posterior limb of internal capsule and end in primary auditory cortex (Hershel’s gyrus)
• each auditory cortex receives from both ears, but input from contralateral side is greater

Question 6

What would result from a unilateral lesion in the auditory pathway after the cochlear nuclei?

Answer 6

• unilateral lesions after the cochlear nuclei don’t result in significant hearing loss, some diminution of hearing in contralateral ear
• most hearing problems most likely occur in external, middle, inner ear and to auditory nerve itself (hearing problem is ipsilateral to lesion)

Question 7

What are examples of conduction deafness and sensorineural deafness?

Answer 7

• conduction: wax, ruptured tympanic membrane, fluid in middle ear, otosclerosis
• sensorineural deafness: damage to organ of corgi, auditory nerve along its course until it enters brainstem. E.g. vestibular schwannoma compressing VIII nerve, aging, drugs

Question 8

What is neuron 1 in the vestibular pathway?

Answer 8

• cell bodies of bipolar neurons of vestibular ganglion

- dendrites contact vestibular receptors in membranous labyrinth and axons converge to form vestibular part of VIII

Question 9

What is neuron 2 in the vestibular pathway?

Answer 9

• most axons synapse in vestibular nuclei, but some enter vestibulocerebellum via ICP
• vestibular nuclear compex occupies area of floor of IV ventricle at lateral angle, partly in medulla, and partly in pons

Question 10

How do axons of the vestibular nuclei connect with nuclei of EOM, LMNs, and vestibulocerebellum?

Answer 10

• medial longitudinal fasciculus (MLF) to control eye movements
• lateral and medial vestibulospinal tracts to LMNs: from vestibular nuclei synapse on LMNs to help maintain balance during changes in head position and movements by adjusting tone in neck, trunk and limb muscles. (lateral-all levels of cord, medial-cervical cord only)
• ICP to vestibulocerebellum: bidirection axons from vestibular nuclei to flocculonodular lobe to coordinate eye movements & balance in response to changes in head position and movement

Question 11

Describe the vestibulo-ocular reflex (VOR) that occurs when turning head to left.

Answer 11

-turning head to left stimulates left vestibular nuclei (ipsilateral)
-axons from left nuclei synapses on right VI nucleus
–>activates lateral rectus on right side
–>VI via MLF activates left CN III nucleus to contract medial rectus
(part of PPRF pathway)
-result: slow conjugate eye to right in response to head rotation to left
(normal individuals: cortex suppresses VOR)

Question 12

What will result from caloric stimulation with cold water to the right ear?

Answer 12

Slow: eyes to same side as cold water, fast: eyes to opposite side

• pushes endolymph away from ampulla, decreasing firing rate
• activates vestibular nuclei on left (opposite side)
• left nuclei activates right CN VI: lateral rectus, and left CN III: medial rectus
• slow movement to right
• fast movement (nystagmus) to left

Question 13

What will result from caloric stimulation with warm water to the left ear?

Answer 13

slow: eyes move to opposite side as warm water, fast: eyes move to same side
- pushes endolymph towards ampulla, and increases firing rate
- activates vestibular nuclei on same side, which activates opposite CN VI nuclei (right)
- slow movement to right
- fast movement to the left

Question 14

What are 3 types of nystagmus that can be elicited in the normal individual? (physiological response)

Answer 14

1. Optokinetic-when you follow a target across the visual field and then quickly rerun to starting point (watching telephones poles in a car)
2. oculocephalic reflex-Rotary nystagmus: quick rotation of head with slow phase of eye movement in opposite direction and fast back
3. caloric stimulation: using hot or cold water irrigation in ear to cause endolymph to move

Question 15

What are pathological causes of nystagmus?

Answer 15

• congenital
• vestibular-damage to semicircular canal
• central-due to disease of brainstem or cerebellum

Question 16

Turning the head to the left has the same effect as:

Answer 16

Turning the head to the left
 = cold water in the right ear 
= warm water in the left ear
= Damage to the right  vestibular sensory organs/Vestibular nerve/nu.
= Nystagmus to the left

Question 17

Turning the head to the right has the same effect as:

Answer 17

Turning the head to the right
 = cold water in the left ear 
= warm water in the right ear
 = Damage to the left  vestibular sensory organs/Vestibular nerve/nu.
= Nystagmus to the right

Question 18

How can the oculocephalic reflex in an unconscious person be tested?

Answer 18

Turning head to either side with eyes held open

• intact brain stem: “doll’s eyes”, slow eye movements to opposite side to head turning. No nystagmus in unconscious person
• bilateral stem stem damage: absent reflex, eyes will look straight ahead-signals lower brain stem lesion and ensuing respiratory and cardiovascular function loss
• can be verified with stronger stimulus, such as caloric testing