Cranial Nerve VIII (Auditory)

membranous labyrinth

sensory organ which sets up the physical conditions that allow for stimulation of the sensory receptor

Pathway of central processing of hearing

3 neurons in ascending pathways

First synapse in cochlear nuclei complex (dorsal or ventral cochlear nuclei)

Some fibres cross to synapse in either the superior olivary nucleus or the nucleus of the trapezoid body

• Info ascends in crossed and uncrossed pathway

In the inferior colliculus at the midbrain level there is integration with proprioception information from neck muscles and visual signals

Crossing in the central processing of hearing

Superior olivary nucleus, nucleus of the trapezoid body, dorsal acoustic stria, intermediate acoustic stria, or trapezoid body

Where auditory info integrates with proprioception info

In the inferior colliculus at the midbrain level

Superior colliculus

Centre for body response to vision

Inferior colliculus

Centre for body response to hearing

Postural responses to sound

Head and neck movements

Communicated at the midbrain level of the inferior colliculus

Spinotectal and tectospinal pathways

Proprioceptive pathways from the tectum (colliculus or bumps) of the midbrain

Auditory reflex pathway

Inferior colliculus → superior colliculus (integration with vision) → medial geniculate body → Heschl’s gyrus in the auditory cortex

area of conscious appreciation of sound

Heschel’s gyrus / primary auditory area

Brodmann’s areas of hearing

Primary hearing - 41 and 42

Secondary hearing - 22

Wernicke’s area - 22 and 39

Tonotopic representation

spatial arrangement of where sounds of different frequency are processed in the brain

Pathway of sound (arriving from the left)

Reaches left cochlea first then right
Some signals will ascend ipsilaterally, others cross then ascend

Superior olivary nucleus and auditory cortex deduce the time of arrival of signals and can figure out where sound is coming from

Localization with cochlear damage

Difficulty localizing sound

Localization with auditory cortex damage

There is both crossed and uncrossed information from both ears arriving at the one intact cortex

Localization is still possible

No change in timing of signal traveling to Heschl’s gyrus, so direction is still perceived accurately (if signal is sufficiently loud)

Perspective of the ear (damage to auditory cortex)

Sound waves that arrive at both ears at the same intensity will be heard as only slightly less loud by the ear contralateral to cortical damage.

Sound is predominantly crossed - equal signal is not registered as well in the damaged cortex

Perspective of the auditory cortex (damage to auditory cortex)

Even if one cortex is damaged, sound will still arrive at the intact cortex from both ears with the qualities of intensity and frequency being controlled by the middle and inner ear.

Sound from the ear contralateral to the damage will be perceived as slightly less loud but the effect is minor

Damage to ear

That ear cannot conduct a normal signal

Auditory radiation

A collection of fibers on which auditory information leaves the medial geniculate body and travels in the sublenticular limb of the internal capsule

(thalamus → auditory cortex)

petrous portion of the temporal bone

the rocky bony ridge that separates the middle and posterior cranial fossa

auditory pathway after integration

travels through the petrous portion of the temporal bone, exits through the internal auditory meatus, enters the posterior cranial fossa traveling with the facial nerve

together they enter/leave the brainstem at the junction between the pons and medulla

acoustic and vestibular schwannomas

Schwann cell tumors

common location is auditory radiation, may extend into the internal auditory meatus

Symptoms of Schwannomas

dizziness, hearing loss, and possibly loss of function of the muscles of facial expression

due to common location, they likely disrupt the area of integration at the petrous portion of the temporal bone where auditory information travels with vestibular and facial nerve.

conductive hearing loss

middle-ear pathologies that affect sound transmission to the cochlea

characterized by fluctuating hearing loss

• good word/speech recognition ability (especially at high intensities)

