Self And The Phenomenon Of Life: A Biologist Examines Life From Molecules To Humanity

Self and Conscious Experience 173

“9x6” b2726 Self and the Phenomenon of Life: A Biologist Examines Life from Molecules to Humanity

8.4.2 Diffuse mode

Distinct from discrete sensations and perceptions, non-specific alertness
is generated by an activating system in the brain stem and thalamus
(Fig. 8.3; Table 8.2). The brain stem reticular formation is an assembly
of diffuse, interconnected neuronal processes forming an ascending
(toward the brain) network. These short fibers are collaterals of the
long sensory neurons from the spinal cord and the brain stem. Retic-
ular formation extends upwards to the thalamus, from where fibers are
projected to the amygdala and hippocampus, and diffusely over the
entire cerebral cortex without focusing on any specific sensory area.
Electrical stimulation along the reticular formation promotes general
wakefulness of an animal, hence the name “reticular activating system”
(RAS). Destruction of RAS leads to coma.
Among the short, diffuse network of reticular interneurons are a
few long projection fibers in the brain stem that are also involved in
general arousal and sleep-wake cycles, the most well studied of which

Fig. 8.3. Schematic drawing of the human brain in coronal section (vertical cross-
section), showing the reticular activating system conveying arousal from the brain stem
to the cerebral hemispheres through a relay in the thalamus. Note that some fibers
bypass the thalamus.