A typical nerve cell has a cell body or ‘soma’ surrounded by dendrites, and an axon that, in humans, can be more than a meter long. Signals are received by the dendrites, passed through the soma and then through the axon, from whence they are transmitted to the next cell’s dendrites. Signals are propagated by extremely fast changes in the ion content of the cells. Special pores along the cell membrane called ‘voltage-gated ion channels’ let specific ions in or out at a prodigious speed, creating an electric impulse that travels through the cell. Each gate is open for milliseconds or less as the signal is passed forward.
That pattern still holds true, but Sheffield and his colleagues have found that this isn’t the only story. In some cases, signals can go back up the line from the axon to the soma. This is actually an amazing finding, given that the ion channels were thought to open and then shut in only one direction along the length of the axon.
Even stranger, the researchers found that axons could fire even when no signal originated from either the dendrites or the soma. And finally, what surprised the scientists the most was that one axon could stimulate another axon without having sent a signal through either dendrites or soma.
The signals sent either back to the soma or straight to another axon were thousands of times slower than the typical axon-dendrite-soma-axon signal processing. The researchers suggest that nerve cells may be using these different types of signal conduction for different purposes.