SENSE OF HEARING EAR. LEARNING OBJECTIVES Describe components and functions of external, middle &inner ear. How sound waves are converted into nerve impulses

SENSE OF HEARING

EAR

LEARNING OBJECTIVES

• Describe components and functions of external, middle &inner ear.

• How sound waves are converted into nerve impulses generated in inner hair cells in cochlea.

• How the rotational linear acceleration are detected by receptors located in semicircular canals, utricle and saccule.

• Know the various type of deafness

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 through auditory ossicles 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

Ear

Parts of the ear

Outer (external) ear Middle ear (ossicles)

for hearing) Inner ear (labyrinth)

for hearing & equilibrium

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

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

Pressure is equalized by the pharyngotympanic (tube ( eustachian or auditory tube

1. Cochlea - hearing

2. Vestibule - equilibrium

3. Semicircular canals - equilibrium

Inner ear = bony “labyrinth” of 3 parts

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

• Is unit of sound expressed in terms of the logarithm of their intensity • Intensity of sound in bels is the log of ratio of the intensity of sound and a standard sound• 0.1 bel is a decibel • 0 decibel=pressure level of0.000204 dyne/cm2 (just auditory threshold for average human)• 0 decibel does not mean absence of sound ,but sound level of intensity equal to that of standard• Human can hear from 20-20,000 Hz • The greatest sensitivity being between 1000-4000Hz range • 0- to 140-dB range from threshold pressure to a pressure potentially

damaging to organ of corti

Chapter 6 The Peripheral Nervous System: Afferent Division; Special SensesHuman Physiology by Lauralee Sherwood ©2010 Brooks/Cole, Cengage Learning

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)

• Total amplification of sound is about 20 times 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

Transmission of Sound Waves

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

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.






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 – Inner hair cells (HEAR)

• Auditory signals are transmitted by the inner hair cells.– outer hair cells are 3-4 times more than inner hair

cells– outer hair cells may control the sensitivity of the

inner hair cells for different sound pitches



Auditory pathway

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

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




Deafness (Hearing impairment/loss)• Is difficulty in hearing• Types of deafness/hearing loss:• Three types of deafness/hearing loss;

1. Conductive deafness/ hearing loss

(impaired sound transmission in external or

middle ear)

2. Sensorineural deafness/hearing loss

(damage to cochlear hair cells or 8th cranial nerve,

rarely ascending aud. pathway or aud. cortex)

3. Mixed hearing loss i.e.

(both conductive & sensorineural hearing loss)

Presbycusis: the gradual hearing loss associated

with aging, affects 1/3rd above 75 yrs of age,

Due to gradual loss of hair cells

Conductive Hearing Loss Sensorineural Hearing Loss

Middle and outer ear infections Age-related hearing loss Collection of fluid in the middle ear (glue ear in children)

Noise exposure

Perforated eardrum Viral infections of the inner ear (e.g. mumps or measles)

Impacted earwax Diseases e.g. multiple sclerosis

Absence or malformation of the outer ear, ear canal, or middle ear

Infections or inflammation of the brain or brain covering – e.g. meningitis

Presence of a foreign bodyAcoustic neuroma, a benign (non-cancerous) tumor affecting the auditory nerve

Otosclerosis (fusion of the bones of the middle ear)

Drugs toxic to the auditory system

Benign (non-cancerous) tumors A brain tumor

• Hearing aids: • Helpful in conductive deafness • Increase the intensity of airborne sounds, • Modify sound spectrum• May help particular pattern of hearing loss at

higher/lower frequencies

• Cochlear implants are available

References

• Human physiology by Lauralee Sherwood, seventh/eighth edition

• Text book physiology by Guyton &Hall, 12th edition• Text book of physiology by Linda .s contanzo,

third edition

Documents

SENSE OF HEARING EAR. LEARNING OBJECTIVES Describe components and functions of external, middle &inner ear. How sound waves are converted into nerve impulses