Sensory function and gating of histaminergic neuron C2 in Aplysia

This paper explores the possible sensory function of the identified histaminergic neuron C2. Mechanical stimulation of a narrow region around the mouth of the animal (perioral zone) elicits brief depolarizing potentials in C2. Extracellular recordings from the peripheral axons of C2 indicate that the depolarizing potentials are due to action potentials that are conveyed from the periphery but do not invade the cell body, since they fail at a region with a low safety factor within the cerebral ganglion. These blocked axonal spikes (A-spikes) function as if they were excitatory synaptic inputs to C2, since the synaptic output of C2 does not occur unless the A-spikes succeed in evoking full action potentials in the soma (or an electrically close initial segment) of C2. Furthermore, like synaptic potentials, the A-spikes exhibit temporal and spatial summation, and facilitation. C2 receives both tonic and phasic inhibitory synaptic potentials, which can decrease the summation of A-spikes and thereby alter the frequency-filtering properties of C2 or block its synaptic output. Thus, C2 appears to be an unusual proprioceptive afferent that has a high degree of integrative function and may provide critical gating that is dependent on a variety of external and internal conditions.