Week 4: Physiology With Electrical Stimulation (PDF)

Week 4: Physiology with Electrical Stimulation A. Anatomical changes with SNHL B. Neural responses  Peripheral Changes with SNHL ~ 35,000 auditory nerve fibers for individuals < age 20 ~2,000 fibers lost per decade d/t aging Fig. 1-4, Hughes boo...

Week 4: Physiology with Electrical Stimulation A. Anatomical changes with SNHL B. Neural responses  Peripheral Changes with SNHL ~ 35,000 auditory nerve fibers for individuals < age 20 ~2,000 fibers lost per decade d/t aging Fig. 1-4, Hughes book, pg. 7 Retrograde degeneration: A. Intact B. Hair cell loss  lack of input to dendrites  C. SGC volume decreases  D. Demyelination  E. Axonal degeneration  cell volume decreases in more central structures  Anatomical Changes with SNHL Degeneration is slowly progressive (years) Peripheral to central Basal to apical progression For most etiologies, basal turn typically has most neural loss. Presence of supporting cells in organ of Corti can delay degeneration. Uneven nerve survival patterns Effect on central pathways Effect on performance with CI Adapted from Wilson et al. (2000) Fig 6.1, pg. 110  Peripheral Changes Factors affecting nerve survival patterns: Examples: Etiologies with higher neural survival: Ototoxicity Sudden SNHL Etiologies with lower neural survival: Bacterial infections of the inner ear Congenital/genetic deafness Postnatal viral labyrinthitis (most severe neural loss) Why the difference?  Peripheral Changes Duration of deafness affects anatomical changes: Longer duration HF SNHL  poorer neural survival in the basal turn. Greater SGC density for duration of deafness. Poorer SGC density for duration of deafness. Zeng, Fig. 4.2, pg. 112  Peripheral Changes Post-CI Chronic electrical stimulation promotes SGC survival. Increase in SGC density Increase in number of surviving neurons Larger cell size More likely to retain normal morphological characteristics Zeng, Fig. 4.3, pg. 115; Fig. 4.4, pg. 116  Peripheral Changes Post-CI Electrode insertion trauma Trauma to spiral ligament, basilar membrane, OSL at insertion site and first basal turn (~8-15 mm) New bone growth Decreases SGC survival Degree of damage is related to amount of SGC loss Insertion trauma seems to offset neurotrophic effects from chronic electrical stimulation. Photos courtesy of J. Thomas Roland Jr., M.D., NYU Medical Center  Peripheral Changes Post-CI How to avoid deleterious effects of insertion trauma? Change electrode array design for less traumatic insertions Soft tip, thinner, more flexible, pre-curved Neurotrophins Protect spiral ganglion neurons and promote survival after various types of insult causing deafness. Neurotrophins + chronic electrical stimulation + atraumatic electrode insertion = optimal neural survival in CI recipients.  Central Changes Reduction in cell size in brainstem structures (PVCN, AVCN, MSO, IC) Reduction in cell density Reduction in overall size of brainstem structures (particularly CN)  Week 4: Physiology with Electrical Stimulation A. Anatomical considerations B. Neural responses  Neural Responses OUTLINE/VOCABULARY: Acoustic versus electric stimulation: Spontaneous activity Frequency tuning Rate-level functions Phase locking Temporal spread Waveform morphology Auditory nerve response properties: Current integration Growth functions (a.k.a. input-output functions) Threshold-distance relationship Summative properties, refractory properties, & adaptation Channel interaction Electrophonic stimulation  Acoustic vs. Electric Stimuli 1.) Spontaneous Activity Acoustic/Normal: Spontaneous neurotransmitter release from IHCs 0-80 spikes/sec Electric/Deafened: Little to no spontaneous activity d/t lack of HCs.  