OHS 314 Chapter 3 Part 2: Physiology of the Auditory System

How do the hair cells of the Organ of Corti transmit neural information? (2)

1) Any movement of the basilar membrane supporting the hair cells will cause the hair cells to bend. 2) The bending of the small hairs produces the nerve impulses in the neurons that are transmitted to the brain.

How can frequency effect the perception of loudness? (5)

1) At any given frequency, loudness varies directly as sound pressure and intensity vary, but not in a simple, straight-line manner. 2) The physical characteristics of a sound as measured by an instrument and the "noisiness" of a sound as a subjective characteristic may bear little relationship to one another. 3) A sound level meter cannot distinguish between a pleasant sound and an unpleasant one. 4) A human reaction is required to differentiate between a pleasant sound and a noise. 5) Loudness is not merely a question of sound pressure level. A sound that has a constant sound pressure can be made to appear quieter or louder by changing its frequency.

What range of frequency can the cochlea detect? (3)

1) Base of the cochlea can detect high frequency sound of 1500-20,000 Hz (0-15mm deep) 2) Deeper portions can detect mid-range (600-1500Hz; 15-20mm deep) and low frequencies (200-600Hz; 20-30mm deep) 3) Different lengths of the cochlea have different audible ranges

What are the 2 major categories of hearing impairment?

1) Conductive 2) Sensorineural

What are features of the inner ear? (3)

1) Contains the receptors for hearing and position sense 2) Includes the vestibular receptive system and the cochlea within the temporal bone 3) Bony labyrinth is filled with fluid called perilymph

What does curve A in the figure show in terms of hearing thresholds for humans? (5)

1) Curve A, the threshold of hearing, represents the level at each frequency where sounds are just barely audible. 2) The curve also shows that human ears are most sensitive around 1-5 kHz. 3) At around 3 kHz a lower-level sound elicits a greater threshold response than higher or lower frequencies. 4) At this most sensitive region, a sound-pressure level defined as 0 dB can just barely be heard by a person of average hearing acuity. 5) The reference level of pressure of 20 μPa was selected to establish this 0-dB level.

What does curve B in the figure show in terms of hearing thresholds for humans? (7)

1) Curve B, the threshold of feeling, represents the level at each frequency at which a tickling sensation is felt in the ears. 2) At 3 kHz, this occurs at a sound-pressure level of about 110 dB. 3) Further increase in level results in an increase in feeling until a sensation of pain is produced. 4) The threshold tickling is a warning that the sound is becoming dangerously loud and that ear damage is either imminent or has already taken place. 5) Between the threshold of hearing and the threshold of feeling is the area of audibility. 6) This area has two dimensions: the vertical range of sound-pressure level and the horizontal range of frequencies that the ear can perceive. 7) All the sounds that humans experience must be of such a frequency and level as to fall within this auditory area.

What level of noise can the ear tolerate? (4)

1) Ear is protected by hard bone 2) Can detect sounds of minute intensity and tolerate sounds of great intensity 3) Loudest sound that can be tolerated is 10^8 (8 bels, 80 dB) 4) Can detect range of 20Hz(low pitched)- 20,000 Hz(high pitched)

How does hearing occur overall? Outline the mechanism

1) External ear collects sound waves and funnels them to the tympanic membrane through the ear canal 2) The tympanic membrane vibrates in response to the sound waves that strike it 3) The vibratory movement transmits to the ossicular chain in the middle ear 4) The vibration of the ossicles creates waves in the inner ear fluid that stimulate microscopic hair cells 5) The stimulation of these hair cells generates nerve impulses that are passed along the auditory nerve to the brains cerebral cortex for interpretation

What is the inner ear made up of? (3)

1) Inner ear is made up of membranous labyrinth structure suspended in perilymph 2) Within membranous labyrinth are sensory organs for hearing: the cochlea; and sensory organs for position: the vestibular system 3) These sensory organs are bathed in endolymph fluid

What is the function of the inner ear labyrinth? (3)

1) Liquid gelatinous substance in cochlea converts sound energy into electrical signals 2) Electrical signals transmitted to the brain 3) Semi-circular canals from vestibular system act for balance and positioning

What is loudness effected by? (3)

1) Loudness is the subjective human response to sound pressure and intensity 2) Depends primarily on sound pressure but is effected by frequency 3) Human ear is more sensitive to high frequency sound than to low frequency sounds

How is the human hearing threshold effected by sound? (4)

