Having sparse links in the hippocampus may maximize memory storage

Our brain’s memory center has a sleek design, new data show.

The finding comes from looking at living human hippocampus tissue. This part of the brain plays a key role in learning and memory. The analyzed tissue revealed fairly few cell-to-cell links between the vast number of nerve cells in the hippocampus. But signals sent through those sparse links proved extremely reliable and precise.

Cellular neuroscientist Peter Jonas led the new research. He works at the Institute of Science and Technology Austria in Klosterneuburg. His team shared its findings January 23 in Cell.

Peeking into the memory center

The brains of all mammals have two hippocampi. One’s in the brain’s left hemisphere. The other is in the right.

Each hippocampus contains a smaller region known as the CA3 area. In humans, some 1.7 million nerve cells — called pyramidal cells — reside there.

But much of what we know about these cells has come from studies in mice. And those animals have only about 110,000 pyramidal cells in each CA3 area.

Past research hinted that animals with more nerve cells in their hippocampi may have fewer links — or synapses — between those cells, Jonas notes. His team wanted to see if this was true.

So they examined the brain tissue taken from eight patients. All had undergone brain surgery to treat epilepsy and agreed to donate the removed tissue for study.

Sparse but strong links

Recordings of electrical activity from pyramidal cells helped the researchers estimate how many cell connections there were in the CA3 area. It came to about 10 synapses for every 800 cell pairs. That’s only about a third as many per cell pair as had been found in the CA3 tissue of mice.

Even though human pyramidal cells had fairly few links, those cells sent steady and strong signals to each other. Mouse pyramidal cells did not.

Math models backed up what the scientists observed in human brain tissue. Having a lot of nerve cells with sparse, strong links maximized memory storage and recall, those models predicted.

Understanding the unique properties of the human hippocampus will teach us more about diseases that affect memory, Jonas says. This work also shows that here, “the human brain is not a scaled, large version of the rodent brain.”