THE LIMBIC SYSTEM
The limbic system has a complex organization and is composed of the limbic cortex and many subcortical nuclei. The cortical part forms a ring at the medial surface of the cerebral hemisphere, including, among other structures, the cingulate and supracallosal gyri, the piriform lobe, and the hippocampus.
The subcortical part is composed of the hypothalamus, septal area, amygdala, habenular nuclei, and dorsal part of the mesencephalic tegmentum. There are numerous associations between these structures and other regions of the brain. The limbic system is often considered to be primarily a “visceral brain” because its major functions are expressed by visceral motor activity.Olfactory impulses passing by way of the piriform lobes may influence many structures of the system. Of all the sensory inputs, olfaction exhibits the most profound effects on visceral motor activities that are associated with emotional behavior such as eating, rage, sexual activity, fear, and drinking. The system also receives optic, auditory, extroceptive, and introceptive stimuli.
The efferent pathways from the cortical regions involve nearly all the subcortical nuclei of the system. A major portion of the influences of the limbic cortex is mediated through the efferent systems of the amygdaloid nuclei. Electrical stimulation of the amygdala produces a wide variety of visceral and somatic reactions and many behavioral reactions such as aggression and anxiety. The types of behavior most influenced by the limbic system are those essential for the preservation of the individual or the species.
The hippocampus probably plays the predominant part in the limbic system’s control of emotional expression and behavior through regulation of autonomic, endocrine, and somatic functions. It is also concerned with memory functions, such as the processing of recently acquired memory and its more permanent consolidation. In its activities the limbic system is closely associated with the reticular formation of the brainstem.