BIME410 Final (Ear and Hearing)

Auditory Perception

mechanosensation that enables us to perceive sounds

• Detect time dependent changes in the pressure (vibrations) of our surrounding medium

Ear

the organ where sound is transduced into nerve impulses that are perceived by the brain

Outer ear

starts with flesh visible pinna, then the ear canal (auditory canal) and ends at the ear drum (tympanic membrane)

Pinna

serves to focus sound waves through the ear canal toward the eardrum

• Asymmetrical shape of the outer ear filters sounds into the ear (modifies after reflection) differently depending vertical direction

Tympanic Membrane

A thin membranous sheet of tissue that seals the ear canal at its internal end and separates the outer ear from the middle ear.

Middle ear

Includes air-filled cavity with three small bones (ossicles) and the opening to the mouth (Eustachian tube)

• Contains bones that transfer the vibrations of the eardrum into waves in inner ear fluid

Eustachian Tube

Normally collapsed to seal off the middle ear, but opens transiently to allow middle ear pressure to equilibrate with atmospheric pressure during mouth functions (chewing, swallowing, and yawning)

Malleus

One end of this middle ear bone is attached to the tympanic membrane

• Known as a hammer bone

Stapes

Stirrup end of the stapes is attached to the thin membrane that separates the middle ear from the inner ear

Inner ear

Complex cavity (membranous labyrinth encased in the bony labyrinth) where mechanoreceptors are responsible for converting vibrations into AP impulses

Vestibular Apparatus (with semicircular canals)

the sensory organ in the temporal bone that provides the sense of balance and spatial orientation for coordinating movement with balance

• canals independently sense rotations

Cochlea

contains sensory receptors for hearing

• Is a membranous tube that lies coiled within a bony cavity

Oval Window

2 membranous disks to which the stapes is attached

• separate the liquid-filled cochlea from the air-filled middle ear

Round window

separates the liquid-filled cochlea from the air-filled middle ear

Scala vestibuli

separates from the Scala media (middle section) by a thin vestibular (Reissner’s) membrane

Scala Media

separates from Scala tympani (lower section) by the basilar membrane, containing the organ of Corti

Organ of Corti

Houses mechanosensitive hair cells responsible for generating nerve impulses

Sound

The combination of one or more compressional waves in a deformable medium

• Pressure fluctuations from a reference pressure oscillate between higher and lower pressures to move sound through the medium

Amplitude

for the sinusoidal pressure function, equal to the maximum change in pressure, pmax, from the reference pressure

Wavelength

Spatial periodicity (fixed time)

Period

Equal to the inverse of the frequency (fixed position)

Sound localization

the process of determining the location of a sound source

Pinna filtering effect

gives us the ability to localize sound vertically

Horizontal Sound Localization

Signaled in one of three ways:

• Difference in arrival times between the ears

• Relative amplitude of high-frequency sounds (shadow effect)

• asymmetrical spectral reflections from various body parts

Interaural Time Difference (ITD)

Sound from the right side reaches the right ear earlier than the left

Tympanic cavity

Hollow space of middle ear

Tympanic Bone

part of the temporal bone, surrounds the tympanic cavity has an even higher impedance than the cochlea fluid

Resonance

a phenomenon in which an acoustic system amplifies or produce sound waves whose frequency matches one of its own natural (resonant) frequencies of vibration determined by its size, shape, and composition

Organ of Corti

the receptor organ that generates nerve impulses in response to vibration of the membrane

Frequency of sounds

determined by the activation of the hair cells, and thus cochlear nerves, from a particular location in the cochlea

Intensity of sounds

Determined by:

• amplitude of vibration of inner hair sounds in response to different intensities

• Recruitment of a higher number of inner hair and nerve cells in basilar membrane (spatial summation)

• Hair cell and nerve cells at fringes of basilar membrane often recruited only at higher amplitudes

Prestin

a motor protein located in the plasma membrane of cochlear outer hair cells (OHCs) which drive electromotility, rapid length changes in response to depolarization (shorten) and repolarization (lengthen)

Subjective Tinnitus

the most frequent type of tinnitus

Objective Tinnitus

Rarer form of tinnitus that creates pulsating/clicking/buzzing sounds generated by internal body mechanisms.

Utricle and Saccule

sense translations of the head