Auditory System

Auditory System

Hearing II: Pathways, Lesions and Tests

Dr. Meenakshi Gupta.

Professor, Physiology

JNMC, AMU, Aligarh

Auditory Pathways

• Nerve impulses pass through 4 order neurons from Hair cell receptor to cerebral cortex:

–

– 2^nd neuron: at lower brainstem (ventral & dorsal cochlear nuclei + SO) – 3^rd neuron: at upper brainstem (inferior colliculus nuclei)

– 4^th neuron: thalamus (medial geniculate nuclei)

• Auditory pathways have bilateral representation:

– input from each ear reaches auditory cortex of both cerebral hemispheres

4

Auditory Pathways

• 1^st order (sensory) neurons

– Located in spiral ganglion (bipolar neurons)

– Peripheral processes synapse with hair cells of spiral organ – Central processes form cochlear division of CN VIII enters

brainstem at lateral end of inferior pontine border to synapse with ventral and dorsal cochlear nuclei in medulla

Cochlear nerve

Cochlear nerve Spiral

ganglion

Dorsal and ventral cochlear nuclei

Medulla- Pons

Internal acoustic meatus

Auditory Pathways

• 2^nd order neurons

– Ventral and dorsal cochlear nuclei send fibers forming 3 bundles:

dorsal, intermediate and ventral acoustic striae. Most axons

decussate and ascend in contralateral lateral lemniscus, but some ascend in ipsilateral lateral lemniscus

– Ventral acoustic stria is also known as trapezoid body

– Fibers of intermediate and ventral acoustic stria project both

ipsilaterally & bilaterally to medial and lateral superior olivary nuclei – Ventral & dorsal cochlear nuclei and superior olivary (SO) nuclei

project to inferior colliculus in mid brain via lateral lemniscus

Auditory pathway

Cochlear nerve Dorsal cochlear nucleus

Ventral cochlear nucleus

Lateral lemniscus

Dorsal acoustic stria

Lateral superior olivary

nucleus Medial superior

olivary nucleus Ventral acoustic

stria

(Trapezoid body) Intermediate acoustic stria Pontomedullary junction

Nucleus of trapezoid body

(LSO)

(MSO) (NTB)

Auditory Pathway

• 3rd order neurons

– Inferior colliculus nuclei send axons to form brachium of inferior colliculus

– Terminate bilaterally in medial geniculate nucleus of thalamus

Auditory pathway:

continued

Lateral lemniscus Inferior colliculus

Commissure of inferior colliculus

Brachium of inferior colliculus

Nucleus of lateral lemniscus

Commissure of lateral lemniscus

Auditory Pathway

• 4th order neurons

– Medial geniculate nucleus project axons forming auditory radiation – Passes through sublenticular part of posterior limb of internal

capsule

– Terminates in primary auditory area (transverse temporal gyri or Brodmann's areas 41 and 42)

Auditory pathway:

Continued

Brachium of inferior colliculus Medial geniculate nucleus of thalamus Sublenticular part of

internal capsule

Primary auditory cortex

(Areas 41, 42)

Auditory association cortex

(Area 22)

Corpus callosum

Auditory association cortex (Area 22)

Auditory pathway

Primary auditory cortex (Areas 41, 42)

Medial geniculate nucleus of thalamus

Inferior colliculus

Cochlear nucleus

Spiral ganglion of cochlear nerve

Cochlear hair cell Peripheral axon of cochlear nerve

Central axon of cochlear nerve

Lateral lemniscus Superior olivary nuclei Brachium of inferior colliculus

Sublenticular internal capsule

Bilateral input

Ipsilateral input

1

2

3

4

Nuclei aid in bilateralism of auditory pathway

– Cochlear nuclei

– Superior olivary nuclei

– Nucleus of trapezoid body – Nucleus of lateral lemniscus – Inferior colliculus

Superior olivary nuclei (medial & lateral)

• Located in anterior part of tegmentum of caudal pons

• Receive fibers from ipsilateral and contralateral ventral cochlear nuclei

• Give rise to fibers joining ipsilateral and contralateral lateral lemnisci

• Play important role in localization of sounds in space

Nucleus of trapezoid body

• Consists of neurons located among fibers of trapezoid body

• Receives fibers from contralateral ventral cochlear nucleus

• Sends fibers to ipsilateral superior olivary nucleus

• Helps superior olivary nucleus in localizing sounds in space

Nucleus of lateral lemniscus

• Groups of neurons located within lateral lemniscus in upper pons

• Receives fibers from lateral lemniscus via commissure of lateral leminiscus

• Sends fibers to ipsilateral and contralateral inferior colliculi

Inferior colliculus

• It sends fibers to contralateral inferior colliculus via commissure of inferior colliculus

• It sends fibers to the ipsilateral medial geniculate nucleus

Primary auditory cortex (Areas 41, 42)

Medial geniculate nucleus of thalamus

Inferior colliculus

Cochlear nucleus

Cochlear hair cells

Periolivary cells

Olivocochlear bundle Cochlear nerve

Cochlear nerve contains efferent axons from the superior olivary nucleus (in lower pons)

Reciprocal connection between auditory nuclei & auditory cortex

permits descending modulation of the ascending auditory activity

Descending auditory modulation

Olivocochlear pathway: Descending modulation of auditory input

Periolivary cells

Medial superior olivary nucleus

Lateral superior olivary nucleus Olivocochlear bundle

Cochlear nerve

Spatial localization of sound by inter-aural time delay

•Separate populations of MSO neurons respond best when:

- sound arrives at the right ear first

- sound arrives at both ears simultaneously

- sound arrives at the left ear first

illustration: sound source on the left side

1 2

3 4

5

Inter-aural intensity difference

• is the only horizontal localization mechanism which is useful for high frequency sounds

