Amygdalar and hippocampal connections with brainstem and spinal cord: A diffusion MRI study in human brain

The limbic system has a central role for the integration of several cognitive and visceral functions through an extended network of connections involving the hippocampus and the amygdala. A number of studies performed in humans have been dedicated to the investigation of supratentorial limbic pathways by means of non-invasive MRI approaches, such as DTI. However, detection of possible limbic connections involving the brainstem and the spinal cord is still missing. Subtentorial limbic pathways have been previously studied in animals by means of invasive approaches, including viral tracing. The detection of limbic connections with the brainstem and the spinal cord has raised several new hypotheses regarding the interaction between the central nervous system and the periphery of the body. We investigated subtentorial limbic connections in twenty-one healthy humans by means of probabilistic constrained spherical deconvolution tractography. Our connectivity analysis showed, for both the hippocampus and the amygdala, a high probability of connections with the midbrain, pons, and bulb. Moreover, hippocampal and amygdalar pathways reaching the cervical spinal cord were also detected. Quantitative evaluation of diffusion parameters was also performed. Findings of the present study are in agreement with the literature and provide the first report of possible limbic connections between the brainstem and the spinal cord in human brain. Since these pathways might also have important implications both in physiological and pathological contexts, further studies should be conducted in order to confirm our data as well as to define functional features of these brain connections.