The synapse is the junction between two neurons, usually between an axon, which gives the signal, and a dendrite, which receives the signal.
This meeting of neurons is absolutely essential to how the brain works. It is where the information gets passed on from one neuron to the next.
The 'magic' at the synapse
When someone talks about neuronal pathways being strengthened, they usually mean a strengthening of this synaptic connection. This strengthening (or weakening) is referred to as "synaptic plasticity." Specifically, when the connection between two neurons is strengthened, it is often referred to as Long Term Potentiation (LTP) and when it is weakened it is is often called Long Term Depression (LTD). Synaptic plasticity is so exciting because it is a feasible biological mechanism for memory formation and storage.
How this 'magic' was discovered
The first paper to show that the connections between neurons could be strengthened was Bliss and Lomo 1973. They were studying the hippocampus, the region that underlies episodic memory and spatial learning.
|Bliss and Lomo, 1973 Fig1a|
|Bliss and Lomo, 1973 Fig4c|
In this figure, the dots represent the size of the signal at each point in time. The arrows represent the high frequency stimulation (here they stimulated 4 times). After each stimulation, the signal grows.
The black dots are the pathway that was stimulated and the open circles are an unstimulated pathway that they used as a control.
"Why did I not pursue and publish a fuller account of my findings in 1966? Because I was overcome by the complexity of the system and my lack of understanding of what was behind the findings. There was also no sense of urgency. Thus, when Tim and I published a full account in 1973 (Bliss & Lømo 1973), it still took years for the significance of the findings to be generally appreciated. "It's hard to imagine 'no rush' to publish something like this and it is refreshing to see a scientist who is hesitant about publishing something that s/he does not fully understand.
Lømo T (2003). The discovery of long-term potentiation. Philosophical transactions of the Royal Society of London. Series B, Biological sciences, 358 (1432), 617-20 PMID: 12740104