The dentate gyrus: fundamental neuroanatomical organization (dentate gyrus for dummies)
Introduction
The dentate gyrus is a simple cortical region that is an integral portion of the larger functional brain system called the hippocampal formation (Fig. 1) (Amaral and Lavenex, 2007). The regular organization of its principal cell layers coupled with the highly ordered laminar distribution of many of its inputs has encouraged its use as a model system for many facets of modern neurobiology. In this chapter, we present the fundamental principles of neuroanatomical organization of the dentate gyrus, its principal cell types and their connectivity, and a summary of the major extrinsic inputs of the dentate gyrus.
One of the unique features of the hippocampal formation is that many of its connections are unidirectional (Fig. 1). Thus, the entorhinal cortex provides the major input to the dentate gyrus via fibers called the perforant path (see Chapter 3). However, the dentate gyrus does not return a projection to the entorhinal cortex. Since the entorhinal cortex is the source of much of the cortical sensory information that the hippocampal formation uses to carry out its functions, and since the dentate gyrus is the major termination of projections from the entorhinal cortex, it is reasonable to consider the dentate gyrus as the first step in the processing of information processing that ultimately leads to the production of episodic memories. Moreover, the unique neuroanatomy of the dentate gyrus predicts that it carries out a specific information-processing task with the information that it receives from the entorhinal cortex and ultimately conveys to the CA3 field of the hippocampus.
In this chapter, we will first give a general overview of the neuroanatomical organization of the dentate gyrus. We will then put the spotlight on three of its most important neurons: the dentate granule cell, the dentate pyramidal basket cell and the mossy cell. We will summarize the major structural features of these neurons as well as the connections they make within and beyond the dentate gyrus. We will then very briefly discuss some of the other neurons located in the dentate gyrus. Finally, we briefly summarize the origin and termination of the major extrinsic inputs to the dentate gyrus. Many of these topics will be discussed in greater detail in other chapters of this book. But, this chapter provides a synoptic view in order to serve as an introduction to the dentate gyrus — this is dentate gyrus for dummies.
Section snippets
Overall organization
The dentate gyrus has three layers (Figs. 1–2). There is a relatively cell-free layer called the molecular layer which, in the rat, is approximately 250 μm thick and is occupied by, among other things, the dendrites of the dentate granule cells. The other major occupants of the molecular layer are the fibers of the perforant path that originate in the entorhinal cortex. There are also a small number of interneurons that reside in the molecular layer and fibers from a variety of extrinsic inputs
Comparative neuroanatomy of the dentate gyrus
The comparative neuroanatomy of the dentate gyrus is covered in detail in Chapter 2. However, we would like to raise a few issues for which there remains substantial confusion in the literature. First, the basic trilaminar structure of the dentate gyrus is common across all species studied. And, relative to other hippocampal structures such as the CA1 field of the hippocampus or the entorhinal cortex, there has not been substantial phylogenetic modification. For example, in the rat, there are
The dentate granule cell
The principal cell type of the dentate gyrus is the granule cell (Fig. 4, Fig. 5). The dentate granule cell has an elliptical cell body with a width of approximately 10 μm and a height of 18 μm (Claiborne et al., 1990). The granule cell bodies are tightly packed together and, in most cases, there is no glial sheath interposed between cells. The granule cell has a characteristic cone-shaped tree of spiny apical dendrites. The branches extend throughout the molecular layer and the distal tips of
Conclusions
In the remainder of this book, the reader will receive far greater detail on many of the neuroanatomical features of the dentate gyrus that were only briefly touched on in this chapter. Every effort will also be made to correlate the neuroanatomy of the dentate gyrus with both its electrophysiological and functional attributes. We will close with a short reflection on the position of the dentate gyrus both within the hippocampal formation and within the brain at large.
It has been fashionable at
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