Acoustic vs. Electric Stimuli 2.) Frequency Tuning Place code Acoustic/Normal: Sharp tuning Electric/Deafened: Broad tuning, not place-specific Clinical application: Pitch is represented by electrode place but is limited by Pickles, Fig. 4.3, pg. 82 (top) ; Fig. 10.10, pg. 313 (bottom; current spread. from Kiang & Moxon, 1972)  Acoustic vs. Electric Stimuli 3.) Rate-Level Functions Rate code Acoustic/Normal: Non-linear/shallower slope Fiber dynamic range ~30 dB Electric/Deafened: Fiber dynamic range ~1.5 – 6 dB Greater maximum discharge rate Steeper rate-level function Clinical application: Smaller behavioral dynamic Pickles, Fig. 10.11, pg. 315 ranges for CI recipients  Acoustic vs. Electric Stimuli 4.) Phase Locking Temporal code Acoustic/normal: Acoustic Fibers fire to positive phase Phase locking saturates at 4-5 kHz Electric/deafened: Fibers fire to cathodic or anodic phase, depending on species Electric More synchronous response to peak of stimulus (B) Clinical application: CIs are not as good at representing music or tonal languages Pickles, Fig. 4.8, pg. 90; Tyler, Fig. 7.3  Acoustic vs. Electric Stimuli 5.) Temporal Spread Ringing Acoustic/normal: Think of BM as “narrower” filter Narrower filter, more ringing Electric/deafened: Rosen & Howell, pg. 192 Not constrained by BM motion, so “wider” filter Virtually no ringing (short PST histograms)  Acoustic vs. Electric Stimuli 6.) Similar in waveform morphology Shape of EABR (pulsatile stimulation) corresponds well to shape of ABR (clicks). Zeng, Fig. 6.5, pg. 230  Auditory Nerve Response Properties Membrane Biophysics Neural membrane: Leaky integrator Myelination Reduces excitation threshold Speeds conduction velocity Action potentials: Fiber excitability varies with repeated electrical stimulation due to stochasticity. Jitter in timing of action potentials Rubinstein et al. (1999), Fig. 1, Hrg Res  Auditory Nerve Response Properties Current Integration Threshold of excitation depends on stimulus duration because neural membranes integrate current over time (capacitive). Strength-duration function Rheobase Chronaxie Zeng, Fig. 5.4, pg. 165 Clinical application: You can manipulate a parameter called “pulse width” in the programming software – this one of the mechanisms behind how CI users’ loudness perceptions change  Auditory Nerve Response Properties Growth (Input-Output) Functions Whole-nerve response properties (eCAP) Amplitude increases with level Latency decreases with level Larger dynamic range for eCAP than SF (why?) Lower threshold & shallower slope when closer to neural elements Clinical application: Thresholds from eCAP growth functions are used to help program the processor for children. Fig. 4, Hughes et al. (2000), E&H  Auditory Nerve Response Properties Threshold-Distance Relationship Lower thresholds and shallower growth of amplitude with level for electrodes closer to modiolus. Why? Clinical application: Expect lower behavioral thresholds for perimodiolar electrode arrays  Auditory Nerve Response Properties Summative Properties 2 successive pulses with a short delay Conditioner pulse Probe pulse Integrative properties of neural membrane Fig. 2a & Fig. 4, Cartee et al. (2000), Hrg Res  Auditory Nerve Response Properties Refractory Properties Refractory time course: Absolute: ~1 ms Relative: ~5-10 ms How fast can you stimulate without effects of refractory periods? Measuring refractory properties: 2 successive pulses with a longer delay Masker pulse Probe pulse Longer time delay  less masking effects Significant level effects Fig. 2b & Fig. 7, Cartee et al. (2000), Hrg Res  Auditory Nerve Response Properties Adaptation Response to sustained stimuli (pulse trains) Decrease in excitability with sustained stimulation Acoustic: neurotransmitter at HC synapse Adaptation exists at level of nerve; time course on order of hundreds of seconds Clinical application: This is why we don’t measure T-levels for really fast stimulation rates. This is also why we use slow rates for ANSD. Zeng, Fig. 5.11, p. 183 (top); Rubinstein et al (1999), Fig. 10  Auditory Nerve Response Properties Spatial Excitation Patterns and Channel Interaction Electrical current spreads in tissue because it is conductive. Degree of spread and neural survival patterns affect spatial excitation patterns Channel interaction: Stimulation on one electrode interferes with the neural responses resulting from stimulation on another electrode (summative or refractory).  Electrophonic Responses alpha response: Direct neural excitation (latency