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The brain may use a mix of analog and pulse (spikes) coding schemes for interneuronal communication

Traditionally, neurons in the brain are thought to collect information from other cells as an analog signal (voltage) that is generated by barrages of synaptic potentials. These synaptic potentials arise from the activity of thousands of inputs that each cell receives on its cell body and dendrites. The traditional theory is that this analog signal is then encoded into a pattern of action potentials that travels down the axon, in a signal that is akin to a digital, or pulse, code. Once the action potentials reach the synaptic terminals, transmitter is released, and thus the message is converted back to an analog form and conveyed to the next cells in the network.

However, in some sensory organs and invertebrate systems, neurons can also communicate in the absence of action potentials by grading their transmitter release according to the presynaptic membrane potential, which is directly determined by the barrages of synaptic activity arriving in the cell. This graded synaptic transmission was thought to be irrelevant at the vast majority of synapses in the brain, because the electrotonic distance between the presynaptic cell and its axonal terminals was considered to large.

In this seminar, we demonstrate that a significant number of synapses in the brain, particularly in the cerebral cortex, may operate through a mode that is best described as a combination of graded and triggered or "analog" and "digital".

This finding overturns a long standing belief about how synaptic transmission operates in the brain and has important implications for not only the normal (e.g. attention, arousal, decisions, memory, etc) but also the abnormal (epileptic seizures, migraine headache, multiple sclerosis) operation of the brain.

Our results demonstrate that information transfer in the brain may be far more efficient and/or flexible than previously appreciated.