Studies show that neurons protect and store certain information with the help of a special zone of stable synapses.

One of the most remarkable qualities of the brain is its adaptability. Changes in neural circuits, whose connections are constantly adjusted as we perceive and interact with the world, are the key to how we learn. But in order to keep knowledge and memories intact, some parts of the chains must be resistant to these constant changes.

"The brain has learned to balance between stability and flexibility so that you can acquire new knowledge and retain memory for life," says neurobiologist Mark Harnett, a researcher at the McGovern Institute for Brain Research at the Massachusetts Institute of Technology.

In a study published in Cell Reports, Harnett and his team show how individual neurons can contribute to both parts of this vital duality. By studying the synapses through which pyramidal neurons in the sensory cortex of the brain interact, they learned how cells retain their understanding of some of the most fundamental features of the world, while maintaining the flexibility they need to adapt to a changing world.

Visual connections

Pyramidal neurons receive input data from other neurons through thousands of connection points. At an early age, these synapses are extremely plastic; their strength can change as a young animal perceives visual information and learns to interpret it. Most of them remain adaptive in adulthood, but Harnett's team found that some cell synapses lose their flexibility when animals are less than a month old. The presence of both stable and flexible synapses means that these neurons can combine input data from different sources to use visual information in flexible ways.

Postdoc Courtney Yeager carefully examined these unusually stable synapses, which are grouped along a narrow area of complex branched pyramidal cells. She was interested in the connections through which cells receive primary visual information, so she traced their connections with neurons in the visual information processing center of the brain thalamus, called the dorsal lateral knee nucleus (dLGN).

Long processes through which the neuron receives signals from other cells are called dendrites, and they branch off from the main cell body into a tree-like structure. Spiky protrusions along the dendrites form synapses that connect pyramidal neurons with other cells. Yeager's experiments showed that all compounds from dLGN lead to a certain area of pyramidal cells - a dense strip inside what she describes as the trunk of a dendritic tree.

Jaeger discovered several ways in which synapses in this area - officially known as the apical oblique dendritic domain - differ from other synapses on the same cells. "In fact, they are not so far from each other, but they have completely different properties," she says.

Stable synapses

In one series of experiments, Jaeger activated synapses on pyramidal neurons and measured the effect on the electrical potential of cells. Changes in the electrical potential of the neuron generate impulses that cells use to communicate with each other. Usually, the electrical effects of the synapse are enhanced when the synapses nearby are also activated. But when the signals were delivered to the apical oblique dendritic domain, each of them had the same effect, regardless of how many synapses were stimulated.

Synapses there do not interact with each other at all, says Harnett. "They just do what they do. It doesn't matter what their neighbors do, they all do about the same thing."

The team was also able to visualize the molecular contents of individual synapses. This revealed the surprising absence of a certain type of neurotransmitter receptors, called NMDA receptors, in apical oblique dendrites.