Session 8: Hearing - the Ear Flashcards
Describe the physics of sound including its two properties
Sound is a compressed air wave – in an elastic medium -that travels at 343m/s in air (its propagation speed depends on the fluid density) and sound is detected by the ear as vibrations of the air, which can be converted to electrical activity.
Travels at over 1,500m/s in water
Sound has two properties:
- The frequency of the pressure wave is measured in cycles per sound or Hertz (Hz).
- The magnitude (volume) is the intensity of the sound and is measured in decibels (which are calculated as the logarithm of the sound pressure relative to a reference pressure, 10xlog(p/p0) = 1dB.
What is meant by Volume and what is the Human Auditory Frequency Range?
Volume
- Loudness defined relative to threshold (a sound just audible).
- Average person: threshold sound (p(o)) is 2x10^-5 Pascals). Pa = N/m2
- This ratio is called the DECIBEL (dB) = 10.log10(I/I(o)) (note no units)
- Since intensity is proportional to square of the pressure I/I(o) = (P/P(o))^2.
- So Sound Pressure Level – (dB-SPL) = 20.log10P/P(o)
- Acoustic threshold = 0 dB-SPL, conversation 60 and painful sound 120
Human auditory frequency range: 20Hz to 20,000Hz
Many animals can hear higher frequencies – up to 160 kHz (Ultrasound).
As you age, you lose your high frequency hearing first
Describe the pathway of sound through the external ear
The ear can detect movements which are on an atomic scale – with thresholds of 1x10^-16 Watts/cm2.
External Ear
The external ear allows sound waves to pass from the pinna/auricle to the external auditory tube, to reach the tympanic membrane located at the end of the tube.
The sound waves vibrate the tympanic membrane.
Describe the pathway of sound through the middle ear
The auditory ossicles (malleus, incus and stapes) articulate with one another in the tympanic cavity and run from the tympanic membrane to the oval window, transmitting the vibrations across (the stapes attaches to the oval window via the annular ligament).
The middle ear is connected to the nasopharynx via the Eustachian tube, acting to equalise air pressure and drain any fluid from the tympanic cavity.
The middle ear consequently allows conversion of sound waves in the air to the fluid of the inner ear. The pressure transmitted to the oval window is amplified due to the area of tympanic membrane being greater than that of the oval window, and due to mechanical efficiency of the ossicles acting as levers.
Describe the inner ear, including the bony labyrinth
The inner ear consists of the bony labyrinth, the membranous labyrinth and the organ of Corti.
The bony labyrinth consists of a complex series of cavities in the petrous part of the temporal bone and surrounds the membranous labyrinth.
- It makes up the vestibule, three semicircular canals and the cochlea, yet it is the cochlea that is responsible for hearing in response to the vibrations. The cochlea detects two sound parameters: frequency and volume
- The cavities of the bony labyrinth contain perilymph, which runs throughout all the bony cavities.
- The cochlea is shaped like a conical snail shell and the cochlea canal is divided into two sections of scala vestibule and scala tympani.
- The vibrations from the middle ear pass through the oval window into the scala vestibule and then out via the round window.
What lies within the bony spiral canal of the cochlea? Describe it, what does it contain?
Within the bony spiral canal of the cochlea lies the cochlea duct (or the scala media). The cochlea duct is part of the membranous labyrinth; the cochlear duct runs along the outer wall of the bony cochlea.
The scala media is filled with endolymph and is bound by vestibular (or Reissner’s) membrane and the Basilar membrane, which separate the endolymph in the membranous labyrinth from the perilymph in the bony labyrinth.
The endolymph contains a high concentration of K+ ions (equivalent inside and outside) and a low concentration of Na+ ions.
Describe the Organ of Corti
The organ of Corti is the sensory area of the cochlear duct and lies on the basilar membrane. It contains hair cells, which are the sensory receptors for sound stimuli, and runs the entire length of the basilar membrane.
The tectorial membrane runs the length of the organ of Corti and interacts with the hair cells.
What is meant by mechanical tuning and tonotopy?
The Travelling Wave Theory: von Bekesy showed that the basilar membrane resonates and so mechanically amplifies sound with progressively lower frequencies along the length of the cochlea.
Place = frequency….this is tonotopy
Higher frequency sounds are detected by hair cells closer to the round window, lower frequency sounds are detected further away.
What are the two types of hair cells?
Inner hair cells’ stereocillia do not attach to the tectorial membrane and their movement is in response to movement of the endolymph. They are responsible for sensing the sound.
- ·1 row
Outer hair cells’ stereocillia are embedded in the tectorial membrane and movement of the basilar membrane relative to the tectorial membrane allowing for generation of sound impulses. They are responsible for amplitude of sound.
