axoaxonal

Axoaxonal describes a type of neural connection in which one axon forms a synapse onto the axon of another neuron. In this arrangement, the presynaptic terminal is located on the axon of the sending neuron and targets the axon of the receiving neuron, rather than its soma or dendritic tree. The term is used to denote axon-to-axon synapses, a specialized form of communication within neural circuits.

Axoaxonal synapses are often chemical and can modulate how the target axon releases neurotransmitter at its downstream synapses. A prominent example involves GABAergic chandelier interneurons that form axoaxonal contacts on the axon initial segment of pyramidal cells. In such cases, the axonal input can influence the initiation and timing of action potentials by altering the target axon’s excitability or neurotransmitter release probability at the AIS.

Functional roles attributed to axoaxonal connections include presynaptic modulation of downstream transmission and fine-tuning of spike timing within networks. The effects can be inhibitory or, in some contexts, excitatory, depending on transmitter systems and ionic gradients at the targeted axon.

Axoaxonal contacts have been observed in several brain regions, including the cortex, hippocampus, and cerebellum, though they are less common than conventional soma- or dendrite-targeting synapses. They are considered important for complex regulation of neural circuit dynamics and information flow.

See also: synapse, axon, axo-dendritic synapse, axo-axonic contact.