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Auditory Pathways And Auditory Function Notes

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05/01/2016

Auditory pathways and auditory function Innervation of hair cells Ribbon synapses
-Hair cells form synapses ith sensory neurons- basolateral membrane of each hair cell contains presynaptic zones at which chemical neurotransmitter is released
-Presynaptic dense body/ synaptic ribbon-lies in cytoplasm adjacent to release site- surrounded by clear synaptic vesicles which are attached to dense bdy by filaments
-Between the presynaptic plasma membrane and dense body are Calcium channes and K+ channels
-Release of glutamate- on depolarisation induced by Calcium
-Unusual features- presynaptic apparatu of hair cells release glutamate continuously at rest-rate of transmitter is increased/ decreased depending if depolarised/ hyperpolarised
-Most hair cells also receive inputs from neurons in brainstem at large boutons on basolateral cell surface-efferent terminal contain numerous clear synaptic vesicles and small larger dense core vesicles- ACH, Calcitonin gene related peptide
-Ach binds with nicotinic ionotropic receptors- increase Ca2+ entry, Na+ and K+, Calcium that enters through channels activates Ca2+
sensitive K channels- hyperpolarisation Cochlear nerve
-From cochlear hair cells-information passed to primary sensory neruonscell bodies lie in cochlear ganglion/spiral ganglion
-central processes of these bipolar neurons form the cochlear division of the vestibulocochlear nerve
-90% of spiral ganglion cells-myelinated- terminate on inner hair cells
-Each axon contacts only a single inner hair cell but each cell directs its output to several nerve fibres- on average about 10-So neuron information from which hearing arises entirely at inner hair cells
-Information from 1 receptor is encoded independently in several parallel channels-divergence in output
-10% of cochlear ganglion cells-unmyelinated contact outer hair cells Acoustic neuroma/ Vestibular schwannoma-Tumour of the schwann cells responsible for myelin formation of the vestibulocochlear nerve

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-Hearing loss, balance issues, loss of balance

Response properties of neurons in auditory pathway
-Each ganglion cell responds best to stimulation at characteristic frequency of presynaptic hair cell-so axon's responsiveness may be characterised by tuning curve
-Tuning curves for nerve fibres with different characteristic frequencies resemble one another but are shifted along the abscissa
-Measure firing rate in response to sounds at different frequencies in single auditory nerve- found that neuron is most responsive to sound at one frequency- characteristic frequency, it is less responsive at neighbouring frequencies-frequency tuning
-spiral ganglion cells receive input from a a single inner hair cell at a particular location of basilar membrane—so only respond to sound within a limited frequency as each hair cell is excited by deformation of basilar membrane, each portion of membrane is maximally deformed to particular range of frequencies
- auditory nerve fibres connected to hair cells near apical basilar membrane have low characteristic frequencies and those connected to hair cell near basal basilar membrane have high characteristic frequencies Cochlear implants, which consist of threadlike multisite microstimulation electrodes driven by digital signal processors, exploit the tonotopic organization of the cochlea, and particularly its eighth nerve afferents, to roughly recreate the patterns of eighth nerve activity elicited by sounds. In patients with damaged hair cells, such implants can effectively bypass the impaired transduction apparatus, and thus restore some degree of auditory function

Phase locking
-Main source of information about sound frequency that complements information from tonotopic maps- timing of neural firing
-Recordings from neurons in auditory nerve show phase locking: temporal code-timing of action potentials follows the sound waveform
-Phase locked neuron would fire action potentials at either peaks, troughs, or some other constant location on the wave

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-Low frequencies- some neurons fire action potentials every time the sound has a particular phase
-Phase locking can also occur even if action potential hasn't fired on every cycle- neuron may respond to 100Hz sound with action potential only 25%
of cycles but will always occur at the same phase of the sound
-Group of such neurons above-each respond to different cycles of input signal-it's possible to have response to every cycle
-Intermediate sound frequencies are represented by pooled activity of a number of neurons- each of which fire in a phase locked manner- volley principle
-Phase locking occurs with sound waves up to about 4kHz- above this, action potentials are fired at a random phase and frequency is represented by tonotopy alone- membrane capacitance of the inner hair cell prevents voltage changing sufficiently rapidly However at higher frequencies-as impulses would be too rapid for an individual axon to fire due to refractory period
-Second frequency lying outside the tuning curve can suppress the response to excitatory tone- two tone suppression
-At low frequency: phase locking, intermediate frequenciesphase locking and tonotopy, high frequencies- tonotopy Stimulus intensity
-Information about sound intensity (loudness) is coded in two ways
-Firing rate of neurons and number of active neurons
-More intense stimulus, basilar membrane vibrates with greater amplitude- causing membrane potential of activated hair cells to be more depolarised/ hyperpolarised
-So nerve fibres with which hair cells synapse fire action potentials at greater rates
-More intense stimuli also produce movements of basilar membrane over greater distances- so increased activation of hair cells
-This causes a broadening of the frequency range to which the fibre responds
-The broadening of the tuning curves and saturation of responses with increasing sound intensity means that frequency resolution becomes poorer above threshold
-The threshold in auditory nerve fibre varies, so firing rate over a population of auditory nerves tells us the intensity

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-Highest audible sound intensity is several million times greater than threshold so a logarithmic scale of decibels (dB) is used to measure sound pressure
-Dynamic range- range of sound intensities that can be perceived between lowest detectable sound pressure and threshold of pain- normally 100dB
-Humans can discriminate 2 sounds differing by 1dB and by 3Hz Efferent neurons
-Mature inner hair cells don't receive efferent input
-Outer hair cells have extensive efferent connections- each outer hair cell receives input from several large efferent terminals-these fill the space between the cell's abse and the Deiter's cell Anatomy Cochlear nucleus
-Afferents from spiral ganglion enter brain stem in auditory vestibular nerve- cochlear nucleus in medulla
-At level of medulla- axons innervate dorsal cochlear nucleus and anteroventral cochlear nucleus and posteroventral nucleus ipsilateral to the cochlea where the axons originated
-Each axon branches so it synapses on neurons in both cochlear nuclei:
-Some lateral inhibition in the cochlear nucleus-first time inhibition occurs in auditory pathways-allows for more complex frequency response areas Tonotopic organisation of cochlear nuclei
-Cochlear nerve fibres terminate in these nuclei in Tonotopic organisationresults in a map of basilar membrane within cochlear nuclei
-Fibres that carry information from apical end of the cochlea-detects low frequencies-terminate ventrally in the ventral and dorsal coclear nuclei
-Those that carry information from basal end of the cochlea detect high frequencies-terminate dorsally
-The location of active neurons in auditory nuclei is one indication of the frequency of the sound
-Frequency must be coded in some way other than site of maximal activation in Tonotopic maps because:
-these maps don't contain neurons with very low characteristic frequencies below 200Hz

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