Outline for today Structures Cell types Circuitry Function

structure c-shaped in coronal or transverse sections outline resembles rams horn (Ammons horn) dorsal and ventral commissures

major components of the hippocampal formation hippocampal proper (CA1,CA2,CA3) dentate gyrus entorhinal cortex Subiculum Presubiculum parasubiculum

How many cell layers are in… Dentate gyrus Hippocampal proper Entorhinal cortex

3 main Layers of DG molecular layer granule cell layer (striatum granulosum) polymorphic (or hilus)

3 main layers of hippocampus molecular layer (stratum radiatum. stratum lacunosum, moleculare) pyramidal layer – stratum pyramidale polymorphic (stratum oriens)

DG cell types diverse variety of interneurons mossy fibers – axons from DG granule cells – unmyelinated axons – synapse on CA3 only excitatory output from DG but not as simple as reduced interneurons - tissue from patients or models of temporal lobe epilepsy found evidence of normal or enhanced granule cell inhibition. GABAA receptor density is greater in acutely isolated granule cells from epileptic rats compared with controls. In epileptic rats, granule cells had prolonged EPSPs, and they discharged more action potentials than controls.

basket cells axons form plexuses surrounding cell body of granule cells GABA

cells and layers of the dentate gyrus 1- granule cell; axons (mossy fiber) – collateralize in PL and then to CA3 2- dentate pyramidal basket cell 3 – stellate cells (give rise to basket plexus) 4- mossy cell (GLU) 5- inhibitory cells

DG studied in relation to kindling, LTP DG has the most consistent cell loss in temporal lobe epilepsy – (in part interneurons in the hilus of DG)

typical CA1 pyramidal cell

some of the connections mossy fibers – axons from DG granule cells – unmyelinated axons – synapse on CA3 sole excitatory output from DG multiple granule cells can synapse on a single pyramidal cell Schaffer collaterals- projections from CA3 to CA1 (ipsilateral and contralateral)

some of the connections perforant pathway – major input to hippocampus axons arise primariy from layers II and III of EC

Entorhinal cortex (perforant path) Recurrent (Schaffer) collaterals of hippocampal pyramidal cells inCA3 Raphe, septal region Commissural axons; intrinsic short axon cells Basket cells Short axon cells; commissural axons Alveus

some more connections alveus - thin layer of fibers originating from pyramidal cells and subiculum; on route to subcortical termination or contralateral hipp covers the ventricular surface of the hippocampus bundle thickens – lateral extreme end of hipp – fimbria; descending into forebrain - fornix.

What are the main afferents to hippocampus and DG? entorhinal cortex*** septum, contralateral hippocampus

Cortical inputs to the hippocampal formation

What are the afferents to the entorhinal cortex? projections from brainstem, raphe, LC, VTA, hypothalamic afferents thalamic nuclei telencephalic cortical areas other hippocampal regions

Hippocampal Efferents Many fibers sent directly back to the entorhinal cortex. Most anatomically prominent output pathway is the fornix

efferents of EC perforant pathway amygdala, cortical nuclei, central nucleus,

Subcortical outputs via the fornix

Cortical projections from hippocampus via entorhinal cortex

subiculum one of most important efferents of hippocampal formation – only its axons leave the telencephalon to terminate in the diencephalon

fimbria/fornix carry efferent from hippocampal formation and subcortical afferent fibers to hippocampus

trisynaptic circuit perforant pathway synapse on granule cells granule cells send their mossy fiber axons to CA3 CA3 pyramidal cells send their shaffer collateral axons to CA1

neurotransmitters? GLU GABA 5HT perforant path, mossy fibers, Schaffer collaterals, excitatory interneurons NMDA and metabotropic R GABA 5HT