Sensory Coding and Transduction Cochlear Mechanotransduction Mammalian Auditory Pathway
6 critical steps Sensory Coding and Transduction
Physical Dimensions of Sound Amplitude - height of a cycle - relates to loudness Wavelength ( λ ) - distance between peaks Phase ( ) - relative position of the peaks Frequency (f ) - cycles per second - relates to pitch Summary Recapitulation previous lectures
Sound is a longitudinal pressure wave: a disturbance travelling through a medium (air/water) The Adequate Stimulus to Hearing Recapitulation previous lectures Summary
The Adequate Stimulus to Hearing Particles do NOT travel, only the disturbance Particles oscillate back and forth about their equilibrium positions Distance from source Duration Recapitulation previous lectures Summary Compression Decompression Compression
Sensory Coding and Transduction
A Sensor Called Ear Sensory Coding and Transduction
Peripheral Auditory System Outer Ear: - Extents up to Eardrum - Visible part is called Pinna or Auricle - Movable in non-human primates - Sound Collection - Sound Transformation Gives clues for sound localization Sensory Coding and Transduction
Peripheral Auditory System Sensory Coding and Transduction Elevation (deg) The Pinna creates Sound source position dependent spectral clues. Frequency “EAR PRINT”
Elevation (deg) Frequency kHz Amplitude (dB) In humans mid-frequencies also exhibit a prominent notch that varies in frequency with changes in sound source elevation (6 – 11 kHz) Elevation Peripheral Auditory System Sensory Coding and Transduction
Barn Owls have Asymmetric Ears and Silent Flight. One ear points upwards, the other downwards. Peripheral Auditory System Sensory Coding and Transduction
Middle Ear: (Conductive hearing loss) - Mechanical transduction (Acoustic Coupling) - Perfect design for impedance matching Fluid in inner ear is much harder to vibrate than air - Stapedius muscle: damps loud sounds Three bones (Ossicles) A small pressure on a large area (ear drum) produces a large pressure on a small area (oval window) Peripheral Auditory System Sensory Coding and Transduction
Inner Ear: The Cochlea is the auditory portion of the ear Cochlea is derived from the Greek word kokhlias "snail or screw" in reference to its spiraled shape, 2 ¾ turns, ~ 3.2 cm length (Humans) Peripheral Auditory System Sensory Coding and Transduction
The cochlea’s core component is the Organ of Corti, the sensory organ of hearing Peripheral Auditory System Sensory Coding and Transduction Cochlear deficits cause Sensorineural hearing loss Its receptors (the hair cells) provide the sense of hearing
The Organ of Corti mediates mechanotransduction: Peripheral Auditory System Sensory Coding and Transduction The cochlea is filled with a watery liquid, which moves in response to the vibrations coming from the middle ear via the oval window. As the fluid moves, thousands of hair cells are set in motion, and convert that motion to electrical signals that are communicated via neurotransmitters to many thousands of nerve cells.
Cochlear anatomy Sensory Coding of Sound
Cochlear anatomy (straightened) Sensory Coding of Sound
Pressure waves distort basilar membrane on the way to the round window of tympanic duct: - Location of maximum distortion varies with frequency of sound - Frequency information translates into information about position along basilar membrane Tonotopic coding Sensory Coding of Sound
Travelling wave theory von Bekesy: Waves move down basilar membrane stimulation increases, peaks, and quickly tapers Periodic stimulation of the Basilar membrane matches frequency of sound Location of peak depends on frequency of the sound, lower frequencies being further away Sensory Coding of Sound Travelling Wave Theory
Cochlear Fourier Analysis High f Med f Low f Periodic stimulation of the Basilar membrane matches frequency of sound BASEAPEX Location of the peak depends on frequency of the sound, lower frequencies being further away Position along the basilar membrane Sensory Coding of Sound
Location of the peak depends on frequency of the sound, lower frequencies being further away Location of the peak is determined by the stiffness of the membrane Travelling wave theory von Bekesy: Waves move down basilar membrane Sensory Coding of Sound Place Theory
Thick & taut near base Thin & floppy at apex TONOTOPIC PLACE MAP Sensory Coding of Sound Sensory Input is Tonotopic LOGARITHMIC: 20 Hz -> 200 Hz 2kH -> 20 kHz each occupies 1/3 of the basilar membrane
Sensory Coding of Sound Sensory input is tonotopic
The COCHLEA: Decomposes sounds into its frequency components Sensory Coding of Sound Sensory Input is Non-linear Has direct relation to the sounds spectral content Represents sound TONOTOPICALLY Has NO linear relationship to sound pressure Has NO direct relationship to the sound’s location in the outside world
Cochlear nonlinearity Active processing of sound BM input-output function for a tone at CF (~9 kHz, solid line) and a tone one octave below (~4.5 kHz) taken from the iso-intensity contour plot. INPUT level (dB SPL) OUTPUT Response in dB CF= 9 kHz ~4.5kHz
Effects of an “active”cochlea Iso-level curves show sharp tuning at low sound levels, broader tuning at high levels. Response is strongly compressive around the so-called characteristic frequency (CF). Requires functioning outer hair cells. Cochlear nonlinearity Active processing of sound The response of the BM at location most sensitive for ~ 9 KHz tone (CF). The level of the tone varied from 3 to 80 dB SPL (iso-intensity contours).
