Cell Types and Physiology in the CANS

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

Trapezoid body

• MedGen Body• Inf Coll • Lat Lemn• SOC• Coch Nuc• VIIIth CN

VIIIth Nerve Afferents

• Bipolar Neurons• Primary Response Type• Synapse in:

– AVCN– PVCN– DCN

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Section Thru Brainstem Shows Cochlear Nucleus

Cochlear Nucleus

• Wide variety of cell types• AVCN >>to ipsi and contra SOC• PVCN >>to contra Lat. Lemniscus & IC• DCN>>contra to Lat Lemniscus

CN Response Types

• Primary-like cell- spherical (bushy) cells of the AVCN

• Chopper cell- identification with any particular cell type is not possible because responses are found throughout the cochlear nucleus

• Onset- located in octopus cells• Pauser cell/ Build up cell- located in the fusiform

layer of the DCN

Pauser cell

Primary-like cell

Chopper cell

Onset cell

Buildup cell

CN Response Types

DCN Networking

• Intranuclear connex– (largely inhibitory)– Enhancing tuning?

• Outputs rising in lateral lemniscus– Predominantly contralateral

The Superior Olivary Complex

Connections To the Superior Olivary Complex

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)

SOC Physiology

• Lateral SOC (EI and predominantly high freq)– Max Response to large SPL diffs between ears– Equal SPLs produce little activation– Larger in animals with smaller heads

• Medial SOC (predominantly EE and low freq)– Max Response to specific interaural time diffs– Larger in man than in animals with smaller heads

Interaural Delay affects MSO Synchronous Firing Rate

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IpsiContra

Lateral Lemniscus

• Tract of axons from just above SOC to IC(originating from cell bodies in several

different structures)• Has a Nucleus (Nucleus of LL)

– Good Temporal Resolution• Involved in Startle Reflex

– Connection from CN to Pontine Reticular Formation

– PRF >>motor neurons and spinal interneurons

Dorsal (back) Side of Brainstem

• Thalamus(medial geniculate)• Inferior Colliculus• 4th Ventricle• Area of Pons

Inferior Colliculus

Inferior Colliculus• Cells respond to characteristic:

– Interaural delays– Interaural amplitude differences– Amplitude modulations– Frequency modulations

• Integration of multi-modal sensory inputs– Adjacent visual nuclei– Proprioception of head and neck– Outputs include oculomotor nuclei– IC stim modifies activity in brain areas involved

in attention and learning.

Medial Geniculate BodyThalamus

• Last Sensory Relay Station prior to Cortex• Complex of nuclei• MGB is mainly auditory

– But has other inputs as well– AND– Some other nuclei respond to aud. stim.

MGB• Three Sub-divisions:

– Ventral MGB – True Auditory Relay nucleus– Medial MGB – Auditory with Somatosensory– Dorsal MGB –Somatosensory

Ventral MGB:

• Relay cells with overlapping dendritic “nests”• Interneurons inhibit relay cells & other

interneurons• Tonotopic Lows-lateral, highs-medial.• Most (90%) Cells are binaural (EE or EI)• 10% Monaural cells: contralateral excitation

Medial MGB:• Cells have CF,

– Some show broad tuning– Some have TWO CFs

• “Characteristic Intensity” in some cells• EE, EI, IE• All respond for duration of stim

– Little Adaptation– But show Habituation

• Influenced by somatosensory inputs

Dorsal MGB

• Inputs– From IC Both Aud. & Mixed– From Superior Colliculus– Somatosensory Afferents

• “Nonspecific” responses

Auditory Radiations Connect

• Medial Geniculate Body (in purple)

to• Primary Auditory

Cortex (in blue)

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

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

Cortical Neurons• Tonotopic

– Lows lateral– Highs medial

• Spatiotopic– (best responses from contralateral hemifield)

• Strong Habituation/Learning• Sensitive to CHANGES in Frequency and

Intensity

Cortical Processing

• Pattern Recognition• Duration Discrimination• Localization of Sounds• Selective Attention

Cerebral Dominance

• Most right-handed individuals show distribution of 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