- ·3 rows
- ·Fewer synapses
Describe the properties of hair cells and the mechanism of sound conduction
Properties of hair cells:
- Mechanically tuned – by their location along the cochlea
- Electrically tuned – by expression of particular ion channels;
Signals produced by the hair cells will eventually conduct their signals to spiral (cochlear) ganglion, which are bipolar neurones, forming at the base of the hair cell. These nerve endings eventually form cochlear division of CN VIII.
Mechanism of Sound Conduction
- Vibrations caused by sound pass via the auditory ossicles from the tympanic membrane to the oval window.
- The compression caused on the perilymph in the scala vestibule and scala tympani produces oscillatory movements in the basilar membrane.
Describe the effect of movement of the basilar membrane?
Movement of the basilar membrane will cause movements to the tips of the stereocilia (stereocilla bending), whether the inner hair cells relative to the endolymph or the outer hair cells relative to the tectorial membrane.
Mechanical displacement of the stereocilia in a lateral direction causes influx of K+ions through their membrane, depolarising the hair cells. The more intense the sound, the more the K+ channels open.
- Depolarisation of the cell causes influx of Ca2+ ions through VOCC, and the rise in [CA2+]i results in release of the neurotransmitter to the spiral ganglion cells.
- The action potential will then propagate along the nerve fibres of the CN VIII.
The hair cells located at the base of the basilar membrane respond to high frequencies and those located at the apical aspect respond to low frequencies. This is known as tonotopic distribution of responding receptors.
Describe Spiral Ganglion Neurones
15,000 Inner Hair Cells + Outer Hair Cells (in 4 rows)
- 3 rows of OHCs are generating force => amplification of signal for IHCs to decode
- 45,000 SGN – afferent axons 8th nerve
SGN Types:
- Type 1 (95%) > Inner Hair Cell (IHC)
- Type 2 (5%) > Outer Hair Cell (OHC)
OHC serves as ‘cochlear amplifier’. This amplification is regulated by the olivocochlear system, with the olivocochlear bundle (OCB) providing efferent feedback to the hair cells.
What is meant by sound is encoded as trains of action potentials?
A louder sound (the higher the volume) produces more APs in a larger number of axons
There is also recruitment of neighbouring fibres.
IHC are the primary sense organ. Transmitter release triggers action potentials which propagate into the brain along the 8th nerve. They innervate the Cochlear Nucleus and the auditory brainstem – concerned with sound localisation.
Describe the beginning of the central auditory pathway
The auditory pathway starts with the bipolar neurones in the vestibulocochlear nerve. The vestibulocochlear nerve passes through the internal acoustic meatus to enter the posterior cranial fossa and divide into vestibular and cochlear branches at the brainstem level to the rostral medulla.
The bipolar neurones synapse with second order neurones in the cochlear nuclei of the medulla, projecting in a tonotropic manner. These second order neurones vary in their pathways; some will pass and synapse in the ipsilateral or contralateral superior olivary complex whereas others will pass contralaterally and synapse directly in the inferior colliculus.
What is the Superior Olivary Complex?
The superior olivary complex is divided into 3 primary nuclei: the medial superior olive, the lateral superior olive and the medial nucleus of the trapezoid body, and several smaller periolivary nuclei
The MSO’s main function is detection of interaural time difference (ITD) cues to binaural lateralisation (helps to locate the azimuth of a sound, that is, the angle to the left or right where the sound source is located).
The LSO employs intensity to localise the sound.
The medial nucleus of the trapezoid body is believed to be integral in timing (?)
What happens after the Superior Olivary Complex?
The superior olivary nuclei allows for localisation of sound in acoustic space, by discriminating differences in time of arrival or intensity of sound to each ear. Neurones passing from the superior olivary nucleus to the inferior colliculus will pass through the lateral lemniscus.
From the inferior colliculus, axons are transmitted to the medial geniculate nucleus, which then are projected to the auditory cortex (Broadman’s Area is the primary site where the auditory inputs are received, found on temporal lobe)
Cochlea => Spiral Ganglion Cells => Ventral Cochlear Nucleus => Superior Olivary Complex => Inferior Colliculus => Medial Geniculate Nucleus => Auditory Cortex
What are the types of hearing impairment and consider assessment?
Types of hearing impairments
- Loud noise
- Congenital defects (inherited)
- Infections (e.g. rubella, glue ear)
- Ototoxic comp. (e.g. some antibiotics such as aminoglycosides)
- Trauma (damage to temporal bone)
- Age
Assessment
- Visual inspection (otoscope)
- Audiograms: plot sensitivity against frequency – provides an overall measure of hearing sensitivity
- Otoacoustic emissions (OAE – this is sound generated by outer hair cells – detects movement of OHCs) tests for amplifier function
- Auditory brainstem response (ABR) also used to assess general hearing function in babies (can extract propagation of action potentials along the auditory canal)
