Psyc 6 Week 4
Popular in Introduction to Neuroscience
Popular in Psychology (PSYC)
This 9 page Class Notes was uploaded by Sabrina Straus on Sunday October 9, 2016. The Class Notes belongs to PSYC 6 at Dartmouth College taught by Catherine Cramer in Fall 2016. Since its upload, it has received 5 views. For similar materials see Introduction to Neuroscience in Psychology (PSYC) at Dartmouth College.
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Date Created: 10/09/16
6. 10-6-16 Intro to Neuro class notes AUDITORY AND VESTIBULAR SYSTEMS I. Auditory system A. Nature of sound: reflects compression and expansion of particles in the air Frequency: cycle / unit time Amplitude: intensity of sound Fourier analysis: breaks down pieces of waveform Timbre: detects additional pieces of sound form B. Anatomy and function of the ear Pinna: external ear Ear canal: sound waves travel down tympanic membrane: eardrum Middle ear: amplifies sound Ossicles: 3 bones which take movement of membrane and amplify it to oval window of cochlea Inner ear Cochlea: location of receptors and transduction organ of Corti basilar membrane: separates fluid / holds hair cells inner hair cells: tip links allow variation in amount of potassium flowing in outer hair cells: more numerous but smaller ~synapse onto spiral ganglion cells tectorial membrane: moves because of fluid stereocilia spiral ganglion cells: auditory nerve leave cochlea and head into the brain Afferents (goes in) vs efferents (out from central nervous system) C. Transduction and amplification Efferent amplifies afferent in inner hair cells Outer hair: inhibited D. Coding of pitch Tonotopy (Place theory): place on basilar membrane where there is maximal distortion represents a particular frequency and therefore pitch } use for high pitch Phase locking (Volley theory): firing pattern} doesn’t work very well for high pitched sounds bc absolute refractory period } use for low pitch E. Central auditory processing} partial decussation auditory nerve cochlear nucleus-1st synapse superior olivary nucleus- 2nd synapse inferior colliculus medial geniculate nucleus (of the thalamus) auditory cortex-on temporal lobe tonotopic organization or cortex : high to low pitch} represents place on basilar membrane F. Coding of location } occurs in brain stem sound shadow: difference in intensity depending on which ear is closer to sound time of arrival partial decussation: helps locate sound II. Vestibular system A. Anatomy semicircular canals: pick up rotation in 3 axis} roll, pitch, yaw hair cells in cupula brainstem vestibular nuclei } helps with balance To motor neurons To cerebellum Finally goes to eyes B. Function -> motion detection balance and reflexes vestibular-ocular reflex (VOR): keep focus on fixed point while in motion motion sickness (conflict theory): vestibular input vs. what you see Chapter Notes Chapter 11: Auditory and Vestibular ~hearing: audition ~vestibular system: regulates balance ~neural responses are generated by auditory receptors from the mechanical energy in sound and by vestibular receptors from the tilts and rotations of the head -nature of sound:audible variations in air pressure ● Frequency: cycles per second + determines pitch -structure of auditory system ● Cartilage: forms funnel called pinna which helps collect sound ○ Helps us hear sound in front of us and folds help us localize sound ● Auditory canal: entrance to internal ear ○ Ends in tympanic membrane: eardrum ■ Connected to ossicles: transfer movements of the tympanic membrane into movements of the second membrane (covering oval window which has fluid filled cochlea behind it and the apparatus for transforming motion of oval window membrane into a neuronal response) ○ pinna->tympanic membrane } outer ear ○ Tympanic membrane-> ossicles } middle ear ○ Apparatus medial->oval window } inner ear ○ Signal -> nuclei in brain stem -> thalamus -> medial geniculate nucleus -> primary auditory cortex (in temporal lobe) -Middle ear: variations of air pressure are converted into movements of ossicles ● Components of middle ear:tympanic membrane, ossicles, muscles ○ Tympanic membrane: conical ○ 3 ossicles: malleus (attached to tympanic membrane), incus (rigid connection with malleus), stapes (flexible connection with incus + bottom called footplate moves in and out at oval window to transmit sound vibrations to the fluids of the cochlea in the inner ear) ○ Air