Interneurons of the crayfish brain: the relationship between dendrite location and afferent input.

This study was undertaken to examine the relationship between the structure and function of the descending interneurons of the crayfish brain. In particular, the dendritic fields were examined to ascertain if the location of an interneuronal dendrite in any of the six cerebral hemineuromeres (which subserve specific sensory modalities) is a necessary or sufficient condition to determine the functional and/or synaptic input to the interneuron. If a neuron projects a dendrite to a hemineuromere of the deutocerebrum or tritocerebrum, the neuron derives sensory input from the corresponding afferent root in 95% of our observations. Most of these inputs (86%) contain at the least a monosynaptic component. Conversely, if a cell derives monosynaptic input from any one of three of the four deutocerebral and tritocerebral roots tested, it has a corresponding dendrite (in 98% of our observations) in the appropriate hemineuromere. Input from the contralateral antennal nerve is an exception to this rule. The presence of a dendrite in the protocerebrum is not sufficient for predicting detectable visual input, but every instance of detectable visual input is associated with a protocerebral dendrite. Polysynaptic inputs are frequently (42%) not associated with corresponding dendrites. In neurons that were repeatedly dye filled in different animals, we observed significant variation only in the number and precise location of the smaller secondary and tertiary neurites. This variation rarely influenced the subset of sensory lobes innervated by the neuron.