Neurobiology of Hearing Flashcards
What do ears allows us gto do?
- Detect sounds and
- Maintain balance via receptors for hearing and equibilibrium in the ear (called hair cell)
The anatomy of human ears based on its lcoation and form of signal inside it (acoustic, vibration or electric) can be divided into 3 parts: outer ear, middle ear and inner ears
parts of the ears
- External ear, middle ear, cochlea of inner ear = involved in hearing
- Semicircular canals, the utricle and saccule of inner ear are involved with equilibrium
- the external ear is composed of the Auricle (pinna) that captures sound waves, the external auditory meatus (ear canal) through which sound waves travel and the tympanic membrane (eardrum) that moves in and out in response to sound (the tympanic membrane marks beginning of middle ear)
Hearing ranges of humans
- The commonly stated range of human hearing is 20 to 20 000Hz
- Under ideal lab conditions humans can hear as low as 12Hz and as high as 28 kHz though the threshold increases sharply at 15kHz in adults, corresponding to the last auditory channel of the cochlea
- The human auditory system is most sensitive to frequencies between 2,000 and 5,000Hz
- Individual hearing range varies according the general condition of a humans ears, age and nervous system.
The outer Ear (function and anatomy)
Pinna:
- Function of pinna or auricle is to gather and focus sound energy to tympanic membrane
- The one portion of auricle that has no cartilage is called lobule, the fleshy lower part of the auricle
- The function of the lobule is not determined yet
- The concha is the “shell-shaped” structure of the cavity of the external ear
- Pinna and concha are selective filter for different sound frequencies in order to provide clues about the elevation of the sound source.
Four grades of Microtia
- Grade I Microtia - small external ear and a small but present external ear canal
- Grade II Microtia - partially developed ear (usually top portion is underdeveloped) with closed external ear canal (atresia) producing conductive hearing loss
- Grade III Microtia - most common form of microtia with absent external ear and small peanut like vestige structure and canal atresia.
- Grade IV Microtia or Anotia (Absent) - Complete absence of external ear with canal atresia.
Describe the function of the ear canal
The concha and external auditory canal acts as a resonator ie, effectively enhance the intensity of sound that reaches the tympanic membrane by about 10 to 15dB
Ear canal (External auditory meatus/ canal) = functions as an entryway for sound waves, which get propelled toward the tympanic membrane, known as the ear drum.
Describe the Tympanic membrane/ ear drum (middle ear)
The tympanic membrane is thin and pliable so that a sound, consisting of compressions and rarefractions of air particles, pulls and pushes at the membrane moving it inwards and outwards at the same frequency as the incoming sound wave.
It is this vibration that ultimately leads to the perception of sound.
- The greater the amplitude of the sound waves, the greater the deflection of the membrane
- The higher the frequency of the sound, the faster the membrane vibrates.
The 3 smallest bones in human body that are in the auditory ossicles
Auditory ossicles are consisted of the following smallest 3 bones in human body to transfer the vibration of tympanic membrane to cochlea
- Malleus (hammer) - forms a rigid connection with incus
- Incus (anvil) - forms a flexible connection with the stapes
- Stapes (stirrup) - connects to oval window.
how do we get a cyclical movement of fluid within the inner ear?
The inward-outward movement of the tympanum displaces the malleus and incus and the action of these 2 bones alternatively drives the stapes deeper into the oval window and retracts it, resulting in a cyclical movement of fluid within the inner ear.
Function of the eustachian tube/ auditory tube
The Eustachian tube or auditory tube helps ventilate the middle ear and maintain equal air pressure on both sides of the tympanic membrane, inside middle ear and outside the body, via nasopharynx (nasal part of pharynx, lying behing the nose and above level of soft palate).
What is the overall function of the middle ear?
The inner ear- describe the vestibular system
The vestibular system is the sensory apparatus of the inner ear that helps the body to maintain its postural equilibrium
- The information furnished by the vestibular system is also essential for the coordinating the position of the head and movement of eyes
- It engages a number of reflex pathways that are responsible for making compensatory movements and adjustments in body position.
