Neuro 3000 Chapter 11: Auditory and Vestibular Systems
Neuro 3000 Chapter 11: Auditory and Vestibular Systems NEUROSC 3000 - 020
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This 14 page Class Notes was uploaded by Victoria Gonzalez on Saturday December 5, 2015. The Class Notes belongs to NEUROSC 3000 - 020 at Ohio State University taught by Robert Boyd in Summer 2015. Since its upload, it has received 66 views. For similar materials see Introduction to Neuroscience in Neuroscience at Ohio State University.
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Date Created: 12/05/15
1 Chapter 11: Auditory and Vestibular Systems Victoria Gonzalez 1. Introduction a. Audition: sense of hearing i. Detect sound ii. Perceive and interpret nuances of sound b. Vestibular system: sense of balance; where we are in space i. Head and body location ii. Head and body movements 2. The nature of sound a. Sounds are audible variations in air pressure b. Frequency: number of waves/cycles per second expressed in units called hertz (Hz) i. Cycle: distance between successive compressed patches c. Range: 20 Hz to 20,000 Hz d. Pitch: i. High pitch = high frequency ii. Low pitch = low frequency e. Intensity: high intensity are louder than low intensity 3. Structure of the auditory system a. Pinna: outer ear; funnels in the sound waves (especially from in front of us) b. Auditory canal: entrance to the internal ear c. Tympanic membrane (eardrum): vibrates with the sound waves and moves the bones in the inner ear d. Ossicles: bones in the inner ear that amplify the sound e. Oval window: connects ossicles to cochlea f. Chochlea: transforms physical motion of the oval window into a neuronal response g. Auditory pathway: i. Sound wave moves tympanic membrane ii. Tympanic membrane moves the ossicles iii. Ossicles move the membrane at the oval window iv. Motion at the oval window moves fluid in the cochlea v. Movement of fluid in the cochlea causes a response in sensory neuron vi. Auditory receptors in the cochlea vii. Brain stem neurons viii. Medial geniculate nucleus in the thalamus ix. Primary auditory cortex (A1) in the temporal lobe h. 2 4. The outer ear a. Pinna and auditory canal 5. The middle ear a. Tympanic membrane and the ossicles b. Eustachian tube: connects the air in the nasal cavities with the air in the middle ear c. Sound is amplified by the ossicles i. Amplification is needed to increase pressure; more pressure is needed to move cochlear fluid vs. moving air ii. Pressure: force/surface area iii. 20x greater pressure at oval window than tympanic membrane d. Attenuation reflex i. Response where onset of loud sound causes tensor tympani muscle and stapedius muscle to contract 1. When muscles contract, the ossicles become more rigid and sound conduction is diminished ii. Sound attenuation is greater at low frequencies iii. Functions: 1. Adapt ear to loud sounds (high intensities) 2. Understand speech better (attenuation suppresses low frequencies more) 3. Turns on when we speak; we don’t hear out own voices as loudly 6. The inner ear a. Includes cochlea and labyrinth (part of vestibular system) b. Cochlea anatomy i. Fluid in the cochlea is divided in three chambers: 1. Scala vestibuli: meets the oval window 2. Scala media 3. Scala tympani: meets the round window ii. Basilar membrane: separates scala tympani and scala media 1. Organ of corti: sits upon basilar membrane; contains auditory receptor cells iii. Perilymph: fluid in scala vestibule and scala tympani 1. Fluid is similar to extracellular fluid iv. Endolymph: fluid in scala media 1. Similar to intracellular fluid v. Endochochlear potential: endolymph’s electric potential is about 80 mV more positive than perilymph 1. Enhances auditory transduction 3 c. Physiology of the cochlea i. Pressure at oval window pushes perilymph into scala vestibuli causing the round window membrane to bulge out ii. Any motion at the oval window must be accompanied by a complementary motion at the round window d. The basilar membrane is flexible and bends in response to sound i. Basilar membrane is wider at the apex than at the base ii. Membrane is more stiff at the base than the apex iii. Endolymph movement bends the basilar membrane near the base creating a wave that moves towards apex 1. Higher frequency wave vibrates the base a lot 2. Low frequency moves farther towards apex 3. Tonotopy: organization of sound frequency within an auditory system e. Organ of corti and associated structures i. Organ of corti: contains auditory receptor cells that change mechanical energy into a change in membrane potential 1. Consists of: hair cells, rods of corti, and supporting cells ii. Each auditory receptor (hair cell) has 10-300 hair- looking stereocilia 1. Hair cells are not neurons 2. Hair cells synapse on neurons in the spiral ganglion in the modiolus 3. Spiral ganglion cells are bipolar; connect hair cells with the auditory nerve f. Transduction of hair cells i. The basilar membrane moves in response to a motion at the oval window causing the whole structure to move 1. The basilar membrane, rods of Corti, reticular lamina, and hair cells are rigidly connected 4 ii. Lateral motion of the reticular lamina relative to the tectorial membrane bends stereocilia on the outer hair cells one way or another 1. All the cilia move as a unit; they are connected 2. When they bend in one direction, cell depolarizes; in the other direction, cell hyperpolarizes iii. The tip of each sterocilium has an ion channel that opens and closes by the bending of stereocilia 1. When cilia are pointing straight up, a small amount of K+ moves from endolymph to hair cell 2. When cilia moves in one direction, channel opens more, increase amount of inward K+ current 3. Displacement in the other direction, closes channels more, reducing inward K+ movement iv. Entry of K+ into hair cells causes depolarization (while it causes hyperpolarization in most neurons; endolymph has a high K+ concentration) 1. Depolarization activates voltage-gated calcium channels 2. Entry of calcium triggers the release of glutamate g. Innervation of