Hippocampus in health and disease: An overview

Abstract

Hippocampus is a complex brain structure embedded deep into temporal lobe. It has a major role in learning and memory. It is a plastic and vulnerable structure that gets damaged by a variety of stimuli. Studies have shown that it also gets affected in a variety of neurological and psychiatric disorders. In last decade or so, lot has been learnt about conditions that affect hippocampus and produce changes ranging from molecules to morphology. Progresses in radiological delineation, electrophysiology, and histochemical characterization have made it possible to study this archicerebral structure in greater detail. Present paper attempts to give an overview of hippocampus, both in health and diseases.

Keywords: AD; atrophy; drug target; early Alzheimer's; hippocampal atrophy; hippocampus; prevention.

Figure 1

Relationship between limbic system and hippocampus. Note that hippocampus is a C-shaped structure having amygdaloid body at its anterior end, culminating deep into mammillary bodies (Figure: courtesy AITBS publishers India)

Figure 2

Microstructure of hippocampus. Note that this C-shaped structure has been divided into distinct histological domains, and it is now possible to radiologically demarcate CA1, which is the dominant histological region of hippocampus (Figure: courtesy AITBS publishers India)

Figure 3

Coruna Ammonia. After the Greek God who had horn and this folded structure resembles to that of hippocampal layers. Historically, it has been linked to silkworm or sea horse. A Danish Anatomist divided it into Cornu ammonis, also in short called CA (Figure: courtesy AITBS publishers India)

Figure 4

Basic pathways of hippocampus.[1] Note that entorhinal cortex (EC) sends projections to dentate gyrus, which in turn sends to CA3. Fibers go from here to CA1 region and are called as Schaffers collaterals (SC). Note that, as Mann and Eichenbaum (2005) have suggested, hippocampus operates in the form of a tricynaptic loop. That means, fibers go from EC to DG (1), CA3 to CA1 (2), and from CA1 to subiculum (3), which project to EC again. Lesion or damage to any component along the trisynaptic loop may result into hippocampal dysfunction. Flow of hippocampus is largely unidirectional, and it flows from EC to dentate gyrus and then to CA3 and onwards to CA1. Importantly, main input to hippocampus is from entorhinal cortex, which receives inputs from multiple cortical areas and all sensory modalities.[1] Cortical input that terminates on the layer I, II, III of entorhinal cortex goes to hippocampus.[1] Eventually, this information goes to entorhinal cortex and then to subiculum. It is important to know that each layer of CA has its own complex circuitry and longitudinal connections (Figure: courtesy AITBS publishers India)

Figure 5

MRI scans showing hippocampal atrophy of both sides in an-85-year-old female with advanced AD