What are the 2 sets of end organis in the inner ear, or labyrinth
- Semicircular canals - these respond to rotational movements (angular acceleration)
- The utricle and saccule within the vestibule, which respond to changes in the position of the head with respect to gravity (linear acceleration)
Proprioceptive vs exteroceptive information - in relation to semicircular canals and utricle and saccule organs
The ifnormation these organs deliver is proprioceptive (dealing with events within the body itself) rather than exteroceptive (dealing with events outside the body) as in the case of responses of cochlea to sound
Functionally these organs are closely related to the cerebellum and to the reflex centres of the spinal cord and brainstem that govern the movements of the eyes, neck, and limbs
Semicircular canals - what they do
Semicircular canals or semicircular ducts respond to angular acceleration and there are 3 pairs of semiciruclar ducts, which are oriented roughly 90degrees to each other for maximum ability to detect angular rotation of the head
- The semicircular canals wont contribute anything and are bony passages that contain the fleshy, fluid-filled semicircular ducts.
- The ducts are the sense organs and the tunnel is not the train; its the drain (the duct) that gets the intended job done.
how does the visual field stay stable as the head tunrs
Semicircular canals mediate interactions between the vestibular system and eye muscles via cranial nerve so plays a smooth movement of eyes toward the left and right, keeping the visual field stable as the head turns.
WHat are Otoliths in the inner ear and what do they consist of?
- otoliths are small particles, composed of a combination of a gelatinous matrix and calcium carbonate in the viscous fluid of the saccule and utricle. The inertia of these small particles causes them to stimulate hair cells when the head moves.
- Otoliths organs consits of saccule and utricle which perpendicular each others and are also called as gravity receptor as they respond to gravitational forces
- The recceptors, called maculae (meaning ‘spot’) ar epatches of hair clels topped by small, calcium carbonate crystals called otoconia.
Inner Ear Anatomy
In the inner ear what monitors the position of the head relative to the vertical
Saccule and Utricle monitors the position of the head relative to the verticle
Due to its position each other that is perpendicular, with any position of the head gravity will bend the cilia of one patch of hair cells, due to the weight of the otoconia to which they are attached by a gelatinous layer.
This bending of the cilia produces afferent activity going through the nerve to the brain stem.
The membranes and compartments of the Cochlea
Cochlea is the main peripherals in the auditory system consisting two membranes and 3 compartments:
- Raissners membrane
- Basilar membrane
- Scala vestibuli (vestibular ducts)
- Scale tympanic (tympanic ducts)
- Scale media (cochlear duct)
Raissners membrane - describe the compartment it creates
Riassners membrane together with the basilar membrane creates a compartment in the cochlea filled with endolymph, which is important for the function of the spiral organ of Corti
Based on experiment evidence, the reissners membrane is believed plays an important role in otoacoustic as a wave on reissners membrane can propagate along whole extent of cochlea.
Basilar membrane in the cochlea
Basilar membrane forms the divison between scala media and tympanic and cause different frequencies to reach maximum amplitutdes at different positions.
BM performs frequency selectivity by its filter bank do is effectiveleu a continuous array of filters whih decompose a complex soound waveform into its constituent frequency components.
Oval window and round window in the cochlea
- Oval window = it receives vibration from stapes and transmit to base of basilar membrane
- Round window = it vibrates with opposite phase to vibrations entering the inner ear through the oval window
It allows fluid in the cochlea to move, which in turn ensures that hair cells of the basilar membrane will be stimulated and that audition will occur.
Frequency waves distirbution over basilar membrane
Organ of Corti - what is it composed of?
