HEARING ANATOMY AND PROCESS

Learning about the auditory system is fascinating and enjoyable but learning about it for the 1st time means that we will face many new terms, concepts and relationships especially for the audiology students.

For this reason it is better that we begin with the general view of how our ear is set up and how a sound can be interpreted by the brain.

Figure 1.0 shows the major parts of the ears

Our auditory system is divided to 3 main sections.

1) The outer ear

The outer ear includes the pinna/auricle, external auditory meatus ending at the tympanic membrane (eardrum). To note that the tympanic membrane is considered part of the middle ear system.

a) Pinna/Auricle- concha, sebaceous glands present in the concha, helix, the crus of the helix, earlobe/lobule, scaphoid fossa, anthelix, crura of antihelix, tragus, antitragus, intratragis notch,

b) External auditory meatus-ear canal has 2 bends forming S shaped pathway, the outer third is composed of cartilage and inner two-thirds is composed of bone, the cartilage part consists of hair, sebaceous (oil) glands and ceruminous (wax) glands.

c) Tympanic membrane-umbo

Function of the outer ear

1) To collect and direct the sound waves to the tympanic membrane
2) To provide directional cues that help in front-back and vertical sound localization
3) Amplifies the sound energy of frequencies important for speech since the resonance of the auricle and ear canal increase the soundpresure in the 2-to-7 kHz range
       

2) The middle ear

The middle ear is the air filled cavity behind the tympanic membrane which is consists of 3 tiny bones (malleus, incus and stapes), known as ossicular chain.

a) Malleus-attached to tympanic membrane at the manubrium
b) Incus- connects the malleus to the stapes
c) Stapes- attached to oval window by the annular ligament of the footplate. The neck of stapes is connected to the stapedius muscle which innervated facial nerve.

To note that:

The middle ear connects posteriorly to the mastoid antrum via the aditus and anteriorly to the nasopharynx by the Eustachian tube (ET). Medial to the middle ear is inner ear. Lateral to the middle ear is outer ear.

I just call MIS bones, MIS stand for (M=malleus, I=Incus, S=stapes). Tips for remember.

Function of the middle ear

a) Overcome the impedance mismatch between the air filled external auditory canal and the fluid-filled inner by acts as a mechanical transformer. But how?so this is the reason, the stapes footplate is smaller that the tympanic membrane, thus it will produce an area ratio hydraulic that provides up to 25 dB of pressure gain.

b) Protection from the loud sound. How?In human, we have stapedius muscle that will make a contraction relatively to loud sounds. The contraction of the stapedius muscle will stiff the ossicular chain and cause the footplate to retract slightly from the oval window, thus produce sound attenuation especially for loud low-frequency sounds.

c) Equalizes the pressure. The middle ear function will be more efficient when its pressure equals that of atmosphere by allowing air to enter/exit the middle ear by the Eustachian tube.



3) The inner ear

The inner ear consists of the cochlear and vestibular organs (balance). The inner ear is located in the petrous part of temporal bone.

a) The cochlea

The cochlea is coiled into a two and three quarters turn and filled with fluid. Reissner's membrane and basilar membrane divide into three partition:

a) scala vestibuli,
b) scala media
c) scala tympani.

Scala vestibuli and scala media are separated by Reissner's membrane while scala tympani and scala media are separated by basilar membrane. Both scala tympani and scala vestibuli contain perilymph and scala media contains endolymph.

Function of the cochlea

Transform the mechanical vibrations of the middle ear into nerve impulses that will be transmitted by the cochlear nerve toward the central auditory pathway.

But how?Ok please see and understand below points

a) A pressure wave in the scala vestibuli will be provoked by the movement of the stapes footplate

b) Then cause basilar membrane to vibrate and the stereocilia on the top of the inner and outer hair cells will be bend.The outer and inner hair cells are located in the organ of corti which is situated in the basilar membrane.

c) Deflection of the stereocilia of the outer hair cells may induces the outer hair cells to change its length.

d) This change of the length causes the tectorial membrane and the basilar membrane to move relative to each other.

e) These movements will enhance the response of the inner hair cells which results in better hearing
sensitivity and better frequency selectivity

f) The place of maximum displacement of the basilar membrane is depend on the frequency. High frequency stimuli will cause maximum displacement at the basal while low frequency stimuli at the apical. This arrangement called tonotopic that is preserved throughtout the auditory system.

The vestibular system is arranged of :

the 3 semicircular canals:

a) lateral/horizontal semicircular canal,
b) anterior/superior semicircular canal
c) posterior semis circular canal

and 2 structures within the vestibule:

a) Utricle
b) Saccule.

The sensory receptors made a contact with neurons that make up the 8th cranial nerve. It connects the peripheral ear to the central nervous system (CNS). The 8th cranial nerve is made up of the auditory branches called cochlear/auditory nerve and the vestibular branches called vestibular nerve. The 8th nerve then connected to the brainstem via the opening of medial side of temporal bone called internal auditory meatus. To note that the auditory nerve go to the cochlear nuclei and vestibular nerve go to vestibuli nuclei.

References: 
1) Gelfand, S. (2009). Essentials of Audiology. (3rd Edition). New York, NY: Thieme. 
2) Katz, J., Medwetsky, L., Burkard, R. & Hood, L. (2009).Handbook of Clinical Audiology (6thEdition).Baltimore, MD: Lippincott, Williams and Wilkins.

audiologis, audiologis, audiologis, audiologis, audiologis, audiologis, audiologis, audiologis, audiologis, audiologis, audiologis, Audiologis is a practitioner in the field of audiology. They have a degree in Bachelor of Audiology and responsible for identifying, preventing, diagnosing, and managing as well as carrying out the rehabilitation services for patients with hearing loss, balance problems as well as problems related to the auditory.