Auditory Pathway Flashcards
What are the 3 main characteristics of the auditory pathway?
Polysynaptic:
- there are many neurones and synapses involved from the cochlea to the primary auditory cortex
Tonotopically organised:
- the frequency of sounds are separated throughout the pathway
Bilateral:
- information that we hear is projected to both sides of the cortex
How can sound be localised?
- sound is localised by comparing 2 inputs:
- timing (allows us to determine how far away a sound is)
- loudness of sound
In brief, what are all of the structures involved in the auditory pathway and where are they located?
- the cochlea is located within the inner ear and it contains the first neurones of the auditory pathway
- the axons of these neurones collect to form the cochlear nerve (VIII)
- the cochlear nerve projects to the cochlear nuclei at the level of the rostral medulla
- from the cochlear nuclei, there are projections to the superior olivary nuclei on both sides at the level of the mid-pons
- from the superior olivary nucleus, there are projections to the lateral lemniscus nucleus at the level of the pons-midbrain junction
- from the lateral lemniscus nucleus, there are projections to the inferior colliculus at the level of the caudal midbrain
- from the inferior colliculus, there are projections to the medial geniculate nucleus (MGN) of the thalamus, which in turn projects to the primary auditory cortex
What is the clinical significance of the auditory pathway being bilateral?
- sound is perceived at the primary auditory cortex
- information from one cochlea travels to both sides of the cerebral cortex
injury to the CNS does NOT lead to loss of hearing on one side
- for there to be complete loss of hearing there would need to be damage to the structures of the ear itself, the cochlea, cochlear nerve or cochlear nuclei on that side
- if the lesion is above the cochlear nuclei, there will not be complete loss of hearing on that side
Where is the cochlea located?
- it is a structure of the inner ear that is located within the petrous part of the temporal bone
What happens when we hear a sound?
How is this converted to an action potential?
- air vibrations are carried from the external ear into the middle ear via vibrations of the tympanic membrane
- within the middle ear, the ossicles will vibrate and amplify the sound to transmit it to the inner ear
- the ossicles are malleus, incus and stapes
- the ossicles amplify the sound sufficiently to cause fluid within the cochlea to move
- movement of fluid within the cochlea is detected by hair cells (sensory receptors)
- hair cells transmit action potentials via the first neurones of the auditory pathway
- the peripheral processes of these neurones connect to the hair cells
- the central processes of these neurones collect together to form the cochlear nerve
- their cell bodies are located within the spiral ganglia of the cochlea
What is the spectrum of frequencies heard by humans?
20 - 20,000 Hz
How is the cochlea tonotopically organised?
- high frequency sounds are perceived at the base of the cochlea
- low frequency sounds are perceived at the helicotrema (ampulla / apex) of the cochlea
How can the vestibulocochlear nerve be identified at the level of the brainstem?
- the abducens nerve is located at the centre of the pontomedullary junction
- the facial nerve is located lateral to the abducens nerve
- the vestibulocochlear nerve is located lateral (and slightly inferior) to the facial nerve
- (the nervus intermedius of VII may be present between VII and VIII)
Where are the cochlear nuclei located?
What fibres do they receive?
- there are both dorsal and ventral cochlear nuclei
- they are located at the level of the rostral (open) medulla and surround the cerebral peduncles
- the cochlear nerve projects to both the dorsal and ventral cochlear nuclei, which contain the cell bodies of the second order neurones
How are the cochlear nuclei tonotopically organised?
- high frequency sounds are projected to the deepest part of the nuclei
-
low frequency sounds are projected to the most superficial part of the nuclei
- this is the outer edge of the nuclei - closest to the body’s surface
What are the projections originating from the cochlear nuclei?
- the dorsal and ventral cochlear nuclei receive afferents from the cochlear nerve
- they project bilaterally via the acoustic striae (dorsal, ventral, intermediate)
Where does the dorsal acoustic stria originate from?
How does this travel?
- it originates from the dorsal cochlear nucleus
- some axons will travel from the dorsal cochlear nucleus to synapse at the ipsilateral superior olivary nucleus
- some axons will travel directly from the dorsal cochlear nucleus to the contralateral inferior colliculus
- these fibres cross the midline and do not synapse at the superior olivary nucleus
Where does the ventral acoustic stria originate?
Where does it travel to?
- originates from the ventral cochlear nucleus
- some fibres travel from the ventral cochlear nucleus to synapse at the superior olivary nucleus on the ipsilateral side
- some fibres travel from the ventral cochlear nucleus to synapse at the CONTRALATERAL superior olivary nucleus
- these fibres cross the midline via the trapezoid body
What is the trapezoid body?
- a visible collection of axons that originate from the ventral cochlear nucleus
- they cross the midline at the level of the pons to reach the contralateral superior olivary nucleus
Where does the intermediate acoustic stria originate from?
What pathway does it take?
- originates from both the dorsal and ventral cochlear nuclei
- crosses the midline and ascends to the contralateral inferior colliculus
- does NOT synapse at the superior olivary nucleus on either side
Where is the superior olivary nucleus located?
What is its key role?
- it is located at the mid-to-caudal pons at the same level as the facial motor nucleus
- it is the key structure involved in the localisation of sound by:
- differences in time of arrival
- differences in sound intensity
- (the trapezoid body and lateral lemniscus are bundles of axons also visible at the level of the SON)
How can the superior olivary nucleus play a role in localisation of sound?
