The limbic system is a diverse collection of cortical and subcortical regions that are crucial for normal human behavior (Martin, 2003). According to Martin (2003), nineteenth century neurologist and anatomist recognized that damage to particular parts of the human brain were associated with disorders of emotion and memory; these lesions , unlike those of the cerebellum, occipital lobe, or cortical regions around the central sulcus, for example, spared perception and movement. This research lead to the understanding that the neural systems of emotions, learning and memory, and their interconnections, are grouped into a single system, called the limbic system. Based on Martin (2003), brain structures for emotions, learning and memory have been conserved throughout much of the vertebrate evolution, reflecting the common and important need for these functions.
The diverse functions of the limbic system include important roles in learning and memory and in emotions – and their behavioral and visceral consequences. According to Martin (2003), many of the structures have a C-shaped configuration; the limbic system has three C-shaped components: (1) the limbic association cortex, (2) the hippocampal formation and fornix, (3) and part of the amygdala (bed of stria terminalis) and the stria terminalis.
Hippocampal circuits are engaged in consolidating explicit memories, such as the conscious recollection of facts, and in forming spatial memories; the hippocampal formation works closely with adjoining entorrhinal cortex, so much so that the two are functionally inseparable (Martin, 2003). According to Martin (2003), these structures receive complex sensory and cognitive information from the limbic association cortex; damage to the hippocampal formation or entorrhinal cortex, depending on the extent, can result in severe and pervasive anterograde amnesia.
As Martin stated, in this form of amnesia, impairments occur in semantic memory, such as knowledge of facts, people, and objects, including new word meaning, and the episodic memory of events that have a specific spatial and temporal context, such meeting a friend last week; and by contrast, patients with hippocampal damage are capable of remembering procedures and actions (i.e., implicit or non-declarative memory), and they retain the capacity for a variety of simple forms of learning and memory.
The output neurons of the hippocampal formation are pyramidal neurons, similar to the neocortex covering most of the cerebral hemisphere, and they are located in the hippocampus and subiculum (Martin, 2003). Two output systems can be distinguished within the fornix, from the subiculum and the hippocampus; although these systems are involved in the cognitive aspects of learning and memory, it is not yet understood how their functions differ.
The innervations of the limbic system by the major neurotransmitter regulatory systems appear to be particularly important for normal thoughts, moods, and behaviors. This conclusion is based on the observation that many of the drugs used to treat psychiatric illness – the disorders of thought, such as schizophrenia, and of mood, such as depression and anxiety – selectively affect one of the neurotransmitters systems; this neurotransmitter system have direct and widespread connections with the limbic system (Martin, 2003).
Pyramidal cells of the entorhinal cortex send their axons to the dentate gyrus to synapses on granule cells, which is the perforrant pathway. Based on Martin (2003), it is not known how the myriad connections of the entorhial cortex and hippocampal formation are organized to play a role in memory consolidation; however, an important clue exists: the strength of many synapses in the hippocampal formation can be modified under various experimental conditions.
The amygdala has three major divisions, which collectively are involved in emotions and their behavioral expression: the basolateral nuclei, the central nuclei, and the corticomedal nuclei
Martin, J. H. (2003). Neuroanatomy: Text and Atlas. New York: McGraw-Hill Professional.