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Glial cell
Astrocytes can be visualized in culture because, unlike other mature glia, they express glial fibrillary acidic protein.
Glial cells, commonly called neuroglia or simply glia (greek for "glue"), are non-neuronal cells that provide support and nutrition, maintain homeostasis, form myelin, and participate in signal transmission in the nervous system. In the human brain, glia are estimated to outnumber neurons by about 10 to 1.[1]
Glial cells provide support and protection for neurons, the other main type of cell in the nervous system. They are thus known as the "glue" of the nervous system. The four main functions of glial cells are to surround neurons and hold them in place, to supply nutrients and oxygen to neurons, to insulate one neuron from another, and to destroy pathogens and remove dead neurons.
Function of the glial cell
Some glia function primarily as physical support for neurons. Others regulate the internal environment of the brain, especially the fluid surrounding neurons and their synapses, and provide nutrition to nerve cells. Glia have important developmental roles, guiding migration of neurons in early development, and producing molecules that modify the growth of axons and dendrites. Recent findings in the hippocampus and cerebellum have indicated that glia are also active participants in synaptic transmission, regulating clearance of neurotransmitter from the synaptic cleft, releasing factors such as ATP which modulate presynaptic function, and even releasing neurotransmitters themselves. Unlike the neuron, which is amitotic, glia are capable of mitosis.
Traditionally glia had been thought to lack certain features of neurons. For example, glia were not believed to have chemical synapses or to release neurotransmitters. They were considered to be the passive bystanders of neural transmission. However, recent studies disproved this. For example, astrocytes are crucial in clearance of neurotransmitter from within the synaptic cleft, which provides distinction between arrival of action potentials and prevents toxic build up of certain neurotransmitters such as glutamate (excitotoxicity). Furthermore, at least in vitro, astrocytes can release neurotransmitter glutamate in response to certain stimulation. Another unique type of glia, the oligodendrocyte precursor cells or OPCs, have very well defined and functional synapses from at least two major groups of neurons. The only notable differences between neurons and glia, by modern scrutiny, are the ability to generate action potentials and the polarity of neurons, namely the axons and dendrites which glia lack.
It is inappropriate nowadays to consider glia as 'glue' in the nervous system as the name implies but more of a partner to neurons. They are also crucial in the development of the nervous system and in processes such as synaptic plasticity and synaptogenesis. Glia have a role in the regulation of repair of neurons after injury. In the CNS glia suppress repair. Astrocytes enlarge and proliferate to form a scar and produce myelin and inhibitory molecules that inhibit regrowth of a damaged or severed axon. In the PNS Schwann cells promote repair. After axon injury Schwann cells regress to an earlier developmental state to encourage regrowth of the axon. This difference between PNS and CNS raises hopes for the regeneration of nervous tissue in the CNS, for example a spinal cord injury or severance.

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Groupthink
is a type of thought exhibited by group members who try to minimize conflict and reach consensus without critically testing, analyzing, and evaluating ideas. During Groupthink, members of the group avoid promoting viewpoints outside the comfort zone of consensus thinking. A variety of motives for this may exist such as a desire to avoid being seen as foolish, or a desire to avoid embarrassing or angering other members of the group. Groupthink may cause groups to make hasty, irrational decisions, where individual doubts are set aside, for fear of upsetting the group’s balance. The term is frequently used pejoratively, with hindsight.
Origin
The term was coined in 1952 by William H. Whyte in Fortune:
“        Groupthink being a coinage — and, admittedly, a loaded one — a working definition is in order. We are not talking about mere instinctive conformity — it is, after all, a perennial failing of mankind. What we are talking about is a rationalized conformity — an open, articulate philosophy which holds that group values are not only expedient but right and good as well.        ”
Irving Janis, who did extensive work on the subject:
“        A mode of thinking that people engage in when they are deeply involved in a cohesive in-group, when the members' strivings for unanimity override their motivation to realistically appraise alternative courses of action. [1]
”
The word groupthink was intended to be reminiscent of Newspeak words such as "doublethink" and "duckspeak", from George Orwell's Nineteen Eighty-Four.
