femme Neuroscience Notes

Some Topics

Thalamus

The thalamus is a cluster of nuclei that contain dense populations of neurons and dense interconnections with every other part of the brain. There are two thalami, lying deep inside each cerebral hemisphere. I compare the Thalamus to a multi-channel mixer using templates to regulate complex interactions of all brain systems. The templates are based on innate patterns and are modified by learning.

Consciousness is a monitor image that samples brain activities. In my discussion of consciousness, I suggested that these monitor images and sensations depend on the recursive interaction of the thalamus and cortex. Other modulating information is fed into the mix from the smaller nuclei surrounding the thalamus contributing feelings, mood and other information.Because the thalamus is so complexly and recursively connected with all other parts of the brain, focusing on the thalamus to explain executive functions will be misleading to some extent. The interaction of frontal lobe and thalamic circuits is essential, for example, to anticipatory planning - one of the more recent attributes of cognition. The intellectual challenge is to become comfortable with complex, self-regulating, recursive systems that do not require an overseer.

The “executive system” is a popular construct, a hypothetical entity that integrates different cognitive processes. The identification of real functional components remains difficult. We would like to explain a variety of manifest properties of the brain such as alerting, orienting mechanisms, goal directed behaviors, strategizing, decision making, planning, sequencing, and social interactions. Patients with frontal lobe damage, for example, display variations of the “dysexecutive syndrome.” As with all broad, metaphorical categories the term, executive function, may obstruct, rather than enhance understand of how different brain modules interact to perform different tasks. All attempts to localize executive function are misleading since brains are inherently recursive, self-regulating, integration systems.

Vertes et al studied the connections between frontal lobes and the thalamus in rats and suggested that the medial prefrontal cortex is involved in high-order cognitive processes such as decision-making, goal directed behavior, and working memory.They showed the infralimbic, prelimbic, anterior cingulate cortices connect to midline/medial structures of the thalamus; the medial agranular cortex connects to the intralaminar nuclei, the ventromedial, and ventrolateral nuclei of thalamus; all four divisions of the medial prefrontal cortex project densely to the nucleus reuniens of the thalamus.

The nucleus reuniens is the major source of thalamic input to the hippocampal formation, transferring information from the medial prefrontal cortex to the hippocampus. Their evidence suggests that thalamic nuclei act as mixers and executive controllers of diverse subcortical and cortical structures in the brain. Although thalamocortical synapses are less dense than corticocortical synapses, their effect on cortical activity is important.

In their review of 10 years of studying the connections of thalamic nuclei in rats, Van der Werf et al stated that the thalamic midline and intralaminar nuclei form awareness, arousing system, involved in cognitive, sensory and motor functions. They proposed that the thalamic nuclei as a whole play a role in awareness,

Vertes et al studied the connections between frontal lobes and the thalamus in rats and suggested that the medial prefrontal cortex is involved in high-order cognitive processes such as decision-making, goal directed behavior, and working memory.They showed the infralimbic, prelimbic, anterior cingulate cortices connect to midline/medial structures of the thalamus; the medial agranular cortex connects to the intralaminar nuclei, the ventromedial, and ventrolateral nuclei of thalamus; all four divisions of the medial prefrontal cortex project densely to the nucleus reuniens of the thalamus.

Van der Werf et al stated that the thalamic midline and intralaminar nuclei form an awareness, arousing system, involved in cognitive, sensory and motor functions. They proposed that the thalamic nuclei as a whole play a role in awareness, with each of the groups subserving a role in different aspects of awareness:

(1) a dorsal group is involved in viscero-limbic functions
(2) a lateral group is involved in cognitive functions
(3) a ventral group is involved with multimodal sensory processing
(4) a posterior group is involved in limbic functions.