What anatomical structures does it contain to account for this?
- the SON contains coincidence detector cells / neurones
- if a sound is heard by the left ear, there is a 700 millisecond delay before this sound is heard by the right ear
- the axon leaving the left ear is longer than the axon leaving the right ear to reach the same coincidence detector cell
- that particular coincidence detector cell detects simultaneous arrival of action potentials from both ears and can interpret the sound as if it is coming from the left side
What is the lateral lemniscus?
What fibres does it contain and what route does it take?
- the superior olivary nucleus has received bilateral auditory information via the acoustic striae
- it now sends ascending fibres via the lateral lemniscus
- some fibres of the lateral lemniscus will ascend directly to the inferior colliculus
- some fibres of the lateral lemniscus will make a stop and synapse at the lateral lemniscus nucleus at the pons-midbrain junction before continuing to the inferior colliculus
- (the lateral lemniscus describes fibres that have originated from the superior olivary nucleus)*
Where is the inferior colliculus located?
How is it tonotopically organised?
- it is located in the tectum of the midbrain
- the more dorsal parts of the nucleus detect low frequency sounds
- the more ventral parts of the nucleus detect high frequency sounds
How does the inferior colliculus send and receive information?
- the inferior colliculus receives the acoustic striae and the lateral lemniscus
- it sends auditory information to the MGN of the thalamus via the brachium of the IC
- inferior colliculi can exchange auditory information via the inferior colliculus commissure
How does the medial geniculate nucleus receive and send information?
- it receives information from the inferior colliculus via the brachium
- it relays precise information regarding intensity, frequency and binaural information to the primary auditory cortex
- it sends fibres to the primary auditory cortex via the acoustic radiations**, which travel in the **sublenticular limb of the internal capsule
Where is the primary auditory cortex located?
What is its function?
- it is located within the transverse temporal gyri (Heschl’s convolutions) on the ventral wall / floor of the lateral sulcus
- the anterior transverse temporal gyri (BA 41) and posterior transverse temporal gyri (BA 42) together make up the primary auditory cortex
- both 41 and 42 receive afferents from the MGN via the auditory radiations
- they are involved in the conscious perception of sound (awareness that we have heard something)
Where is the association auditory cortex located?
What is the role of this?
- this is BA 22 and it is located in part of the temporal and parietal lobes
- it surrounds the primary auditory cortex
- in the left hemisphere, the most posterior part of BA 22 is Wernicke’s (receptive speech) area
- it is involved in giving meaning to the sound that is heard
What are the roles of Wernicke’s and Broca’s areas?
How are they connected?
Wernicke’s area (BA 22):
- located in the left superior temporal lobe and adjoining parietal cortex
- has a role in the interpretation of spoken word and the understanding of written and spoken language (receptive speech area)
Broca’s area (BA 44, 45):
- located in the left inferior frontal gyrus
- has role in regulating the pattern of breathing and vocalisation needed for normal speech
- it has a role in the motor production of speech (expressive speech area)
They are connected via the arcuate fasciculus
What is the result of a lesion to Wernicke’s area?
receptive / fluent aphasia
- the patient is unable to understand written or spoken language
- they are still able to produce words, so their speech is fluent but incoherent
What is the result of a lesion to Broca’s area?
motor / non-fluent aphasia
- patient is still able to understand written and spoken language
- they are unable to produce words and speak fluently
What happens if there is a lesion to the arcuate fasciculus?
conduction aphasia
- Wernicke’s area is intact, so they are still able to understand language
- Broca’s area is intact, so they are still able to speak fluently
- they are unable to repeat phrases
Why is someone with a lesion to the arcuate fasciculus unable to repeat phrases?
- the patient is able to understand the command “repeat after me” as Wernicke’s area is still intact
- in order for the patient to repeat the words, this information needs to travel to Broca’s area as this is involved in the motor production of speech
- the lesion in the arcuate fasciculus prevents information from reaching Broca’s area
- the patient is unable to repeat a word / phrase
How is the auditory cortex tonotopically organised?
-
low frequency sounds reach the anterolateral part of the transverse temporal gyri
- this corresponds to sound detected at the apex of the cochlea
-
high frequency sounds reach the posteromedial part of the transverse temporal gyri
- this corresponds to sound detected at the base of the cochlea
Describe the auditory reflex that leads to movement of the neck
- if a sound is heard (e.g. someone calls your name) and you need to move your neck to look in that direction
- acoustic fibres from the inferior colliculus pass to the superior colliculus
- the superior colliculus gives rise to the tectospinal tract, which travels from the tectum of the midbrain to the cervical spinal cord
- the tectospinal tract connects to motor neurones that innervate the muscles of the neck, allowing rapid movement
What auditory reflex results in movement of the eyes?
- when a sound is heard and the eyes move in the direction of the auditory stimulus
- neurones relay information to the motor nuclei of III, IV, VI at the level of the brainstem
What is involved in the reflex that leads to reduced vibration of the tympanic membrane upon hearing a loud noise?
- acoustic fibres from the superior olivary nucleus travel to:
motor nucleus of V:
- this sends motor fibres to tensor tympani, which tenses the tympanic membrane and prevents amplification of sound
motor nucleus of VII:
- this sends motor fibres to stapedius
- stapedius is connected to stapes, so when it contracts it prevents vibration of the ossicles and amplification of sound
