Perforant path input from EC layer II enters the dentate gyrus and is relayed to region CA3 (and to mossy cells, located in the hilus of the dentate gyrus, which then sends information to distant portions of the dentate gyrus where the cycle is repeated). The main output pathways of the hippocampus are the cingulum bundle and the fimbria/ fornix, which arise from field CA1 and the subiculum. Layer II of entorhinal cortex (EC) brings input to the dentate gyrus and field CA3, while EC layer III brings input to field CA1 and the subiculum. The perforant path, which brings information primarily from entorhinal cortex (but also perirhinal cortex, among others), is generally considered the main source of input to the hippocampus. CA2 represents only a very small portion of the hippocampus and its presence is often ignored in accounts of hippocampal function, though it is notable that this small region seems unusually resistant to conditions that usually cause large amounts of cellular damage, such as epilepsy.
Hippocampus anatomy free#
The majority of its output is via the alveus into the fimbria which breaks free to the hippocampus to form the fornix. Information flows through the hippocampus proceeds from the dentate gyrus to CA3 to CA1 to the subiculum, with additional input information at each stage and outputs at each of the two final stages. NB: CA is an abbreviation of cornu Ammonis Pathways The hippocampus proper is made up of CA1, CA2 and CA3 fields (Figure 1).
It has a head (posterior to the amygdala), a body, and a tail (which follows the upwardly curving lateral ventricle).Īlthough there is a lack of consensus relating to terms describing the hippocampus and the adjacent cortex, the term hippocampal formation generally applies to the dentate gyrus, fields CA1-CA3 (CA4 is frequently called the hilus and considered part of the dentate gyrus), and the subiculum ( parahippocampal gyrus). Connectional and comparative studies, including the use of kainic acid excitotoxicity, suggest that the V-shaped layer is comparable to the dentate gyrus of the mammalian hippocampal formation and DM to Ammon's horn and subiculum.The hippocampus lies in the hippocampal sulcus immediately below the floor of the temporal horn of the lateral ventricle, and in cross section (coronal) has appearances that are reminiscent of a seahorse. The neural pathways indicate that the hippocampal formation plays a central role in the limbic system, which also includes the dorsolateral corticoid area, nucleus taeniae of the amygdala, posterior pallial amygdala, septum, medial part of the anterior dorsolateral nucleus of the thalamus, and the lateral mammillary nucleus. Sensory inputs from higher order visual and olfactory stations enter DL and DM, are modified or integrated by intrinsic hippocampal circuitry, and the outputs are sent, via DL and DM, to various telencephalic nuclei, septum, and hypothalamus. Neurons in the V-shaped layer appear to be intrinsic neurons. In the hippocampal formation, reciprocal connections are found between DL-DM, DL-Tr, DL-Ma, DM-Ma, DM-V, and Tr-V. DL and DM can be further divided into dorsal and ventral, and lateral and medial portions, respectively.
Evidence obtained by a combination of Nissl staining and tract-tracing shows that the pigeon hippocampal formation can be divided into seven subdivisions: dorsolateral (DL), dorsomedial (DM), triangular (Tr), V-shaped (V), magnocellular (Ma), parvocellular, and cell-poor regions. This review therefore describes the functional neuroanatomy of the avian hippocampal formations, i.e., its subdivisions, cytoarchitecture, and afferent and efferent connections.
Knowledge of the neural circuits in the hippocampal formation and its related areas or nuclei is important for the understanding of these functions.
Increasing knowledge of the avian hippocampal formation (hippocampus and parahippocampal area) suggests that it plays a role in a variety of behaviors, such as homing, cache retrieving, visual discrimination, imprinting, and sexual behavior.