Above is a diagrammatic representation of a midsagittal view of the upper regions of the central nervous system. Outlined in light blue is the cingulate gyrus which comprises the majority of the limbic lobe. Its primary responsibilities include olfaction and emotions. The yellow area shown centrally located within the brainstem is the reticular formation. The reticular formation's major responsibility is the "arousal" of the nervous system as a whole. It sends both excitatory and inhibitory signals to higher and lower centers to either "excite" or "relax" the nervous system as a whole. As you will see in the final unit on "muscle tone", the reticular formation can actually be separated into the medial and lateral reticular areas. The lateral reticular formation is much larger and is an excitatory center while the medial reticular area is smaller and is an inhibitory center. There are neuronal pathways that lead from each to both higher and lower centers. The pathways that lead to lower centers to impact on lower motoneurons are the lateral and medial reticulospinal tracts. These tracts go to all levels of the spinal cord to impact on "overall" muscle tone (either to increase or decrease resting tone).

The limbic lobe has no direct neuronal pathways leading to lower centers (viz., lower motoneurons). Therefore, the limbic lobe must impact on other higher centers which do have lower center influences! The limbic lobe (cingulate gyrus) interacts very deeply with the reticular formation as represented by the green arrows above. Olfaction and emotions arising in the limbic lobe may either excite or inhibit an organism. Excitatory and inhibitory effects are seen on lower motoneurons due to the impact of the limbic lobe on the reticular formation and, therefore, on the reticulospinal tracts. The limbic lobe and the reticular formation are so densely intertwined that they may collectively be referred to as the "reticulolimbic system".

(Review or go on to next page.)