The first-order giant neurons of the giant fiber system in the squid: electrophysiological and ultrastructural observations

The giant fiber system controlling mantle contraction used for jet propulsi on in squid consists of two sets of three giant neurons organized in tandem . The somata of the 1st- and 2nd-order giant cells are located in the brain , while the perikarya of the 3rd-order giant cells are encountered in the s tellate ganglia of the mantle. The somata and dendrites of one fused pair o f 1st-order giant cells are thought to receive synaptic input from the eye, statocyst, skin proprioceptors, and supraesophageal lobes. To define the c ellular properties for integration of such an extensive synaptic load, espe cially given its diversity, intracellular recordings and electron microscop ic observations were performed on 1st-order giant cells in an isolated head preparation. Spontaneous bursts of action potentials and spikes evoked by extracellular stimulation of the brachial lobe were sensitive to the Na+ ch annel blocker TTX. Action potentials were also abolished by recording with microelectrodes containing the membrane-impermeant, use-dependent Na+ chann el blocker QX-314. The small action potential amplitude and the abundant sy naptic input imply that the spike initiation zone is remotely located from the recording site. The high spontaneous activity in the isolated head prep aration, as well as the presence of synaptic junctions resembling inhibitor y synapses, suggest; that afferent synapses on 1st-order giant neurons migh t represent the inhibitory control of the giant fiber system. The character ization of the electroresponsive properties of the 1st-order giant neurons will provide a description of the single cell integrative properties that t rigger the rapid jet propulsion necessary for escape behavior in squid.