Rat Reticular System

In most elementary account of the brain, the brain stem structure is described as an arrangement of well-defined reflex centers. The centers are highly specialized in function and can operate independently of the forebrain: a “midbrain animal” is one in which the forebrain has been destroyed; it still has an array of reflex responses. But paradoxically the midbrain also has a component of which the function are generalized and not specific and which operates in intimate relation to the forebrain. This component, the reticular activating system (RAS), consists of large numbers of cells, most of them without long axons, whose functions were for long an enigma. In 1949 Moruzzi & Magoun published observation which have given the RAS a prominent position in neurophysiology. The subject has been given reviewed by Jasper and colleagues and by Magoun himself.

The RAS is a matrix or network of cells extending from the anterior part of the spinal cord into the thalamus. It receives collateral fibers from afferent tracts as they proceed toward the thalamus, and all the receptors contribute inputs to it, directly or indirectly; hence it is a center, or group of centers, on which information from every sensory modality converges. The are also cortico-reticular tracts, both from the isocortex and the limbic system of fibers from the reticular substance to the whole of the cortex; this is in contract to the more familiar specific projection system carrying the sensory inputs, via the thalamus, to the special sensory regions.