Primitive nervous systems: electrical activity in ventral nerve cords of the flatworm, Notoplana acticola.
Electrical activity evoked in the major cords of the ventral submuscular nerve plexus were measured. Recording and stimulation utilized suction electrodes attached directly to exposed nerve cords. Four categories of potentials have been recorded: (a) short latency spikes which have relatively high thresholds and appear to be single units; (b) short duration fast compound spikes that are made up of a small number of large units; (c) long duration compound potentials that are made up from a large number of smaller units, and (d) small amplitude potentials with long latencies and a characteristic shape. These can be conducted diffusely through the nerve plexus. The first two categories of spikes are called "fast" potentials because of their characteristic rise and fall times and the last categories are known as "diffuse" potentials. The spikelike fast potentials were only recorded from the main trunks (nerves VI), while diffuse potentials could also be recorded from side branches of these nerves. The diffuse potential appears to be conducted throughout the plexus but preferential conducting pathways occur around lesions. Both diffuse and fast potentials show facilitation of response to repeated stimulation. Facilitation can be demonstrated in the presence of high Mg2+ concentrations. Conductance of the diffuse potential also occurs in the presence of high ambient Mg2+. In Ca2+-free medium containing 1-mM EGTA one can also observe facilitatory events. The possibility of Mg2+-insensitive synapses is discussed.