Causes of groupthink
Highly cohesive groups are much more likely to engage in groupthink. The closer they are, the less likely they are to raise questions to break the cohesion. Although Janis sees group cohesion as the most important antecedent to groupthink, he states that it will not invariably lead to groupthink: 'It is a necessary condition, but not a sufficient condition' (Janis, Victims of Groupthink, 1972). According to Janis, (a) group cohesion will only lead to groupthink if one of the following two antecedent conditions is present: (b) Structural faults in the organisation: insulation of the group, lack of tradition of impartial leadership, lack of norms requiring methodological procedures, homogeneity of members' social background and ideology. (c) Provocative situational context: high stress from external threats, recent failures, excessive difficulties on the decision-making task, moral dilemmas.
Social psychologist Clark McCauley's three conditions under which groupthink occurs:
•        Directive leadership.
•        Homogeneity of members' social background and ideology.
•        Isolation of the group from outside sources of information and analysis.
Symptoms of groupthink
In order to make groupthink testable, Irving Janis devised eight symptoms that are indicative of groupthink (1977).
1.        Illusions of invulnerability creating excessive optimism and encouraging risk taking.
2.        Rationalising warnings that might challenge the group's assumptions.
3.        Unquestioned belief in the morality of the group, causing members to ignore the consequences of their actions.
4.        Stereotyping those who are opposed to the group as weak, evil or stupid.
5.        Direct pressure to conform placed on any member who questions the group, couched in terms of "disloyalty".
6.        Self censorship of ideas that deviate from the apparent group consensus.
7.        Illusions of unanimity among group members, silence is viewed as agreement.
8.        Mindguards — self-appointed members who shield the group from dissenting information.
Classic cases of groupthink
Two classical cases studies by sociologists and psychologists are NASA prior to the Challenger disaster and the presidential cabinet during crisis periods. Both of these cases were government organizations under extremely high stress, with direct leadership, a situation some theorists have stated contributes to groupthink. NASA actually funded sociologists in the aftermath of the Space Shuttle Challenger disaster to examine how the groups failed in preventing the disaster (Giddens 114-15).
Space Shuttle Challenger disaster (1986)
The Space Shuttle Challenger disaster is a classic case of groupthink. The Challenger exploded shortly after liftoff on January 28, 1986 (Vaughan 33). The launch had been originally scheduled for January 22, but a series of problems pushed back the launch date. Scientists and engineers throughout NASA were eager to get the mission underway.[2] The day before the launch an engineer brought up a concern about the o-rings in the booster rockets.
Several conference calls were held to discuss the problem and the decision to go ahead with the launch was agreed upon. The group involved in making the Challenger decision exhibited several of the symptoms of groupthink. They ignored warnings that contradicted the group’s goal. The goal was to get the launch off as soon as possible. They also suffered from a feeling of invulnerability, and therefore failed to completely examine the risks of their decision. Another factor that had suppressed the few engineers who were "going against the grain" and "sounding the alarm" was that all eyes were on NASA not to delay the launch and that Congress was seeking to earmark large funding to NASA given the large amount of publicity on the Teacher in Space program. These misjudgments led to the tragic loss of several astronauts, and a huge black mark on the space shuttle's (then) near perfect safety record.
Bay of Pigs invasion (1959-1962)
Another closely-studied case of groupthink is the 1961 Bay of Pigs invasion[3]. The main idea of the Bay of Pigs invasion was to train a group of Cuban exiles to invade Cuba and spark a revolution against Fidel Castro’s communist regime.
The plan was fatally flawed from the beginning, but none of President Kennedy’s top advisers spoke out against the plan.[citation needed] Kennedy’s advisers also had the main characteristics of groupthink; they had all been educated in the country's top universities, causing them to become a very cohesive group. They were also all afraid of speaking out against the plan, because they did not want to upset the president. The President's brother, Robert Kennedy, took on the role of a "mind guard", telling dissenters that it was a waste of their time, because the President had already made up his mind.[4]
Preventing groupthink
According to Irving Janis, decision making groups are not necessarily doomed to groupthink. He also claims that there are several ways to prevent it. Janis devised seven ways of preventing groupthink (209-15):
1.        Leaders should assign each member the role of “critical evaluator”. This allows each member to freely air objections and doubts.
2.        Higher-ups should not express an opinion when assigning a task to a group.
3.        The organization should set up several independent groups, working on the same problem.
4.        All effective alternatives should be examined.
5.        Each member should discuss the group's ideas with trusted people outside of the group.
6.        The group should invite outside experts into meetings. Group members should be allowed to discuss with and question the outside experts.
