A map of neural connections obtained by performing sophisticated imaging on a sample of a woman’s brain has revealed some strange and surprising new connections and structures. This new map was a joint project of Harvard University and Google. The sample was taken from the cerebral cortex of a 45-year-old woman undergoing surgery for epilepsy. It was smaller than a sesame seed and about a millionth of the volume of the average human brain.
After it was removed, the brain sample was quickly preserved and stained with heavy metals that revealed cellular structures. The sample was then sliced into more than 5,000 wafer-thin pieces and imaged with powerful electron microscopes.
Computational programs stitched the resulting images back together and artificial intelligence programs helped scientists analyze them. A short description of the resulting view was published as a preprint May 30 to bioRxiv.org. The full dataset is freely available online.
The data set is huge, and it has only just become available, so scientists are nowhere near having analyzed it in its entirety. However, some new and fascinating structures have already become apparent.
[Short primer on neuron (nerve cell) structure and function. A typical neuron has many short branches protruding from its body called dendrites, and one long branch called the axon. Dendrites pick up signals from the axons of other neurons. If the signals from other neurons are strong enough, the neuron will “fire,” sending a signal through the axon to the dendrites of other neurons, which in turn may be stimulated to fire. The gap between the axon of one neuron and the dendrite of another is called the synapse; neurotransmitters are released from the axon of one neuron, which cross the synapse, and provide a signal to the next neuron by binding to sites on its dendrite.]
One such curiosity concerns synapses, connection spots where signals move between nerve cells. Usually, most message-sending axons touch a message-receiving dendrite just once. In the new dataset, about 90 percent of the connections were these one-hit contacts. Some pairs of cells have slightly more contacts. But every so often, researchers spotted cells that connect multiple times, including one pair that were linked by a whopping 19 synapses.
Further, neurons with whorled tendrils were found, looking just a bit psychedelic. No one yet knows for sure what the purpose of these structures is, but there is plenty of speculation. For example, multiple neuron connections might be coded for information that is heavily used (such as information learned by rote), thus needing strong triggering.
It will take many years of study for this new map to yield reliable interpretations, but the images are quite stunning, and a reminder of just how little we know about the functioning of the brain.
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