• impaired auditory reflex

• air-bone gap

Otosclerosis

common cause of CHL
abnormal bone growth of bone near the oval window impedes the movement of the stapes

dominant autosomal condition

Otitis media

common cause of CHL

an accumulation of fluid in the middle ear causes the eustachian tube to malfunction
fluctuating hearing loss

sensorineural hearing loss

associated with damage to the cochlear hair cells and/or the auditory nerve

usually permanent

can range from mild to severe in the affected ear

characteristics:
- difficulty in understanding speech, especially in noise
- accompanied by tinnitus
- patients usually speak loudly

tinnitus

perception of a ringing or hissing sound in the absence of an environmental acoustic stimulus

often associated with hearing loss subsequent to the inner ear lesion

Rhine test

stem of vibrating tuning fork (514Hz) is placed on the mastoid process and the patient is asked to listen to the tone by bone conduction

When the tone is no longer heard, the fork is placed in front of the ear to determine if the patient can still hear the sound by air conduction

positive Rinne

clinical information: patient with normal hearing or SNHL should hear the tone longer by air conduction than by bone conduction

negative Rinne

clinical information: patient with CHL hears the sound longer through bone conduction

Weber test

vibrating tuning fork is placed on the scalp at the vertex and the patient is asked to lateralize the sound by indicating if the tone is loud in one ear than the other

clinical information:

• patient with normal hearing or bilaterally symmetrical hearing loss - sound is sensed at the midline

• patient with unilateral CHL hear the sound in the affected ear

• patient with a unilateral SNHL heard the sound mainly in the unaffected ear

tympanometry

measures the compliance (elastic deformation) of the tympanic membrane and middle-ear pressure under the conditions of changing air pressure (relative to atmosphere) in the external auditory meatus

impaired tympanic compliance is an indicator of middle-ear pathology'

• eg. middle-ear fluid, ossicular abnormalities, or eustachian tube dysfunction

pure tone audiometry

establishes threshold of hearing across the human communication frequency range (250-8000)

generates pure tones at various frequencies and intensities

hearing is tested by determining air and bone conduction thresholds for each frequency

hearing loss = need to increase in intensity above the normal sensitivity required to reach the threshold

• eg. 50dB loss at 1000Hz means that patient requires 50dB of sound pressure above normal to obtain the threshold

pure tone calibration

calibration takes into consideration the differential sensitivity of the human ear

make 0dB HL at each frequency equal to the lowest intensity level in SPL decibels required by the average listener to hear the frequency

cupula

a gel which is disturbed by motion in the ampulla

otolithic membrane

gelatinous membrane lying over the hair cells of the saccule and utricle in the vestibular sac

cellular mechanisms of cupula & otolithic membrane

motion activates the hair cells and causes transduction of physical to electrical energy to primary neurons

neurons have their cell body in the vestibular ganglion and synapse in the vestibular nuclei in the brainstem

nystagmus

series of reflexive rhythmic conjugate eye movements (vestibular reflexes)

function: maintain a stable, conjugate visual fixation point

involve the vestibular nuclei and their ocular projections through the MLF

eg. eyeballs

• slow phase: eye slowly drifts away from the central filed of gaze toward periphery

• fast phase: sudden jerk returns to the central field of gaze

• nystagmus is identified according to the direction of its quick phase

induced nystagmus

opticokinetic nystagmus

vision dependent

activated by visual fixation on a moving pattern

vestibular nystagmus

independent of visual input

Doll’s eye reflex

Tests the reflex of eyes responding to head position - the status of the medial longitudinal fasciculus (MLF)

On an unconscious patient, turn their head and observe the movement of their eyes

Doll’s eye absent

If the area/connection (MLF) is damaged, the eyes will remain in the same position as the head

Doll’s eye present

If the area/connection (MLF) is intact, the eyes will move in the opposite direction to the head

Caloric (Barany) test

Water is placed in the external ear and eyes are monitored closely for nystagmus

• The water alters the temperature of the middle ear and induces a current (by convection) in the endolymph → stimulates head rotation

Normal nystagmus response (caloric test)

Warm water causes the eyes to drift away from the tested side, then snap back quickly to the tested side

Cold water causes the eyes to drift to the right and then snap left, hence the vestibular nystagmus is to the left

COWS - cold opposite, warm same side

vestibulospinal tract

constant input to the limbs to keep supporting reflex activity, produces extension of the upper and lower limbs

medial longitudinal fasciculus (MLF)

coordinate eye muscle with vestibular input

the reticular system

to keep constant input to the alertness centre (consciousness)