Week 9+10 - Anatomy Physiology Hearing

FINAL EXAM https://create.kahoot.it/share/anatomy-and-physiology-of-hearing/3e1f92e2-e6e4-4d76-aca0-69e3ff640016

The hearing organ (“ear”) is composed of series of what structures:

The hearing organ (“ear”) is composed of series of what structures:

1. The external ear

2. The middle ear

3. The internal ear

• The neural impulses registered by the sensory cells are then analyzed by the brain– The external earzed by the brain

**Label the ear structure**

The external ear consists of:

• Everyday use of the word “ear” means technically the external ear

• External ear consists of:

  • Auricle (pinna)

  • Meatus (ear canal)

  • Tympanic membrane (ear drum)

• Task of the auricle: localize the sound source (important: evolution)

• Tympanic membrane works like a loudspeaker or microphone membrane

Whats the external auditory meatus

• External ear canal

• 7 mm in diameter and 2.5 cm long

• this generates resonance frequencies at 3400 Hz.

• From an acoustic point of view: ear canal is a filter that amplifies frequencies between 2kHz and 5 kHz.

• Terminates at the tympanic membrane

  • Two-thirds of ear canal housed in bone (osseous portion)

  • One-third of ear canal composed of cartilaginous parts

• Resonating cavity that contributes to hearing

  • Determine resonant frequency

• Outer third- line with hair cells and cerum (ear wax)-protects by trapping dirt and insects

Whats the tympanic membrane?

• Also known as the eardrum

• Separates the middle ear from the outer ear

• Oval shaped, 10 mm in diameter

• Thin three layered sheets of tissue

• Landmarks:

  • Umbo - point of attachment for malleus, middle ear bone- location is cone of light (reflects light from otoscope)

• Responsible for initiating mechanical

  impedance-matching process of middle ear

  • First layer: outer (cuticular) layer

  • Second layer: Intermittent (fibrous) layer

  • Third layer: inner (mucous) layer

The middle ear consist of:

• The middle ear consists of the tympanic cavity

  • This cavity contains the smallest moving bones of the human body – the ossicles:

    • Malleus (hammer): touches the tympanic membrane and transmits to

    • the Incus (anvil) which transmits to the

    • Stapes (stirrup) which transmits to internal ear (oval window)

  • Malleus and stapes are attached to muscles (may attenuate to transmission of sound by these bones)

• The stapes connect directly to the internal ear through the oval window → transmission of stapes movement to the lymphatic fluid inside the internal ear

What is the malleus?

• Largest of the ossicles

  • Nine mm long and weighs only 25 mg

• Provides point of attachment with tympanic membrane

• Bulk of bone is the head or caput

What is the incus?

• Shaped like an anvil

• Weighs 30 mg and is around 7 mm long

• Provides intermediate link of ossicular chain

• Incus and malleus articulate by means of a saddle join

What is the stapes (stirrup)?

• Third bone of ossicular chain

• Weighs 4 mg with an area of 3.5 mm 2

• Helps to transmit sound vibrations from eardrum to oval window

• Articulation of the incus and stapes of ball and socket type

• Ossicular chain is held in place by ligaments

What are the Tympanic Muscles? (2)

• Muscles of middle ear attached to ossicles

• Smallest muscles of human body

  • Stapedius muscle

    • Imbedded in posterior wall of middle ear

    • Pulls stapes posteriorly

  • Tensor tympani

    • Pulls malleus anterior and medial

What’s the sound attenuation in the middle ear?

• The middle ear performs a kind of “volume control” → the muscles of malleus can be tensed, resulting in a low frequency damping

• It MUST be activated though by neural impulses to activate these muscles → in order to be activated as noise control, the noise has to be processed by the internal ear → damage could have been occurred already

• Additionally, these muscles are activated just before a person starts to speak → damping mechanism to protect against own voice

How does pressure increase in the middle ear form?

• Sound waves are mechanically transmitted by ossicles of the middle ear to the internal ear, which is filled with watery liquid

• Ossicles perform conversion of pressure changes from an elastic medium (air) to pressure changes of an incompressible liquid (water)

• Ossicles function like a cone: from large surface (tympanic membrane) to smaller surface (stapes)

  • This leads to a pressure increase -> pressure variations at the internal ear are about 20 times stronger than original air pressure variation

  • This pressure increase is necessary to generate the necessary activation of the liquid (otherwise reflection would occur)

How does pressure equalization in the tympanic cavity form?

