“Descending” neuronal units in the commissure of the crayfish central nervous system; and their integration of visual, tactile and proprioceptive stimuli

The integrative properties of “descending” neuronal units in respect to visual, tactile and proprioceptive stimuli were studied at the commissure level of the crayfish central nervous system and 41 entities were established, bringing the total number of established descending entities at this level to 69. All but three of these are interneurons, over half of which have an entirely homolateral input. Altogether 227 interneurons have so far been established in the crayfish central nervous system, 73 of which extend through more than one of the studied levels. Compared to the thoracic‐abdominal level, interneurons transmitting both anteriorly and posteriorly are very scarce. The properties of high order interneurons are discussed. In addition to those which respond to visual stimuli, or are multimodal, a number react only to mechanorecptive stimulation, and some of this latter type have peripheral sensory fields exclusively posterior to the recording level. A few interneurons, both visual and non‐visual, exibit inhibitory phenomena of various kinds, again mirroring the findings in the mammalian central nervous system. Visual interneurons were shown to respond to specific types of optic stimulation such as movement of objects or changes in light intensity in correspondence with units found in the optic nerves of other decapod crustaceans.