Sensory neurons bring peripheral sensory information to the CNS via specific pathways that ascend and synapse with specific nuclei of the thalamus, which, in turn, innervate primary sensory areas of the cerebral cortex. These pathways involve three to four synapses, starting from a receptor that responds to a specific sensory modality—such as touch, hearing, or vision. Each modality has, in addition, a nonspecific form of sensory transmission, in that axons of the ascending fibers send collateral branches to cells of the reticular formation (Fig. 7.4). The latter, in turn, send their axons to the intralaminar nuclei of thalamus, which innervate wide areas of the cerebral cortex and limbic system. In the cerebral cortex and limbic system, the influence of the nonspecific projections from the reticular formation is arousal of the organism. This series of connections from the reticular formation through the intralaminar nuclei of the thalamus and on to the forebrain is termed the ascending reticular activating system.
The reticular formation also houses the neuronal systems that regulate sleep/wake cycles and consciousness. So important is the ascending reticular activating system to the state of arousal that a malfunction in the reticular formation, particularly the rostral portion, can lead to a loss of consciousness and coma.
The brainstem reticular formation and reticular activating system. Ascending sensory tracts send axon collateral fibers to the reticular formation. These give rise to fibers synapsing in the intralaminar nuclei of the thalamus. From there, these nonspecific thalamic projections influence widespread areas of the cerebral cortex and limbic system.
Was this article helpful?
This ebook provides an introductory explanation of the workings of the human body, with an effort to draw connections between the body systems and explain their interdependencies. A framework for the book is homeostasis and how the body maintains balance within each system. This is intended as a first introduction to physiology for a college-level course.