A major allocortical structure (see ALLOCORTEX) in the medial TEMPORAL LOBE is so named because the interlocking arrangement of GRANULE CELLS in the DENTATE GYRUS and PYRAMIDAL NEURONS of areas CA1 and CA3 (see CA1–CA3) of the hippocampus proper (see AMMON’S HORN) together form the shape of a seahorse whose Latin name is hippocampus. Prompted by the observation that patients with brain damage in the medial temporal lobe, including the hippocampus, display AMNESIA (see for example, patient H.M.), it is widely thought to play a critical role in certain types of MEMORY.
The human hippocampus contains approximately 10 million CA1 pyramidal cells (about 30 times more than in the rat brain) and about the same number of CA3 cells. These cells form sheets within an essentially three-layered structure having an intriguing pattern of in trinsic circuitry. The hippocampus processes information projected from many regions of the NEOCORTEX, via the ENTORHINAL CORTEX which also serves as one route through which the output of hippocampal processing is fed back to the neocortex. CA3 axons project via the SCHAFFER COLLATERALS to stratum radiatum of area CA1 and to the lateral septal nucleus (see SEPTAL NUCLEI). CA3 receives input from the MOSSY FIBRES of the dentate gyrus and a direct projection of the PERFORANT PATH emanating from layer II of entorhinal cortex. These intrinsic connections are excitatory and have GLUTAMATENEUROTRANSMITTERS. Excitatory interconnections are modulated by a complex array of inhibitory synapses at DENDRITE, CELL BODY and initial AXON segments, and neuromodulatory input from MIDBRAIN structures involving MONOAMINE and other neurotransmitters. This input reaches the hippocampus via the FORNIX which also provides a route for output from the SUBICULUM to reach the MAMILLARY BODIES and DIENCEPHALON. The hippocampus is also a major site of stress-related CORTICOSTEROID receptors.
Electrophysiological studies have revealed prominent ELECTROENCEPHALOGRAM (EEG) rhythmicity (see HIPPOCAMPAL THETA) and patterns of hippocampal SINGLE-UNIT activity that, in the rodent, reflect some kind of processing of spatial information (see PLACE CELLS). Place cells are pyramidal cells whose firing patterns are influenced by the location of a freely moving (rat or mouse) in a familiar environment, an observation that lies at the root of the COGNITIVE MAP theory of hippocampal function. Other complex cell properties are also observed that probably reflect other contributions to memory processing, including the registration of events. These include activity-dependent changes in synaptic efficacy such as the phenomenon of LONG-TERM POTENTIATION (LTP). LTP is a widely studied model of the synaptic and cellular changes that underlie information storage in the brain, having physiological properties that are ideal for rapid, associatively induced and input-specific changes in synaptic strength. Behavioural studies have provided substantial evidence that an LTP-like mechanism could be involved in
The Hippocampus
Source: Figure reproduced, with permission, from Carlson, N.R. (1998) Physiology of Behavior, 6th edn, Allyn & Bacon: Needham MA.
hippocampal-dependent memory; physiological, pharmacological and TRANSGENIC techniques have provided supportive evidence.
There are numerous theories of hippocampal function, most emphasizing its role in some aspect of MEMORY. Prominent amongst these is the proposal that hippocampus is the site of spatial or cognitive maps of the environment used by rodents for NAVIGATION and by PRIMATES (including humans) for organizing the encoding of EPISODIC MEMORY. A LESION of the hippocampal formation in rodents will cause striking impairments of spatial navigation, but will also cause other nonspatial deficits, and certain types of navigation can occur successfully in hippocampal lesioned animals. Another theory is the proposal that the hippocampus plays a critical but time-dependent role in DECLARATIVE MEMORY—the memory for facts and events. This theory is supported by the existence of severe impair ments in PROPOSITIONAL or EXPLICIT MEMORY in amnesic patients, including for events that occurred shortly before brain damage, in the absence of any impairment in non-propositional or IMPLICIT MEMORY. However, the claim that this dissociation has been adequately modeled in primates with damage to the hippocampal formation has been undercut by the finding that the integrity of the neighbouring PERIRHINAL CORTEX is critical for RECOGNITION memory. The use of single tasks such as DELAYED NON-MATCHING TO SAMPLE is giving way to the development of new behavioural tasks that point to dissociations within the realm of what has hitherto been considered to be a single unitary process of declarative memory. Other theories of hippocampal function have been developed including the idea that it is critical for the creation of stimulus configurations, for the use of contextual information, for scene memory, and for the flexible use of relational representations. These ideas are a current focus of research.
New insights into the function of the hippocampus are emerging from FUNCTIONAL NEUROIMAGING work with human subjects using PET and FMRI. These indicate that activation of the human hippocampus and PARAHIPPOCAMPAL GYRUS occur during the recall of navigation through real or virtual environments, supporting the spatial theory. Episodic and declarative memory tasks seem, surprisingly, to activate the PREFRONTAL CORTEX preferentially and to do so differentially on the left or right side of the brain during encoding and recall respectively. However, a developing suspicion is that the hippocampus is metabolically continuously active and so hard to activate differentially using the subtractive methodology widely adopted in imaging work. The hippocampus may therefore play a critical role in some aspect of episodic memory, probably in the more automatic types of encoding that do not depend on intentional or effortful processing characteristic of the FRONTAL LOBE.
Reference
Schacter D.L. and Tulving E. (1994) Memory Systems 1994, MIT Press: Cambridge MA.
RICHARD G.M.MORRIS
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