• when sound is coming from straight ahead, both ears receive equal intensities

• when sound is coming from one side, one ear is in a “sound shadow”, and intensity is decreased

Sound localization by inter-aural intensity difference

illustration: sound source on the left side

Loud sound on left:

● left lateral superior olive (LSO) receives:

- strong excitatory input from loud ear

- weak inhibitory input from the quiet ear (via the inhibitory

medial nucleus of the trapezoid body (MNTB) pathway)

● right LSO receives:

- strong inhibitory input from the loud ear (via the inhibitory MNTB pathway)

- weak excitatory input from the quiet ear

• result: auditory information projected to higher centers is

stronger on the side receiving the louder sound (left side)

* Sound information from each cochlear nucleus is sent:

- to the ipsilateral LSO - to contralateral MNTB

- MNTB inhibits LSO (ipsilateral)

Deafness

( Partially or wholly lacking the sense of hearing)

• Unilateral lesions to auditory cortex or

auditory pathways distal to cochlear nuclei:

– Cause no loss of hearing

– May impair ability to localize direction and distance of

sounds

Deafness

• Deafness is of two types:

– Conductive deafness

– Sensorineural or perceptive (nerve) deafness

Common causes of conductive and nerve deafness

conductive deafness nerve deafness

Results from any interference with passage of sound waves through external or middle ear (e.g. serous otitis media, otosclerosis)

Results from damage to receptor cells in spiral organ or to cochlear nerve (e.g.

ototoxic drugs, acoustic neuroma)

Conductive deafness

• Interruption of sound waves through external or middle ear

• Three causes:

– Otosclerosis: Overgrowth of labyrinthine bone at oval window fixes footplate of stapes

• Common cause of progressive conductive deafness

• Found in elderly

– Otitis media

• Inflammation of middle ear

– Obstruction by wax or foreign body in external

auditory meatus

Sensorineural deafness

• Due to pathology of cochlear hair cells, cochlear nerve, or rarely central auditory pathways

• Damage to organ of Corti, cochlear nerve, or cochlear nuclei:

– Ipsilateral total hearing loss

• Damage to higher central auditory pathways (central deafness):

– Bilateral diminished hearing

Sensorineural deafness

1. Caused by drugs or toxins (ototoxicity):

• Aspirin

• Quinine

• Antibiotics

(aminoglycosides)

2. Prolonged exposure to loud noise

3. Rubella infections in utero or syphilis

Loss of hair cells indicated by lack of stereocilia

Sensorineural deafness

• Presbyacusis

– Most common form of deafness.

– Loss of hearing in elderly due to degeneration of hair cells in organ of Corti in basal part of cochlear duct

– High frequency sound detection most affected

• Acoustic Neuroma (Schwannoma) of vestibulocochlear nerve

– Tinnitus (ringing in ears)

Rinne’s test

• Air conduction is more sensitive than bone conduction by 35 dB

1. Test bone conduction by tuning fork (512 Hz) for end of the tone sound 2. Then immediately test air conduction

while tuning fork is still vibrating

• Normally, air conduction recognition should last an additional 10 -15

seconds (seen in partial nerve deafness)

• If patient can’t hear by air conduction, indicates conductive deafness (as due to middle ear dysfunction)

Weber’s test

Bilateral test of hearing loss

• Fork is placed over the vertex of skull The patient is asked to localize the sound Three possibilities:

1. Both ears are normal

The sound is localized to the middle of the head.

2. Unilateral nerve deafness

The sound is localized to the side of the good ear (bad ear hearing impaired) 3. Unilateral conductive deafness

The sound is localized to the side of the bad ear (air & bone conduction interfere with each other in the good ear partially diminishing the sound)

Schwabach’test

• Bone conduction of patient compared with that

of normal subject .

Weber Rinne Schwabach Method Base of vibrating tuning

fork placed on vertex of skull

Tuning fork placed on mastoid process until subject no longer hears it, then held in air next to ear

Bone conduction of patient compared with that of normal subject.

Normal Hear equal on both sides.

Hear vibration in air after bone conduction is over. (Rinne

positive).

Both subject &

examiner hear equally .

Conduction deafness in one ear

Sound louder in ds ear (masking effect of

environmental noise is absent on ds side)

Lateralized to Ds ear

Vibration in air not heard after bone conduction is over . (Rinne Negative)

Bone conduction better than normal (conduction defect excludes masking noise)

Nerve deafness (one ear)

Sound louder in normal ear (lateralized to

healthy ear )

Vibration heard in air after bone conduction is over, as long as nerve deafness is partial.

Bone conduction is worse than normal

Ménière's disease

• Damage to hair cells occurs from increase in endolymphatic fluid pressure – Overproduction or mal-reabsorption of endolymph

– Blockage of endolymphatic duct (drains to subarachnoid space)

• Periodic rupture of membranous labyrinth → potassium rich endolymph contaminates perilymph → depolarization of CN VIII

• Vestibular effects: Vertigo, nausea, vomiting, horizontal nystagmus (fast- phase opposite to side of pathology), dysmetria (past-pointing) and falling to side of pathology.

• Auditory effects: Tinnitus, hearing loss

Brain stem auditory evoked responses (BAER)

• Non-invasive technique for evaluating auditory pathways in infants and comatose patients

• Multiple calibrated clicks (square wave auditory signal) are delivered to ear

• Surface electrodes record electrical events evoked in brainstem along central auditory pathway

• Five peaks are identified from background noise

• Delay or absence of peaks indicate location of auditory system lesion

Brainstem Auditory

Evoked Responses