ANTENNULAR WITHDRAWAL MOTONEURONS IN THE LATERAL ANTENNULAR NEUROPIL OF CALLINECTES SAPIDUS

ABSTRACT Antennular withdrawal behavior in Callinectes sapidus can be initiated by stimulating antennular sensory nerve IIIc, and monitored as fictive withdrawal in antennular motor nerve IA. Five motoneurons have been identified with neurites located predominately in the lateral antennular neuropil that contribute to this behavior. These motoneurons exhibited a range of evoked response following stimulation of nerve IIIc. The duration of the excitatory component of the evoked response was appropriate for the role of each motoneuron in producing antennular withdrawal. When depolarized by the intracellular injection of current, each of these cells produced a stereotypical movement of one antennular segment. Motoneurons 26-f and 26-s displaced the medial antennular segment laterally, positioning the entire antennule in a protective cephalothoracic groove. Motoneuron 27 rotated the medial segment about its long axis, while motoneuron 28 displaced the medial segment toward the midline. The movement of the medial segment produced by motoneuron 28 was antagonistic to movements produced by motoneurons 26-f and 26-s. Motoneurons 26-f, 26-s, 27, and 28 were active during the early phase of withdrawal and may establish a consistent initial position of the medial segment prior to placement of the antennule in its final protected location. Motoneuron 30―1f, also active during the early phase of withdrawal, elevated the distal segment as the initial position of the medial segment was being established.