Decussating interneurons mediate antennular withdrawal in the blue crab, Callinectes sapidus

Four large (Type I) interneurons with cell bodies in the anterior cluster o f the brain and decussating axonal processes in an oesophageal connective i nitiate antennular withdrawal behavior in Callinectes sapidus. Recurrent ne urites from the oesophageal process of each interneuron project into antenn al and lateral antennular neuropils. These recurrent processes have varicos ities on their terminal processes that are hypothesized to possess output s ynapses onto antennular withdrawal motoneurons. Secondary neurites of Type I interneurons in the medial antennular neuropil receive tertiary branches from antennal and lateral antennular neuropils. These secondary neurites in the medial antennular neuropil also receive short latency, excitatory inpu t from mechanosensory afferent fibers from the antennule base. Intracellula r depolarization of Type I interneurons produces antennular withdrawal ipsi lateral to the descending axonal process. Intracellular recordings from ide ntified withdrawal motoneurons during depolarization of Type I interneurons produces excitatory postsynaptic activity in these motoneurons that is of short latency. Hyperpolarization tests of Type I interneurons during stimul us-evoked fictive antennular withdrawal show that each is sufficient but no t necessary for the production of withdrawal. Type I interneurons in Callin ectes are morphologically and physiologically similar to interneurons previ ously described in Carcinus maenas.