The brain is made up of two main kinds of cells, neurons
and glia. Neurons form the biological basis of mental processes such as memory, while
glia have mainly a supporting role (delivering nutrients, recycling molecules, providing
electrical insulation, etc.). Each neuron is connected to many other neurons through
projections called dendrites and axons, forming large networks. The point where two
neurons contact each other is called a synapse, and neurons are constantly forming new
synapses and pruning old ones. A synapse can also become stronger or weaker over time,
making the communication between the two contacting neurons more or less efficient. When
we learn something, or form a new memory, the brain stores this new information by
changing the strengths of some of the synapses. Thus, memories are encoded in the
connections between neurons, and different kinds of memories are stored in different
kinds of neurons in different parts of the brain.
A brain
structure that has been identified as critical for the formation of several kinds of
memories is the hippocampus, which is a small, elongated structure located inside the
temporal lobe. People whose hippocampus is damaged, due to stroke, infection or brain
surgery, lose the capacity to form long-term memories. They can remember things for a
few seconds, but as soon as they get distracted by something else, the memory disappears
completely. They can remember things that happened and things they learned before their
hippocampus was damaged, but cannot remember anything that happened since. This
condition is called anterograde amnesia. Neuroscientists think the hippocampus
participates in memory formation by facilitating the changes in the synapses between
neurons in different parts of the brain.
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