1) Need certain sound pressure at certain frequency for sound to be audible 2) Higher amplitude sound can be heard at higher frequencies 3) Certain sound intensities are not sufficient to be heard are lower frequencies 4) More sensitive to higher frequency sounds

What is the organ of Corti within the Cochlea? (3)

1) Organ of Corti is the essential receptor end organ for hearing 2) Nerve endings contained in a complex, slightly elevated structure over the floor of the tube containing the cochlea 3) Supporting network over on which the hair cells rest

What is loudness? (5)

1) Subjective human response to sound pressure and intensity. 2) At any given frequency, loudness varies directly as sound pressure and intensity vary, but not in a simple, straight-line manner. 3) A sound that has a constant sound pressure can be made to appear quieter or louder by changing its frequency 4) A sound level meter cannot distinguish between a pleasant sound and an unpleasant one. 5) A human reaction is required to differentiate between a pleasant sound and a noise

Outline the mechanism of hearing

1) The ear is responsible for translating variations in air pressure from sound sources into the neural perception of sound 2) 3 principle sections of the Ear: Outer Ear, Middle Ear and Inner Ear 3) Outer Ear: consists of the Pinna(external ear), Concha (sound gather), External auditory meatus (ear canal)-function to gather sound energy from environment and focuses it on the Tympanic membrane (eardrum) 4) The configuration of the external outer ear amplifies sound particular at the frequency range of 2-5 kHz, a range that is important for speech perception 5) Middle Ear: from the ear canal the sound waves vibrate the eardrum which in turn vibrates 3 tiny bones in the middle ear: The Malleus (hammer), Incus (Anvil) and Stapes (stirrup) 6) The Stapes vibrates a small membrane at the base of the Cochlea, the Oval Window. 7) The Oval Window of the Middle Ear transmits amplified vibrational energy to the fluids of the Cochlea 8) Because the entire structure of the Cochlea is filled with non-compressible fluid, movement within the Cochlea in response to a push on the Oval Window requires the presence of a moveable outlet membrane, the Round Window which seperates the Scala tympani from the middle ear 9) Inner ear: convert sound energy into neural activity, in mammals the auditory portion of the inner ear is a coiled structure called the Cochlea. The region closest to the Oval Window membrane is the base of the spiral while the other end(snail like) is referred to as the Apex (top) of the Cochlea. 10) Along the length of the Cochlea are 3 parallel canals: Scala tympani, Scala vestibuli, Scala media. The principle elements for converting sounds into neural activity are found on the Basilar membrane; a flexible structure that separates the Scala tympani from the Scala media 11) The Basilar membrane is about 5 times wider at the apex of the Cochlea then that of the base, even though the Cochlea narrows toward the apex. It vibrates in response to sound transmitted to the fluid filled Cochlea by deflections of the Oval window initiated by the bones of the middle ear. 12) Acoustical stimuli initiate a travelling wave in the Cochlea which propagates from the base to the apex of the Basilar Membrane growing in amplitude and slowing in velocity until a point of maximum displacement is reached 13) High frequencies displace the base of the Basilar membrane where its stiffer and low frequencies maximally displace the apex giving rise to a topographical mapping of frequency (high, medium, low frequencies the deeper you go into cochlea) 14) Within the Scala media, and atop the Basilar membrane is the Organ of Corti(the collective term for all the elements involved in the transduction of sounds) 15) The Organ of Corti includes 3 main structures: the sensory cells called Hair Cells, elaborate framework of Supporting Cells, terminations of the auditory nerve fibers 16) Each human ear contains about 1 row of 3500 inner Hair cells and 3 rows of outer hair cells (totaling about 12000 cells) 17) Afferent nerve fibers running from the inner hair cells account for about 95% of the afferent nerve fibers in the auditory nerve and give rise to the perception of sound 18) The outer hair cells receive mostly efferent nerve inputs from the brain that help sharpen the frequency resolving power of the Cochlea. 19) From the upper end of each hair cell protrude relatively stiff tiny hairs called Stereocilia. Each hair cell has roughly 50-200 stereocilia. The heights of the stereocilia increase progressively across the hair cell so the tops approximate an incline plane. 20) Atop the Organ of Corti is the Tectorial membrane. The stereocilia of the outer hair cells extend into indentations at the bottom of the Tectorial membrane. 21) The movements of fluid in the Cochlea cause vibrations of the Basilar membrane. These vibrations bend the stereocilia inserted into the Tectorial membrane. 22) Depending upon the direction of the bend, ion channels in the hair cells either open or close. 23) Ultimately a change in ion conductance in hair cells will either increase or decrease the firing rate of auditory nerve fibers