• The middle ear is not completely airtight, a connection with the Eustachian tube allows for pressure equalization (e.g. meteorological pressure changes)

  • The Eustachian tube leads from the middle ear to the nasopharynx

  • Without pressure equalization, the meteorological changes would “push” the ear membrane inwards → feeling of “pressure on the ear” (felt e.g. when going downhill, or in airplane, or fast elevator)

The internal ear consists of:

• Cochlea: part of the inner ear relevant for hearing-> sound waves are transformed into neural impulses

  • is shaped like a snail shell

  • contains two passages, separated by basilar membrane:

    • Upper: scala vestibul

    • Lower: scala tympani

    • These two passages meet at the apex (the tip) in the helicotrema

• Scala vestibuli connects to middle ear (stapes) through the oval window

What’s the physiology of hearing

• The pressure waves from the middle ear (stapes) reach the cochlea through the oval window -> longitudinal pressure waves are generated in internal ear fluid through scala vestibuli to the apex

• These pressure waves return via scala tympani to the round window

• Round window serves as pressure release, since the fluids are incompressible

The basilar membrane: How does frequencies occur on different locations?

• The travelling (longitudinal) wave meets differing conditions along the basilar membrane:

1. Cochlea is thinner (i.e. fluid is less deep) near the apex

2. Basilar membrane is softer near the apex

• These two conditions are responsible for a mechanical frequency transformation:

  • High frequencies reach high displacement (of the transversal wave) near the base (just behind the stapes and oval window)

  • Low frequencies reach high displacement (of the transversal wave) near the end of the basilar membrane (apex)

What’s the structure of the Basilar membrane? pt1

• The basilar membrane contains the organ of Corti, which is linked to the auditory nerve.

• The organ of Corti consists of the tectorial membrane

  • Unlike the basilar membrane, the tectorial membrane does not cover the whole width of the cochlea, but only partly overlaps the basilar membrane the hair cells a number of additional supporting cells

Explain the edges of the tectorial membrane? pt2

• One edge of the tectorial membrane is attached to the basilar membrane, close to the point of attachment of Rossner’s membrane

• The other edge is supported by about 20000 outer hair cells, which are coded into three rows along the basilar membrane

• These outer hair cells rest upon the basilar membrane at their lower end, with, at their upper end, the stereocilia, reach into the tectorial membrane

What are outer hair cells?

• Outer hair cells add another mechanical source of frequency differentiation: Stiffer, shorter hair cells at the base are more responsible to high frequencies, while the long, more flexible hair cells at the apex bend more easily to low frequencies

• The outer hair cells have only few neural pathways to the brain (and these are slow) → it is assumed that they do not transmit any detailed information about the sound signal to the brain

• The actual transmission of information to the brain is done by the inner hair cells

• Outer hair cells can be controlled by the brain via a feedback loop (see explanation oto-acoustic emissions), it is assumed that outer hair cells help to control the movement of the basilar membrane, e.g. for damping or frequency amplification purposes

What are inner hair cells?

• Around 3500 inner hair cells are responsible for the conversion of the mechanical movement of the basilar membrane into neural impulses

• The inner hair cells are grouped in a single row inside the organ of Corti

• About 30000 nerve fibers lead from the inner hair cells into the brain → each inner hair cell is linked to about 10 nerve cells

• The inner hair cell consist of a main cell body with thin stereocilia on top

• The upward movement of the basilar membrane causes the inner hairs cells to fire when touching the tectorial membrane

Where is frequency information encoded along the basilar membrane?

• in the tonotopic location along the basilar membrane:

  • displacements at the base correspond to high frequencies,

  • displacements at the apex correspond to low frequencies

• In other words, different areas of the basilar membrane reach their maximum displacement for different frequencies → causing the nerve cells to fire at different frequencies

What is Auditory Mechanism

• Processes the acoustic signals of speech

• Has an amazing range of sound pressures

• Has a frequency range of around 10 octaves spanning 20 to 20,000 Hz

  • An octave is a doubling in frequency

Organize the Physiological Principles of the Auditory System

1. Outer ear: Collects sound; “shapes” frequency components

2. Middle ear: Matches airborne acoustic signal with fluid medium of cochlea

3. Inner ear: Performs temporal and spectral analysis on ongoing acoustic signal

4. Auditory pathway: Conveys and further processes the signal

5. Cerebral Cortex: Interprets the signal

What’s the function of the Outer Ear?

