GSU NEUR 3000 - NEUR 3000 - Chapter 11 (40 pages)

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NEUR 3000 - Chapter 11



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NEUR 3000 - Chapter 11

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Pages:
40
School:
Georgia State University
Course:
Neur 3000 - Hon Principles of Neuroscience
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THE AUDITORY AND VESTIBULAR SYSTEM NEUR 3000 Dr Joseph J Normandin AUDITION SOUND Audition is hearing Sound is pressure waves in the air AUDITION SOUND We can measure sound by its intensity in decibels dB or its frequency in Hertz Hz We can hear sounds in the range of 20 Hz to 20 000 Hz What do different frequencies sound like The perception of frequency is called pitch and the perception of intensity is loudness AUDITORY STRUCTURES The outer ear consists of the disc shaped pinna and an auditory canal The pinna helps to collect sound waves and can also modify sound The human pinna enhances the reception of sounds from 2000 5000 Hz a range important for speech perception Pinnae are unique to mammals and there is a great diversity in shape related to the environments each species evolved in Sound waves travel through the ear canal to the middle ear AUDITORY STRUCTURES AUDITORY STRUCTURES Through a series of structures the middle ear amplifies sound The tympanic membrane eardrum vibrates with each sound wave The ossicles malleus incus and stapes transmit the vibration to the oval window of the cochlea in the inner ear The ossicles amplify sound waves in order to be able to affect the fluid filled cochlea AUDITORY STRUCTURES AUDITORY STRUCTURES The inner ear detects sound waves The cochlea of the inner ear is a coiled fluid filled structure The oval window is a membrane covered opening in the cochlea that is connected to the stapes Sounds causes the stapes to push and pull on the oval window creating waves of mechanical energy in the fluidfilled cochlea The organ of Corti in the cochlea contains the sensory receptors for sounds hair cells Small muscles in the inner ear dampen sounds via the attenuation reflex AUDITORY STRUCTURES AUDITORY STRUCTURES AUDITORY STRUCTURES The scala tympani and scala vestibuli are continuous with one another The fluid inside is called perilymph Similar to cerebrospinal fluid Low K and high Na The fluid in the scala media is called endolymph Unusual extracellular fluid High in K and low in Na Maintained by the stria vascularis Active transport of K out and Na in against their concentration gradients AUDITORY STRUCTURES AUDITORY STRUCTURES The basilar membrane is not rigid and will move in response to sound waves passing through endolymph Narrower at base than at the apex Stiffness decreases from base to apex floppy at the end These properties allow different parts of the basilar membrane to respond to different frequencies of sound pressure waves The stiff narrow base will vibrate and dissipate high frequencies The floppy wide end will vibrate at low frequencies AUDITORY STRUCTURES SOUND TRANSDUCTION The organ of corti contains auditory receptor cells hair cells that transduce movements of the basilar membrane into electrical signals SOUND TRANSDUCTION The movement of the basilar membrane alters hair cells mechanically SOUND TRANSDUCTION The back and forth movement of the stereocillia produces alternating depolarizations and hyperpolarization in hair cells SOUND TRANSDUCTION A special type of cation channel the TRPA1 channel is on the tips of stereocillia TRPA1 channels are mechanically gated Movement of the stereocillia can alternately open and close the channels SOUND TRANSDUCTION Hair cells release neurotransmitter at synapses with spiral ganglion neurons 95 of spiral ganglion neurons form synapses with inner hair cells The inner hair cells are therefore responsible for 95 of the auditory information coming into the auditory system These synapses are a 1 1 relationship one inner hair cell spiral ganglion neuron 5 of spiral ganglion neurons form synapses with outer hair cells What the hell are the outer hair cells for SOUND TRANSDUCTION Outer hair cells form the cochlear amplifier The motor protein prestin is found on the membranes of outer hair cells Receptor potentials cause the prestin molecules to compress pulling the tectorial membrane toward the basilar membrane This results in an amplification of low intensity sounds Efferent axons from the brainstem synapse with outer hair cells and are thought to regulate this process THE AUDITORY PATHWAY THE AUDITORY PATHWAY Each spiral ganglion neuron receives information from one hair cell which is in a particular place on the basilar membrane Each spiral ganglion neuron will respond to a characteristic frequency based on this relationship This tuning is maintained throughout the auditory pathway ENCODING SOUND INTENSITY AND FREQUENCY Sound intensity is coded in two ways The firing rate of neurons The number of active neurons The experience of loudness is thought to be a function of how many neurons are firing and how fast they are firing throughout the auditory pathway ENCODING SOUND INTENSITY AND FREQUENCY Frequency or as we experience it pitch is represented tonotopically and by phase locking in the CNS Tonotopic maps are spatial maps of sound frequencies Such a map is found at the very beginning the basilar membrane Tonotopic maps also exist at each level of the auditory pathway ENCODING SOUND INTENSITY AND FREQUENCY Because of tonotopy the location of an active neuron in any given auditory structure indicates a particular frequency being heard An issue Tonotopic maps do not contain frequencies below 200hz Solved Phase locking The timing of neural firing compliments tonotopy ENCODING SOUND INTENSITY AND FREQUENCY SOUND LOCALIZATION Localization of sound requires processing of auditory information at higher levels of the auditory pathway Localization of sound in the horizontal plane is via a different mechanism than localization in the vertical plane We localize sound in the horizontal plane by comparing the time delay or intensity differences of sounds entering the right vs left ears We localize sound in the vertical plane as a function of the shape of the pinna SOUND LOCALIZATION Localization of sound in the horizontal plane Interaural time delay Sounds may reach the ears at different times because the ears are not in the same location Neurons sensitive to the timing difference can use this information to localize sound Localization of sound can be difficult with this method at high frequencies 2000 Hz as the delay is shorter SOUND LOCALIZATION Localization of sound in the horizontal plane Interaural intensity difference A sound coming from one direction will be louder in one ear vs the other because of our big fat head sound shadow The intensity of a sound is encoded in the number of action potentials


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