Tools to investigate the anatomy of the central autonomic systems

The functional specificity of the autonomic regulation of target organs and the neurophysiological recordings from peripheral autonomic neurons (see Chapter 4) argue that central autonomic systems must be differentiated. It seems likely then that the central organization is reflected in the micro- and macroanatomy of the central autonomic systems. Originally, central stimulation and lesion studies, involving recordings from autonomic nerves or autonomic effector responses (e.g., blood pressure, heart rate, gastrointestinal motility), gave clues about the location of the autonomic centers in the neuraxis. However, the results from these studies were imprecise so that it was not possible to identify anatomically distinct populations of central neurons as being associated with distinct autonomic output systems. Furthermore, focal electrolytical lesioning and focal electrical stimulation did not discriminate between destruction or excitation of cell bodies and passing axons. With the introduction of chemical stimulation of cell bodies (by microionophoretic application of excitatory amino acids, such as glutamate, aspartate or dl-homocystic acid, or inhibitory amino acids, such as γ-aminobutyric acid [γ-aminobutyric acid] or glycine) it was possible to topically excite or inhibit small populations of central neurons selectively. This technique turned out to be a valuable tool in physiological experimentation.