7.        At least one group member should be assigned the role of Devil's advocate. This should be a different person for each meeting.
By following these guidelines, groupthink can be avoided. After the Bay of Pigs fiasco, John F. Kennedy sought to avoid groupthink during the Cuban Missile Crisis.[5] During meetings, he invited outside experts to share their viewpoints, and allowed group members to question them carefully. He also encouraged group members to discuss possible solutions with trusted members within their separate departments, and he even divided the group up into various sub-groups, in order to partially break the group cohesion. JFK was deliberately absent from the meetings, so as to avoid pressing his own opinion. Ultimately, the Cuban missile crisis was resolved peacefully, thanks in part to these measures.
Criticism
Robert S. Baron contends that recent investigation and testing has not been able to defend the connection between certain antecedents with groupthink. [6] This may simply be due to the fact that the groupthink theory is very difficult to test in a lab situation using the scientific method. Alfinger and Esser also came to the same conclusion.[7] After ending their study, they stated that better methods of testing Janis' symptoms were needed. It is impossible to create in labs the same conditions under which important government groups work. It is impossible to create the same levels of stress and pressure experienced by high level government officials, with the future of an entire nation hanging in the balance. Baron also contends that the groupthink model applies to a far wider range of groups than Janis originally concluded. This contention remains to be tested.
Resource partitioning

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Niche differentiation
The term niche differentiation (synonymous with niche segregation, niche separation and niche partitioning), as it applies to the field of ecology, refers to the process by which natural selection drives competing species into different patterns of resource use or different niches. This process allows two species to partition certain resources so that one species does not out-compete the other as dictated by the competitive exclusion principle; thus, coexistence is obtained through the differentiation of their realized ecological niches.
Niche differentiation is a process which occurs through several different modes and on multiple temporal and spatial scales. In most cases, niche differentiation has created a relationship between two species where current competition is small or non-existent. Because of this, the presence of niche differentiation can be methodologically difficult to prove or disprove. The lack of evidence for current or past competition can blur the line between 1) two competitive species differentiating their niches to allow coexistence as opposed to 2) two non-competing species which occupy similar niches. It is important to keep in mind that niche differentiation and inter-specific competition cannot always be considered linked.
As an example of resource partitioning, seven Anolislizards in tropical rainforest share common food needs — mainly insects. They avoid competition by occupying different sections of the rainforest. Some live on the leaf litter floor while others live on shady branches, thereby avoiding competition over food in those sections of the forest. All resources are subject to partitioning, for example; space, food, nesting sites. This minimizes competition between similar species.
Niche differentiation: detection and quantification
The Lotka-Volterra equation states that two competing species can coexist when intra-specific (within species) competition is greater than inter-specific (between species) competition (Armstrong and McGehee 1980). Since niche differentiation concentrates competition within-species, due to a decrease in between-species competition, the Lotka-Volterra model predicts that niche differentiation of any degree will result in coexistence.
In reality, this still leaves the question of how much differentiation is needed for coexistence (Hutchinson 1959). A vague answer to this question is that the more similar two species are, the more finely balanced the suitability of their environment must be in order to allow coexistence. There are limits to the amount of niche differentiation required for coexistence, and this can vary with the type of resource, the nature of the environment, and the amount of variation both within and between the species.
To answer questions about niche differentiation, it is necessary for ecologists to be able to detect, measure, and quantify the niches of different coexisting and competing species. This is often done through a combination of detailed [natural history] studies, controlled experiments (to determine the strength of competition), and mathematical models (Strong 1982, Leibold 1995). To understand the mechanisms of niche differentiation and competition, much data must be gathered on how the two species interact, how they use their resources, and the type of ecosystem in which they exist, among other factors. In addition, several mathematical models exist to quantify niche breadth, competition, and coexistence (Bastolla et al. 2005). However, regardless of methods used, niches and competition can be distinctly difficult to measure quantitatively, and this makes detection and demonstration of niche differentiation difficult and complex.
Development of niche differentiation
Over time, two competing species can either coexist, through niche differentiation or other means, or compete until one species becomes locally extinct. Several theories exist for how niche differentiation arises or evolves given these two possible outcomes.
