Auditory and Vestibular Pathways Flashcards
CN VIII ENTERS THE BRAIN AS 2 ROOTS AT THE JUNCTION OF THE _______ and _________.
PONS & MEDULLA
Vestibular schwannoma: internal acoustic meatus gets much larger because the growing tumor gets larger. True or False
True
Where are the special organs of hearing and balance found?
In the petrous part of the temporal bone
*•They are embedded in dense petrous temporal bone
There are three ganglia associated with vestibulocochlear nerve
•Inferior and superior vestibular ganglia w/ bipolar neurons
•Spiral ganglion – related to cochlea and functions of hearing.
•These axons from the vestibular nerve, superior and inferior ganglion have projections to 2 major structures.
The bipolar cells from the 2 vestibular ganglion project to the:
1)Cerebellum
- flocculonodular lobe
- vermis
2)4 Vestibular nuclei
- lateral
- medial
- superior
- inferior (descending)
Label
What are the vestibular outputs?
1.Extraocular motor nuclei via MLF
2. Cerebellum (reciprocal connections)
3. Ventral posterior nucleus of thalamus
5.Reticular formation
6.Spinal Cord
*Outputs of vestibular nuclei are carrying info that are important for balance/posture and gaze (coordinated eye movements, keep eyes fixed on particular object as you move your head).
Major outputs from vestibular nuclei to extraocular motor nuclei. The pathway by which these fibers reach abducens and oculomotor is _______________.
medial longitudinal fasciculus
Descending vestibular outputs go through spinal cord, which is important for ___________,__________ and __________.
balance, muscle tone, and posture.
All four vestibular nuclei are found beneath the floor of the ____________ in the medulla and pons, lateral to the sulcus limitans.
fourth ventricle
•2 major vestibular nuclei involved in coordinating the eye movements are ________ and ________ vestibular nuclei.
medial and superior
The medial and superior vestibular nuclei reach the extraocular motor nuclei via the __________
MLF
Explain the vestibulo-ocular reflex
- A reflex, where activation of the vestibular system causes eye movement.
- Functions to stabilize images on the retinas during head movement by producing eye movements in the direction opposite to head movement, thus preserving the image on the center of the visual field.
- The VOR is ultimately driven by signals from the vestibular apparatus in the inner ear.
The superior vestibular nuclei goes through the _________ nerve.
Oculomotor
What are the four basic steps to the VOR?
- Endolymph flow stimulates hair cells
- Increases nerve firing rate
- Stimulates vestibular nuclei (in brainstem)
- Both eyes look in the same direction
The second order axons from the medial vestibular nuclei will project contralaterally to the ___________nucleus.
abducens
- These axons will make synaptic connections with 2 different types of cells in abducens nucleus. The first one is the motor neuron, which will contract lateral rectus. Receiving mixed in with motor neurons of abducens nucleus are these interneurons that cross over the midline and go to oculomotor nuclei. They will drive the motor neurons that control medial rectus muscle. So if these interneurons become active they cause motor neurons of medial rectus to contract so you get parallel contraction of medial/lateral rectus.
How does the VOR work?
- Move your head in space
- Move head left to right causing change in endolymphatic flow in semicircular canals, stimulating hair cells. Those hair cells become excited, this increases the neuronal firing of the first order neurons in the vestibular ganglion.
- These axons target is the medial vestibular nucleus. The neurons of the medial vestibular nucleus become increased. They send signals to motoneurons controlling lateral recuts and sending excitatory signals to internuclear interneurons that go to oculomotor nucleus
Stimulation of the semicircular canals cause imbalance. There are there ways in which you can stimulate the semicircular canals. What are they?
- Move head way to do this is by moving head
- By squirting warm water into one ear, warm water increases endolymph flow in semicircular canal, this increases the neuronal activity in ganglion cell and vestibular nuclei. This causes slow drifting of the eye in the opposite direction and horizontal gaze. Eyes will slowly drift in opposite direction as far as you possibly can (lateral excursion), then you bring the eyes back to central position (via the cortex)
- Increase imbalance by causing a lesion. Lesions on one side suddenly there is an imbalance because the intact side will have tonic activity, the lesion side cells will have no activity. It is lower than it should be, the eyes will drift to the side with decreased activity. Stimulate vestibular nuclei producing eye movement in opposite direction or cause lesion of vestibular nuclei (producing imbalance of vestibular nuclei on the left and right hand side)
- Give warm water in right ear: increase activity on right ear, eyes drift to the left (afterwards, the cortex will move eyes back to center)
- Give cold water in left ear: decrease activity of left vestibular nuclei so eyes drifting to the left. (Afterwards, cortex will move eyes in opposite direction.
What is the normal result of the caloric test?
Nystagmus
If the caloric test is performed and no nystagmus results, what does this mean?
Deep coma
If the caloric test is performed and nystagmus results, what does this mean?
The brainstem is still in tact and recovery is possible
If the caloric test is performed and the eyes move and stay to one side, what does this mean?
The cortex is impaired
*The cortex is responsible for bringing the eyes back to center after nystagmus
INTERNUCLEAR OPHTHALMOPLEGIA
- A disorder of conjugate lateral gaze in which the affected eye shows impairment of adduction.
- When an attempt is made to gaze contralaterally (relative to the affected eye), the affected eye adducts minimally, if at all. The contralateral eye abducts, however with nystagmus.
- Additionally, the divergence of the eyes leads to horizontal diplopia.
NOTE: Convergence is normally in tact
What is the cause of internuclear ophthalmoplegia?
Injury or dysfunction in the medial longitudinal fasciculus
Why does damage to the MLF cause internuclear ophthalmoplegia?
