Contrary to popular belief, brain cells use a mix of analog and digital coding at the same time to communicate efficiently, according to a study by Yale School of Medicine researchers published in Nature.
This finding partially overturns a longstanding belief that each of the brain's 100 billion neurons communicate strictly by a digital code. Analog systems represent signals continuously, while digital systems represent signals in the timing of pulses. Traditionally, many human-designed circuits operate exclusively in analog or in digital modes.
"This study reveals that the brain is very sophisticated in its operation, using a code that is more efficient than previously appreciated," said David McCormick, professor in the Department of Neurobiology and senior author of the study. "This has widespread implications, not only for our basic understanding of how the brain operates, but also in our understanding of neuronal dysfunction."
"It's as if everyone thought communication in the brain was like a telegraph, but actually it turned out to be more similar to a telephone," he said.
Neurons receive input from other cells largely through synaptic contacts on their dendrites and cell bodies. The release of neurotransmitters at these synapses causes the voltage inside the cell receiving the transmitters to fluctuate continuously. Once this voltage passes a threshold, an action potential is generated. The action potential is a specialized waveform known to be able to travel down the axon, or output portion of the cell.
Due to its length and thinness, the nerve axon has been believed to be impassable to the smaller analog voltage deflections that gave rise to action potential. As this action potential reaches the synaptic terminals of the axon, it causes the release of a transmitter onto the next neurons in the chain. So, although signals in the cell body are represented in an analog fashion, they were thought to be transmitted between cells solely through the rate and timing of the action potentials that propagated down the axon, that is, in a digital fashion.