Dendritic spines are a popular subject these days. One theory is that they exist, at least in part, to allow axons to synapse with dendrites without deviating from a nice straight path through the brain. But, as Anderson and Martin point out in Nature Neuroscience, axons can also form spine-like structures, known as terminaux boutons. Might these also be used to maintain economic, straight axonal trajectories?

Anderson and Martin proposed that, if this were the case, the two types of axonal connection — terminaux boutons and en passant boutons — would each make synapses preferentially with different types of dendritic site. Terminaux boutons would be used to 'reach out' to dendritic shafts, whereas en passant boutons (where synapses are formed on the shaft of the axon) would occur when dendritic spines were able to bridge the gap between dendrite and axon. They carried out a detailed morphological study of spiny neuron axons in cat cortex to test this hypothesis.

Perhaps surprisingly, they found that the two types of synaptic bouton did not differentiate between dendritic spines and shafts. Axons were just as likely to use terminaux boutons to contact dendritic spines as they were to use en passant boutons, even in cases where it would seem that the path of the axon would allow it to use an en passant bouton. So, why do axons bother to construct these delicate structures, if not simply to connect to out-of-the-way dendrites?

The authors propose that terminaux boutons might share some of the physiological properties of dendritic spines. For example, spines are known to compartmentalize calcium transients. If these axonal structures also prevent calcium from diffusing rapidly into the parent axon during an action potential, terminaux boutons might show more presynaptic facilitation than en passant boutons. Of course, there is no evidence yet that terminaux boutons have this kind of influence on synaptic dynamics and plasticity, but the demonstration that they seem not to be simple bridges does compel us to consider alternative functions.