MLF is responsible for connecting the paramedian pontine reticular formation (PPRF)-abducens nucleus complex of the contralateral side to the oculomotor nucleus of the ipsilateral side.
The two major vestibular nuclei that send descending activity are __________ vestibular and _________ vestibular nuclei
medial; lateral
What is the difference between the medial vestibulospinal tract and the lateral medial vestibulotract?
A. medial vestibulospinal tract (descending MLF)
a. medial vestibular nucleus
b. bilateral (via descending MLF)
c. extension of neck
d. vestibulo-cervical reflexes
B. lateral vestibulospinal tract
a. lateral vestibular nucleus
b. ipsilateral
c. antigravity muscles
d. vestibulo-spinal reflexes
The medial vestibulospinal tract originates from the ____________, terminates medially in the ____________, and affects ___________.
medial vestibular nucleus; ventral horn of cervical regions; axial musculature
The lateral vestibulospinal tract originates from the ___________, terminates in the ___________, and excitatory to _________.
lateral vestibular nucleus; intermediate, ventral horn; antigravity muscles
Medial vestibular spinal tract is located in __________funiculus.
anterior
*These axons will be terminated in medial motor nuclei of the cervical spinal cord. Motor nuclei that control axial musculature, muscles in the neck.
•This input coming from the lateral vestibular nuclei, is very __________ (excitatory/inhibitory).
Excitatory
What is interesting about this lateral vestibular spinal tract is normally speaking most cases, the lateral vestibular nucleus is being tonically inhibited by cortex or other parts of brain. So that the pathway going to __________ muscles is not working (being inhibited), as if you have a lesion of the cortex.
antigravity
Decerebrate posturing
- It describes the involuntary extension of the upper extremities in response to external stimuli. In decerebrate posturing, the head is arched back, the arms are extended by the sides, and the legs are extended.
- Decerebrate posturing indicates brain stem damage
- It is exhibited by people with lesions or compression in the midbrain and lesions in the cerebellum.
Decorticate rigidity: You have brainstem lesions, between _________ and _________ colliculus.
superior; inferior
*When you remove this descending inhibition of lateral vestibular nucleus, you get overexcitation of antigravity muscles.
Brainstem or cortical lesions after the cochlear nuclei will NOT produce monaural deafness. True or False.
True
Spiral ganglion-embedded in the ___________
cochlea
•The pathway is tonotopically organized
- The tones for different frequencies have specific locations in the relay nuclei of the auditory system.
- These are specific bands in the ______________.
transverse gyrus of Heschl
Auditory pathway
- Bipolar neurons in the spiral ganglion (1st order neurons)
- Dorsal cochlear nucleus (2nd order neuron)
* The dorsal cochlear nucleus decussates at a distinct part of the brainstem called the trapezoid body - Axons decussate and relay in the inferior colliculus
* The axons decussate and form the lateral leminiscus - Auditory signals terminate in the medial geniculate nucleus
- Project to the auditory cortex
Axons from the lateral leminiscus terminate in the ___________
inferior colliculus
Auditory signals terminate in the ____________
medial geniculate nucleus
Auditory nerve axons synapse ipsilaterally on secondary neurons that are found in two cochlear nuclei:
- dorsal and ventral
Label
At the pontomedullary junction
•Dorsal cochlear nucleus has axons that project predominantly ____________and cross in the trapezoid body
contralaterally
•Ventral cochlear nucleus has axons that project __________
bilaterally
NOTE:
- Any lesion distal to this hearing can be persevered (there won’t be monaural deafness) because of this bilateral projection
- A lesion proximal to the cochlear nuclei will produce monaural deafness
AICA comes off the __________ artery. PICA comes off the _________ artery.
basiliar; vertebral
•Major symptom of a stoke or obstruction involving AICA is __________.
hearing loss
*•If you see a clinical scenario involving monaural deafness chances are the lesion involves AICA.
PICA supplies the_______, and __________.
spinothalamic tract, trigeminal spinal tract and nucleus
*a lesion here results in contralateral pain and temp loss in the body and hemisensory loss in the head and neck.
•If you have the above symptoms you know that you’re in the caudal part of the medulla involving PICA
Label
Trapezoid body
*The trapezoid body is where the axons of the dorsal and ventral cochlear nuclei decussate
The ____________ is formed after the axons of the dorsal and ventral cochlear nuclei cross at the trapezoid body.
lateral lemniscus
Lateral lemniscus axons terminate in the ___________.
inferior colliculus
Label
Axons from inferior colliculus terminate in the _____________.
medial geniculate nucleus
Label
The red circles represent the medial geniculate nucleus
Label
•Axons from the MGN travel up through the internal capsule and terminate in the _____________.
transverse gyrus of Heschl (primary auditory cortex)
____________are secondary auditory areas that surround the gyrus of Heschl
Planum temporale
- Larger of the left than on the right- one area of the brain where the left side has an anatomical difference from the right
- Involved in sound localization and recognition of perfect pitch.
The location of sound is mapped in the brainstem:
1) Low frequencies (<3 kHz): interaural time differences
2) High frequencies (>3 kHz): interaural intensities differences
•Some axons terminate into the ____________ for sound localization
superior olivary nucleus
*These axons will join the lateral lemniscus and go to the inferior colliculus
What nucleus is being described below?
- sends bilateral inhibitory efferents to cochlea
- form olivocochlear fibers (to hair cells)
- reduces background noise
Superior Olivary Nucleus
What are the inputs and outputs for the superior olivary nucleus?
Inputs: bilateral - ventral cochlear n.
Outputs: inferior colliculus
