Something interesting about tonotopic mapping is that it is similar to the concept of somatosensory mapping (something we go over in every biology class probably). One difference is that the mapping doesn't start on the surface on any of our ear structures but rather it is the neurons that are mapped tonotopically from hair cell afferent neurons to the neurons in the cochlear nucleus to higher levels in teh cortex. Also, I believe that every frequency signal has the same amount of space on the cochlear nuclei. However in a somatosensory map, some things such as the fingers, are over-represented because of the immense amount of neuronal pathways leading from there to the cortex in the brain compared to our toes for example.
What does an Audiograph tell us? (ex: tells sensitivity decreases as you go lower than 20Hz)
What are tuning curves?
- info that goes to the brain by the auditory nerve (type 1 AP spikes) must be extracted and processed to form a perception of the stimulus
- info = which fibers are responding, rate, time patterns of the spikes in each fiber
- response area for a fiber is plotted sound pressure level vs. frequency = tuning curve
CF: the frequency that evokes a response at the lowest sound pressure level , at CF, auditory nerve fibers can respond to sound levels as low as 0dB in the most sensitive range of hearing
- likely generated by active motility of the OHCs
- narrow at low sounds b/c the fiber responds only a narrow band of frequencies near CF
- wider at higher sounds, this reflects the passive mechanical characteristics of basilar membrane motion w/ little contribution
- fibers w/ low CFs innervate the apex of the cochlea and high CFs = more basal places (as expected from the pattern of BM vibration)
Tonotopic Mapping: the precise mapping of frequency to position
- it is preserved as the auditory nerve projects into the cochlear nucleus
- this suggest that the frequency is coded via a place code, w/ neurons at different places coding for different frequencies
- spontaneous APs = no sound stimulation, what causes these?
- 20% voltage-gated channels are open at rest and Na+ is entering, APs are “smaller”
- hair cells RMP = (-55mV)
- but if you get a stronger signal (NT) then you would fire more frequent spikes, more “prominent” AP
- represents how this neuron responds to different frequences, the most sensitive response is @CF
- sharp-tuning, afferent neuron can separate frequencies very well b/c everything happens before the auditory fiber