Audition OHMS Revision

Question 1

What is sound?

Answer 1

A pressure wave transmitted through a medium made ip of alternate compression and rarefaction of the medium caused by vibration of the object

Question 2

What is the overall function of the auditory system?

Answer 2

To collect information about frequency composition, intensity of sound and direction of source of sound

Question 3

What are the three main compartments of the ear?

Answer 3

Outer ear
Middle ear
Inner ear

Question 4

What are the components of the outer ear and what does it do?

Answer 4

Pinna and external auditory meatus
Changes the intensity of frequencies in the range of 2-7kHz which is important for front/back localisation
Separated from middle ear by tympanic membrane (ear drum)

Question 5

What are the components of the middle ear and what does it do?

Answer 5

Has ossicles (malleus, incus and stapes) for mechanical impedance matching between tympanic membrane and oval window as outer ear is filled with air and inner ear is filled with fluid. Ossicles tap against the oval window and cause vibrations in fluid-filled components.
Tensor tympani and stapedius muscles alter efficiency of impedance matching so can protect inner ear from very loud sounds (are also active during speech).
Separated from inner ear by oval window and joined to nasopharynx by eustachian tube.

Question 6

What is impedance matching?

Answer 6

Impedance describes a medium’s resistance to movement so means boosting the pressure to stop energy being lost when low impedance airborne sounds transferred to high impedance fluid

Question 7

What two portions can the inner ear be divided into and what components make up the inner ear?

Answer 7

Divided into bony and membranous labyrinths

Contains cochlea which is made up of three scalae in a spiral - scala vestibuli, scala media and scala tympani

Question 8

Where are the three scala located and what do they contain?

Answer 8

Scala vestibuli - begins at oval window then joins scala tympani at helicotrema (head of spiral) and goes back to oval window
Scala media - runs between scala vestibuli and scala tympani, enclosed by Reissner’s membrane towards scala vestibuli and basilar membrane towards scala tympani
Scala tympani - located under scala media
Scala vestibuli and scala tympani are filled with perilymph (similar composition to CSF) and scala media filled with endolymph (similar to intracellular fluid)

Question 9

Where is the Organ of Corti located? How are sound waves transduced into neuronal impulses in the Organ of Corti?

Answer 9

Located on basilar membrane of inner ear
Oval window is moved by ossicles which sends travelling wave along basilar membrane (which is thin and stiff at base at oval window and gets wider/less stiff towards its apex at the helicotrema - this means high frequency waves have maximum amplitude near oval window and low frequency have maximum aplitude near helicotrema).

Question 10

What does the Organ of Corti contain?

Answer 10

Rows of hair cells with stereocilia in linear rows (inner hair cells) or in V/W formations (outer hair cells)
Along the basilar membrane there are three rows of OHCs and one row of IHCs

Question 11

What is the role of stereocilia?

Answer 11

They rest against the tectorial membrane (found above Organ of Corti) which is stiff so when basilar membrane vibrates the stereocilia are bent sideways.
If basilar membrane moves down, they bend in one direction and links between tips slacken, whereas when basilar membrane moves up the stereocilia bend the other way and tip links tighten

Question 12

What does tightening of stereocilia tip links cause?

Answer 12

It opens stretch-activated cation channels so potassium ions move in from the endolymph in the scala media causing oscillatory membrane potential potential and therefore oscillatory glutamate release at the basal end of hair cells

Question 13

How is the inner ear innervated?

Answer 13

By the 8th cranial nerve (vestibulocochlear nerve) which carries both afferent and efferent fibres.
Most afferent fibres are from inner ear hair cells which synapse one-to-one except a few that innervate outer hair cells and are polled approx. 20 OHCs per afferent fibre
Efferent fibres are from superior olive and mostly innervate OHCs (involved in frequency tuning of basilar membrane)

Question 14

How can outer hair cells contribute to sharpening frequency tuning?

Answer 14

By changing the length of their stereocilia

Question 15

What do afferent fibres in the auditory nerve do?

Answer 15

They have a characteristic frequency depending on the hair cells they innervate - nearby frequencies can also stimulate the nerve fibre but require greater sound intensity so there is a V-shaped tuning curve
Afferent fibres can carry sound frequency in two ways - the combination is known as Duplex theory

Question 16

What is Duplex theory?

Answer 16

The two ways in which afferent fibres can carry sound frequency
Frequencies up to 10kHz - phase-locking is used, where firing of nerve fibre coincides with rising of basilar membrane that depolarises hair cell so firing is same frequency of sound.
Frequency also encoded by position of hair cell along basilar membrane (tonotopicity - tonotopically means that different frequencies stimulate different parts of structure).

Question 17

How is sound intensity encoded?

