Text 1 Flashcards
How sounds is processed
The pressure waves of sound are collected by the pinna and funneled to the tympanic membrane by
the external auditory canal. The tympanic membrane vibrates
in response to the sound, which sets the ossicular chain into motion. The mechanical movement of the ossicular chain then sets
the fl uids of the cochlea in motion, causing the hair cells on the
basilar membrane to be stimulated. These hair cells send neural
impulses through the VIIIth cranial nerve to the auditory brainstem. From the brainstem, networks of neurons act on the neural
stimulation, sending signals to the auditory cortex.
Pinna/Auricle
Acts as a funnel and collects and directs sound further into the ear
visible portion of the ear, skin covered cartilage
serves as a resonator, enhancing sound by 4500 Hz
Head shadow effect – The head may attenuate speech intensity by 6.4 dB • Pinna effect – Each individual's pinna creates a distinctive imprint on the acoustic wave traveling into the auditory canal
Function of the outer ear
Collection of sound, localization, resonance and protection
Concha
the bowl at the entrance of the external auditory meatus
Helix
Upper rim of the ear, The helix has a 2 dB peak at 4000 Hz.
The concha has a 9 dB increase at 5300 Hz.
Auricle/pinna
-collects sound waves and
ITD (Interaural time distance)
The difference in the two ears sound wave arrival time (pinna/auricle) part of the localization part
Ear canal/external auditory meatus
- 5bcm long, 23-29mm. Outer 1/3 is cartilaginous and contains hair, sebaceous and ceruminous glands. Inner 2/3 is bone and skin. S shaped
- Resonator: provides about 10 dB of gain to the eardrum at around 3,300Hz
- Sounds in the 2,000 to 4,000 Hz region are amplified by up to 20db, Noises in this range are the most hazardous to hearing
Cerumen
A mixture of skin, sweat, hair, and debris held together with a fluid secreted by glands inside the ear canal.
Purposes: To repel water, trap dust, sand, micro organisms, odor discourages insects, antifungal, moisturizes the epithelium
Tympanic membrane
At the end of the ear canal, membranous, cone-shaped, 10mm in diameter. Movements are small about one billionth of a cm.
Made up of 2 sections: Pars flaccid (smaller) and pars tensa (longer)
Vibrates with a magnitude that’s the same as the intensity of the sound wave
Creates a barrier that protects the middle and inner areas from foreign objects
Separates outer and middle ear
Muscles of the middle ear
Tensor tympani connects to cranial nerve V - the trigeminal nerve, connected to the malleus
Stapedius muscle - connects to the stapes and cranial nerve V - the facial nerve
Function of the middle ear muscles
They help maintain ossicles in proper position, protects the inner ear from excessive sound
levels. When ear is exposed to sound levels above 70 dB, the muscles contract, decreasing amount of
energy transferred to inner ear. This protective reflex is termed “acoustic reflex
Middle ear
Air filled space within the temportal bone of the skull and functions as an impedance matching device. Contains the ossicular chain and the eustachian tube
Functions of the middle ear
Protects by creating a barrier that protects the middle and inner areas from foreign objects.
Protection from loud sounds.
Conductor: Conducts sound from the outer to the inner ear
Transducer: converts acoustic energy to mechanical energy. Converts mechanical energy to hydraulic energy.
Amplifier: impedance matching of the middle ear.
(only about 1/1000 of the acoustic energy in air would be transmitted to the inner-ear fluids (about 30dB hearing loss)
-Bridge between pressure waves in the tympanic membrane and the fluid waves of the cochlea
Matches the energy transfer from air to fluid
Impedance matching of the middle ear
Enhances the transfer of acoustical energy:
1. The area of the eardrum is about 17 times larger than the oval window. The effective pressure is increased by this amount [the force].
2. The ossicles produce a lever action that further amplifies the pressure.
Without the transformer action of the middle ear 1/1000 of acoustic energy in air transmitted to the inner ear meaning 30db loss
Eustachian tube
-Connects the front wall of the middle ear with the
nasopharynx.
• Operates like a valve, which opens during
swallowing and yawning
• Equalizes the pressure on either side of the eardrum,
which is necessary for optimal hearing
– Without this function, a difference between the static
pressure in the middle ear and the outside pressure may
develop, causing the eardrum to displace inward or
outward and reduces the efficiency of the middle ear and
less acoustic energy will be transmitted to the inner ear.