Current competition
Niche differentiation can arise from current competition. For instance, species X has a fundamental niche of the entire slope of a hillside, but its realized niche is only the top portion of the slope because species Y, which is a better competitor but cannot survive on the top portion of the slope, has excluded it from the lower portion of the slope. With this scenario, competition will continue indefinitely in the middle of the slope between these two species. Because of this, detection of the presence of niche differentiation (through competition) will be relatively easy. It is also important to remember that there is no evolutionary change of the individual species in this case; rather this is an ecological effect of species Y out-competing species X within the bounds of species Y’s fundamental niche.
Via past extinctions
Another way by which niche differentiation can arise is via the previous elimination of species without realized niches. This asserts that at some point in the past, several species inhabited an area, and all of these species had overlapping fundamental niches. However, through competitive exclusion, the less competitive species were eliminated, leaving only the species that were able to coexist (i.e. the most competitive species whose realized niches did not overlap). Again, this process does not include any evolutionary change of individual species, but it is merely the product of the competitive exclusion principle. Also, because no species is out-competing any other species in the final community, the presence of niche differentiation will be difficult or impossible to detect.
Evolving differences
Finally, niche differentiation can arise as an evolutionary effect of competition. In this case, two competing species will evolve different patterns of resource use so as to avoid competition. Here too, current competition is absent or low, and therefore detection of niche differentiation is difficult or impossible.
Types of niche differentiation
Resource partitioning
When two species partition [divide] a resource based on behavioral or morphological variation, it is termed differential resource utilization or resource partitioning. There are three types of differential resource utilization.
Temporal partitioning
Temporal resource partitioning is when two species eliminate direct competition by utilizing the same resource at different times. This can be on a daily scale (e.g. one species of spiny mouse feeds on insects during the day while a second species of spiny mouse feeds on the same insects at night, Kronfeld-Schor and Dayan 1999) or on a longer, seasonal scale. An instance of the latter would be reproductive asynchrony, or the division of resources by the separation of breeding periods. An example of reproductive asynchrony would be two competing species of frog offsetting their breeding periods. By doing this the first species’ tadpoles will have graduated to a different food resource by the time the tadpoles of the second species are hatching (Lawler and Morin 1993).
Spatial partitioning
Spatial resource partitioning occurs when two competing species use the same resource by occupying different areas or habitats within the range of occurrence of the resource. Spatial partitioning can occur at small scales (microhabitat differentiation) or at large scales (geographical differentiation). Microhabitat differentiation occurs when two competing species with overlapping home ranges partition a resource. Two examples would be different species of fish feeding at different depths in a lake or different species of monkey feeding at different heights in a tree. Geographical differentiation is when two competing species have non-overlapping home ranges and thus partition resources. An example might be given with monkeys again; two competing species of monkey using the same species of fruit trees, but in different areas of the forest.
Morphological differentiation
The final type of differential resource utilization is morphological differentiation or niche complementarity. Morphological differentiation happens when two competing species evolve differing morphologies to allow them to use a resource in different ways. A classic example of this is a study detailing the link between bumblebee proboscis lengths and flower corolla lengths (Pyke 1982). In this study, the long-proboscis bee species would preferentially feed on the long-corolla plants, the medium-proboscis bee species would feed on the medium-corolla plants, and so on. By evolving different proboscis lengths, several competing bee species are able to partition the available resources and coexist.
Conditional differentiation
The second form of niche differentiation is conditional differentiation, which occurs when two competing species differ in their abilities to use a resource based on varying environmental conditions. One species may be more competitive in one set of environmental conditions, but another species is more competitive in another set of conditions. Therefore, in a varying environment, each species is sometimes a better competitor and they can coexist. Differentiation based on environmental conditions is often difficult to separate from resource differentiation, and often conditional differentiation includes one or more types of resource partitioning.
