1. Neurophysiology Neurons Gross Anatomy The Central Auditory Nervous System Frequency and Intensity encoding Central Auditory Processing Binaural Processing Central Control (Descending Systems) Cortical Processing

2. The Neuron Dendrites receive synaptic stimulation (neurotrans.) Action Potential generated in soma near axon AP conducted along axon from Node to Node (saltatory conduction) AP produces release of neurotransmitter at terminal boutons

3. An Action Potential (or Spike)

4. Two Descriptors for Neurons Afferent (sensory)-- carrying signals toward the brain Efferent (motor) -- carrying signals from brain to periphery

5. Afferent Afferent & Efferent Neurons

6. 4 Types of Cochlear Neurons INNER HAIR CELLS >Multiple (10 to 20) Afferent synapses >(Efferents synapse on afferent dendrites) OUTER HAIR CELLS: >Large Efferent synapses engulf base of cell >Small (& not very active) Afferent synapses

7. IHC Innervation Pattern

8. OHC Innervation Pattern

9. Inner hair cells Synapse at the base with up to 20 afferent neurons “Divergence” Efferents synapse on afferent dendrites under IHCs

10. IHC activation alters firing rate

11. Afferent neurons have their cell bodies in the Spiral Ganglion (4)

12. Central Nervous System Structures Nucleus = a group of nerve cell bodies Fiber Tract = a group of axons

13. Major Components of the Central Auditory Nervous System (CANS) VIIIth cranial nerve Cochlear Nucleus Superior Olivary Complex Lateral Lemniscus Inferior Colliculus Medial Geniculate Body Primary Auditory Cortex Brainstem Thalamus Mid-brain Temporal Lobe

14. Mid-Saggital View of Brain Pons Cerebellum 4th Ventricle Thalamus Corpus Callosum

15. MedGen Body Inf Coll Lat Lemn SOC Coch Nuc VIIIth CN

16. Neural Web-Site http://rprcsgi.rprc.washington.edu/ neuronames/hierarchy.html

20. Section Thru Brainstem Shows Cochlear Nucleus

21. The Superior Olivary Complex

22. Connections To the Superior Olivary Complex

23. Superior Olivary Processing Supports Localization Lateral SO-- Interaural Intensity Differences Medial SO-- Interaural Time Differences (These are the two primary acoustic cues for localizing sounds)

24. Dorsal (back) Side of Brainstem Thalamus (medial geniculate) Inferior Colliculus 4th Ventricle Area of Pons

25. Inferior Colliculus

26. Thalamus in Purple

27. Auditory Radiations Connect Medial Geniculate Body (in purple) to Primary Auditory Cortex (in blue)

28. Lateral-Superior view of brain

29. Primary Auditory Cortex (AI): superior surface of the temporal lobe

30. Brain Photos Web-Sites http://rpiwww.mdacc.tmc.edu:80/se/anatomy /brain/ http://www.ets.uidaho.edu/med532/start.htm

31. Neurophysiological Measures Gross Evoked Potentials-- Voltage changes in response to auditory stimulation recorded from the scalp Single-Unit Measures-- Voltage (or other) changes recorded within a neuron

32. Auditory Evoked Potentials Recorded in different time intervals (“epochs”) following a sound Earlier epochs come from lower in the system Later epochs come from higher in the system

33. Examples of AEP Epochs Electrocochleography-- within 5 milliseconds Auditory Brainstem Response-- thru 10 ms Middle Latency Response-- thru 75 ms Auditory Late Response-- thru 200 ms

34. Auditory Brainstem Response Time (ms) Amplitude (  V) Wave V Latency I II III IV V Amp V 0 10

35. GENERATORS of ABR WAVES I II III IV V =Distal VIIIth nerve =Medial VIIIth nerve =Cochlear Nucleus =Superior Olivary Complex =Lateral Lemniscus & Inferior Colliculus

36. Single-Unit Measures Post-Stimulus Time Histogram-- Shows firing rate changes over time Period or Interval Histograms-- Show phase-locking of neural firing

37. Tuning Curves Iso-Rate Function -- Shape similar to what we’ve already described (Fig 6.12 b) Iso-level Function -- Shows spike rate as a function of frequency-- peak at a single frequency (Fig 6.12a)

38. Two-Tone Suppression The response to one tone can be reduced or eliminated by introducing a second tone near the neuron’s CF. (Fig 6.16) Second tone can be either one which normally would excite the neuron or not

39. Two-tone Suppression Regular Tuning Curve

40. Frequency Coding The Place Code-- each neuron has a characteristic frequency Periodicity Pitch-- neurons phase-lock to stimuli

41. Intensity Coding Firing rate increases in single neurons Spread of activation to a wider range of neurons-- “Density of Discharges” Latency of Firing (shorter delay at higher levels)

42. Efferent (Descending) Control Cochlear Efferents come from Superior Olivary Complex --The Olivo-Cochlear Bundle (OCB) Uncrossed OCB-- synapses on dendrites under inner hair cells Crossed OCB-- synapses on outer hair cells Both use inhibitory neurotransmitters

43. Uncrossed OCB-- synapses on dendrites under inner hair cells

44. Crossed OCB-- synapses on outer hair cells

45. Efferent Control (cont’d) The Acoustic Reflex Auditory Cortex and Thalamus also send descending fibers to auditory brainstem locations

46. The Acoustic Reflex Afferent: VIIIth nerve Cochlear Nucleus Superior Olivary Complex Efferent: VIIth nerve nucleus VIIth nerve Stapedius muscle

47. Primary Auditory Cortex (AI): superior surface of the temporal lobe

48. 6 Cortical Layers Thalamic inputs >IV project to pyramidal cells in layer III Divergence from III –within AI –other cortical areas –contra AI V and VI >>thalamus &IC

49. Cortical Neurons Tonotopically and Spatiotopically organized Highly Adaptable Sensitive to CHANGES in Frequency and Intensity –Coding virtual pitch –demodulating complex signals (e.g. speech)

50. Cortical Processing Pattern Recognition Duration Discrimination Localization of Sounds Selective Attention

51. Cerebral Dominance/Laterality Language Processing in the left hemisphere. (Remember the right ear has the strongest connections to the left hemisphere) Most people show a right-ear advantage in processing linguistic stimuli