The crayfish second antennae. II—motoneuron structure as revealed by cobalt chloride backfilling

Abstract

1. 1. The location of both the crayfish antennal motoneuron somata, and dendrites were determined using a cobalt chloride backfilling technique. The cobalt technique is especially useful in analysing unstructured neuropile. 2. 2. The antennal motoneuron somata (AMS) occur in several distinct regions of the brain. A majority of the somata belong to the angular cell group (Helm, 1928) located posterio-laterally in the tritocerebrum. Another prominent group of AMS form an extension of Helm's superior, lateral cell group. 3. 3. Most of these cells occur over the posterio-medial portion of the accessory neuropile, with a few cells located along the medial edge of the accessory neuropile. One cell body is located between the circumesophageal connectives. 4. 4. All the motoneurons are unipolar, with the somata being located ipsilaterally and dorsally in the brain. Most of the somata are 40 μm long by 30 μm wide. 5. 5. The antennal motoneuron dendrites form a dorsal, diffuse motoneuron neuropile that overlies the medial portion of the antennal neuropile and the lateral part of the tegumentary neuropile. 6. 6. The dendritic structure of individual motoneurons is highly complex. The motoneurons exhibit profuse branching, with primary, secondary and higher order branches being easily distinguished. 7. 7. In addition, there are alternating thick and thin diameter segments along the larger individual dendritic branches. 8. 8. The potential effects of this structural complexity on integration are discussed.