entorhinal cortex (Brodmann's area 28)

Structure

• temporal cortex closely associated, if not considered part of the ventral hippocampal formation
• it extends rostrally to about 5 mm short of the rostral end of the amygdala & caudally to the anterior edge of the lateral geniculate nucleus (about midway back through the hippocampus).

• rostrally, the entorhinal cortex borders the periamygdaloid cortex medially
• as the other fields of the hippocampal formation appear caudally, the entorhinal cortex merges medially with the subiculum (pre or para-subiculum)
• there is no easily identifiable lateral border of the entorhinal cortex; it merges with the perirhinal cortex on the banks of the collateral sulcus

• cytoarchitecturally, the entorhinal cortex transforms the 4 layers of the allocortex into the 6 layers of the neocortex
• the allocortical entorhinal region does not gradually transform into the temporal isocortex
• instead, there is an extended stretch of "transentorhinal" cortex with interdigitation of allocortical & isocortical laminae

• stellate nerve cells of layer 2 (pre-alpha) are transformed into pyramidal cells with the transition of allocortex to isocortex

The transentorhinal region thus consists of 6 layers, distinct from other neocortical regions.

• Layer 1: acellular, plexiform layer
• Layer 2: islands of pyramidal & stellate* neurons
• Layer 3: medium sized pyramidal cells (homogenous population)
• Layer 4: often indistinct acellular region
• Layer 5: large pyramidal cells (5-6 cells deep)
• Layer 6: often indistinct

Afferents

• subiculum
• CA1 region of the hippocampus
• cortical (see hippocampal afferents, cortical)
  • perirhinal cortex (areas 35 & 36)
  • parahippocampal gyrus (areas TF & TH)
  • cingulate cortex
  • piriform cortex
  • temporal polar cortex
  • superior temporal gyrus
  • ventral insular cortex
  • orbitofrontal cortex

Efferents

• hippocampal formation
  • dentate gyrus*
  • subiculum
  • cornu ammonis CA1#
  • cornu ammonis CA3
• subcortical (see hippocampal efferents, subcortical)
  • claustrum
  • amygdala
  • septal nuclei
  • nucleus accumbens
  • striatum (caudate & putamen)
  • hypothalamus
  • anterior thalamic nucleus
  • mammillary bodies
• cortical
  • perirhinal cortex
  • parahippocampal gyrus
  • orbitofrontal cortex

* Neurons of layers 2 & 3 of the lateral entorhinal cortex send axons into the dentate gyrus to synapse on granule cell dendrites in the outer 2/3 of the molecular layer in the perforant pathway of the trisynaptic circuit.

# Neurons of layers 2 & 3 of the medial entorhinal cortex send axons into the hippocampus to synapse on basilar dendrites of hippocampal CA1 pyramidal cells in the alvear pathway.

Pathology

• stellate neurons of layer 2 are among the most vulnerable & 1st injured in Alzheimer's disease; they are particularly prone to development of neurofibrillary tangles
• the isocortical pyramidal cells of layers 3 to 5 are far less inclined to develop neurofibrillary changes.

More general terms

• brain structure

Additional terms

• alvear pathway
• Alzheimer's disease (ADD)
• hippocampal afferents, cortical
• hippocampal efferents, subcortical
• perforant pathway

Component of

• hippocampus (hippocampal formation)