Hippocampal formation

Hippocampal formation (oldest part of the cerebral cortex)

Hippocampal formation

1. hippocampus
2. dentate gyrus
3. subiculum

Extensive connections with the entorhinal cortex and the fornix

1. entorhinal cortex (input to hippocampus; lies inferior to the hippocampus) – receives inputs from complex multisensory and higher order association areas of the cerebral cortex via the cingulum
2. fornix (input from just superior to the hippocampus) – recives cholinergic fibers from the basal forebrain (septal nuclei and basal nucleus of Meynert); dopaminergic fibers from the ventral tegmental area of the midbrain; serotonergic fibers from the raphe nuclei (midline pons and midbrain); and noradrenergic fibers from the locus ceruleus (floor of fourth ventricle)

Structure

The hippocampus and the contiguous subiculum are three layered cortices with a prominent middle pyramidal layer while the dentate gyrus is a three layered cortex with a prominent middle granular layer

Divided into four pie shaped sectors

1. ventrolateral Cornu Ammonis (CA)1
2. dorsolateral CA2
3. dorsomedial CA3
4. ventromedial CA4

Afferent fibers

Afferent fibers from the fornix and enterohinal cortex terminate on either the pyramidal cells of the hippocampus directly or on the graunule cells of the dentate gyrus which then project to the pyramidal cells of the hippocampus

1. CA1 and CA3 are the major efferents of the hippocampal formation
2. CA1 pyramidal cells are very sensitive to anoxia, degenerate early in Alzheimer’s disease and are probably responsible for temporal lobe epilepsy
3. some of the efferent fibers feed back to the entorhinal cortex and fornix to provide feedback to the association cortices, thalamus, nucleus accumbens, basal forebrain, mammillary bodies, hypothalamus and midbrain retiruclar formation
   1. the Papez loop describes an anatomic course where the emotional processes of cortical origin are built up and prepared prior to discharge in the hypothalamus – this includes hippocampal formation to mammillary body to anterior nucleus of the thalamus to cingulate gyrus to entorhinal cortex and back to the hippocampal formation

Hypothalamic secretion

Efferents of the hippocampal formation can also control hypothalamic secretion of corticotropin releasing hormones in response to stress and also has efferents to the reticular activating system

Function

Most important function of hippocampal formation is in learning and memory

1. unilateral or bilateral injury of the hippocampal formation can cause an inability to learn new information and anterograde amnesia
2. PCA occlusion (major blood supply to the hippocampus) can cause anterograde ammesia as well
3. hippocampal formation also provides a spatiotemporal cognitive map that allows integration of our position in space and time