The neural basis for consciousness has fascinated scientists and philosophers for centuries. The reticular formation of the brain was recognized in the 1950s as a master regulation of consciousness. The reticular formation plays a central role in the regulation of the state of consciousness and arousal. It consists of a complex network of interconnected circuits of neurons in the tegmentum of the brain stem, the lateral hypothalamic area, and the medial, intralaminar, and reticular nuclei of the thalamus (Fig 18–1). Many of these neurons are serotonergic (using serotonin as their neurotransmitter), or noradrenergic. Axons from these nonspecific thalamic nuclei project to most of the cerebral cortex, where they modulate the level of activity of large numbers of neurons.
Ascending reticular system.
The term reticular formation derives from the characteristic appearance of loosely packed cells of varying sizes and shapes, embedded in a dense meshwork of cell processes, including dendrites and axons. The reticular formation is not anatomically well defined because it includes neurons located in diverse parts of the brain. However, this does not imply that it lacks an important function. Indeed, the reticular formation plays a crucial role in maintaining behavioral arousal and consciousness. Some authorities refer to it as the reticular activating system.
In addition to sending ascending projections to the cortex, the reticular formation gives rise to descending axons, which pass to the spinal cord in the reticulospinal tract. Activity in reticulospinal axons modulates spinal reflex activity and may also modulate sensory input by regulating the gain at synapses within the spinal cord. The reticulospinal tract also carries axons that modulate autonomic activity in the spinal cord.
Regulation of arousal and level of consciousness is a generalized function of the reticular formation. The neurons of the reticular formation are excited by a variety of sensory stimuli that are conducted by way of collaterals from the somatosensory, auditory, visual, and visceral sensory systems. The reticular formation is, therefore, nonspecific in its response and performs a generalized regulatory function. When a novel stimulus is received, attention is focused on it while general alertness increases. This behavioral arousal is independent of the modality of stimulation and is accompanied by electroencephalographic changes from low-voltage to high-voltage activity over much of the cortex. The nonspecific thalamic regions project to the cortex, specifically to the distal dendritic fields of the large pyramidal cells. If the reticular formation is depressed by anesthesia or destroyed, sensory stimuli still produce activity in the specific thalamic and cortical sensory areas, but they do not produce generalized cortical arousal.
Many regions of the cerebral cortex produce arousal when stimulated. Because different attributes of the external world (eg, color, shape, location, ...