Cochlear nonlinearity Active processing of sound The response of the BM at location most sensitive for ~ 9 KHz tone (CF). The level of the tone varied from 3 to 80 dB SPL (iso-intensity contours). BM input-output function for a tone at CF (~9 kHz, solid line) and a tone one octave below (~4.5 kHz) taken from the iso-intensity contour plot. INPUT level (dB SPL) OUTPUT Response in dB CF= 9 kHz ~4.5kHz Frequency [kHz]
Cochlear nonlinearity 1) Reduced gain: Higher thresholds in quiet; loss of audibility as measured with pure-tone audiogram 2) Loss of nonlinearity: Reduced dynamic range; quiet sounds lost but loud sounds just as loud: Loudness Recruitment GAIN equals Amplitude of motion divided by Amplitude of stimulus pressure No nonlinearity post mortem Basilar-membrane intensity-velocity coding functions for a chinchilla using a tone at the 10 kHz Rugero et al. (1997)
The Auditory Nerve FTC versus FRC FTC data indicate the characteristics of the cochlea from which has been eliminated the non-linear response characteristics of the cochlear nerve excitation process. Response Rate versus Frequency Curve (FRC) FRC data indicate the limits which may be set upon the central representation of the cochlear filtering by the non-linear rate behavior of the cochlear fibers.
Place Theory: Place of maximum vibration along basilar membrane correlates with the place of the Tuning curve (or FTC=Frequency Threshold Curve) along the frequency axis. Shown are tuning curves measured by finding the pure tone amplitude that produces a criterion response in an 8th nerve fiber (cat). Tuning curves for four different fibers (A-D) are shown. The Auditory Nerve Frequency Selectivity: CF & place theory
Cochlear nonlinearity OHC motor driven by the Tectorial membrane A virtuous loop. Sound evoked perturbation of the organ of Corti elicits a motile response from outer hair cells, which feeds back onto the organ of Corti amplifying the basilar membrane motion.
The Problem of Hearing “Sound has no dimensions of space, distance, shape, or size; and the auditory periphery of all known vertebrates contains peripheral receptors that code for the parameters of the sound pressure wave rather than information about sound sources per se.” William A. Yost (Perceiving sounds in the real world, 2007; p. 3461) William A. Yost (Perceiving sounds in the real world, 2007; p. 3461)
The Problem of Hearing Tonotopie blijft in het auditief systeem tot en met de auditieve hersenschors behouden. “De samenstelling van een geluid uit afzonderlijke tonen is te vergelijken met de manier waarop wit licht in afzonderlijke kleuren uiteenvalt wanneer het door een prisma gaat.” John A.J. van Opstal (Al kijkend hoort men, 2006; p. 8)
The Problem of Hearing Neurons within a brain area may be organized topographically (or in a map), meaning that neurons that are next to each other represent stimuli with similar properties. Mapping can be an important clue to the function of an area. If neurons are arrayed according to the value of a particular parameter, then that property might be critical in the processing performed by that area. Neurons do not need to be arranged topographically along the dimensions of the reference frame that they map, even if its neurons do not form a map of that space.
The Problem of Hearing Problem I: Sound localization can only result from the neural processing of acoustic cues in the tonotopic input! Problem II: How does the auditory system parse the superposition of distinct sounds into the original acoustic input?
Organ of Corti Basilar Membrane Auditory nerve Inner Hair cell Outer Hair cells Sensory Coding of SoundSummary
Mechanotransduction: Step 5: Vibration of basilar membrane causes vibration of hair cells against Tectorial membrane (TM): Movement displaces stereocilia/kinocilia, opens ion channels in hair cell membranes Rush of ions depolarizes hair cells, which initiates the release of neurotransmitters NEXT WEEK Cochlear Innervation & Auditory Nerve
Neural responses in the AN: Step 6 Information about region and intensity of cochlear stimulation is relayed to CNS over cochlear branch of vestibulocochlear nerve (VIII): Called the auditory nerve (AN): Has sensory neurons in spiral ganglion of cochlea Carries neural information to cochlear nuclei (CN) of midbrain for distribution to other (more higher) brain centers. NEXT WEEK Cochlear Innervation & Auditory Nerve
NEXT WEEK Cochlear Innervation & Auditory Nerve type 1 type 2 Inner hair cells: Main source of afferent signal in auditory nerve. (~ 10 afferents per hair cell) Outer hair cells: Primarily receiving efferent inputs. Type I neurons (95% of all ganglion cells) have a single ending radially connected to IHCs. Type II small, unmyelinated neurons spiral basally after entering the organ of Corti and branch to connect about ten OHCs, in the same row.