Update on Limbic Connections in Human Brain: A Possible Closer Relationship Between Brain Processes and Visceral Information

Abstract

The limbic system includes cortical and subcortical brain structures involved in several
functions, first of all emotional and memory processes and integration. Two key structures
of the limbic network are hippocampus and amygdala; their connections with the other brain
regions are allowed through a number of white matter pathways, including cingulum, uncinate
fasciculus and fornix.1 All these pathways were studied by means of invasive approaches in animals
as well as by means of MRI techniques, e.g. diffusion based tractography. These methods
resulted very useful for the non-invasive study, in vivo, of these limbic brain connections,1 as
well as to show new possible pathways, e.g. the cerebellar limbic one.2 Recently, new insights
regarding possible limbic functions came from a study conducted by means of advanced tractographic
algorithms.3 The main goal of the latter paper was to investigate subtentorial limbic
connections in healthy humans; this was an interesting point, since these connections were previously
investigated only in animals by means of viral tracing techniques.4 These previous studies
revealed extensive connections of both amygdala and hippocampus with brainstem nuclei,
as well as connections with the periphery of the body through spinal projections.5,6 By means
of constrained spherical deconvolution (CSD)7 based tractography, Arrigo and colleagues3 reported
and described hippocampal and amygdalar connections with midbrain, pons and bulb
as well as connections with cervical spinal cord (Figure 1). These represented novel findings in
humans, suggesting that functional speculation based on animals studies might be adopted also
in human brain. Amygdalar connections with brainstem were proposed to be related with faster
answers to fear stimuli, in order to establish a more efficient alerting mechanism.8-10 Based on
findings provided by animal studies, other authors proposed a role of amygdalar-brainstem
pathways in the visceral control as well as in the control of the appetite.11,12 With respect to
hippocampal-brainstem pathways, these were previously reported in animals13,14; those results
allowed to advance the hypothesis of a larger learning and memory connectivity network, involving
also brainstem structures, which might influence the limbic system during memory
and learning elaboration.15 In this context we might advance the hypothesis that information