14.2 Neurobiology of Hearing Flashcards
What is the purpose of the ear?
- Detect sounds
- Maintain balance
What are the 3 main parts of the ear?
- Outer ear
- Middle ear
- Inner ear
Label the diagram
What are the main structures of the inner ear?
- 3 semi-circular canals
- Utricle
- Saccule
Are involved with equilibrium in the ear
What makes up the external ear?
-
Auricle (pinna)
- Captures sound waves
-
External auditory meatus (ear canal)
- Sound waves travel through this
-
Tympanic membrane (ear drum)
- Moves in & out in response to sound
(tympanic membrane marks the beginning of the middle ear)
What is the range of human hearing & the most sensitive range?
- The commonly stated range of human hearing is 20 to 20,000 Hz.
- The human auditory system is most sensitive to frequencies between 2,000 and 5,000 Hz
What is the function of the pinna (auricle)?
- Gather and focus sound energy to tympanic membrane
- The one portion of the auricle that has no cartilage is the called lobule, the fleshy lower part of the auricle
- 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
What is microtia & what are the different grades?
Microtia is a congenital deformity where the pinna (external ear) is underdeveloped
What is the function of the ear canal?
- Functions as an entryway for sound waves, which get propelled toward the tympanic membrane, known as the eardrum
- The concha and external auditory canal acts as RESONATOR, i.e. effectively enhance the intensity of sound that reaches the tympanic membrane (when look in the ear examination see this)
Briefly describe the tympanic membrane (ear drum)
- Is thin and pliable so that a sound, consisting of compressions and rarefactions 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
Talk about the 3 small bones in the middle ear
- Auditory ossicles are consisted of the following smallest three bones in human body to transfer the vibration of tympanic** **membrane to cochlea
- Malleus (hammer): forms a rigid connection with the incus
- Incus (anvil): forms a flexible connection with the stapes
- Stapes (stirrup): connects to oval window
- The inward-outward movement of the tympanum displaces the malleus and incus and the action of these two bones alternately drives the stapes deeper into the oval windowand retracts it, resulting in a cyclical movement of fluid within the inner ear
Explain what the eustachian tube(s) do
- 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 nasopharyx (the nasal part of the pharynx, lying behind the nose and above the level of the soft palate)
What is the function of the middle ear?
- The overall function of middle ear is to amplify the vibrations of tympanic membrane to oval window
- It also matches low acoustic impedance of air to high acoustic impedance of fluid inner ear (impedance matching)
What is the vestibular system?
- Is the sensory apparatus of the inner ear that helps the body maintain its postural equilibrium
- The information furnished by the vestibular system is also essential for coordinating the position of the head and the movement of the eyes
What are the two sets of organs found in the inner ear & explain how they work
- The semicircular canals, which respond to rotational movements (angular acceleration)
- 3 semi-circular ducts at 90 degrees from each other to detect angular rotation of the head
- Are bony passages that contain the fleshy, fluid-filled semicircular ducts
- The utricle and saccule within the vestibule, which respond to changes in the position of the head with respect to gravity (linear acceleration)
The information 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 the responses of the cochlea to sound
Organs functionally related to cerebellum, brainstem, reflex centres of the spinal cord - govern movements of eyes, neck & limbs
How do semicircular canals work?
Semicircular canals mediate interactions between the vestibular system and eye muscles via cranial nerve so plays a smooth movement of the eyes toward the left and right, keeping the visual field stable as the head turns
Briefly explain otoliths organs
- Otoliths organs consist of saccule and utricle which perpendicular each others (and are also called as gravity receptor as the respond to the gravitational forces
- The receptors, called maculae (meaning ”spot”), are patches of hair cells topped by small, calcium carbonate crystals called otoconia
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 brainstem
What are the membranes & compartmements in the cochlea?
- Raissner’s membrane
- Basilar membrane
- Scala vestibuli (vestibular ducts)
- Conducts sound vibrations to the cochlear duct
- Scala tympani (tympanic ducts)
- Together with vestibular duct to transduce the movement of air that causes the tympanic membrane and the ossicles to vibrate, to movement of liquid and the basilar membrane.
- Scala media (cochlear duct)
- Houses of organ corti that transform fluid vibration into nerve impulse
Explain what the Raissner’s membrane does?
- Raissner’s membrane together with the basilar membrane it creates a compartment in the cochlea filled with endolymph, which is important for the function of the spiral organ of Corti
What does the round window do?
- 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
What does the organ of Corti do?
- Organ of corti transduces auditory signals and minimize the hair cells’ extraction of sound energy and consist of two 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 human is not clear yet, but TM may be involved in the longitudinal propagation of energy in the intact cochlea
Explain the process of conversion of a sound wave to an ACTION POTENTIAL
- Sensory signal is the sound wave and the responsibility of converting vibrations into action potentials falls upon the inner row of hair cells in the cochlea
- The apical end of the hair cell contains the stereocilia and that they are arranged in order of ascending lengths from one side of the cell to the other
- The membranes of the stereocilia contain mechanically gated cation channels
- Extending from the gate of the ion channel to the adjacent, taller, stereocilium is a fibrous protein called a tip link
- When the stereocilia bend toward the longest stereocilium the tension in the tip link increases, pulling the gates on the ion channels open, and when they bend in the opposite direction the tension decreases and the gates close
- The stereocilia are bathed in the endolymph of the cochlear duct which is similar to intracellular fluid and has a high K+ concentration
- When the gates on the cation channels open, K+ rushes into the cell, depolarizing the membrane and depolarization opens voltage gated Ca2+ channels on the basal membrane of the hair cell allowing Ca2+ to enter
- The influx of Ca2+ stimulates the release of neurotransmitter by the hair cell triggering an action potential in the neuron that synapses with the hair cell
- The axons of these neurons form the cochlear nerve that transmits the action potential to the auditory cortex of the brain
- In hair cells at rest, about 10% of the K+ ion channels are open resulting in a low frequency of action potentials traveling to the brain when it is perfectly quiet
- This allows for both an increase in action potential frequency when hair cells bend toward the longest stereocilium, and a decrease in frequency of action potentials when the hair cells bend the other way
Explain the process of 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 a 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, pitch is determined by the region of the brain that receives input from the cochlea
Explain the process of perception of sound INparticular LOUDNESS
- LOUDNESS, on the other hand, 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 a greater depolarization of the hair cell, more Ca2+ entry through the voltage-gated ion channels and more neurotransmitter release
- The end result is a greater frequency of action potentials going to the auditory cortex, which is perceived as a louder sound
- Frequency of action potentials ≠ frequency of the sound waves
- The frequency of action potentials is a function of the amplitude of the sound wave whereas the frequency of the sound waves determines which portion of the auditory cortex receives the action potentials
What are the main categories of HEARING LOSS or DEAFNESS & explain them
-
Sensorineural
- 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 eighth cranial nerve or within central auditory pathways
- It often impairs the ability to hear certain pitches while others are unaffected
-
Conductive
- Refers to impaired sound transmission in the external or middle ear and impacts all sound frequencies
- Severe conductive deafness can result from otosclerosis in which bone is resorbed and replaced with sclerotic bone that grows over the oval window
What is bone conduction testing? (& what would happen in these tests)
This is another type of pure-tone test that measures your inner ear’s response to sound
- Weber’s test
- Rinne test
What is auditory brainstem response testing? (ABR)
This type of testing is used to determine whether a specific type of hearing loss (sensorineural) exists (common test in new borns)
