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Thought has been linked to existence in René Descartes's famous proposition : "I think, therefore I am" (Figure 31h). However, it is not so easy to understand the nature of thought. It was considered as something intangible existing on another level beyond this material world. Nowadays, a search for its definition in dictionary would turn up something like : "Thought is a mental conception whether an opinion, a judgment, an idea, a fancy, a conceit, purpose, intention, deliberation, reflection, solicitude, ...etc". The adjective "mental" is defined as "Of or relating to the mind". Finally, it reveals that the "mind" is "Human consciousness that originates in the brain". Surgery in modern time has established conclusively the link between thought and brain by observing the relationship between the lesion of a certain brain part and the lost function of a particular mental task. |
Figure 31h Thought |
Further study of thought can be facilitated by considering the three facets of mental life : |
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The concept of cell assembly was introduced by Donald Hebb in 1949 to explain the adaptation of neurons in the brain during the learning process. The theory is often summarized as "Cells that fire together, wire together." As emphasized by its original author, this statement should be qualified by the rule of causality, i.e., the group of cells fire one after the other (not simultaneously). It is now used loosely to describe a group of neurons that perform a given action or represent a given percept or concept in the brain. Such group have strong internal synaptic interactions which set them apart from other groups of neurons. This is actually the base of memory. Thus, thought can be considered as a process |
Figure 31i Cell Assembly |
of memory retrieval one after the other. Intracranial recordings in subjects suffering from intractable epilepsy in 2012 has yielded evidence for this cell assembly concept. The studies showed that group of neurons in the human medial temporal lobe fires in a remarkably selective manner to particular objects. |
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There are many types of glial (gluey in Greek) cells (Figure 31j). They make up around 90% of cells in the brain, but used to be considered as playing a supporting role for maintaining the integrity of the neurnal system (this is the base for the idea of only 10% of our brain is being used). The traditional functions of the glial cells are summarized in Table 09. Research in 21st century has now discovered that the glial cells, especially the astrocytes, play a more active role in conducting the bio-electric signals. Our thoughts and imagination may actually be more related to the astrocytes than the neurons, i.e., the neurons may be playing the supporting role instead. |
Figure 31j Glial Cells [view large image] |
| Location | Type | Function(s) |
|---|---|---|
| CNS | Astrocyte | provide both mechanical and metabolic support for neurons, regulate the environemnt, contribute to the blood-brain barrier, control transport of substances from the blood to neural tissue, help coordinate nerve pathway development |
| CNS | Oligodendrocyte | wrap some axons to form an insulating coat known as the myelin sheath |
| CNS | Microglia Cell | protect against microorganisms, remove cellular waste |
| CNS | Ependymal Cell | form the linings of the ventricles of the brain and central canal of the spinal cord, contribute to the production of cerebrospinal fluid (CSF) |
| PNS | Schwann Cell | form part of the myelin sheath for axon |
| PNS | Satellite Cell | encapsulate, protect, and support the peripheral neurons in many peripheral ganglia |