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Nervous System


Thought

Characteristics, Cell Assembly, Glial Cells

characteristics

Think 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
[view large image]

Further study of thought can be facilitated by considering the three facets of mental life :
  1. Mental Event - Thought is different from other types of mental events such as perceptual experience, and bodily sensations. It involves the deployment of concepts. It can be triggered by external stimulus or aroused from within. It is very private - nobody knows what's going on within another person's head. That is, until recently, neuroscientists are starting to peek into a person's brain to determine "what is being thought" using fMRI - albeit with only a very simple mental task of whether to "add" or to "subtract" two numbers, i.e., whether the subject is thinking about "add" or "subtract" (with 70% accuracy).


  2. Mental Faculty - While perceptions are limited by the physical properties of the sensing organs, thought comes up for all kinds of situations and the scope is almost unlimited in space and time, real or fancy. The power of thought is especially evident in its problem-solving ability. Indeed, the term "thinking" is sometimes reserved for this activity. Anyway, this is not a very good definition because there are evidences that some animals can "think" as well, the human just has a lot more complicated mental wiring evolved from those simpler ones. The use of symbol and language helps a lot to disseminate the "thought" through generations without having to re-think (re-invent) the "thought" anew in each generation.


  3. Mental Activity - Thinking comes up in trains - one thought leads to another, e.g., in daydream or other forms of reverie. It may be delightful but highly disorganized. Repetitive task does not require very much thinking, most likely we do think a lot during our daily life whether logical or illogical. It is the scientists who do the most logical thinking following a set of rules (e.g., mathematics) to perform their work. BTW, "falling in love" has no logic most of the time.


Cell Assembly

Nuron Assembly 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.
For example, the cell assembly on the left of Figure 31i is a hypothetical cell assembly encoding the concept "apple". It represents the neural association involved in thought - thinking about the apple in this case. The activation of the "apple cell assembly" can then trigger other associated concepts, such as "pineapple" or "Newton", through the firing of the neurons with an overlapping and excitatory connections. The thought about "apple" can be triggered from sense organs or other cell assembly. However, thought sometimes occurs randomly without any stimulation. It is through the study of glial cells in recent years that sheds some light on such phenomena.

See "Origin of Consciousness" for the role of each individual neuron cell within the assembly, e.g, one for the perception of color (an optical signal), another one for smell (a chemical signal), or taste (another chemical signal), ...

Glial Cells

Glial Cells 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

Table 09 Traditional Functions of Glial Cells

The first hint came from the study of the astrocytes in leech and frog. It was found that they exhibited electrical potential in potassium solution much like neurons, and that they were influenced by neuronal ion exchange, a process long held to be the chemical counterpart to thought. Since then many researchers have completed experiments on the communicatory ability of glial cells with neurons. It was discovered in late 1980s that glial cells respond to and release neurotransmitters.

Calcium wave was discovered in 2006 when several lines of evidence indicate that calcium ions (Ca++) are released randomly and without stimulation from astrocytes' internal storages in small bursts called "puffs". It is suggested that the random thought may be related to this calcium wave. It is known from all over the world and since ancient time that practitioner of meditation are all bothered by random thought when they are trying to relax the mind as well as the body. Some advise to chant mantra, others try to focus on something internally or externally - efforts to cover the random thought with something else. These methods are not very effective as the mind is stressed to overcome such thought. The practice of ignoring whatever occurs in the mind may be more natural because such calcium wave can never be suppressed - it occurs as natural as breathing.

See "Default State" for the restless brain; also see un-finished work on "Electrical Conduction by Schwann Cell and Acupuncture".

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