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Auditory nerve & Cochlear NucleusAmanda M. Lauer, Ph.D.Associate ProfessorCenter for Hearing and Balance

Dept. of Otolaryngology-HNSJohns Hopkins University SOM

www.lauerlab.com Overview

Auditory nerve Ventral Cochlear Nucleus

Dorsal Cochlear Nucleus

Physiology: in situ cochlea vs. slices and in vivo single cell recordings

200 µm

DCNAVCN

VIIIMouseChanda, Oh, & X-F (2011)

PVCN

Anatomical techniques: staining and microscopy

Basic structure of theauditory nerve

Auditory nerve in the cochlea (Spiral Ganglion Neurons)

Spiral ganglion neurons (SGNs)

Ribbon synapse

Nouvian et al. 2005

Lauer lab: CBA/CaJ mouse

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Type I and type II SGNs

Spoendlin 1979

Muniak et al. 2016

Two types of auditory nerve fibers: • I-myelinated; ~90%; form synapses

with IHCs; extensive projections to CN

• II-unmyelinated; ~5-10%; form synapses with OHCs; limited projections

Central projections of the auditory nerve labeled with tracers, antibodies, genetic tags

M. A. MERCHAN AND OTHERS

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Fig. 4(a-c). HRP tracing. Transverse sections of the DCN and the VCN (posterior division).(a) General pattern of distribution of primary cochlear afferents. VCN (*) shows HRP labellingalthough the higher concentration of label appears in the DCN, in a semilunar area (S) placedat the boundary with the VCN The remaining areas of the DCN (arrowheads) are practically

devoid of label. x 200. (b) Detail of the HRP-positive areas in Fig. 4(a). Axons in the VCN

turn parallel when they arrive at the interface with the DCN (DN). The DCN shows higherpositivity and an irregular distribution pattern. x 4160. (c) Labelled axons in the DCN. Primarycochlear afferents are distributed over the deep layer (1), but some of them reach the centrallayer (fusiform cell layer; 2) of the nucleus. No primary cochlear afferents appear at the molec-

ular layer (3). Arrows point to mossy enlargements of the fibres. x 1500.

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Merchan, Merhcan, Saldana 1994 HRP, rat Lauer Lab, VGLUT1 reporter mouse

Lauer Lab, VGLUT1 antibody, mouse

Auditory nerve projections to CN are tonotopically organized

Muniak et al. 2016 Auditory nerve fibers bifurcate and send tonotopic projections to dorsal (DCN) and ventral (VCN) cochlear nucleus. The nerve projects anteriorly and posteriorly in VCN.

These projections form synapses with several CN principal neuron types, which we will cover later.

How do you think these data were obtained (experiments)?

3-D Frequency Map of Mouse CN

Normal auditory nerve physiological

responses

Thresholds and frequency tuning (spectral coding)

Adapted from Palmer and Evans 1975

Base

From Pickles (2008)Do you recall why there is a big onset response followed by a smaller sustained response?

Thresholds and spontaneous activity

Auditory nerve fibers with high spontaneous activity have low thresholds.

Low spontaneous activity fibers have high thresholds.

Bharadwaj et al. 2014

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Spontaneous activity and rate-level functions (dynamic range/intensity coding)

Auditory nerve fibers increase firing rate with increasing sound levels up to a saturationpoint.

Low spontaneous activity fibers have higher saturation points than high spontaneous fibers..

Bharadwaj et al. 2014

Temporal coding: phase locking and the volley theory

Phase locking Volleying

How does the auditory nerve encode stimuli with complex

stimuli that change in frequency and intensity over time?

A short lesson about speech

Time waveform

Spectrogram

A short lesson about speech Speech coding

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The auditory nerve must convey the temporal fluctuations in amplitude and frequency from the cochlea to the brain.

How does it represent this complex information?

Rate coding plus place coding: auditory nerve fibers with best frequencies closest to the speech formant will response at the highest rates.

