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.