Neurophysiology

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 & 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 Mid-brain Thalamus Temporal Lobe

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

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 IV V III II I Amp V Amplitude (V) Wave V Latency Time (ms) 10 0

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

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

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