On the origin of the extracellular potential in the nucleus laminaris of the barn owl

Abstract

The barn owl is a good night hunter and mainly localizes the prey with its auditory system. The auditory localization in the horizontal plane, based on interaural time differences, depends on the auditory brainstem circuit consisting of nucleus magnocellularis (NM) and nucleus laminaris (NL). An extracellular field potential (EFP), named neurophonic, can be recorded in the NL. It has a very high temporal precision of below 10 microseconds and replays the stimulating sound up to 9 kHz. In this thesis I study how an EFP with such a precision can be generated. Furthermore, what can we learn about the system and about the origin of the neurophonic in NL from these recordings? The answers will help connecting the neural activity to the EFP also in general. Firstly, hundreds of sources, all firing with a high rate and in a highly phase-locked manner, are needed to generate the neurophonic in NL. The number of the neurons in NL and the magnitude of their output currents are not high enough to alone give rise to the neurophonic. The majority of the neural sources conveys the input from NM to NL, i.e., the currents from the nodes of Ranvier in the afferent axons from NM, and the synaptic currents to the dendrites of the NL neurons. Furthermore, the neurophonics in response to monaural stimulation sum up linearly and predict accurately the neurophonics in response to binaural stimulation. This implies that the non-linear response of the NL neurons usually cannot be detected in the neurophonic, but that there might be a minor contribution from a single NL neuron when in the immediate vicinity of the electrode. All in all, the neurophonic in the barn owl’s NL seems to reflect the inputs to the nucleus, whereas usually the output is well represented in the EFP. Even in the homologue nuclei in chick and mammals the neurophonic is thought to reflect the output instead of the input. Thus, the exceptionality of the barn owl might be needed for the high precision in its NL.