In your typical brain cell, the branched dendrites receive electrical signals from other neurons, and once they’re processed, these signals are passed on to other neurons along the axon, a long extension of the cell. Now, researchers have discovered a brain cell shape that has never been seen before -- where the axon arises directly from one of the dendrites. The findings were published in Neuron last week.
The axon and dendrites are usually connected by the neuron's cell body. Rather than conduct signals via the neuron’s center, these unusual nerve cells take a shortcut to transmit information: The signals go around the cell like a bypass road. "Input signals at this dendrite do not need not be propagated across the cell body," explains study author Christian Thome of Heidelberg University in a news release. And they’re more efficiently forwarded than signals coming from elsewhere.
The team colored the places of origin of axons of pyramidal cells in the hippocampus, a brain region involved in memory. In more than half of the cells, they found that the axon doesn’t emerge from the cell body but originates at a dendrite instead. Then, to study the effect of signals received at these axon-carrying dendrites, the team injected a neurotransmitter called glutamate into the brain tissue of mice that can be activated by light pulses. Using a high-resolution microscope, the team directed the light beam directly to a specific dendrite, and then activated the neurotransmitter to simulate an input signal.
“Our measurements indicate that dendrites that are directly connected to the axon, actively propagate even small input stimuli and activate the neuron," says study author Tony Kelly from University of Bonn. They call it “privileged synaptic input.”
Using computer simulations, the team predicts that this effect is particularly pronounced when the information flow from other dendrites to the axon is suppressed by inhibitory input signals at the cell body. "That way,” Kelly adds, “information transmitted by this special dendrite influences the behavior of the nerve cell more than input from any other dendrite.”
To figure out why these unusual shapes are there, the team is now looking for the biological functions that are strengthened through these sorts of dendrites.