Speech

ANF

Speech (highertime res)

ANF (highertime res)

Representation of vowel formants

(Fourier transform)Young 2008

Five women played basketball

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Submit answers to the question.

Type in questions you have about the auditory nerve.

Cochlear nucleus: First stage in central auditory processing

Ascending central auditory system

System tasks:• Receives frequency, timing, intensity

cues from cochlea• Requires coordinated effort of many

structures & neuron types to transform these cues into perceptions – Detect a sound– Where is the sound coming from?– What is the source of the sound?– What does the sound mean?– Separate simultaneous sound

streams in an acoustic scene (cocktail party)

Kubke and Wild 2018

Brainstem

Midbrain

Cortex

Descending central auditory system

System tasks:• What to pay attention to• Modulation of afferent information

depending on behavioral state• Hearing in noise, separating out

competing sounds

Kubke and Wild 2018 Fuchs and Lauer 2018

Early studies of auditory brainstem structure were performed in Spain!

Santiago Ramon y Cajal

Which theory of nervous system organization is illustrated in this drawing? What was the other prevailing theory?

Early studies of auditory brainstem structure were performed in Spain!

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Auditory nerve input to cochlear nucleus

First place in brain that processesacoustic information (not just a relay)

Multiple jobs:

• Encoding cues used for sound localization (timing and intensity in AVCN with inhibition from DCN, monaural/spectral in DCN with input from VCN)

• Differentiating self-generated vs. external sounds and coordinating with other senses (DCN)

• Encoding spectrotemporal cues in complex sounds such as vocalizations

Muniak et al. 2016

How does cochlear nucleus do all of these jobs?

Different cell types: Ventral cochlear nucleus

Major input from auditory nerve.

Diverse types of neurons in different regions are optimized to encode different acoustic cues:• Bushy cells: timing,

intensity

• Multipolar/stellate: spectral shape

• Octopus cells: broadband onsets

A dapted from O sen, 1969

Spherical bushy cell

Globular bushy cell

Octopus cell

Multipolar cell

Break?

VCN response types

Auditory nerve endbulb/bushy cell interface

Bushy cell receives ~1-20 endbulbs.

Endbulbs: specialized axosomaticauditory nerve synapses with many release sites for high-fidelity transmission of information.

Dendrites of other bushy cells form synapses with endbulbs, forming nests of cells—for what?

Lauer et al. 2013, PLOS ONE

Lauer & Xu-Friedman labs

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Bushy cell responses

Pri-NPri

Blackburn and Sachs 1990

Responses to tones are similar to auditory nerve responses and are narrowly tuned.

Primary-like=spherical bushy cells (low frequency). Primary-like with notch=globular bushy cells (high frequency). Slightly different inputs.

Responses shaped by glycinergic (and GABAergic) input from ~5 sources (most from DCN, VCN), cholinergic inputs from multiple sources.

The endbulbs are very plastic in response to acoustic experience!

ExcitatoryInhibitory

Lauer et al., 2013

Bushy cells are very good at encoding temporal cues

Lauer Lab

Young and Oertel

Bushy cells are very good at processing temporal information.

Some units actually show better phase locking than auditory nerve fibers.

Endbulbs & bushy cells can fire at high rates and show activity-dependent plasticity

Ngodup et al. 2015, PNAS (Xu-Friedman and Lauer labs)

Early age exposure to 1 week ~85 dB SPL background noise facilitates endbulb synapses and improves the fidelity of bushy cell action potential firing by increasing the number of release sites in juvenile and young adult.

These changes are recoverable.

Facilitation ofEPSCs

More reliable spiking with high frequency

stimulation

Larger synapses and more release sites

Auditory brainstem responses (ABRs) as an assay of auditory nerve/bushy cell-driven pathways

ABRs are a rapid, high through-put assessment of synchronous activity in auditory nerve/bushy cell-driven pathways.

Also a standard estimate of hearing status in small animals and babies.

A useful tool to compare hearing across many species and after experimental manipulations.

Visit my lab for a demo at JHU!

