axonaxon

Axonaxon is not a standard term in current neuroscience reference works. It is sometimes used informally to refer to a class of neural connections in which one axon forms synapses on another axon's segment, rather than on dendrites or the soma. In formal literature, the prevailing term for this arrangement is axo-axonal synapse or axoaxonal contact.

An axo-axonal synapse forms when the presynaptic bouton of one neuron comes into close apposition with the axon of a second neuron. These contacts can occur at various regions of the postsynaptic axon and may influence the timing and probability of action potential propagation and neurotransmitter release. Mechanisms include classical chemical signaling and, in some cases, ephaptic or other non-synaptic influences.

Functional roles attributed to axo-axonal connections are diverse and remain an active area of investigation. They are thought to contribute to precise temporal coordination, gain control, or synchronization within neural circuits. The rarity and small size of axo-axonal contacts pose technical challenges for identification and measurement, and estimates of their prevalence vary across brain regions.

Terminology varies: many researchers avoid the shorthand axonaxon and instead use axo-axonal synapse or axoaxonal contact. The concept highlights the broader understanding that axons can participate directly in intra-circuit signaling, not only as conduits for postsynaptic targets but as active participants in circuit modulation.