is continuous in middle ear with that in the nasal cavities via Eustachian tube ● Sound force amplification by the ossicles ○ pressure at the oval window will become greater than the pressure at the tympanic membrane if (1) the force on the oval window membrane is greater than that on the tympanic membrane, or (2) the surface area of the oval window is smaller than the area of the tympanic membrane ● Attenuation reflex:muscles attached to ossicles = tensor tympani muscle (anchored to bone and malleus) + stapedius muscle (fixed anchor of bone and stapes) ○ Contraction results in rigid ossicles, reducing sound conduction ○ Attenuation reflex: contraction because of the onset of a loud sound } protects ear} allows high frequency noise to be heard better -Inner ear: cochlea (auditory) + labyrinth (vestibular) ● Anatomy of the cochlea: hollow tube w/ walls made of bone ○ Base:two membrane covered holes ■ Oval window: met by scala vestibuli ■ Round window: met by scala tympani ○ Three fluid filled chambers: ■ Scala vestibuli ■ Scala media ■ Scala tympani: continuous w/ scala vestibuli at helicotrema ○ Reissner’s membrane separates the scala vestibuli from the scala media ○ Basilar membrane separates the scala tympani from scala media ■ Has organ of corti: contains auditory receptor neurons ■ Tectorial membrane hangs over this ■ Stria vascularis: place of generation of active transport by ion differences ● Perilymph: fluid in scala vestibuli and scala tympani (high Na+) ● Endolymph: fills scala media (high K+) } has electrical potential that is higher than the perilymph } endocochlear potential ● Physiology of Cochlea ○ Inward motion at the oval window pushes perilymph into the scala vestibuli ○ Any motion at the oval window must be accompanied with a round window ○ Response of basilar membrane to sound ■ Membrane is wider at apex ■ Stiffness decreases base to apex ■ Movement of endolymph makes the basilar membrane bend near its base to start a wave} distance travelled depends on frequency (low frequency travels farther) ● Place code: different locations of membrane are deformed at different frequencies ■ Tonotopy: systematic organization of sound frequency within an auditory structure ○ Organ of corti and associated structures: changes mechanical energy into change of membrane polarization ■ Hair cells: have stereocilia ● Not neurons ● Lack axons ● Specialized epithelial cells ● Bending of cilia causes transduction of sound into a neural signal ● In between basilar membrane and reticular lamina ● In between rods and modiolus: inner hair cells ● Cells farther out than the rods of Corti: outer hair cells ● Extend above reticular lamina into endolymph end in tectorial membrane (outer) or below (inner) ● Basilar: base ● Tectorial: top ● Reticular: middle ● Form synapses on neurons in spiral ganglion ○ Spiral ganglion: bipolar / axons enter the auditory nerve (which projects to cochlear nuclei in medulla) ■ Rods of corti: provide structural support ■ Supporting cells ○ Transduction by hair cells: basilar membrane moves up -> reticular lamina moves toward modiolus which bends stereocilia on tectorial membrane and actin filaments make cilia move as a unit (direction of hair bending determines whether cell is depolarizing or hyperpolarizing) ■ Tip of stereocilium has a special type of ion channel that opens and closes based on bending ● Open: inward ionic current flow-> receptor potential ● Tip link: connects channel to upper wall of adjacent cilium and when cilia are up, K+ goes into cell-> depolarization (unlike most neurons which hyperpolarize) ○ Hair cells and axons of auditory nerve ■ Located in spiral ganglion which are the first neurons to fire action potentials and provide all the auditory info sent to the brain ● Communicate with inner hair cells ○ Amplification by outer hair cells of movement of basilar membrane ■ Cochlear amplifier: amplify basilar membrane ● Motor proteins: can change the length of outer hair (driven by receptor potential) ○ Ex: prestin: used to move outer hair cells in response to sound ● Hair bundles ○ myosin : enhances movement of hairs in response to weak sounds ○ Outer hair cells: change the physical relationship between cochlear membranes } cause stereocilia to bend more and increase transduction ■ Affected by efferents which synapse on outer hair -central auditory processes: ● Anatomy of auditory pathway ○ Medulla: axons innervate