Entorhinal cortex

The entorhinal cortex (EC) is an important memory center in the brain. The EC forms the main input to the hippocampus and is responsible for the pre-processing (familiarity) of the input signals. In the reflex nictitating membrane response of classical trace conditioning, the association of impulses from the eye and the ear occurs in the entorhinal cortex. The EC-hippocampus system plays an important role in memory consolidation and memory optimization in sleep.

Entorhinal cortex is one of the first areas to be affected in Alzheimer's Disease, and one of the first symptoms is impaired sense of direction. In 2005, it was discovered that entorhinal cortex contains a neural map of the spatial environment.^[1]

The entorhinal cortex show a modular organization, with different properties and connections in different areas. Neurons in the lateral entorhinal cortex exhibit little spatial selectivity^[2], whereas neurons of the medial entorhinal (MEA) cortex exhibit multiple "place fields" that are arranged in an hexagonal pattern, and are therefore called "grid cells." These fields and spacing between fields increase from the dorso-lateral MEA to the ventro-medial MEA.^[3][1]

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Anatomy

In rodents, EC is located at the caudal end of the temporal lobe and is usually divided into medial and lateral regions (with three bands with distinct properties and connectivity running perpendicular across the whole area). A distinguishing characteristic of EC is the lack of cell bodies where layer IV should be; this layer is called the lamina dissecans.

Inputs and outputs

The superficial layers - layers II and III - of EC project to the dentate gyrus and hippocampus: Layer II projects primarily to dentate gyrus and hippocampal region CA3; layer III projects primarily to hippocampal region CA1 and the subiculum. These layers receive input from other cortical areas, especially associational, perirhinal, and parahippocampal cortices, as well as prefrontal cortex. EC as a whole, therefore, receives highly-processed input from every sensory modality, as well as input relating to ongoing cognitive processes, though it should be stressed that, within EC, this information remains at least partially segregated.

The deep layers, especially layer V, receive one of the three main outputs of the hippocampus and, in turn, reciprocate connections from other cortical areas that project to superficial EC.

Brodmann's areas

• Brodmann area 28 is known as the "area entorhinalis"
• Brodmann area 34 is known as the "area entorhinalis dorsalis"

References