SENSE OF HEARING EAR. Ear Consists of 3 parts –External ear Consists of pinna, external auditory...

SENSE OF HEARING

EAR

Ear

• Consists of 3 parts– External ear

• Consists of pinna, external auditory meatus, and tympanum• Transmits airborne sound waves to fluid-filled inner ear• Amplifies sound energy

– Middle ear• Transmits airborne sound waves to fluid-filled inner ear• Amplifies sound energy

– Inner ear • Houses 2 different sensory systems

– Cochlea » Contains receptors for conversion of sound waves into

nerve impulses which makes hearing possible– Vestibular apparatus

» Necessary for sense of equilibrium

Parts of the ear

Outer (external) ear

Middle ear (ossicles) for hearing)

Inner ear (labyrinth) for hearing & equilibrium

Sound in external acoustic meatus hits tympanic membrane (eardrum) – it vibrates

Pressure is equalized by the pharyngotympanic tube (AKA eustachian

or auditory tube

The Tympanic Membrane and the Ossicular System

• Tympanic membrane functions to transmit vibrations in the air to the cochlea

• Amplifies the signal because the area of the tympanic membrane is 17 times larger than the oval window

• Tympanic membrane connected to the ossicles– malleus– incus– stapes

Attenuation of Sound by Muscle Contraction

• two muscles attach to the ossicles– stapedius– tensor tympani

• a loud noise initiates reflex contraction after 40 - 80 milliseconds

• attenuates vibration going to cochlea• serves to protect cochlea and damps low

frequency sounds i.e., your own voice

1. Cochlea - hearing

2. Vestibule - equilibrium

3. Semicircular canals - equilibrium

Inner ear = bony “labyrinth” of 3 parts

Organ of Corti

• receptor organ that generates nerve impulses• lies on the surface of the basilar membrane,

contains rows of cells with stereo cilia called hair cells

• the tectorial membrane lies above the stereo cilia of the hair cells

• movement of the basilar membrane causes the stereo cilia of the hair cells to shear back and forth against the tectorial membrane

The Organ of Corti

Nerve Impulse Origination

• The stereo cilia, when bent in one direction cause the hair cells to depolarize, and when bent in the opposite direction hyperpolarize.– this is what begins the neural transduction of the

auditory signal• Auditory signals are transmitted by the inner

hair cells.– 3-4 times more outer hair cells than inner hair cells– outer hair cells may control the sensitivity of the

inner hair cells for different sound pitches

Hearing

• Neural perception of sound energy• Involves 2 aspects

– Identification of the sounds (“what”)– Localization of the sounds (“where”)

• Sound waves– Traveling vibrations of air– Consist of alternate regions of compression and

rarefaction of air molecules

Formation of Sound Waves

Hearing

• Pitch (tone) of sound– Depends on frequency of air waves

• Intensity (loudness)– Depends on amplitude of air waves

• Timbre (quality) – Determined by overtones

Decibel

• unit of sound• expressed in terms of the logarithm of their

intensity• a 10 fold increase in energy is 1 bel• 0.1 bel is a decibel• 1 decibel is an increase in sound energy of 1.26

times

Sound Wave Transmission

• Tympanic membrane vibrates when struck by sound waves

• Middle ear transfers vibrations through ossicles (malleus, incus, stapes) to oval window (entrance into fluid-filled cochlea)

• Waves in cochlear fluid set basilar membrane in motion

• Receptive hair cells are bent as basilar membrane is deflected up and down

• Mechanical deformation of specific hair cells is transduced into neural signals that are transmitted to auditory cortex in temporal lobe of brain for sound perception

Bending of Hairs on Deflection of Basilar Membrane

Determination of Sound Frequency and Amplitude

• Place principle determines the frequency of sound perceived.– Different frequencies of sound will cause the

basilar membrane to oscillate at different positions.– Position along the basilar membrane where hair

cells are being stimulated determines the pitch of the sound being perceived.

• Amplitude is determined by how much the basilar membrane is displaced.

The “Place Principle”

Determining the Direction of Sound

• superior olivary nucleus divided into lateral and medial nuclei

• lateral nuclei detects direction by the difference in sound intensities between the 2 ears

• medial nuclei detects direction by the time lag between acoustic signals entering the ears


Auditory pathway

Deafness

• Nerve (sensorineural) deafness– impairment of the cochlea or the auditory nerve– Acoustic neuroma– Cerebello-pontine angle tumor

• Conduction deafness– Obstruction of External auditory canal e.g. wax

or foreign body– impairment of tympanic membrane or ossicles

Equilibrium

• Vestibular apparatus– In inner ear– Consists of

• Semicircular canals– Detect rotational acceleration or deceleration in any

direction• Utricle and saccule

– Detect changes in rate of linear movement in any direction

– Provide information important for determining head position in relation to gravity

Equilibrium

• Neural signals generated in response to mechanical deformation of hair cells by specific movement of fluid and related structures

• Vestibular input goes to vestibular nuclei in brain stem and to cerebellum for use in maintaining balance and posture, controlling eye movement, perceiving motion and orientation

Equilibrium

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