Claustrum

The claustrum (Latin for: to close or shut) is a thin, bilateral structure which connects to cortical (ex. pre-frontal cortex) and subcortical regions (ex. thalamus) of the brain. It is located between the insula laterally and the putamen medially, separated by the extreme and external capsules respectively. The blood supply to the claustrum is fulfilled via the middle cerebral artery. It is considered to be the most densely connected structure in the brain allowing for integration of various cortical inputs (ex. colour, sound and touch) into one experience rather than singular events. The claustrum is difficult to study given the limited number of individuals with claustral lesions and the poor resolution of neuroimaging. The claustrum (Latin for: to close or shut) is a thin, bilateral structure which connects to cortical (ex. pre-frontal cortex) and subcortical regions (ex. thalamus) of the brain. It is located between the insula laterally and the putamen medially, separated by the extreme and external capsules respectively. The blood supply to the claustrum is fulfilled via the middle cerebral artery. It is considered to be the most densely connected structure in the brain allowing for integration of various cortical inputs (ex. colour, sound and touch) into one experience rather than singular events. The claustrum is difficult to study given the limited number of individuals with claustral lesions and the poor resolution of neuroimaging. The claustrum is made up of various cell types differing in size, shape and neurochemical composition. Five cell types exist and a majority of these cells resemble pyramidal neurons found in the cortex Within the claustrum, there is no organization of cell types compared to the cortex and the somas of the cells can be a pyramidal, fusiform or circular shape. The principal cell type found in the claustrum is Type 1 cells, which are large cells covered in spiny dendrites. The claustrum usually connects to the cortex in an ipsilateral manner; however, the few that travel contralaterally are considerably weaker than the former. The claustrum acts as a conductor for inputs from the cortical regions so these respective areas do not become unsynchronized. Without the claustrum, one could respond to stimuli that are familiar to the individual but not to complex events. Additionally, the claustrum is essential in combining sensory and motor modalities so that various anatomical patterns are present. One of the proposed functions of the claustrum is to differentiate between relevant and irrelevant information so that the latter can be ignored. Cortical components of consciousness include the fronto-parietal cortex, cingulate and precuneus. Due to the claustrum's widespread connectivity to these areas, it is suggested that it may play a role in both attention and consciousness. The neural networks that mediate sustained attention and consciousness implicate numerous cortical areas, many of which overlap in connectivity with the claustrum. Previous clinical reports suggest that conscious processes are lateralized to the left hemisphere in humans. The claustrum is a small bilateral gray matter structure (comprising roughly 0.25% of the cerebral cortex) located deep to the insular cortex and extreme capsule, and superficial to the external capsule and basal ganglia. As mentioned, its name means “hidden or shut away” and was first identified in 1672, with more detailed descriptions coming later on during the 19th century. Although the regional neuroanatomical boundaries of the claustrum have been defined, there remains a lack of consensus in the literature when defining its precise margins. Despite this long history of reports on the claustrum, descriptions of its overall connectivity have been sparse. However, recent work has suggested that this mysterious structure is present in all mammals, with extensive connections to cortical and subcortical regions. More specifically, electrophysiological studies show extensive connections to thalamic nuclei and the basal ganglia, while isotopological reports have linked the claustrum with the prefrontal, frontal, parietal, temporal and occipital cortices. Additional studies have also looked at the relationship of the claustrum to well-described subcortical white matter tracts. Structures such as the corona radiata, occipital-frontal fasciculus and uncinate fasciculus project to the claustrum from frontal, pericentral, parietal and occipital regions. Reciprocal connections also exist with motor, somatosensory, auditory and visual cortical regions. Altogether, these findings leave the claustrum as the most highly connected structure per regional volume in the brain and suggest that it may serve as a hub to coordinate activity of cerebral circuits. Interestingly, even with this extensive connectivity, most projections to and from the claustrum are ipsilateral (although there are still contralateral projections), and little evidence exists to describe its afferent or efferent connections with the brainstem and spinal cord. In summary, the cortical and subcortical connectivity of the claustrum implies that it is most involved with processing sensory information, as well as the physical and emotional state of an animal. Inputs to the claustrum are organized by modality, which include visual, auditory and somatomotor processing areas. In the same way that the morphology of neurons in the spinal cord is indicative of function (i.e. rexed laminae), the visual, auditory and somatomotor regions within the claustrum share similar neurons with specific functional characteristics. For example, the portion of the claustrum that processes visual information (primarily synthesizing afferent fibers concerned with our peripheral visual field) is comprised by a majority of binocular cells that have “elongated receptive fields and no orientation selectivity. This focus on peripheral sensory system is not an isolated occurrence, as most sensory afferents entering the claustrum bring peripheral sensory information. Moreover, the claustrum possesses a distinct topological organization for each sensory modality. For example, there is a retinotopic organization within the visual processing area of the claustrum that mirrors that of visual association cortices and V1, in a similar (yet less complicated) manner to the retinotopic conservation within the lateral geniculate nucleus. The claustrum is made up of various cell types that differ in size, shape and neurochemical composition. Five types of cells exist and the majority of these cells are structurally similar to pyramidal neurons found in the cortex. Within the claustrum, the somas of the cells can be found with a pyramidal, fusiform or circular shape. The principal cell type found in the claustrum is Type 1 cells, which are large neurons covered in spiny dendrites. These cells receive inputs from the cortex, and their axons will then leave in a medial or lateral fashion and send reciprocal projections back to the cortex. GABAergic interneurons represent only 10%-15% of the neurons within the claustrum. Finally, many studies show that the claustrum is best distinguished structurally by its prominent plexus of parvalbumin-positive fibers formed by local interneurons.