Chapter 11 The Auditory System PDF

CH APT E R 1 1 T H E A U D I T O RY SYSTEM I NT RO D UC TI O N Ear contains array of miniature acoustical detectors packed in space size of pea Can transduce vibrations as small as diameter of an atom! Respond 1000 times faster than visual photoreceptors Critical for...

CH APT E R 1 1 T H E A U D I T O RY SYSTEM I NT RO D UC TI O N Ear contains array of miniature acoustical detectors packed in space size of pea Can transduce vibrations as small as diameter of an atom! Respond 1000 times faster than visual photoreceptors Critical for both analysis of rapidly varying sounds such as music, speech, and for sound location Audition represents very important mode of sensation  T H E N AT U R E O F SOUND Sound Audible variations in air pressure Waveform of sound stimulus is amplitude plotted against time Cycle: Distance between successive compressed patches Sound frequency (Pitch): Number of cycles per second expressed in units called hertz (Hz) Human Range: 20 Hz to 20,000 Hz Overall amplitude of wave corresponds to loudness (logarithmic decibel scale, dB)  THE ST RUCTU RE OF THE A U D I T O RY SYSTEM  THE STRUCTURE OF THE A U D I T O RY S Y S T E M Auditory pathway stages Sound waves Gathered by pinna, concha, and the auditory canal (auditory meatus) Tympanic membrane (eardrum) Ossicles Malleus, incus, and stapes Oval window Site where bones contact inner ear Cochlea Sensory neuron response  THE MIDDLE EAR Components of the Middle Ear Sound Force Amplification by the Ossicles Pressure: Force by surface area Greater pressure at oval window than tympanic membrane, moves fluids The Attenuation Reflex Response where onset of loud sound causes tensor tympani and stapedius muscle contraction Function: Adapt ear to loud sounds, understand speech better  THE INNER EAR Anatomy of the Cochlea Site where energy from vibrating pressure waves transformed into neural impulses Includes oval and round window Bisected from basal end almost to apical end by flexible structure that supports basilar membrane and tectorial membrane Fluid filled chambers on either side (scala vestibuli and scala tympani) Perilymph: Fluid in scala vestibuli and scala tympani (high in sodium, low potassium) Endolymph: Fluid in scala media (high in potassium) Endocochlear potential: Endolymph electric potential 80 mV more positive than perilymph  THE INNER EAR Physiology of the Cochlea Pressure at oval window, pushes perilymph into scala vestibuli, round window membrane bulges out The Response of Basilar Membrane to Sound Structural properties: Wider at apex Stiffness decreases from base to apex Perilymph movement bends basilar membrane near base, wave moves towards apex Base of basilar membrane tuned for higher frequencies Apex of basilar membrane tuned for lower frequencies THE INNER EAR  THE INNER EAR Mechanoelectrical Transduction Channels of Hair Cells Research: A.J. Hudspeth Hair cell resting potential between -45 and -60mV Cochlear hair cells in humans One row of inner hair cells (sensory receptors) Three rows of outer hair cells (Descending input from brain, modulation) Project into scala media Each bundle contains 30-few hundred stereocilia Graded in height and arranged in bilateral, symmetric manner Connected by tip links Basilar membrane upward, reticular lamina up, and stereocilia bends outward (depolarization) Movement in opposite direction compresses tip links, closing the channels (hyperpolarization) Eventual transmitter release leads to action potentials in Cranial Nerve VIII fibers (spiral ganglion neurite)  THE INNER EAR The Innervation of Hair Cells One spiral ganglion fiber: One inner hair cell, numerous outer hair cells Amplification by Outer Hair Cells Otoacoustic emissions (self generated sounds) Outer hair cells contract and expand in response to small electrical currents Receive descending signals from brain Brain can control these contractions and expansions Modulate response of cochlea to incoming sound Sharpens frequency sensitivity and enhancing responsiveness to low intensity sounds Such emissions can occur spontaneously and can lead to tinnitus Prestin: Required for outer hair cell movements CENTRAL A U D I T O RY P RO CE S S ES  CENTRAL A U D I T O RY PROCE SSE S  CENTRAL A U D I T O RY P RO CE S S ES Response Properties of Neurons in Auditory Pathway Characteristic frequency: Frequency at which neuron is most responsive - from cochlea to cortex Response Properties more complex and diverse beyond the brain stem ENCODING SOUND INTENSITY AND FREQUENCY Encoding Information About Sound Intensity Firing rates of neurons Number of active neurons Stimulus Frequency Tonotopic maps on the basilar membrane Tonotopy Tonotopic maps on the basilar membrane, spiral ganglion, and cochlear nucleus ENCODING SOUND INTENSITY AND FREQUENCY Phase Locking Consistent firing of a cell at the same phase of a sound wave Low frequencies: phase-locking on every cycle or some fraction of cycles High frequencies: not fixed Volley Principle Intermediate sound frequencies are represented by pooled activity of a number of neurons, each of which fires in a phase locked manner MECHANISMS OF SOUND L O C A L I Z AT I O N Techniques for Sound Localization Horizontal: Left-right, Vertical: Up-down Localization of Sound in Horizontal Plane Interaural time delay: Time taken for sound to reach from ear to ear Duplex theory of sound localization: Interaural time delay: 20-2000 Hz Interaural intensity difference: 2000-20000 Hz  MECHANISMS OF SOUND L O C A L I Z AT I O N Delay Lines and Neuronal Sensitivity to Interaural Delay Sound from left side, activity in left cochlear nucleus, sent to superior olive Sound reaches right ear, activity in right cochlear nucleus, first impulse far Impulses reach olivary neuron at the same time→ summation→ action potential IN T E R AU RA L INTENSITY DIFFERENCES  MECHANISMS OF SOUND L O C A L I Z AT I O N Localization of Sound in Vertical Plane Vertical sound localization based on reflections from the pinna  A U D I T O RY C O R T E X Primary Auditory Cortex Axons leaving MGN project to auditory cortex via internal capsule in an array Tonotopy, columnar organization of cells with similar binaural interaction Cells respond more to stimulation of both ears than to either ear separately Lesion in auditory cortex: Normal auditory function, inability to locate sound