The inner ear
Sensorineural receptor organ that analyzes sound stimulus
for frequency, intensity and temporal properties
• Transmits information to CNS for further processing and interpretation
Divided into the vestibule, cochlea and semicircular canals
Inner ear is enclosed in bony labyrinth inside is a membranous labryinth of the same shape. The space between membrane and bone is filled with perilymph (scala tympani and scala vestibule) and the inside the membranous labryinth is endolymph (scala media > different fluid composition = electrochemical enviornment
Function of the inner ear
Converts mechanical sound waves to neural impulses (electrochemical energy) that can be recognized by the brain for: – Hearing – Balance
Perilymph
cochlear fluid high in sodium and calcium
Endolymph
Cochlear fluid high in potassium and low in sodium
Scala tympani
In the cochlear duct, filled with perilymph, bottom, round window
Scala media
middle of the cochlear duct, filled with endolymph
Its the cochlear partition > separates the scala tympani and the scala vestibule
Scala Media has a large positive potential known as
endocochlear potential (EP) and may be the driving force for signal transduction
• +80 mV relative to scala tympani - this voltage difference is called the
endocochlear potential (EP)
Scala vestibule
top, terminates basally at oval window, filled with perilymph
Resner’s membrane covers the partition separating it from the scala vestibuli
Basilar membrane base of the partition separating it from the scala tympani
Heliotrema
Passage connecting scala vestibuli and scala tympani
Vestibular mechanism
Shares inner ear bony labyrinth with the auditory system and the vestibular and auditory nerves form the VIII cranial nerve
-provides accurate info about air position in space and about the direction and speed of our movement
Consists of 2 groups = otolithic organs (utricle and saccule = linear motion and head positioning) and 3 semicircular canals (rotary motion)
The cochlea
Sensory organ of hearing Resembles a snail shell and spirals for about 2 3/4 turns around a bony column • Within the cochlea are three canals: – Scala Vestibuli – Scala Tympani – Scala Media
Organ of Corti
-On the basilar membrane and contains the 2 sensory cells of hearing: Outer and inner hair cells
Outer hair cells
elongated or tube shaped, embedded in the tectorial membrane, 3-5 rows of outer hair cells, mostly efferent or motor fibers of the nervous system, receives most efferent fibers from VIII cranial nerve, Stereocilia form letter W
There’s 12- 13,000 outer hair cells in the cochlea
Aid in frequency discrimination
• Assist in hearing of soft sounds
• May play a role in hearing of loud sounds
Inner hair cell
3500 IHCs, One row, Flask-shaped
• Receive most afferent 8th nerve fibers
•Stereocilia form a line
• Stereocilia not attached to Tectorial membrane
Rely of OHC for soft sounds
• Transmit sound wave information to hearing nerve
Stereocilia (cilia)
Approx. a hundred on top of each cell
• Three rows of graded lengths
• Attached by transverse (lateral) links, both in the same row and from row
to row
• thin tip links (involved in the mechano-transduction process)
Physiology of the Cochlea
Cochlea receives sound wave from ME
– Stapes moves annular ligament of the oval window which stimulates sensory cells and causes neural impulses
– Perilymph displaced at the basal end of cochlea
– Wave propagates toward the apex
– Round window moves out of phase with the oval window
– Energy converts from mechanical to hydraulic
Higher frequencies = closer to the oval window
lower frequencies = further from the oval window
Basilar membrane
Bottom of the cochlear duct
arranged tonotopically, each frequency stimulates a different place
The Beskey traveling wave theory
Basilar membrane displacement as a function of frequency
Beskey’s Traveling Wave Theory
Each location of the basilar membrane responds best to a small range of
frequencies
• The place on the BM with the biggest displacement
dictates the frequency response
• Frequency responses are arranged from high (base) to low
at the (apex= the middle of the cochlear), or tonotopically
• Intensity of sound depends on the magnitude of wave displacement or it’s amplitude
• The greater is the amplitude of sound wave the larger is
the effected area of BM and more nerve fibers fire
Traveling wave theory is too simplistic
• Describes cochlea as passive
• Does not fully explain frequency discrimination ability of cochlea
• Bekesy’s observations were made in cadavers
• Active mechanisms take part in activation of
living cochlea – Johnstone and Boyle, end of
1990’s
Physiology of the Cochlea
Soundwave propagation inside cochlea
– Vibration of scala vestibule via perilymph →
