Neuromodulation of arthropod mechanosensory neurons.

Arthropod mechanosensory afferents have long been known to receive efferent synaptic connections onto their centrally located axon terminals. These connections cause presynaptic inhibition by attenuating the action potentials arriving at the axon terminals, thus reducing the synaptic potentials in the postsynaptic neurons. This type of inhibition can specifically reduce the excitation of selected postsynaptic neurons while leaving others unaffected. However, recent research has demonstrated that sensory signals detected by arthropod mechanosensory neurons can also be synaptically modulated before they ever arrive at the axon terminals. In arachnids and crustaceans, wide and complex networks of synapses on all parts of the afferent neurons, including the somata and dendrites, provide mechanisms to inhibit or enhance the responses to mechanical stimuli as they are being detected. This modulation will affect the signal transmission to all axonal branches and postsynaptic cells of the affected receptor neuron. In addition to the increased complexity of mechanosensory information transmission produced by these synapses, a variety of circulating neuroactive substances also modulate these neurons by acting on their postsynaptic receptors.