Utilization of two resources (Tilman’s R*)
The final type of niche differentiation is based on Tilman’s (1990) notion that if two species are competing for the same exact resource then the ultimate winner will be the species which can deplete the resource the lowest, surviving on the lowest amount of the resource. This alone does not allow coexistence. However, if two species rely on two resources differentially, then coexistence is possible when each species can tolerate a lower amount of only one resource compared to its competitor. An example would be if grass species 1 is more limited by nutrient B than A and grass species 2 is more limited by nutrient A than B. Then, if species 1 uses more of nutrient B than A and species 2 uses more of nutrient A than B, species 1 can out-compete species 2 for nutrient A and species 2 can out-compete species 1 for nutrient B. Additionally, the starting point for each nutrient’s availability must be roughly an equal distance from each species limits (i.e. no species can quickly lower one resource while the other is still in abundance). Coexistence is now possible because each species uses more of the nutrient that limits its own growth, but each still requires both nutrients to survive. This subtle form of niche differentiation is dependent on two conditions: 1) the habitat needs to be such that one species is more limited by one resource, and the other species is more limited by the other resource, and 2) each species must consume more of the resource that strongly limits its own growth.
Coexistence without niche differentiation: exceptions to the rule
Some competing species have been shown to coexist on the same resource with no observable evidence of niche differentiation and in “violation” of the competitive exclusion principle. One instance is in a group of hispine beetle species (Strong 1982). These beetle species, which eat the same food and occupy the same habitat, coexist without any evidence of segregation or exclusion. The beetles show no aggression either intra- or inter-specifically. Coexistence may be possible through a combination of non-limiting food and habitat resources and high rates of predation and parasitism, though this has not been demonstrated.
This example illustrates that the evidence for niche differentiation is by no means universal. Niche differentiation is also not the only means by which coexistence is possible between two competing species (see Shmida and Ellner 1984). However, niche differentiation is a critically important ecological idea which explains species coexistence, thus promoting the high biodiversity often seen in many of the world’s biomes.

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Declarative memory
Declarative memory is the aspect of human memory that stores facts. It is so called because it refers to memories that can be consciously discussed, or declared. It applies to standard textbook learning and knowledge, as well as memories that can be 'travelled back to' in one's 'mind's eye'. It is contrasted with procedural memory, which applies to skills. Declarative memory is subject to forgetting, but frequently-accessed memories can last indefinitely. Declarative memories are best established by using active recall combined with mnemonic techniques and spaced repetition.[1]
Types of declarative memory
There are two types of declarative memory:
Semantic memory
Factual knowledge independent of time and place
Episodic memory
Theoretical knowledge of a specific moment in time and place
Some people believe that episodic memory and semantic memory are really just one type of memory. However, most believe they are quite different, and indeed distinct. [2]

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Neuropsychology
Physically speaking, declarative memory requires the medial temporal lobe, especially the hippocampus and related areas of the cerebral cortex. The famous amnesiac H.M. had a great deal of his medial temporal lobe removed and had a primarily declarative impairment (specifically, his episodic memory).
Procedural memory, also known as implicit memory or unconscious memory, is the long-term memory of skills and procedures, or "how to" knowledge (procedural knowledge).
As compared with declarative memory, it is governed by different mechanisms and different brain circuits. Procedural memory is often not easily verbalized, but can be used without consciously thinking about it; procedural memory can reflect simple stimulus-response pairing or more extensive patterns learned over time. In contrast, declarative memory can generally be put into words. Examples of procedural learning are learning to ride a bike, learning to touch type, learning to play a musical instrument or learning to swim. Procedural memory can be very durable.
In cognitive psychology, the term procedural knowledge denotes knowledge of how to accomplish a task, and often pertains to knowledge which unlike declarative knowledge cannot be easily articulated by the individual, or knowledge that is nonconscious. For example, most individuals can easily recognize a specific face as "attractive" or a specific joke as "funny," but they cannot explain how exactly they arrived at that conclusion or they cannot provide a working definition of "attractiveness" or being "funny." Research by a cogntive psychologist Pawel Lewicki has demonstrated that procedural knowledge can be acquired by nonconscious processing of information about covariations.
Studies of people with certain brain injuries (such as damage to the hippocampus) suggest that procedural memory and episodic memory use different parts of the brain, and can work independently. For example, some patients are repeatedly trained in a task and remember previous training, but do not improve in a task (functioning declarative memory, damaged procedural memory). Other patients put through the same training can't recall having been through the experiment, but their performance in the task improves over time (functioning procedural memory, damaged declarative memory).
Damage to the cerebellum and the basal ganglia seems to particularly affect procedural learning.

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