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 Sound OnSound Off

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

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

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 Body Thalamus 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