Neural circuits are characterized as interconnecting neuron networks connected by synapses. Some kinds of gene expression and/or functional changes of neurons and synaptic connections may result in aberrant neural circuits, which has been recognized as one crucial pathological mechanism for the onset of many neurological diseases. Gradual advances in single-cell sequencing approaches with strong technological advantages, as exemplified by high throughput and increased resolution for live cells, have enabled it to assist us in understanding neuronal diversity across diverse brain regions and further transformed our knowledge of cellular building blocks of neural circuits through revealing numerous molecular signatures. Currently published transcriptomic studies have elucidated various neuronal subpopulations as well as their distribution across prefrontal cortex, hippocampus, hypothalamus, and dorsal root ganglion, etc. Better characterization of brain region-specific circuits may shed light on new pathological mechanisms involved and assist in selecting potential targets for the prevention and treatment of specific neurological disorders based on their established roles. Given diverse neuronal populations across different brain regions, we aim to give a brief sketch of current progress in understanding neuronal diversity and neural circuit complexity according to their locations. With the special focus on the application of single-cell sequencing, we thereby summarize relevant region-specific findings. Considering the importance of spatial context and connectivity in neural circuits, we also discuss a few published results obtained by spatial transcriptomics. Taken together, these single-cell sequencing data may lay a mechanistic basis for functional identification of brain circuit components, which links their molecular signatures to anatomical regions, connectivity, morphology, and physiology. Furthermore, the comprehensive characterization of neuron subtypes, their distributions, and connectivity patterns via single-cell sequencing is critical for understanding neural circuit properties and how they generate region-dependent interactions in different context.

Visual systems appear like homogenous structures, where identical functional units repeat themselves across the eye. This architecture is thought to ensure a uniform sampling of the surrounding environment. Furthermore, anatomically and functionally identical properties of single units belonging to the same cell type, yet located across retinotopical positions are thought to ensure translational invariance. At the same time, regional differences and stochastic variations in microcircuit architecture have been linked to the processing of specific visual features. Recent access to connectomic datasets has revealed heterogeneity in visual circuitry that is at odds with these criteria: Cells considered to belong to the same type are variable in number and identity of connected partners, as well as in the relative number of synapses. This variable connectivity suggests that heterogeneous computations, even within defined cell types, is the rule, rather than the exception. It is therefore an exciting question whether these network properties increase functional variability, or even functional robustness, of visual processing.

The extrinsic motor innervation of the large intestine consists of both sympathetic and parasympathetic pathways. The sympathetic supply originates from neurons in the prevertebral, paravertebral and pelvic ganglia while the parasympathetic supply arises only from the pelvic ganglia. Experiments investigating the sympathetic component of the pelvic innervation of the bowel are scarce although it is assumed that the control of the bowel by sympathetic neurons in the pelvic ganglia is similar to that provided by the prevertebral ganglia. However, several observations regarding the synaptic inputs to pelvic neurons, their immunohistochemical and morphological features suggest that pelvic neurons are not similar to prevertebral neurons. The aims of this thesis were to characterise pelvic pathways to and from the bowel and to compare these with the prevertebral innervation of the bowel in the same species. This was carried out in the male rat as the anatomy of its pelvic plexus is particularly simple. A number of experimental techniques were used to investigate the pelvic pathways supplying the bowel. Retrograde tracing from the distal colon was employed to identify the population of pelvic colon-projecting neurons. The spinal connections and the immunohistochemical features of these neurons were then investigated. Half of the pelvic colon-projecting neurons are sympathetic, comprising both noradrenergic and cholinergic neurons. Hence pelvic sympathetic neurons differ from prevert