  Spatial learning across many species is impaired by lesions in the
hippocampus, a subcortical brain structure whose cellular composition changes
substantially over its 5 to 6 week lifetime from mainly excitatory neurons
during development to equal proportions of inhibitory interneurons (gamma
Amp/Arcs) as well as pyramidal cells early in life, but which later on
comprises only about 10% Arches+ projection spiny cortical projecting principal
cells that are located within discrete cytoarchitectonic patches known as CA3
or just hilus granular layer 2 (CG2). While other structures may contribute
importantly in certain situations e.g., perirhinal cortex when using visual
cues with no reference frame for location), these remaining cell types also
change their proportion through time with gamma APs forming 30 to 35 percent),
fast firing parvalbumin immunoreactive basket or axoaxonic synapses into
somatic spines at 40 days after birth (DABs formed into spine membranes by 15
DAB), and CG2 pyramids forming relatively slowly but strengthening synapses
onto nearby dendritic shafts made up largely (>80 %) by GABAergic terminals
(+spines) until then that were still synaptically silent.
