In anatomy of animals, the archicortex, or archipallium, is the phylogenetically oldest region of the brain's pallium or cortex. It is often considered contiguous with the olfactory cortex, but its extent varies among species. In older species, such as fish, the archipallium makes up most of the cerebrum. Amphibians develop an archipallium and paleopallium, whereas reptiles develop an archipallium, paleopallium and a primitive neopallium. In humans, the archipallium makes up the hippocampus. The archicortex is also defined as a type of cortical tissue that consists of three cortical laminae. It has fewer laminae than both the neocortex, which has six, and the paleocortex, which has either four or five. The archicortex, along with the paleocortex and periallocortex, is a type of allocortex. Because the number of laminae that compose a type of cortical tissue seems to be directly proportional to both the information-processing capabilities of that tissue and its phylogenetic age, the archicortex is thought to be the oldest and most basic type of cortical tissue.
Location
The archicortex is most prevalent in the olfactory cortex and the hippocampus, which are responsible for processing smells and forming memories, respectively. Because olfaction is considered to be the phylogenetically oldest sensory modality, and the limbic system, of which the hippocampus is a part, is one of the oldest systems in the brain, it is likely that the archicortex was one of the first types of tissue to develop in primitive nervous systems. Archicortical precursor cells are also present in the dentate gyrus of the developing mammalian embryo.
Structure
The archicortex is largely made up of memorizing cells with two types of afferent synapses: excitatory and unmodifiable inhibitory synapses. Memorizing cell inhibition serves two functions; one is controlling synaptic modification conditions in the memorizing cell dendrites during learning and the other is controlling cell thresholds during recall. The archicortex may also contain codon cells. Unlike the neocortex, the archicortex lacks climbing fibers. Consequently, the archicortex is not adapted for this type of classification.
Memory
Unlike the neocortex, current theories of the archicortex argue that it performs simple memorization without changing the input's format in any complex manner. The archicortex is unable to classify inputs. It has two main uses: free simple memory and directed simple memory.
