The Anatomical and Functional Heterogeneity of the Mediodorsal Thalamus

Abstract

The mediodorsal nucleus (MD) represents just one piece of a complex relay structure situated within the brain, called the thalamus. MD is characterized by its robust interconnections with other brain areas, especially with limbic-related structures. Given the close anatomo-functional relationship between the MD and the limbic system, this particular thalamic nucleus can directly influence various affective behaviors and participate in cognition. In this work, we review data collected from multiple anatomical studies conducted in rodent, human, and non-human primates, highlighting the complexity of this structure and of the neural networks in which it takes part. We provide proof that the MD is involved in the unification of several anatomical structures, being able to process the information and influence the activity in numerous cortical and subcortical neural circuits. Moreover, we uncover intrinsic and extrinsic mechanisms that offer MD the possibility to execute and control specific high functions of the nervous system. The collected data indicate the great importance of the MD in the limbic system and offer relevant insight into the organization of thalamic circuits that support MD functions.

Keywords: anatomy; behavior; cognition; emotion; limbic system; mediodorsal thalamus; prefrontal cortex.

Figure 1

Mediodorsal thalamic neurons have bidirectional connections with prefrontal cortex pyramidal neurons. The pyramidal neurons of the prefrontal cortex (PFC), represented in green, distribute their soma in layer 3, 5, and 6. The deep layer pyramidal neurons project back to the mediodorsal thalamus (MD), forming a loop. The mediodorsal thalamocortical neuron is represented in blue and synapses with the pyramidal neurons. The parvalbumine-expressing inhibitory interneurons (PFC-PV+) are represented in dark purple, while the excitatory connections are represented as a triangle [24].

Figure 2

Mediodorsal thalamus establishes numerous connections with the prefrontal cortex, medial temporal regions, subcortical regions, and cerebellum. Due to the highly complex connectivity of the mediodorsal thalamus to other brain regions, only the most relevant connections (black arrows) are shown, making this a non-exhaustive representation.

Figure 3

Graphical representation of the mediodorsal thalamus (MD) in human and non-human primates (a) and rodents (b).