dorsal cochlear nucleus and ventral cochlear nucleus ipsilateral to cochlea ○ Cochlear nuclei-> auditory cortex ■ Cells in ventral cochlear nucleus send axons that project to superior olive ■ Axons of olive ascend in lateral lemniscus and innervate inferior colliculus of midbrain (dorsal path bypasses superior olive) ● All ascending auditory pathways converge onto inferior colliculus ■ Neurons in inferior send axons to medial geniculate nucleus of thalamus (projects to auditory cortex) ■ Important notes: 1. Projections and brain stem nuclei other than the ones described contribute to the auditory pathways. For instance, the inferior colliculus sends axons not only to the MGN but also to the superior colliculus (where the integration of auditory and visual information occurs) and to the cerebellum. 2. There is extensive feedback in the auditory pathways. For instance, brain stem neurons send axons that contact outer hair cells, and auditory cortex sends axons to the MGN and inferior colliculus. 3. Each cochlear nucleus receives input from just the one ear on the ipsilateral side; all other auditory nuclei in the brain stem receive input from both ears. This explains the clinically important fact that the only way by which brain stem damage can produce deafness in one ear is if a cochlear nucleus (or auditory nerve) on one side is destroyed. ● Response properties of neurons in the auditory pathway: ○ Spiral ganglion: receives input from inner hair cells on basilar membrane->fire action potentials } neuron is most responsive to sound at one frequency = characteristic frequency ■ In MGN there are cells that have frequency selectivity ■ Superior olive: helps with sound localization -encoding sound intensity, frequency, and location: ● Intensity: ○ Firing rates: more intense->basilar membrane vibrates with greater amplitude, causing the membrane potential of the activated hair cells to be more depolarized or hyperpolarized-> greater firing rate ○ Number of active neurons: more intense->movements of the basilar membrane over a greater distance->activation of more hair cells ● Frequency, tonotopy, and phase locking ○ Tonotopy: basilar membrane is deformed by sound of different frequencies} high ■ base->apex:progressive decrease in frequency that produces a maximal deformation of membrane ■ Tonotopic representation in auditory nerve ● Nerve fibers connected to hair cells near apical basilar membrane=low ● Nerve fibers connected to hair cells near basal basilar membrane=high ■ Auditory axons in auditory-vestibular nerve synapse in cochlear nuclei in an organized pattern based on frequency ■ Frequency is coded because: ● Maps don’t have neurons w/ low frequencies->need another coding device ● Basilar membrane depends on intensity and frequency ○ Phase locking: timing of neural firing } low ■ The consistent firing of a cell at the same phase of a sound wave ■ Makes it easy to determine frequency because it is the same as that of the neuron’s action potentials ■ Only works at low stages bc when frequency is higher, the action potentials can’t catch up -Mechanisms of sound localization: ● Localization of sound in the horizontal plane: based on timing (sound takes longer to reach further ear) } delay = i nteraural time delay which enables horizontal location } l ow ○ Hard for continuous tones } use time at which the phases differentiate ○ Also hard for high frequency bc short time in between peaks ■ When this happens the brain uses i nteraural intensity difference: sound shadow} high ● Doesn’t work for sound waves at low frequencies because these diffract around the head ○ Duplex theory of sound localization: both together ○ Sensitivity of binaural neurons to sound location ■ Neurons in cochlear nuclei receive afferents from ipsilateral auditory-vestibular nerve} monaural neurons (only respond to sound present in one ear) ■ Binaural: respond to sound at both ears } help with horizontal localization ● In superior oliver} receive input from cochlear nucleus ○ interaural time delay bc phase locked to lower frequency} each neurons encode a particular position in the horizontal plane ○ Sound location cue } for high frequency ■ One neuron gives response when both ears are stimulated ■ Other neuron is excited by sound in one ear but inhibited by the sound in the other ear ● Delay lines: measure small time differences but action potentials arrive at postsynaptic neurons in