- Limbic and Olfactory Systems 359
Hippocampus: Circuitry
■ Th e Papez circuit (extrahippocampal circuitry):
- Th e entorhinal cortex projects to the hippocampus.
- Th e hippocampus projects to the mammillary
nuclei. - Th e mammillary nuclei project to the anterior
thalamic nuclei. - Th e anterior thalamic nuclei project to the
cingulate gyrus. - Th e cingulate g yrus projects back to the entorhinal
cortex.
■ Th e Papez circuit is the cornerstone of memory
consolidation.
■ Th e perforant pathway (intrahippocampal
circuitry):- Th e entorhinal cortex projects through the
subiculum to the dentate g yrus. - Th e dentate g yrus projects to CA3.
- CA3 projects to CA1 via Schaff er collaterals.
- CA1 projects to the subiculum.
- Th e subiculum projects along the alveus to the
fi mbria, which passes posteriorly and becomes the
crus of the fornix. - Th e subiculum also projects back to the entorhinal
c o r t e x.
Olfactory System
■ Th e olfactory neurons lie within the olfactory
epithelium and project through the cribriform plate
to innervate the olfactory bulb.
■ Th e olfactory bulb and tract are telencephalic
extensions of the cerebrum.
■ Th e medial olfactory stria innervates the medial
olfactory area in the subcallosal region.
■ Th e lateral olfactory stria innervates the primary
olfactory cortex in the basal frontal and anteromedial
temporal lobes.
■ Two principle classes of secondary olfactory neuron
exist: tuft ed cells and mitral cells.
■ Dendrites from secondary olfactory neurons
communicate with the primary olfactory axons
within the glomerular layer of the olfactory bulb.
■ Th e olfactory system bypasses the thalamus in its
projection to the cerebral cortex.Olfactory Cortex & Basal Forebrain
■ Th e basal forebrain structures with the most notable
cholinergic properties are the medial septal nuclei,
the diagonal band of Broca, and the basal nucleus of
Meynert.
■ Th e medial, intermediate, and primary (aka lateral)
olfactory cortices are innervated by the medial,
intermediate, and lateral olfactory striae,
respectively.
■ Th e medial olfactory cortex comprises, most notably,
the subcallosal and paraterminal g yri.
■ Th e intermediate stria terminates in the olfactory
tubercle within the anterior perforated substance.
■ Th e primary olfactory cortex comprises the piriform
cortex, periamygdaloid cortex, corticomedial
amygdala, and a small portion of the entorhinal
cortex, anteriorly.