In one theory, as people learn things throughout the day, connections between neurons get strengthened, but during sleep then all synapses are weakened, tenuous connections are ... and only the strongest bonds could remain.
(1) reinforced
(2) reproduced
(3) replaced
(4) stimulated
(5) severed 
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New Words
synapse: the place where nerve cells meet, especially in the brain
tenuous: a situation or relationship that is tenuous is uncertain, weak, or likely to change
(Collocation)
(1) tenuous link/ connection etc
(Example)
(1) The link between her family and the King's is rather tenuous.
(2) For now, the band's travel plans are tenuous.
bond: something that unities two or more people or groups, such as love, or a shared interest or idea (Synonym) tie, relationship
(Example)
- the emotional bond between mother and child
reinforce: to give support to an opinion, idea, or feeling, and make it stronger
(Example) The film reinforces the idea that women should be pretty and dumb.
severe: (verb) to cut through something completely, separating it into two parts, or to become cut in this way (Synonym) cut off
(Collocations) sever ties/ relations/ connections/ links etc (with/ between somebody)
(Example)
- Martin's hand was severed in the accident.
