Mikiel-Hunter, JAD; (2013) The synaptic and electrophysiological properties of principal neurons in the lateral superior olive of two Mammalian species. Doctoral thesis (PhD), UCL (University College London).

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Abstract

The lateral and medial superior olives (LSO and MSO respectively) receive inputs from the two ears and represent the first station in the binaural localization pathway. The need for two discrete nuclei to locate sounds sources on the horizontal axis was long assumed to reflect the binaural acoustic cue that each nucleus’ principal neurons employed when processing sounds in a restricted frequency range. This arrangement proves to be overly simplistic however: even though the MSO is restricted to processing temporal information within low-frequency sounds, LSO principal neurons are sensitive to interaural discrepancies in both the intensity and arrival time of high-frequency auditory stimuli. The ability of these neurons to extract temporal information from the envelope of amplitude-modulated carrier stimuli raises the possibility that responses to binaural stimuli may be continuous across the two nuclei. The purpose of this thesis was to determine whether synaptic and biophysical properties are specialised in guinea pig LSO principal cells for temporal and intensity difference processing. Whole cell patch clamp recordings demonstrated that LSO principal neurons responded to superthreshold current injection with a continuous range of phasic and tonic firing patterns whilst maintaining low input resistances and fast membrane time constants across the neuronal population. The importance of these fast passive properties was made clear when it was revealed that LSO principal neurons rely more on their passive properties to integrate excitatory synaptic potentials than do MSO principal neurons. Furthermore the passive properties of principal neurons appeared fastest in the lowfrequency, lateral limb of the LSO where contralateral excitation and band-pass filitering were also present. These findings suggest that many lateral guinea pig principal neurons may actually retain the biophysical and synaptic machinery to process pure tone and envelope temporal processing whilst still being sensitive to rapidly changing intensity differences.

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