- Organ of Corti transduces auditory signals and minimise the hair cells extraction of sound energy and conisst of 2 hair cells and tectorial membranE:
- Inner hair cells (IHC) = detect the sound and transmit it to the brain via the auditory nerve
- Outer Hair cells (OHC) = perform an amplifying role
- Tectorial membrane = the function for the human is not clear yet, but TM may be involved in longitudinal propagation ofenergy in the intact cochlea
- The stereociliar on the tip of the hair cells repsond to fluid motion when the basilar membrane displaces to the right or left
Innervation of the organ of corti
Endolymph and Perylymph, what are they and their function
- Endolymph = fluid contained in the membranous labyrinth of the inner ear
- Perilymph = ECF inside perilymphatic space
Their function is to regulate electrochemical impulses of hair cells
Perilymph resembles ECF in composition (sodium salts are the predominate positive electrolyte) while endolymph resembles Intracellular fluid in composition (Potassium is main cation)
Functions of Endolymph
(Regulate electrochemical impulses of hair cells) and
- Hearing - fluid waves in the endolymph of the cochlear duct stimulate the receptor cells, which in turn translate their movement into nerve impulses that the brain perceives as sound
- Balance - Angular acceleration of the endolymph in the semicircular canals stimulate the vestibular receptors of the endolymph
How does sound go into vibration
- Sound is mechanical wave that needs medium to propagate and itself is created by something that vibrates, for example fork
- Then the vibration creates air particles movement due to change of air pressure
- The osund wave through sair reaches human air pinna
Describe the Perception of sound
- Once the action potential is generated and sent to the brain it is the function of the auditory cortex to convert that action potential into perception and each region of the cochlea is hardwired to its own specific region of the auditory cortex
- When that particular region of the brain receives input from the ear we perceive the unique pitch associated with that frequency of sound wave (Like a piano where each key is like a different segment of the cochlea)
- Each time an action potential reaches that specific segment of the auditory cortex we perceive the same sound
- Therefore, the pitch is determined by the region of the brain that receives input from cochlea.
How is loudness of sound determined
- Loudness is determined by the number of action potentials that reach the brain (sound is a function of the amplitude of the sound wave)
- Sound waves of higher amplitude cause the hair cells to vibrate more vigorously, which would cause more ion channels to open
- This would result in greater depolarisation of the hair cell, more Ca2+ entry through voltage gated ion channel and more NT released.
- The ened result is a greater frequency of action potentials going to the auditory cortexm, which is perceived as a louder sound
What are the 2 major categories of hearing loss or deafness?
Sensorineural and conductive
- Sensorineural hearing loss (deafness) is the most common type of hearing loss, it is usually due to the loss of cochlear hair cells but can be result from damage to the eight cranial nerve or within central sauditory pathways. It often impairs the ability to hear certain pitches while others are unaffected
- Conductive hearing loss refers to impaired sound transmission in external or middle ear and impacts all sound frequencies.
- Among the causes of conduction hearing loss are plugging of the external auditory canals with wax ( cerumen) or foreign bodies, otitis external (inflammation in outer ear “swimmers ear”) and otitis media (inflammation of middle ear) causing fluid accumulation or scarring or perforation of eardrum. Severe conductive deafness can result from otosclerosis in which bone is resorbed and replaced with sclerotic boen that grows over oval window
Types of Hearing loss
Quantitative information about hearing loss
With regard to degree of hearing loss, the audiologist is looking for quantitative information
Hearing levels are expressed in decibels (dB) based on the pure tone average for the frequencies 250 to 8000 Hz and discussed using descriptors related to severity:
▪ normal hearing (up to 20 dB HL)
▪ mild hearing loss (21 to 40 dB HL)
▪ moderate hearing loss (41 to 70 dB HL)
▪ severe hearing loss (71 to 95 dB HL)
▪ profound hearing loss (95 dB HL or greater)
Bone conduction testing
- Another type of pure-tone test that measures your inner ears response to sound
- A conductor will be placed behind your ear, it will send tiny vibrations through the bone directly to the inner ear
- This is different than the traditional version which uses air to send audible sounds
- If the results of this test are different than the pure tone aduiometry, your green valley audiologist can use this information to determine your type of hearing loss.
Common tests with a tuning fork to distinguish between sensorineural and conduction hearing loss
Acoustic reflex testing- what does this measure
This measures involuntary muscle contractions of the middle ear and is used to determine the location of hearing problem (the ossicles, cochlea, auditory nerve, etc) as well as the type of hearing loss.
Auditory Brainstem Response (ABR) - what is this type of testing used for
- This type of testing is used to determine whether a specific type of hearing loss (sensorineural) exists
- It is also frequently used to screen newborns for hearing problems
- In an ABR test, electrodes are attached to the head, scalp or earlobes, and headphones to wear.
- The brainwave activity is measured in response to sounds of varying intensities.
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