Lecture 20: Hearing 3 Flashcards
Types of afferent fibres innervated by hair cells?
Spiral ganglion cells because their nerve bodies make up a spiral ganglion that goes around the central column with the auditory nerve.
TYPE 1: innervate IHC (90-95% of nerve fibres)
TYPE 2: Innervate OHC (5-10% of nerve fibres)
Efferent fibres to the cochlear?
These descending fibres mostly deal with the OHC that allows a large amount of central control regulating the sensitivity of the ear though these cells.
Tuning of auditory fibres?
These nerve fibres have a very narrow response area and are only activated by a very discrete frequency. Lower frequencies have a slightly larger range but still narrow.
High frequency nerves are localised closer to the base/stapes of the cochlea and lower frequencies further away.
Methods of frequency coding?
- Place principle - All the frequencies are tonotopically organised and frequency is detected by spatial representation from base to apex. Has evolved as we can not detect high frequencies due to firing limitations in the volley principle.
- Volley principle - Low frequencies are detected by temporal firing of nerve fibres locked to the stimulus frequencies = phase locking. Can be summed between a number of nerve fibres to increase this again.
Intensity coding in the Auditory nerve?
Rate of firing of individual nerve fibres increases with sound intensity until saturation
Number of fibres increases as energy spreads along the organ of corti giving us a measure of power.
There are also 3 types of nerve fibres that have increasing threshold levels and these saturate (except the high threshold ones) at different (/increasing) levels of loudness.
Processing of sound as we move up the system?
Ascending audtory system shows increasing complexity of auditory processing.
Cochlea: extraction of frequency, intensity and timing of sounds
Brainstem and cortex neurons: Response is more complex, such as to noise, speech sound (esp. at brainstem), stop and start noises.
Most responses maximal with binaural stimulation
Summary of Auditory pathways?
- Cochlear
- Cochlear nucleus
- Superior olivary complex
- Inferior colliculus
- Medial geniculate body (thalamus)
- Primary auditory cortex
Function of the cochlear nucleus?
Relay nucleus - all afferents terminate in here from the cochlear. Uniaural so just bring in from one ear.
Somatosensory integration with trigeminal nerve and vestibular system.
Fucntion of superior olivary complex?
Binaural integration begins here
Sound localisation and detection of speech in noise
Inferior colliculus function?
Sensory integration especially for hearing and vision enhanced sound localisation
Recieves descending inputs from the auditory cortex
Medial geniculate body of the thalamus function?
Radiates to the Auditory cortex in Heschel’s gyrus of the temporal lobe
Arrangement of the primary auditory cortex?
Not quite as clear cut but is said to be organised tonotopically
Cochlear inplant function?
Insertion and electrical tonotopic activation of the cochlea restores the tonotopic map, this may be what underpins the success of the implant.
Divisions of the cochlear nucleus?
Dorsal Cochlear nucleus
- Dominate part of human Cochlear nucleus. Projects directly to midbrain (inferior colliculus and cortex).
Ventral Cochlear nucleus (anterior and posterior)
- Projects directly to SOC (superior olivary complex -brainstem). Important for sound localisation
Main typs od neurons in the cochlear nucleus?
Ventral cochlear nucleus = bushy cells
- Similar response to type 1 primary auditory afferents
- Project to SOC
- Precise relay of timing information about onset of sound essential for measuring “inter-ear” differences of sound arrival for localisation.
