Figure 1.

Overview of long and short loop connections from the hippocampus via mammillary area to the frontal cortex and back present in mammals ranging from rodents to primates [28,51,71,123-128]. The mammillary bodies (MB) and supramammillary area (SuM) have aligned medial, mediolateral, and lateral parts. MB targets prefrontal and anterior cingulate cortex, which target the hippocampal formation, completing the Papez circuit. Tonic arousing reticular input to medial (p = parvicellular [28]) SuM is converted to phasic theta rhythmicity (θ), passed to mediolateral (g = grandicellular) SuM, then diagonal band of Broca (DBB)/medial septum (MS) complex then hippocampal formation. Lateral (s = shell) SuM project to entorhinal cortex (EC). The fimbria (fi), fornix (fx), and internal capsule (ic) return hippocampal formation output to SuM/MB in onion-like, nested loops. EC, dentate gyrus (DG), CA3, CA1, subiculum (SUB), and retrosplenial cortex (RSp) connect unidirectionally. Successive loops are similar, but outside loops have greater delays and more highly processed information. There is a similar ‘onion’ with mammillothalamic tract (mt) output from MB and output from AMT/AVT/ADT to infralimbic (IFL), prelimbic (PRL) and anterior cingulate (ACC) cortex. Dorsal and ventral prefrontal (PRFd, PRFv) then perirhinal (Peri) and parahippocampal (Para) cortex complete the Papez circuit in EC. We have not included, e.g., the AMT-CA1 connection [129], to keep the fundamental architecture of the loop circuits clear. Abbreviations: ADT, AMT, AVT = anterior thalamus, dorsal, medial, ventral, respectively. ML, MM_L, MM_M = mamillary nucleus, lateral, medial pars lateralis, medial pars medialis, respectively.