Abstract
The thalamocingulate tract is a key component of the Papez circuit that connects the anterior thalamic nucleus (ATN) to the cingulum bundle. While the other white matter connections, consisting of the fornix, cingulum bundle and mammillothalamic tract, were well defined in Papez’s original 1937 paper, the anatomy of the thalamocingulate pathway was mentioned only in passing. Subsequent research has been unable to clarify the precise anatomical trajectory of this tract. In particular, the site of thalamocingulate tract interactions with the cingulum bundle have been inconsistently reported. This review aims to synthesize research on this least studied component of the Papez circuit. A systemic approach to reviewing historical anatomical dissection and neuronal tracing studies as well as contemporary diffusion magnetic resonance imaging studies of the thalamocingulate tract was undertaken across species. We found that although inconsistent, prior research broadly encompasses two differing descriptions of how the ATN interfaces with the cingulum after passing laterally through the anterior limb of the internal capsule. The first group of studies show that the pathway turns medially and rostrally and passes to the anterior cingulate region (Brodmann areas 24, 33, and 32) only. A second group suggests that the thalamocingulate tract interfaces with both the anterior and posterior cingulate (Brodmann areas 23 and 31) and retrosplenial region (Brodmann area 29). We discuss potential reasons for these discrepancies such as altering methodologies and species differences. We also discuss how these inconsistencies may be resolved in further research with refinements of terminology for the cingulate cortex and the thalamocingulate tract. Understanding the precise anatomical course of the last remaining unresolved final white matter tract in the Papez circuit may facilitate accurate investigation of the role of the complete Papez circuit in emotion and memory.
Highlights
The Papez circuit was described in 1937 by James Wenceslaus Papez in his paper “A Proposed Mechanism of Emotion” (Papez, 1937)
The anterior cingulate cortex (ACC) corresponds to Brodmann Areas (BA) 32, 33, and 24, while the posterior cingulate cortex (PCC) and retrosplenial cortex (RSC) corresponds to BA 23, 29, and 31 (Figure 1)
The reason for compiling this review was to properly identify the trajectory of the thalamocingulate tract for the purposes of future diffusion-based research of the Papez circuit. To resolve these unreconciled findings surrounding the thalamocingulate tract we suggest the following: (a) Future research should make use of more consistent and current cytoarchitectural subdivisions for the cingulate cortex, and that this more precise terminology be implemented when discussing the thalamocingulate tract; (b) Dividing the tract into pre- and post- anterior capsular components, and known terminations may aid accurate resolve conflicting descriptions of the tract; (c) Further anatomical detailed staining and microdissections of the thalamocingulate to delineate its precise trajectory in cadaveric humans and clarify the terminal fiber distribution across the cingulate gyrus and cingulum
Summary
The Papez circuit was described in 1937 by James Wenceslaus Papez in his paper “A Proposed Mechanism of Emotion” (Papez, 1937). This pioneered a new understanding of how the limbic lobe directed emotions and memory through its connections with the hypothalamus. An integral component of the Papez circuit, the thalamocingulate tract connecting the anterior thalamic nuclei to the cingulum bundle is less consistently described in both the anatomical and neuroimaging literature
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