• “Collector” of sounds

• Shapes frequency components of sound

  • Enhances certain midrange frequencies

• Pinna of outer ear

  • Funnels acoustical information to the external auditory meatus; localizes sounds

• External auditory meatus

  • Funnels sound to the tympanic membrane

  • Acts as a resonator to amplify frequencies between 2kHz and 4kHz

What’s the function of the Middle Ear?

• Increases the pressure arriving at the cochlea

  • Cochlea is a fluid-filled cavity

• Acts to overcome impedance

  • Impedance is resistance to the flow of energy

• Designed as an impedance-matching device

  • Increases pressure of signal arriving at cochlea

Primary function:

• Match the impedance of two conductive systems

  • The outer ear and the cochlea

• Three impedance-matching mechanisms

  • Area ratio provides a 25 dB gain

    • From tympanic membrane to small oval window

  • Lever advantage provides a 2 dB gain

    • Ossicular chain

  • Buckling effect of tympanic membrane provides a 4-6 dB gain

• These three impedence matching devices combined cause a signal gain of around 31 dB.

• Disorders that impede middle ear function reduce audition

  • Otitis media, otosclerosis, glomus tumors

What’s the function of the Inner Ear?

• Performs frequency and temporal acoustic analysis of incoming acoustical signal

Inner Ear Function: The cochlea

• Establishes first level of auditory processing of incoming acoustic signals

  • Determines frequency components of signal

  • Determines amplitude of signal

  • Identifies temporal aspects of signal

Inner Ear Function: The traveling wave (4)

• Wave-like action of basilar membrane

  • Determines frequency data going to brain

  • Arises from stimulation of perilymph of vestibule

  • Moves along basilar membrane until it reaches point of maximum growth

  • Wave damps after reaching maximum growth

Inner Ear Function: Basilar membrane

• Basilar membrane

  • Determination of ability to analyze frequency

• Characteristics

  • Thicker, stiffer, and wider at base than at apex

  • Traveling wave always travels from base to apex due to impedance gradient of basilar membrane and then quickly damps

  • High frequency sounds are processed at the base whereas low frequency sounds are processed at apex-tonotopic representation

Inner Ear Function: Excitation of hair cells

• Hair cells displaced as traveling wave moves along basilar membrane

• Excitation of outer hair cells displaced relative to tectorial membrane

  • Result of shearing effect on cilia

  • Important for coding intensity in intensity less than 40 dB

• Excitation of inner hair cells

  • Results from fluid flow and endolymph turbulence

  • Essential for coding frequency

What’s the Auditory loudness scale?

• A normal acoustic scale weights all frequencies equally strong, i.e. assumes a linear frequency scale

• Psychophysical experiments have shown that humans perceive very low and high frequencies not as well as frequencies in the range of 2-4kHz

  • The lower frequencies are damped by the middle ear

  • The higher frequencies are attenuated by the resonance characteristics of the ear canal (or rather the lack of resonances)

What’s the phon-scale (and also dBA- weighting)

• Was developed to solve this problem:

  • 0 phon are defined as having the same dBSPL value as the hearing threshold at 1kHz (sine tone)

  • Loudness at other frequencies is related to this perceived loudness at 1kHz → the hearing threshold at other frequencies

What are Audiograms?

• Registration of patient’s hearing threshold over (selected) octave frequencies (i.e. doubling of frequencies)

• Sine waves are (traditionally) used between 250 Hz and 8kHz

• Determines hearing loss (deviation from “normal” threshold) for certain frequencies, the most common is hearing loss in high frequencies (with age) or mid frequencies (due to noise exposure)

What are Oto-acoustic emissions

• It is generally assumed that the nerves first transmit the sound stimulus (the impulse projected into the ear) to the brain → the brain then tells the outer hair cells to react

• As a result, the basilar membrane produces oto-acoustic emission → the change of shape of the outer hair cells results in a specific movement of the basilar membrane

• Discovered by Kemp (1978) who projected short signal impulses into the ear → with some delay the ear produced an echo of the impulses

• The echo occurred much later and much stronger (higher amplitude) than expected by the reflective loop of the ear (external ear → internal ear → external ear) → OAE’s are a reaction of the ear itself (including the brain)

What’s spontaneous OAE’s

• Spontaneous OAE’s were discovered (i.e. no stimulus sound needs to be present) → the basilar membrane may move by itself, even without being stimulated by a sound

• Spontaneous OAE’s are seen as a kind of “convulsion” of the outer hair cells → OAE’s provide evidence that the basilar membrane is actively controlled (and does not only passively react to external sound input)

• Apparently the movements of the basilar membrane can be actively controlled by a feedback process from the brain