Copyright ©Mark Nelson, 2002. All rights reserved.
Chapter 12: Hearing
What you need to know
(exam questions will be a drawn from this subset of material)

Where are the receptor cells that transduce sound information? What are they called?  What form of energy do they transduce  (p. 287-289)
    the receptors are located in the inner ear (cochlea); they are called "hair cells"; they transduce mechanical energy

What is the general structural organization of the cochlea?  (p.291, Fig. 12-3)
    study Fig. 12-3

What are the two main classes of hair cells in the mammalian cochlea? Which are responsible for sound transduction (p. 290-291)
     inner and outer hair cells; inner hair cells are responsible for sound transduction

Where are the cell bodies of the hair cells in the cochlea?  (p. 291)
     the cell bodies are attached to the basilar membrane

Where are the tips of the stereocilia?  (p. 290-291)
     tips of the stereocilia make contact with the tectorial membrane

What differential motion do hair cells detect?  (p. 290-291)
     differential motion between the basilar and tectorial membranes

Which of the two membranes is stiffer?  (p.290)
     the tectorial membrane is stiffer; sound causes the basilar membrane to vibrate relative to the tectorial membrane

What forms the extracellular fluid around the hair cells?  What's unusual about it? (p. 290)
     the fluid inside the scala media of the cochlea is called endolymph; it has a high K+ concentration

Where are the transduction channels located? what ion flows through the channel? (p.290)
     on the walls and tips of the stereocilia; K+ ions

Is the ionic current through the channels hyerpolarizing or depolarizing?  (p. 290)
   surprisingly, the K+ current is depolarizing (because of the high K+ concentration in the endolymph)

Are hair cells spiking or non-spiking? (p. 290)
     non-spiking

What mechanisms contribute to frequency tuning of cochlear hair cells?  (p. 292-296)
    1) mechanical tuning of the basilar membrane (low freq excites the broad end of the cochlea; high freq excites the narrow end)
    2) mechanical tuning of stereocilia (high-freq hair cells have short, stiff stereocilia)
    3) electrical tuning of the hair cells (membrane potential oscillations)
    4) active control of tectorial membrane tension by efferent control of outer hair cells
What does a typical threshold tuning curve look like for an auditory neuron ?  (p. 294, Fig. 12-6)
     v-shaped plot of threshold versus frequency;
     location of the tip of the "v" (minimum threshold) defines the neuron's "best frequency"

How does the auditory nerve code what frequencies are present in a complex sound?  (p.296-297)
    1) a rough labeled line code; afferents from different parts of the cochlea respond best to different frequencies
    2) a temporal code for low frequencies; APs tend to synchronize with low frequency stimuli (below about 1 kHz)
If the frequency of APs is related to the frequency of the sound, then how is intensity coded by auditory afferents?  (p. 296-297)
     by the number of fibers that are active

What is two-tone suppression?  (p. 296-297)
     a reduction of the response to one tone in the presence of a second nearby tone;
    conceptually similar to lateral inhibition; provide "contrast enhancement" in frequency space

What two physical cues does a barn owl use to localize the direction of a sound in space?  (p. 300-302)
    1) the difference in intensity of sound arriving at the two ears (interaural intensity difference - IID)
    2) the difference in time of arrival between the two ears (interaural time difference - ITD)

What physical cue provides information about azimuthal (left-right) position of the source?  elevation (up-down)?  (p. 300-302)
     ITD - timing diff - azimuthal (left-right);
     IID    - intensity diff - elevation (up-down);  (because the barn owl's left ear is higher than its right ear)

What stage of processing contains a topographic map of sound direction in the barn owl?  (p. 300-302)
     the midbrain;  the homologue of the mammalian inferior colliculus (called the mesencephalicus lateralis dorsalis, MLD)

Why is the topographic map of auditory space in the barn owl considered to be a computational map?  (p. 300-302)
     because spatial topography was not implicitly coded in the receptor array (as it is for the visual and somatosensory systems)
    a spatial topography had to be constructed "computationally" from ITD and IID information

What additional properties of a sound source do bats analyze (in addition to frequency and spatial direction)?  (p. 302-304)
     distance (echo delay); velocity (doppler shift)

Where in the bat's brain would you find topographic maps of target distance and target velocity?  (p. 302-304)
     cerebral cortex (telencephalon); near primary auditory cortex

Which class of  invertebrates have "true ears" that are capable of detecting high-frequency sound?  (p. 304-306)
     insects

What different types of sound receptor organs are found among insects?  (p. 304-306)
     1) specialized vibration sensors (mosquito Johnston's organ)
    2) cerci; air movement detectors found at the rear-end of the abdoment some insects (crickets, cockroaches)
    3) tympanic organs (true ears); typically found on the body or legs of some insects (grasshoppers, crickets, moths)

What similarities are there in the organization of vertebrate and invertebrate auditory systems?  (p.305-307)
     both exhibit tonotopic organization; v-shaped tuning curves; ability to estimate sound direction and distance