94 dB click

2 ms5 ms

P1

N1

P2P3 P4P5

N2N3 N4

N5

Auditory Brainstem Response (ABR)

T stellate cells (type I multipolar, planar, chopper)

Oertel et al. 2011

Chanda and Xu-Friedman 2010

T stellates are most populous in the area around the the auditory nerve root and near the granule cell domain. “T” because axons pass through the trapezoid body.

Most inputs are to the (multiple) dendrites..

T stellates-responses and non-AN sources of inputs

Oertel et al. 2011

T stellates are excitatory and show tonic (chopping) responses to tones. Narrow frequency tuning.

They receive inputs from many sources..

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T stellates specialize in coding the spectral shapes of sounds such as speech

T stellates encode spectral features of sound very well).

Why do the peaks look sharper than in the auditory nerve?

Eric Young,

D Stellate cells: Inhibitory interneurons

D (dorsally projecting) stellates are inhibitory (glycinergic) interneurons that showtransient responses to onset of tones.

Broad tuning. Many axosomatic and axodendritic inputs.

Source of broadband inhibition within VCN and DCN..Inhibit bushy and T stellate cells in VCN.

Lauer lab

Smith and Rhode 1989

Stellate cell projections within CN

Malmierca 2013, based on work by Ryugo

Integration between VCN and DCN.

T (planar) stellates project to DCN in a frequency-specific manner.

D stellates (radiate) have more diffuse terminal fields.

Octopus cells

Blackburn and Sachs 1990

Lauer lab

Octopus cells (excitatory) receive auditory nerve inputs via small bouton terminals covering their soma and dendrites.

They fire at the onset of broadband sounds, requiring many simultaneously active auditory nerve inputs.

Occasional inputs from other octopus cells or inhibitory sources (rare) are observed..

Octopus cells compensate for cochlear delay

McGinley et al. 2012

Octopus cells receive high frequencies on their dendrites and low frequencies on their soma, which compensates for the auditory nerve delay.

VCN seems to take care of most of the encoding of important acoustic cues. So what does DCN do?

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Laminar organization of DCN compared to VCN

Osen

DCN cell layers and types

Layer 1-Molecular (Superficial)• Granule cell axons and small

stellate interneurons• Cartwheel cell and

pyramidal/fusiform cell dendrites

Layer 2-Pyramidal (Fusiform)• Pyramidal cell bodies-most

numerous principal cells• Cartwheel cells• Granule cells

Layer 3&4-Deep • Auditory nerve fiber axons

(tonotopic)• Giant cells (2nd principal cell type)• Vertical/tuberculoventral cells

From Osen 1990

Cerebellum-like circuit in DCN

DCN, Oertel & Young Cerebellum

What does this suggest about DCN’s role in hearing?DCN response

types

Response types and role of inhibition

There are many response types in DCN-complex circuitry.

Note: DCN neurons do not all respond to sound, do not normally show much spontaneous activity.

Responses depend on stimulus level

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Processing monaural spectral cues

DCN neurons are good at processing monaural spectral cues from the pinnae.

These cues are mainly used to localize sound in the median vertical plane, where binaural cues are absent.

DCN neurons also cancel responses to self-generated sounds.

DCN may tell your brain what sounds to pay attention to

Hernandez-Peon et al (1956)

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Answer the questions!

End of ANF/CN lecture!

Questions?

In case of questions

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Coronal sections of ventral (VCN) and dorsal (DCN) cochlear nucleus

Muniak et al 2013

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Major outputs of VCN neurons

Young and Oertel

Main--DCN projections

A closer look at the granule cell domain

Fluorescence: Yaeger & Trussell 2015; Borges-Merjane & Trussell 2015

Unipolar brush cellWeedman and Ryugo 1995 Granule

cell

Golgi cell

Responses are shaped by synaptic integration/inhibition

Fusiform and giant cell responses to sound are shaped by multiple excitatory and inhibitory inputs.

Combined, these two types of responses allows detection of sounds with spectral peaks and notches.