The claustrum: a historical review of its anatomy, physiology, cytochemistry and functional significance.

The claustrum (Cl) is a subcortical structure located in the basolateral telencephalon of the mammalian brain. It has been a subject of inquiry since the mid-nineteenth century. The Cl can be identified in a number of species, and appears as a phylogenetically related nucleus in Insectivores, Prosimians and Marsupials. Ontogenetic investigations have been the subject of much debate over the years. There are three hypotheses for claustral development. To date, the "hybrid theory" has garnered the most support. Pathological conditions specifically associated with the Cl, while few in number, are of interest from a functional perspective. Several cases of claustral agenesis have been reported. The implications of these clinical reports are discussed. Claustral neuroanatomy at the light-microscopic and electron-microscopic level is reviewed. The morphology of the claustral neuron consists of several types, which roughly corresponds to the neuron's location within distinct claustral subdivisions. The interconnectivity of the Cl with the cerebral cortex is rather complex and reflective of complex functional interrelationships. Several researchers have investigated the angioarchitecture of the Cl. It appears that vessels permeating the insula also vascularize the Cl. Literature investigating the neurotransmitters and overall chemical neuroanatomy of the Cl is extensive. These studies clearly demonstrate that the Cl is richly innervated with a wide and diverse array of neurotransmitters and neuromodulators. Lesion, stimulation and recording experiments demonstrate that the functional and physiologic capacity of the Cl is quite robust. A recurring theme of claustral function appears to be its involvement in sensorimotor integration. This may be expected of the Cl, given the degree ofheterotopic, heterosensory convergence and its interconnectivity with the key subcortical nuclei and sensory cortical areas. The Cl remains a poorly understood and under investigated nucleus. Therefore, a review of the world literature through 1986 prior to the advent of the "molecular revolution" is presented. This diverse and extensive body of knowledge is reviewed in the areas ofphylogeny, ontogeny, pathology, angioarchitecture, cytochemistry, anatomy and physiology. Theories of possible claustral function are also noted. It is hoped that this work will stimulate research scientists to further investigate the functional interrelationships of the Cl as well as to aim with far greater precision and accuracy towards a deeper understanding of its raison d'etre. The recent efforts in neurosciences by Sir Francis Crick and Christof Koch implicating the Cl in visual consciousness, is an important step in understanding just what its functions could encompass. Efforts in molecular neurosciences will be indispensable for a mechanistic understanding of these functions. Currently research efforts are underway from many perspectives. In considering the past scientific literature on the Cl, it is interesting to regard that this once obscure brain structure, may serve as a model system for the study of one of the most interesting and complex brain functions-consciousness.