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CSD 3103 anatomy of speech and hearing mechanisms Hearing mechanisms Fall 2008 Central Pathways Cells of the nervous system The primary cell of the nervous system is the neuron Non-replaceable Typical neuron Important Parts Cell body Dendrites Axon Neuron specialization The three major types of neurons, depending on their specialization: Sensory Neurons Motor Neurons Interneurons Sensory neurons Sensory neurons conduct nerve impulses from the ear and deliver sensory information to the brain for processing and interpretation Afferent refers to this direction of travel and this kind of pathway or system Neuron classification by structure How neurons communicate Communication between neurons is achieved by the release of small packets of neurotransmitters into the synapse If the release of neurotransmitters reaches a critical level to the receiving neuron, it will cause an action potential to be generated in the cell body “All-or-none” behavior How neurons communicate The action potential is an electrical event The action potential travels down the axon to reach another neural cell body Neurotransmitters are released at the synapse and the process is repeated in a new neuron The viii. Cranial nerve (vetibulocochlear) Sensory neuron Acoustic Portion Vestibular Portion The central auditory pathways Landmarks: Auditory nerve Cochlear nucleus Superior olivary complex Lateral lemniscus Inferior colliculus Medial geniculate body Auditory cortex The auditory nerve VIII cranial nerve Bipolar neurons Cell bodies from these neurons lie right outside the cochlea and form the spiral ganglion One end innervates the individual inner and outer HCs of the cochlea and the other end synapses with the neurons of the cochlear nucleus The auditory nerve Auditory Portion--30,000 fibers Vestibular Portion--20,000 fibers Evidence of tonotopic organization Spiral Ganglion Cell bodies of the first order neurons Bipolar Cerebello-Pontine angle--where the cerebellum, medulla oblongota and pons meet The auditory nerve Evidence of tonotopic organization The auditory nerve The individual fibers pass from the modiolus of the cochlea through the internal auditory meatus, which exits at the base of the brain The IAM also carries fibers from the utricle, saccule, and semicircular canals that form the vestibular portion of the VIII nerve The vestibular and auditory portions of the VIII N. separate at the cerebellopontine angle The branch of the facial nerve that courses through the middle ear also exits here The cochlear nucleus Two major parts Dorsal and ventral Tonotopic organization “must-synapse” station--second order fibers Preserves, but does not enhance, information received from the auditory nerve Superior Olivary Complex Most of the fibers from the cochlear nuclei cross and project to the contralateral SOC Plays a role in the acoustic reflex Analyzes intensity and time-of-arrival differences between the two ears to help with localization tasks The superior olivery complex Most (about 80%) of the fibers from the cochlear nuclei cross and project to the contralateral SOC via the trapezoid body Evidence of tonotopic organization Plays a major role in the acoustic reflex Analyzes intensity and time-of-arrival differences between the two ears to help with localization/lateralization tasks The lateral lemniscus Highway of axons that arise from the SOC and terminate in the midbrain The lateral lemniscus Tonotopic organization is evident Nuclei within the lateral lemniscus have a large proportion of cells that are sensitive to interaural time differences, binaural input, and interaural intensity differences The inferior colliculus “must-synapse” station at the level of the midbrain Highly tonotopic First evidence of neurons that are sensitive to sound duration Active in binaural processing Auditory Cortex Areas of auditory reception are in the temproal lobes on both sides of the cerebral cortex in an area called the superior temporal gyrus or Heschl’s gyrus The medial geniculate body Located in the auditory thalamus Last subcortical relay in the pathway Evidence of tonotopic organization Very active in localization and lateralization The medial geniculate body Pathways of neurons projecting from the medial geniculate to cortical areas The human brainstem Structures: Cochlear nuclei (1) Lateral lemnisci (2) Inferior colliculi (3) Superior colliculi (4) Medial geniculates (6) Auditory thalmi (7) The auditory cortex Areas of auditory reception are in the temproal lobes on both sides of the cerebral cortex The Sylvian or lateral fissure is the focal point Primary and secondary auditory areas are above and below this point Central Auditory Pathways The corpus callosum Large fiber tract that connects the two hemispheres of the brain Allows information (like auditory) to be transferred from one side of the brain to the other Very important for normal dichotic listening and pitch pattern perception Behavior of the auditory nerve This figure shows how the single neuron responses are tied to the temporal characteristics of the signal “phase-locking” Input-output function Input-output function and histogram This figure shows the input-output function of a single fiber. Notice that the firing rate of the neuron increases as the stimulus intensity increases within its dynamic range, and eventually plateaus. Single fiber tuning curve Characteristic frequency (tip) Tail Difference in slope between the low frequency and high frequency sides Shaded area is the response area Single fiber tuning curve Portions of response areas for each of three cochlear neurons with CFs of 100, 1000, and 10,000 Hz Single fiber tuning curves of many frequencies Tuning curves of auditory neurons with a variety of characteristic frequencies Post-stimulus time histogram Pst histograms Selected histograms showing that neural firing during a pure tone is timed to the period of the tone. The dots along each x-axis correspond to the multiples of the period of the tone.