hair cells i. One spiral ganglion fiber connects to one inner hair cell ii. One spiral ganglion fiber connects to many outer hair cells iii. Even though there are more outer hair cells, most of the information leaving the cochlea comes from inner hair cells 5 6 h. Amplification by outer hair cells i. Cochlear amplifier: outer hair cells amplify the movement of the basilar membrane during low- intensity (quiet) sound stimuli 1. Motor proteins: change the length of outer hair cells a. Prestin: primary motor protein required to move outer hair cell ii. Soft sounds are amplified more than loud sounds 7. Central auditory processes a. Spiral ganglion ventral cochlear nucleus superior olive inferior colliculus medial geniculate nucleus auditory cortex b. All ascending auditory pathways converge onto the inferior colliculus c. Response properties of neurons in auditory pathway i. Characteristic frequency: frequency at which neuron is most responsive (from cochlea to cortex) ii. Response properties become more complex and diverse beyond the brain stem iii. Binaural neurons (both ears) are present in the superior olive 1. Important in sound localization 8. Encoding sound intensity and frequency a. Information about sound intensity is coded by: i. Firing rates of neurons (higher with higher intensity/louder) ii. Number of active neurons (more with higher intensity/louder) b. Stimulus frequency i. Neurons are most sensitive at their characteristic frequency 7 8 c. Tonotopic maps on the basilar membrane i. Auditory nerve fibers connected to hair cells near the apex (top) of the hair cell have low frequencies ii. Auditory nerve fibers connected to hair cells near the base of the hair cell have high frequencies iii. There are tonotopic maps on: 1. Basilar membrane 2. Spiral ganglion 3. Cochlear nucleus d. Phase locking: consistent firing of a cell at the same phase of a sound wave i. Low frequencies: phase-locking on every cycle or some fraction of cycles 1. Makes it easy to determine the frequency of the sound ii. High frequencies: the response is not fixed to a particular phase of the wave 1. Sound waves travel too fast to represent their timing 9 e. Indicating sound frequency: i. At low frequencies: phase locking is used ii. At intermediate frequencies: phase locking and tonotopy iii. At high frequencies: tonotopy is used 9. Mechanisms of sound localization a. Localization of sound in the horizontal plane i. Interaural time delay: the time it takes for a sound to reach from ear to ear ii. Interaural intensity difference: sound at high frequency from one side of the ear 1. Our heads cast a sound shadow 2. There is no sound shadow at low frequencies because sound waves diffract around the head iii. Duplex theory of sound localization: time delay + intensity 1. Interaural time delay: lower frequencies (20- 2,000 Hz) 2. Ineraural intensity difference: higher frequencies (2,000-20,000 Hz) b. Binaural neurons are present in the superior olive; they receive information from both ears and play a role in sound localization i. Sound wave enters ear, activates cochlear nucleus, and is sent to the superior olive ii. Delay lines: arrival of the spike from one side to the olivary neuron is delayed just enough that it coincides with the arrival of the spike from the other side; spikes summate to make EPSP 10 1. Phase locking is essential for comparing timing of inputs (only at low frequencies) c. Localization of sound in vertical plane i. Comparing input to both ears is not useful in vertical plane 1. Interaural delay and interaural intensity do not change ii. Vertical sound localization is based on reflections from pinna 10. Auditory cortex a. Primary auditory cortex (A1) is in the temporal lobe i. Secondary auditory cortex surrounds A1 b. Principles of auditory cortex i. Tonotopy, columnar organization of cells with similar binaural interaction ii. Bilateral ablation of auditory cortex = deafness 11 1. Ablation of one side preserves function; both ears send output to cortex in both hemispheres a. Lesion in striate cortex: complete blindness in one visual hemifield 2. Unilateral loss leads to impaired sound localization iii. There are different frequency bands that are processed in parallel by tonotopic structures c. Neuronal response properties i. Frequency tuning: similar characteristic frequency ii. Isofrequency bands: group similar characteristic frequency, diversity among cells 12 11. The vestibular system a. Importance of the vestibular system: balance, equilibrium, posture, head, body, eye movement b. Vestibular labyrinth: structure near cochlea; contains hair cells i. Vestibular system also uses hair cells to detect changes ii. Vestibular labyrinth includes two structures: 1. Otholith organs: gravity, tilt/angle of the head, linear acceleration a. Each otolith organ contains a sensory epithelium called a macula i. Macular hair cells respond to tilt ii. When the head tilts, gravity pulls on otoconia which moves the gelatinous cap and causes the cilia to bend 2. Semicircular canals: changes in head rotation a. Structure: three arching structures angled 90 degrees between them in each ear i. Each arching structure is paired with another on opposite side of head ii. Helps sense all the possible head rotation angles b. Head rotation: shaking or nodding head c. Angular acceleration: caused by sudden rotational movements 13 c. Vestibulo-ocular reflex (VOR): retinas remain stable independent of head motions i. Function: line of sight is fixed on a visual target ii. Mechanism: senses rotations of head and commands compensatory movements of eyes in the opposite direction iii. Works well even in the dark or when your eyes are closed iv. Connections from semicircular canals, to vestibular nucleus, to cranial nerve nuclei excite extraocular muscles 12. Hearing and balance a. Nearly identical sensory receptors (hair cells) 14 b. Movement detectors: periodic waves, rotational, and linear force c. Auditory system: senses external environment d. Vestibular system: senses movements of itself
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