Publikationsserver der RWTH Aachen University
The tectal-isthmic loop of the chicken (Gallus gallus) in a slice preparation : electrophysiology of its elements
Abstract
dc:descriptionThe signal flow in the brain is not just feedforward. Rather, most neural pathways in the brain are dominated by feedback projections. One prominent feedback system in the vertebrate midbrain is the tectal-isthmic loop. The optic tectum (superior colliculus in mammals) is a multimodal structure and it is the primary visual processing area in the mesencephalon of non-mammals. The isthmic system (nucleus parabigeminalis in mammals) is a complex of midbrain nuclei being spatially separated from the tectum. It contains three substructures having reciprocal connections with the optic tectum: the nucleus isthmi pars magnocellularis (Imc), the nucleus isthmi pars parvocellularis (Ipc) and the nucleus isthmi pars semilunaris (Slu). Tectal input to the isthmic nuclei arises from radial neurons located in tectal layer 10 (L10 neurons). Since both the anatomy and electrophysiology of this system have been extensively studied and since the reciprocal connections between the optic tectum and the isthmic system are preseved in slice preparations, this system is a very appropriate model to study feedback properties. However, a thorough knowledge of the somatic electrophysiology of the elements of the loop and the delays within the system was still missing. In this study, the somatic electrophysiological properties of all relevant network components were studied with whole-cell patch recordings in midbrain slice preparations of chick hatchlings (Gallus gallus). Basic electrophysiological features, such as resting potential, input resistance, response to current injection, initial AP threshold and initial AP amplitude were investigated for all elements of the loop. The firing behavior to increasing positive current injected into the cells was found to be tonical in all cell types, with a linear relationship between input current and firing frequency. Only one cell type in the Imc showed phasic response that, upon current increase, resulted in an oscillatory firing behavior. It was, however, not possible to attribute these physiological cell types of Imc neurons to the different anatomical cell types found in the Imc. In summary, despite the varying input resistances and spontaneous activity, the somatic properties of the isthmic cells were rather uniform. A second aspect of this study was the investigation of the delays between elements of the tectal-isthmic feedback loop. Retinal afferents to the upper tectal layers were stimulated extracellulary, and postsynaptic responses were measured in all elements of the feedback system. The delays between the tectum and the isthmic nucleis were found to be distributed around average delays of less than 10 ms and greater than 3 ms. The postsynaptic responses to synaptic input were found to be of different durations. Synaptic stimulation of the Slu by retinal ganglion cells was not possible, since projections between the tectum and the Slu were rarely preserved in the slice. To study the delays from the isthmic system to the tectum the Imc and the Ipc were stimulated extracellulary. Since the postsynaptic targets in the tectum are hitherto unknown, extracellular recordings were made in the tectum that however could only reveal axonal latencies. In a third series of experiments, the long-lasting response of all L10 neurons to stimulation of retinal afferents was studied. Deactivation of the Ipc by either microsurgical removal or by blocking of acetylcholine receptors was performed to reveal the impact of the cholinergic isthmic system on the L10 neurons. Under both conditions the responses of L10 neurons to stimulation by retinal afferents changed from long-lasting (90 ms) with many APs to only one AP, pointing towards an excitatory input of the Ipc neurons onto the L10 cells. In summary, the basic electrophysiological features of the tectal-isthmic loop in the chicken were elaborated in this thesis. Differences and similarities of somatic electrophysiological features were demonstrated, and the delays between components of the system were investigated. Together, these results were and will be the basis for modeling mathematical neural feedback sytems.
Degree
thesis:*- Grantor dc:publisher
- Publikationsserver der RWTH Aachen University
- Year dc:date
- 2008
Author and committee
dc:creator, dc:contributor.*- Author dc:creator
-
- Meyer, Ulrike
- Contributors dc:contributor
-
- Luksch, Harald
Subjects
dc:subject × 8Rights
dc:rights- Statement dc:rights
-
- info:eu-repo/semantics/openAccess
- Language dc:language
- eng