Neocortical connections with perihippocampal and periamygdalar regions in the hedgehog tenrec

Abstract

The perihippocampal fields represent the most important regions connecting the neocortex and the hippocampus in rat, cat and monkey but little is known about their presence and connectivity in species with poorly differentiated brain. Using axonal tracer substances we have recently studied the distribution of cortical cells projecting to the hippocampus in the hedgehog tenrec. In the present study we determined the regions of the paleocortex and the rhinal cortex connected with the neocortex, and provide a tentative view of the site and the extent of the tenrec's entorhinal, perirhinal and postrhinal/parahippocampal fields. It is shown that only the dorsal portions of the posterior rhinal cortex may be considered equivalent to the perirhinal and postrhinal fields of higher mammals, while a considerable expanse of the ventral rhinal cortex may be part of the entorhinal area (its so-called dorsal portion) connected with both the dentate gyrus and the neocortex. A few cells projecting to the neocortex were also noted in the dorsal-most portion of the three-layered paleocortex (ventral entorhinal portion). These cells were linearly arranged and reminiscent of the neocortical projecting cells in the entorhinal layer 4/5 in more differentiated mammals. The main portion of the paleocortex caudal to the corpus callosum remained unlabeled following neocortical and hippocampal tracer injections. Unexpectedly, the area in the most ventral paleocortex adjacent to the amygdala also projected to the neocortex, particularly the tenrec's somatosensorimotor cortex.

Amygdalar connections in the lesser hedgehog tenrec

“…This projection involves the dorsal hemisphere, the shoulder region and the medial wall including the prefrontal area, the somatosensorimotor field as well as limbic/anterior cingulate regions (rat: Verwer et al 1996;cat: Room et al 1985;monkey: Carmichael and Price 1995;Morecraft et al 2007). Due to its connectivity, one may consider the tenrec's area Mv of zone 1 as the equivalent to the infralimbic cortex in higher mammals (Musil and Olson 1988;Hoover and Vertes 2007;Roberts et al 2007; see also Künzle 2003). The Mv of zone 2 does not appear to receive an amygdalar input nor does the most rostral tip of the tenrec's hemisphere (cases 01, 02) unlike the rat (McDonald 1998;Gabbott et al 2006).…”

“…10b-d, g, i; Table 2). The superficial layers of the caudal RCx and the adjacent area X (XCx; Künzle 2003) were labeled most consistently in the cases of group C (Fig. 10i-n).…”

“…9d, e). In the cases 33-35 with the injection sites restricted to the zones 4-6 (including area X caudally adjacent to RCx; Künzle 2003) the presence of labeled fibers and perikarya was largely restricted to the ipsilateral AyLd (Fig. 9j, k, m), partly extending into the subventricular region of the AyP (Fig.…”

“…Due to the tracer contamination along the electrode tract (see ''Materials and methods''), the labeling of the PCx was difficult to judge with regard to heavily (particularly, its ventral portion) and weakly labeled regions (such as the dorsal third of PCx representing in part entorhinal cortex; see Künzle 2003).…”

Organization of the somatosensory cortex in elephant shrews (E. edwardii)

“…Its members have a wide range of brain sizes and sensory specializations and may provide important insights into mammalian brain evolution. Recently, tenrecs have been the focus of investigations because their small brains with little neocortex are thought to resemble the brains of ancestral mammals in a number of respects (Krubitzer et al, 1997;Kunzle, 2003). Manatees are of interest because of their unique ecological niche and unusual body form (Marshall and Reep, 1995;Reep et al, 2001Reep et al, , 2002.…”