Chapter 2: Anatomy and physiology of the conductive auditory mechanism

Objectives (1)Two subsystems of the human auditory systemThree main functional mechanisms of the peripheral auditory systemMajor outer ear structures and their functions in the auditory systemThree layers of tissue and two types of fibers in the TMStructures of the middle earSpecific nerves and musclesDifference between epitympanic recess and tympanic cavity proper(tympanum)

Objectives (2)Ossicular chain and three ossicular bonesFunctions of the auricle and external auditory meatusAuditory mechanism in terms of energy conductionImpedanceEffect of atmospheric pressure and potential complications of differential pressureThree functions of the auditory tubeEffects of muscles surrounding the middle ear

Auditory system

Peripheral system and central systemPeripheral system: the outer ear, the middle ear, the innerConductive mechanism: the outer and middle earsSensory mechanism: the inner ear

Temporal bones

Four portions: squamous portion, mastoid portion, tympanic portion, and petrous portionSquamous portion: the opening of the external auditory meatus (outer ear canal)Mastoid portion: behind the auricle and contains numerous air-filled spacesTympanic portion: a section of the external auditory meatusPetrous portion: at the base of the skull and houses the essential parts of the organs of hearing and equilibriumSquamous: 비늘로 덥인, petrous: 바위의, 딱딱한

Outer earAuricle (pinna)External auditory meatus (outer ear canal)Tympanic membrane

Auricle (pinna)

Soft tissue and cartilage forming a cup around the entrance to the external auditory meatus

Surface: eneven and contains pits, depressions, ridges, and grooves

Portions: concha, helix, antihelix, scphoid fossa, triangular fossa, tragus, lobule (lobe)

Functions: help to collect and direct sound energy into the external auditory meatus

Its musculature: vestigial and of limited functional usefulness

External auditory meatus (outer ear canal)Canal leading from the pinna to the tympanic membrane, approximately 25 to 35 mm long, very narrow (5-9 mm), and S shaped.Two sets of glands, ceruminous (wax secreting) and sebaceous (oil producing), line the skin of the cartilaginous portion of the canalCerumen: wax produced by ceruminous glands in the external auditory meatus that helps to keep the canal cleanLined with hairs in the external ear canal: protection for the tympanic membrane (TM) against insects and other foreign bodiesFunctions: receives and directs airborne sound to reach to the tympanic membrane, convert from the acoustic sound into mechanical vibrations of the TM

Tympanic Membrane (TM 1)

Tympanic Membrane (TM): elastic structure separating the outer ear from the middle ear cavity

Surrounding by fibrous tissue called the annulus

Normally concave

Three layers of tissue:

(1) An outer, cutaneous layer-lining of the external auditory meatus

(2) A middle, fibrous layer-compliance of the TM

(3) An inner, mucous membrane layer- lining of the middle ear cavity

Tympanic Membrane (TM 2)

Two types of fibers in the TM:

(1) Radial fibers: originate near the center of the tympanic membrane, dense and spread toward the periphery

(2) Circular fibers: surround the TM and sparse near the center and dense toward the periphery

Pars tensa and pars flaccida:

(1) Pars tensa: largest portion of the tympanic membrane, consisting of fibers that contributes to the taut nature of the membrane

(2) Pars flaccida: small portion on the superior portion of the TM containing few fibers and is flaccid

Tympanic Membrane (TM 3)

Cone of light

Malleus: the most lateral bone of the ossicular chain in the middle ear, attached to the fibrous layer of the TM, responsible for pulling the TM medially toward the middle ear, thus causing its concave shape

Middle Ear (ME1)A hollow, air-filled cavity approximately 0.5 inch high and wide and about 0.25 inch depthTwo opening into the inner ear

(1) Fenestra vestibuli (oval window)-CN VII (facial nerve)(2) Fenestra rotunda (round window)

The chorda tympani: a branch of the facial nerve, passes the middle ear and carries information about the sense of taste from anterior portion of tongue to the CNSPyramidal eminenceTwo muscles:

(1) Stapedius muscle(2) Tensor tympanic muscle

Middle Ear (ME2)Two middle ear cavity areas:

(1) Tympanic cavity proper (tympanum)

(2) Epitympanic recess (attic)

Eustachian (auditory) tube: 35mm long and connects the middle ear cavity to the nasopharynx. The muscles of the nasopharynx open the tube during swallowing, yawning, and sneezing.

Tympanic cavity proper (tympanum):

(1) Ossicular chain: Malleus (hammer), incus(anvil, 모루), and stapes (stirrup, 등자)

(2) Joints: incudomalleolar and incudostapedial joint

Middle Ear (ME3)

Two ME muscles

(1) Tensor tympani: located in the anterior portion of the middle ear cavity, and innervated by a branch of cranial nerve V (trigeminal nerve)

(2) Tensor stapedius: located in the posterior of the middle ear cavity and connects to the neck of the stapes, and innervated by cranial nerve (CN) VII. When the stapedius muscle contracts in response to acoustic sounds-acoustic reflex

Middle Ear

Three middle ear bones:

(1) Malleus: Attached directly to the TM, composed of a head, neck, and three processes (manubrium, anterior, and lateral)

(2) Incus: consist of a body and two processes (a short and a long), transmit an energy to the stapes

(3) Stapes: consist of a head, neck, crura, and footplate, coupling the middle ear to the inner ear, and causing movement of the fluid in the inner ear

Function of the Conductive Mechanism

Nonacoustic Function

Acoustic Function

Impedance: Impedance Mismatch

Condensation Effect (Areal Ratio)

Lever action of malleus and incus

Curved-membrane buckling principle

Auditory(Eustachian) tube

Nonacoustic FunctionIn some species, the auricle helps to collect and direct sound into the external auditory meatus, however, its function is limited in that capacity in humans

The external auditory meatus: protect the TM from the outside atmosphere in several ways

(1) S shaped

(2) The skin of the cartilaginous portion of the canal has two sets of glands: ceruminous and sebaceous. The ceruminousgland-produces a wax called cerumen- keeps the canal supple and clean and reduces dryness and an insect repellent

(3) The lateral cartilaginous portion of the canal: hairs-provide additional protection for the TM against insects and other foreign bodies

Acoustic FunctionConduct sound energy from outside the head, through the outer and middle to the cochleaThe external auditory meatus: natural amplification for some sounds by acting as a closed-tube resonator- gain of 10 to 15 dB at around 2700 HzClinical implications of the canal resonance:

(1) When hearing is tested under earphones, canal resonance is reduced or eliminated: different from its normal test results without earphones

(2) When an ear mold is placed in an ear for fitting a hearing aid, canal resonance is reduced or eliminated-insert ion lossThe acoustic energy of the sound is transduced to mechanical energy through the ossicular chain, which is transferred to the inner ear through the fenestravestibuli

ImpedanceWhen sound reaches a low impedance medium (air)- small reflection but big absorption

When sound reaches a high impedance medium (fluid)- big reflection but a little absorption

Low impedance air to high impedance fluid: mismatch impedance

The middle ear bones: middle ear transformer (impedance matching function)

(1) Condensation effect (areal ratio)

(2) Lever action of malleus and incus

(3) Curved membrane buckling mechanism of the tympanic membrane

Auditory (Eustachian) tubeThree functions of eustachian tube:

(1) Equalize air pressure on the lateral and medial sides of the tympanic membrane (swallow, yawn, or sneeze)

(2) Provide air supply needed for metabolism of the middle ear tissues

(3) Drain middle ear secretions into nasopharynx

Conductive Hearing Loss:Hearing loss resulting from impairment to any portion of the conductive auditory mechanism