Figure 1.

Schematic summary of the vertebrate reticular formation's anatomical organization. Directional arrows apply to all panels. (a) Sagittal section of cat brain, showing relative size and location of reticular formation (RF) and medial core. Abbreviations: CPu, caudate-putamen; SC, superior colliculus; SN, substantia nigra. (b) Sagittal section of the brainstem; the dendritic trees (grey lines) of the projection neurons (one cell body shown, open circle) extend throughout the medial RF along the dorso-ventral axis but extend little along the rostro-caudal axis. These dendritic trees contact axon collaterals of both ascending sensory systems (black dashed line) and far-reaching axons of the projection neurons (the axon of the depicted cell body is shown by the solid black line); ST is the spinothalamic tract. (c) The cluster model of RF organization. The medial RF comprises stacked clusters (three shown) containing medium-to-large projection neurons (open circles) and small-to-medium inter-neurons (filled circles); cluster limits (grey ovals) are defined by the initial collaterals from the projection neuron axons. Their radial dendritic fields allow sampling of ascending and descending input from both other clusters (solid black lines) and sensory systems (dashed black line). The interneurons project predominantly within their parent cluster.