1 - Auditory and Vestibular Function 1 (Overview, Pathway)

Question 1

Overview of Auditory Pathways

Input

Cell bodies / axons

Output

Answer 1

Input: Cochlear hair cells

Bodies: Spiral ganglion

Axons: Cochlear division of CN VIII

Output: Synapse in dorsal and ventral cochlear nuclei (rostral medulla, lateral to inferior cerebllare peduncle)

Question 2

Pathway of Cochlear Nuclei (Second-order Neurons)

Pathway?

Location of Ascending Pathway

Synapse location?

Answer 2

Some fibers cross midline, enter contralateral lateral lemniscus

Ascending pathway located in lateral midbrain

Synapse in Superior Olivary Nucleus (trapezoid body)

Question 3

Superior Olivary Nucleus

Location

Function

Answer 3

Location: Rostral end of facial motor nucleus

Function: Sound localization, compares arrival time of sound waves and relative intensities of sound between the ears

Aons enter lateral lemniscus

Question 4

Inferior Colliculus

Answer 4

Relay Function: projects to medial geniculate nucleus (thalamus)

Also key for Sound Localization and multi sensory integration

Question 5

Medial Geniculate Nucleus

Answer 5

Relay to primary auditory cortex

(Hescl’s Transverse gyri in Temporal lobe)

Question 6

Primary Auditory Cortex

Answer 6

Hescl’s Transverse Gyri (Temporal Lobe)

Brodmann 41

Question 7

What is the acoustic reflex and what enables it?

Answer 7

Reflex - Contraction of Stapedius and Tensor Tympani muscles

Enabled By - Cochlear nuclei connecting to facial and trigeminal motor nuclei

Question 8

Olivocochlear Pathway

Answer 8

Efferent function

Some neurons in Superior Olivary Nucleus project back out through CN VIII to innervate auditory hair cells

***Feedback to input to modulate function***

***Improve sound discrimiation, suppress noise, enhance speech, etc***

Question 9

Clinical: Due to the bilateral nature of the auditory pathway, what can be the causes of unilateral hearing loss?

Clinical: What can lesions above cochlear nuclei cause?

Answer 9

Damage to cochlear nuclei, CN VIII, cochlear, middle, and outer ears

Deficits in sound localization, separating sounds from background noise, and sound interpretation

Question 10

Overview of Vestibular Pathways

Areas of cerebellum?

Where else does info go?

Answer 10

Vestibular Afferents, most synapse in Vestibular Nuceli neurons (second-order)

Vestibulocerebellum - Flocculocerebelar lobe, direct input from afferents (relayed from nuclei above)

MLF to spinal levels (vestibulospinal pathway)

MLF to nuclei of CNs III, IV, VI (Vestibulooccular)

Thalamus

Question 11

Vestibular Afferents

Input

Location of Cell Bodies / Axons

Synapse

Output

Answer 11

Input: Vestibular Hair Cells

Bodies: Vestibular Ganglion (Scarpa)

Axons: Vestibular Div. of CN VIII

Synapse: in Vestibular Nuclei (sup, inf, med, lat)

Output: Some go direct to vestibulocerebellum (modulation)

Question 12

Vestibular input to cerebellum

Path?

Two way traffic?

Answer 12

Relayed and Direct: Via Inferior Cerebellar Peduncle (juxtarestiform body)

Vestibulocerebellum projects back to vestibular nuclei; modulates vestibular reflexes

Question 13

Vestibulo-Ocular Reflexes

Answer 13

Vestibular input goes to motor nuclei of extraocular muscles

(CNs III, IV, VI)

Eye movements that compensate for head movememt, stabilize visual imagery on retina

Question 14

Vestibular input down spinal cord

Answer 14

Descend down Medial Longitudinal Fasciculus (MLF)

Maintain posture against gravity

Two components: Medial to cervical, lateral to all levels

Question 15

How does the Vestibular input reach the cerebral cortex, and what is the importance of this?

Answer 15

Relay through thalamus

No true primary vestibular cortex, since most are multimodal

Key areas centered around parietal cortex, allow conscious perception of vestibular sensations

Question 16

Clinical: Dizziness vs Vertigo

Answer 16

Dizziness - faintness, unsteadiness, sensation of floating, sensation of spinning

Vertigo - Pathological illusion of movement (usually spinning)

Cause: Imbalance of vestibular signal; usually accompanied by nystagmus, nausea

Question 17

Clinical: Central / Peripheral Causes of Vertigo

Answer 17

Central: Lesion affecting vestibular nuclei, cerebellum

Peripheral:

1. BPPV (Benign Paroxysmal Positional Vertigo) - displaced otolith in posterior semicircular canal
2. Meniere’s Disease (Vestibular and Auditory effects)
3. Vestibular Neuritis, Labyrinthitis
4. Vestibular Schwannoma (acoustic neuroma) - Schwann Cell Tumor CN VIII (vestibular / auditory effects)