Audition Flashcards
Which characteristics of sound does frequency and amplitude determine?
Frequency - pitch or tone sound
Amplitude - loudness of sound
What is the function of the external ear?
It funnels sound into the middle ear and it also helps determine where sound is coming from
What is the function of the external ear together with the middle ear?
It collects sound waves and amplifies their pressure so that the sound energy in air can be successfully transmitted to the fluid filled cochlea
Inside the cochlea, what are the fluid filled spaces called? And which types of fluid is in each space?
Scala vestibuli (top) - filled with perilymph
Scala media (middle) - filled with endolymph
Scala tympani (bottom) - filled with perilymph
Describe the structures in the Organ of Corti.
It is divided into 2 from its basal end to its apical end by the basilar membrane and the tectorial membrane.
- basilar membrane is constituted of a single row of inner hair cell and three rows of outer hair cells
- the hair cells have sterocilia at their apical ends
Describe the phenomenon of tonotopy when it refers to the inner ear.
High frequencies are picked up at the base of the basilar membrane whereas low frequencies are picked up at the apex.
What are the functional differences between the inner and outer hair cells?
Inner hair cells - sensory receptors which perceive the sound
Outer hair cells - involved in the modulation of the basilar membrane, therefore controls how much sound is perceived
What is the tallest stereocilium referred to as?
Kinocilium
What are the results of displacing a hair cell bundle?
If the hair bundle is displaced in the direction of kinocilium, the hair cells are depolarised. While displacement in the opposite direction hyperpolarises the hair cell.
Receptor potentials lead to transmitter release which triggers action potentials in cranial nerve 8 fibres
Describe how a traveling sound wave initiates sensory transduction.
The sound wave displaces the sensory hair cells on top of the basilar membrane.
Basilar membrane and tectorial membrane are not anchored at the same positions, therefore the vertical component of the traveling wave is translated into a shearing motion between the 2 membranes which then bends the stereocilia at the apical ends of hair cells. This is leads to voltage changes across the hair cell membrane.
Outline the auditory transduction in the hair cell.
- Displacement of the stereocilia towards the kinocilium.
- Tip links are stretched and force open the cation channels to open near the tips the stereocilia.
- K+ enters and causes a depolarisation
- Voltage gated Ca+2 channels open leading to an influx of Ca+2
- A neurotransmitter is released by hair cells into the afferent nerve
Detail how information travels in the central auditory pathways.
• Spiral ganglion houses the cell bodies of the first order neurons → these neurons receive information from hair cells in Organ of Corti and travel within the osseous spiral lamina (their central axons form the main component of the cochlear nerve)
• Vestibular nerve joins the cochlear nerve entering the internal acoustic meatus to form the vestibulocochlear nerve
• After entering the meatus, it travels a short distance to enter the brainstem at the cerebellopontine angle
• The first order neurons synapse at the ipsilateral cochlear nuclei
• Fibres from cochlear nerve bifurcate and information is sent to the cochlear nuclei on each side of the brainstem:
◦ Ventral (anterior) cochlear nucleus → located in area where nerve enters brainstem
◦ Dorsal (posterior) cochlear nucleus → located posterior to the inferior cerebellar peduncle
• From the dorsal nucleus, most fibres decussate and ascend in the contralateral lateral lemniscus while other fibres ascend in the ipsilateral lateral lemniscus
• From the ventral nucleus, some fibres ascend in the lateral lemniscus bilaterally
◦ Most decussate to the contralateral superior olivary nuclei in the trapezoid body
◦ Some synapse at the ipsilateral superior olivary nucleus → located just next to the trapezoid body (it also projects upwards through the lateral lemniscus)
• Fibres ascending from both cochlear nuclei and superior olivary nuclei (through the the lateral lemniscus) arrive at the inferior geniculate colliculus → where all these fibres carrying auditory information converge
• fibres project to the ipsilateral medial geniculate body (MGB) in the thalamus
◦ MGB does not act as a simple relay centre → it has reciprocal connections with auditory cortex and ,educates refinement of the incoming information
• Projections from MGB proceed then to the primary auditory cortex
◦ Primary auditory cortex located in the superior temporal gurus, under the lateral fissure → it is organised tonotopically
On the central auditory pathways, where is the first point where information from the two ears interacts?
Superior olive - helps localise sound in space
On the central auditory pathways, where is the first point at which sound can interact with the motor system?
Inferior colliculus
What is beneficial about the fibres of the central auditory pathway travelling both ipsilaterally and contralaterally?
Because supranuclear lesions (above cochlear nucleus) will not lead to serious hearing loss