– Cochlear duct receives the wave due to compressions of Reissner’s membrane →
– Endolymph is displaced →
– Wave-like motion of the basilar membrane →
– Vibrations transmitted to organ of Corti →
– Movement of tectorial membrane →
– Shearing of hair cells
Hair cell transduction
Shearing motion between the hair cells & Tectorial membrane converts sound energy and transmit it to the nerve
• Shearing opens up potassium ion channels. The current
flowing through these channels alters the cell’s membrane
potential (this is the electrical response-depolarization):
– Resting potential of cell membrane changes to receptor potential
(endocochlear potential)
• Calcium channels open & calcium ions rush into HC →
• Neurotransmitter is released at the basal end of HC
• Action potential is elicited & stimulates the nerve endings
Auditory Nerve (AN)
AN is a bundle of axons (nerve fibers) that synapse(communicate) with hair cells
• Consists of 30,000 neurons
• Most fibers connect to IHCs
• AN maintains the tonotopic organization of the basilar membrane
• Each AN neuron has a characteristic frequency to which it is sensitive
From the Auditory nerve to the brain
The auditory nerve sends sound information up through a chain of
nuclei in the brainstem
• Sound information receives further processing at each nuclei
• Final processing occurs in the auditory cortex, located at the top of the temporal lobe in the Sylvian fissure
• Additionally, descending neural pathways exist to
allow higher centers to control lower centers
Ascending Auditory Pathway
Organ of corti to the cochlear nerve to the cochlear nucleus to the superior olivary complex to the lateral lemniscus to the inferior colliculus to the medial geniculate nucleus to the auditory cortex
Auditory cortex - organized tonotopically - by frequency
Auditory Cortex- Transverse Temporal Gyrus
Endocochlear potential
Perilymph has a similar composition to CSF
• Rich in sodium (Na) and poor in potassium (K) and calcium (Ca)
• Endolymph has a unique composition
• Rich in K and poor in Na and almost lacking in Ca
• Differences in electrolyte contents of the cochlear fluids
create electrochemical potentials
• Scala Media has a large positive potential known as
endocochlear potential (EP) and may be the driving
force for signal transduction
• +80 mV relative to scala tympani - this voltage difference is called the
endocochlear potential (EP)
Audiologist
An audiologist is a healthcare professional conceerned with the assessment, rehabilitation and habilitation of hearing and balance disorders
The role of an audiologist
Diagnostician, therapist, dispenser, consultant and researcher
Hearing loss in the US statistics
1 in 8 people over the age of 12, 13%, 30 million have hearing loss in both years
2 out of 3 children were born with a detectable level of hearing loss in 1 or both ears
The father of audiology
Dr. Raymond Carhart
Sound
vibratory energy transmitted by pressure waves in the air or other media
Hearing
The preception of sound
Where does sound come from?
The compression of molecules in the medium through which it’s traveling
Air is a sound medium
Requirements for sound:
- There must be a source of vibrational energy
- The energy has to then be delivered to and cause a disturbance in the medium
- Medium needs to have mass and be compressible or elastic
- The disturbance is spread in the form of sound waves and they form a compression of a medium or condensation and expansion of the medium or rarefraction
Condensation
Density of air moleules are increased, causing increased pressure
Rarefraction
density of air molecules decrease, causing decreased pressure
Simple Harmonic Motion (sinusoidal motion)
The back and forth movement of an object
Cycle
One complete period of compression and rarefraction of a sound wave
Phase
Any stage of a cycle
Intensity
the magnitude of a sound = loudness or the distance a molecule moves
Audiogram
A graph of thresholds of hearing sensitivity as a function of frequency
Frequency
The number of cycles occured at 1 speed, the speed of vibration
Pitch
the perception of speech
Displacement
The movement of a molecule
A period
The length of time for a sine wave to complete a cycle
Sinusoid
A periodic wave that repeats itself at regular intervals ata time
Spectrum
Different frequencies in sounds , complex sounds
Blood supply to the auditory nervous system
2 sources: One that supplies the brainstem structures (basiliar artery) and the other supplies cortical structures (middle cerebral artery)
Processing of speech info
occurs throughout the central auditory system
The left temporal lobe
Primary location for processing information
The right EAR is dominant for processing info too
dynamic range
the range between the threshold of sensitivity and the treshold of discomfort
