The auditory pathway consists of relay stations from the inner ear hair cells through the spiral ganglion, cochlear nuclei, superior olivary nucleus, lateral lemniscus, inferior colliculus, medial geniculate body, and auditory cortex. Sound is transmitted through these relay stations with the first order neurons in the spiral ganglion and higher order neurons at each subsequent station. There are three main types of hearing loss - conductive, sensorineural, and mixed. Conductive hearing loss interferes with sound conduction to the cochlea, sensorineural involves lesions of the cochlea/vestibulocochlear nerve, and mixed has both conductive and sensorineural components. Hearing
2. AUDITORY PATHWAY
RELAY STATIONS:
spiral ganglion
superior olivary nucleus
,trapezoid n
nucleus of lateral
lemniscus
inferior coliiculus
medial geniculate body
auditory cortex
from hair cells through cochlear division of
8th cranial nerve
3. SPIRAL GANGLION:
first order neurons –bipolar cells of spiral
ganglion.
dendrites of bipolar cells constitute afferent
fibres innervating the hair cells.
axons form the cochlear division of eight
cranial nerve
cochlear nerve ends in cochlear nuclei in
medulla.
4. COCHLEAR NUCLEI:
second order neurons
two cochlear nuclei-dorsal and ventral
axons pass medially in pons.
most of them cross to opposite side, some
remain uncrossed.
the crossing fibres of two sides form –
trapezoid body
5. SUPERIOR OLIVARYNUCLEUS,TRAPEZOID
NUCLEUS :
third order neurons
s o n-recieves large majority of fibres from
cochlear nucleus. form as ascending bundle
of lateral lemniscus.
few fibres relay in trapezoid nucleus before
reaching lateral lemniscus.
fibres of lateral lemniscus ascend in midbrain
to terminate in inferior colliculus.
6. INFERIOR COLLICULUS:
fourth order of neurons.
fibres terminate in medial geniculate body.
MEDIAL GENICULATE BODY:
FIFTH ORDER OF NEURONS
fibres form acoustic radiationsend in
auditory cortex.
7.
8.
9.
10. AUDITORY CORTEX:
temporal lobe
primary auditory cortex(41,42)
auditory association area(22,21,20)
PRIMARY AUDITORY
AREA/AUDIOSENSORY AREA:
centre of hearing
location:middle of superior temporal gyrus on
upper margin on its deep insular aspect.
11. APPLIED PHYSIOLOGY
:
HEARING LOSS :IMPAIRMENT OF
HEARING
DEAFNESS:LITTLE OR NO HEARING
TYPES OF HEARING LOSS:
conductive
sensory
mixed
12. 1.CONDUCTIVE HEARING LOSS:
interferes with conduction of sound from
external ear to cochlea.
CAUSES:
external ear
tympanic membrane
middle ear cavity
ear ossicles
eustachian tube obstruction
14. SENSORINEURAL HEARING LOSS:
results from lesions of cochlea/8th nerve and
its central connections.
causes:
congenital and acquired
acquired hearing loss later in life
infection of labyrynth,injury to labyrynth,noise
trauma,ototoxicity,acoustic neuroma,d m
,htn,senile degeneration of hair cells-
presbycusis, meniers disease.
15. LOSS OF HEARING –complete
speech discrimination –poor
management:
treat the underlying cause
hearing aids-cochlear implants
3.MIXED TYPE:
16. TINITUS:
Ringing sensation in ears caused either due to
irritative stimuli to inner ear or vestibulocochlear
nerve.
HEARING TESTS:
TO ASSESS THE TYPE OF HEARING
LOSS,DEGREE,SITE OF LESION, CAUSE OF
HEARING LOSS
TWO TYPES
CLINICAL TYPES AND AUDIOMETRIC TYPES.
CLINICAL TYPES:
1.FINGER FRICTION TEST-
20. AUDIOMETRIC TESTS:
ASSESMENT OF HEARING USING
ELECTRONIC EQUIPMENT.
PURE TONE AUDIOMETRY
SUBJECTIVE TEST.
AUDIOMETER –MACHINE
AUDIOGRAM –RECORDING
TO DETECT OR RULE OUT CONDUCTIVE
DEAFNESS
21. 2.SPEECH AUDIOMETRY:
.
3.TONE DECAY TEST:
4.BRAIN STEM EVOKED RESPONSES:
POTENTIALS RECORDED FROM
AUDITORY PATHWAY IN RESPONSE TO
BRIEF AUDITORY STIMULATION.
ASSESS FUNCTIONAL STATUS OF
AUDITORY PATHWAY UPTO MID BRAIN