How does the frequency of sound effect the Cochlea? (3)

1) The frequency (pitch) of the sound determines which part of the cochlea responds 2) High frequencies stimulate the base of the cochlea near the oval and round windows 3) Low frequencies stimulate the nerve endings at the small end of the cochlea

What is the Cochlea? (3)

1) The main part of the inner ear is the cochlea, which is a bony tube about 34 mm (1.34 in) long, filled with liquid and coiled like a snail's shell. 2) The cochlea makes about 2 3/4 turns around a central hollow passage that contains the nerve fibers going to the brain. 3) Terminated in the audiotory nerve which sends electrical impulses to the brain

What is the Organ of Corti of the Cochlea?

1) The organ of corti is mounted about halfway along the spiral of the cochlea on the basilar membrane. 2) The organ of corti is made up of about 30,000 hair cells, arranged in four rows, which are attached to the tectorial membrane in contact with the upper surface of the organ of corti.

Why are the outer and middle sections of the ear considered a conductive hearing mechanism?

1) The outer and middle sections of the ear conduct sound energy to the deeper structures of the ear 2) They act together as a conducter to funnel and propogate sound

How does the vestibular branch of the cochlear (auditory) nerve transmit positional information for balance? (2)

1) The vestibular branch of the auditory nerve (combined called the eighth cranial nerve) transmit impulses to the cerebral cortex 2) Allows us to recognize the position of our head in space as it relates to the pull of gravity

What is the basilar membrane of the Cochlea? What is the scala vestibuli? What is the scala tympani? What is the helicotrema? (4)

1) There is a bony projection or shelf and a membrane called the basilar membrane that runs the length of the cochlea. 2) The basilar membrane divides the cochlea into two chambers, the upper chamber or scala vestibuli, and the lower chamber or scala tympani. 3) There is a small opening at the end of the cochlea, called the helicotrema, which provides a connecting passage between the upper and lower chambers. 4) The basilar membrane varies in width from 0.2 mm (0.008 in) at the oval window to about 0.5 mm (0.020 in) at the end of the cochlea chamber.

What is the Cochlea of the inner ear? (4)

1) Tubular snail-shaped structure lined with basilar membrane 2) Basilar membrane contains thousands of hair cells tuned that vibrate at different sound frequencies 3) Nerve endings contained over the floor of the tube forming the cochlea: organ of Corti 4) Organ of Corti is the essential receptor end organ for hearing

What levels of loudness can damage hearing? (4)

1) Up to 80 dB (green): there is no risk for the ear, regardless of the duration of the sound exposure 2) From 80 to 90 dB (yellow): we are getting closer to the danger zone, but the risks are limited to very long exposures. 3) From 90 to 115 dB (red): the danger zone: the louder the sound the less time is needed for damage to occur. 4) Above 115 dB (brown), very brief sounds immediately cause irreversible damage.

What is the upper limit of frequency at which a sound can be heard? (3)

1) Upper frequency at which airborne sounds can be heard depends on the conditions of persons hearing and on the intensity of the sound 2) For young adults the upper limit is between 16,000 to 20,000 Hz 3) People tend to lose sensitivity for higher frequency sounds as they grow older (presbycusis)

How does the vestibular or semicircular canal system function in the inner ear? (4)

1) Vestibular system is responsible for sense of balance; acts as an organ of equilibrium 2) Located near the cochlea as 3 semicircular canals lying in perpendicular planes to each other 3) Canals contain endolymph fluid that responds to movement of the head 4) Additional positional information is provided by receptors located in the vestibule

How does the Cochlea process sound? (4)

1) Vibrations of stapedial footplate set in motion the inner fluids of the inner ear 2) Basilar membrane displaced, creates shearing movement on tectorial surface 3) Drags the hair cells attached to the nerve endings 4) Sets up electrical impulses that are coded and transmitted to the brain via the cochlear (auditory) nerve

Why are the deeper structures of the ear such as the inner ear and auditory nerve considered a sensorineural mechanism?

Inner ear structures are responsible for sound transduction into a nerve impulse that is propogated to the brain via sensory nerve systems. This includes the inner ear cochlea, vestibular system and auditory nerve.