Answer 17

By the number of action potentials, and increased amplitude of sound wave causes increased depolarisation in hair cells and increased transmitter release

Question 18

What two processes mediate sound localisation?

Answer 18

Sound-shadow of the head - causes intensity differences between the two ears, intensity differences are measured by cells in lateral superior olive that receive excitatory inputs from contralateral cochlea and inhibitory inputs from ipsilateral cochlea - means cells are activated if sound source is on contralateral side
Phase differences - distance to sound source is different from each ear when head is turned towards/away from source, measured by coincidence detectors in medial superior olive

Question 19

When is sound localisation mediated by sound-shadow of the head and when is it by phase differences?

Answer 19

Sound-shadow of head - for higher frequencies 1.5-20kHz

Phase differences - for lower frequencies 40Hz-3kHz

Question 20

Where is the primary auditory cortex (A1) located and how is it arranged?

Answer 20

Found on superior temporal gyrus
Contains tonotopic maps in a columnar organisation
Columns are arranged in patterns of summation (binaural) and suppression (one ear dominant)

Question 21

What i an overview of the auditory pathway?

Answer 21

Auditory nerve enters cochlear nucleus of brainstem, projects to superior olivary complex to inferior colliculus to medial geniculate body to auditory cortex

Question 22

How do hair cells communicate with the cochlear nucleus?

Answer 22

Hair cells form synapses with spiral ganglion cells that travel through auditory nerve (vestibulocochlear nerve) to cochlear nucleus.
Auditory nerve has to travel through internal acoustic meatus

Question 23

What are the two types of hair cells?

Answer 23

Inner hair cells - type 1, thick and myelinated
Outer hair cells - type 2, minor role
They connect to different types of auditory nerve fibres that differ in number/transmission/connectivity etc.

Question 24

What sections of the basilar membrane project to which parts of the cochlear nucleus?

Answer 24

The base of the basilar membrane projects to the medial cochlear nucleus
The apex of the basilar membrane projects to the lateral cochlear nucleus

Question 25

What are the different types of neurons in the cochlear nucleus?

Answer 25

Primarylike (bushy) cells - in anterior ventral cochlear nucleus, responsible for temporal information from phase locking, project to the superior olivary nucleus (both ipsilateral and contralateral)
Chopper (multipolar) cells - in posterior ventral cochlear nucleus
Pyramidal cells - in dorsal cochlear nucleus, use lateral inhibition to detect contrast, project to the inferior colliculus or lateral lemniscus
Principal cells and stellate cells - in dorsal cochlear nucleus, send axons to contralateral nucleus of lateral lemniscus

Question 26

Where does the superior olive receive input from and what is it involved in?

Answer 26

Medial superior olive receives excitatory input from each side
Lateral superior olive receives excitatory input from ipsilateral side and inhibitory input from contralateral side
Superior olive important in sound localisation as depending on which side detects sound first indicates source of sound
In particular, lateral superior olive involved in determining sound intensity and medial superior olive involved in determining timing of sound

Question 27

How does auditory information move from auditory brainstem nuclei (cochear nucleus, superior olivary nucleus and lateral leminiscus nucleus) to inferior colliculus?

Answer 27

Info from all these nuclei integrates in lateral lemniscus pathway and inputs to IC (most IC inputs are from contralateral ear).

Question 28

How is the inferior colliculus arranged and what does it do?

Answer 28

Has three main divisions:
Central nucleus - largest, tonotopically arranged
Dorsal cortex
External nucleus
No unifying theory for function but contains a descending tract that can move head/neck muscles in response to sound (part of tectospinal tract that is involved in auditory reflexes)

Question 29

Where does the inferior colliculus project to?

Answer 29

To the medial geniculate body, via the brachium of the inferior colliculus

Question 30

What does the medial geniculate body consist of?

Answer 30

Three major nuclei - the ventral, medial and dorsal nuclei
Ventral nucleus - receives information from central nucleus of inferior colliculus, is tonotopically organised
Dorsal and medial nuclei - receive input from the outer parts of inferior colliculus and and non-auditory structures so function to integrate information from sensory and motor systems
MGB has projections to auditory cortex (A1) in superior temporal gyrus

Question 31

How is the auditory cortex organised?

Answer 31

Tonotopically organised (although into A2 and other higher cortical areas this organisation breaks down)

Question 32

What do higher cortical areas do and what are some examples?

Answer 32

They integrate acoustic information with contextual information (sensory input, motor input, memories, expectations) to produce complex responses
Wernicke’s area - found in left temporal lobe, involved in speech perception
Broca’s area - found in left frontal lobe, involved in speech production
There are descending projections from auditory cortex that originate from L5 and L6 and target thalamus/inferior colliculus and brainstem - function unclear but thought to be involved in attention