the olive at the same time to produce an excitatory potential ● Phase locking is essential for a precise comparison of timing inputs ● Localization of sound in vertical plane ○ Affected by curves of outer ear as bumps and ridges produce reflections of entering sound which causes delays + outer ear allows higher frequency sounds to enter the auditory canal more effectively from elevated source ■ Impairment occurs if pinna are covered -Auditory cortex ~axons leaving MGN project to auditory cortex via internal capsule in an array called acoustic radiation (layer 2+3+5+6=pyramidal cells/ layer 4=termination of medial geniculate axons ● Neuronal response properties: columnar organization on basis of frequency ○ tonotopic=low frequencies are represented rostrally and laterally ○ High frequencies=caudally and medially ○ Isofrequency bands running across A1 contain similar neurons ○ But considerable diversity ○ Types of organization ■ Tonotopic representation ■ Columns of cells w/ similar binaural interaction ● Effects of auditory cortical lesions and ablation ○ Bilateral ablation of auditory cortex leads to deafness ○ But lesions usually are ok just messes w/ location and only for specific frequencies -Vestibular system: monitors position and movement of head and gives a sense of balance and helps communicate body posture ● Vestibular labyrinth: use hair cells to transduce movements ○ Evolved from lateral line organs: small pits or tubes along an animal’s sides ■ Each pit has clusters of hairlike sensory cells whose cilia project into a gelatin substance ■ Purpose:sense vibrations or pressure changes in water (also sometimes are sensitive to temp or electrical fields) ○ Evolved into vestibular labyrinth: purpose is to transmit mechanical energy ■ Otolith organs: detect force of gravity and tilts of head / large chambers near center + makes excitatory synapse w/ end of sensory axon from vestibular nerve (cell bodies lie in Scarpa’s ganglion) ~both detect changes of head angle and linear acceleration of head ~each contains a sensory epithelium} macula=vertically oriented within the saccule and horizontally oriented within the utricle when the head is upright ~vestibular macula contains hair cells, which lie among a bed of supporting cells with their cilia projecting into a gelatinous cap ~otoconia: calcium carbonate crystals} key to tilt sensitivity of macula ~each hair cell has one especially tall cilium } kinocilium >bending toward: depolarizing ● Saccule ● utricle ■ Semicircular canals: sensitive to head rotation ● Detect angular acceleration ● Hair cells are clustered within a sheet of cells} crista located along ampulla ● Cilia project into gelatinous cupula ● Filled w/ endolymph} stays relatively still while canal rotates-> bows cupular->bends cilia->excites or inhibits->stop motion-> counter-rotation ● Push pull arrangement ○ At constant rate, it all moves at the same rate ● Central vestibular pathways and reflexes: coordinate and integrate info to control output of motor neurons to adjust the head, eye, and body positions ○ Primary: connect to medial and lateral vestibular nuclei on same side of brain as well as cerebellum ■ Nuclei: receive inputs from cerebellum, visual and somatosensory systems, etc. -> project (ex: otolith->lateral vestibular->vestibulospinal tract))(semicircular->medial vestibular->medial longitudinal fasciculus) -> thalamus (ventral posterior nucleus)->neocortex ○ Vestibulo-ocular reflex:helps w/ straight vision ■ Moves eyes in opposite direction as head ■ Axons from side that you turn to horizontal canal innervate the vestibular nucleus (sends excitatory axons to contralateral cranial nerve)->motor axons from abducens nucleus excite lateral rectus muscle of right eye and another crosses midline and ascends to excite the ipsilateral cranial nerve II nucleus-> excites medial rectus muscle of that eye ■ Medial rectus muscle also gets excited via projection from vestibular nucleus directly to the left oculomotor nucleus ■ Speed is also maximized by activation of inhibitory connections to muscles that opposes movement ● Vestibular pathology ○ Damage: antibiotics, bilateral lesions ~summary: ● 3 inner ear structures to give selectivity of different kinds of mechanical energy ○ Periodic waves of air pressure=sound ○ Rotational forces=head turns ○ Linear forces=head or tilt acceleration
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