Comparative strategies in the investigation of neural networks

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.

Release of 3H-gamma-aminobutyric acid (GABA) by inhibitory neurons of the crayfish

Inhibitory neurons innervating the muscle receptor organ (MRO) of crayfish were used to study the uptake and release of tritiated GABA. MROs that have been directly exposed to 3H GABA for 60--75 min release radioactivity during low-frequency electrical stimulation. When ganglia containing the inhibitory cell bodies are exposed to 3H GABA, the isotope travels along a pathway unique to the inhibitory axon, at rates that range between 160 and 240 mm per day. Electrical stimulation of inhibitory axons whose cell bodies have been exposed to 3H GABA for 4--5 hr produces release of isotope from isolated MROs. Low calcium, high magnesium exposure prevents the stimulus-dependent release of radioactivity. Thin layer chromatographic analyses indicate that GABA comprises at least a major fraction of the radioactivity collected from stimulated preparations. A number of unidentified radioactive compounds are usually present with GABA, and it is suggested that most of these are catabolites of GABA.

Lobster claw motorneurons respond to contralateral sensory stimuli

Unilateral sensory stimuli in an isolated lobster claw--ganglion preparation elicits soma and axon spikes in homologous motorneurons in the ipsilateral as well as the contralateral hemiganglion. This cross excitation of the motorneurons provides a basis for bilateral reflexes in the claw and is likely mediated through interneuron(s).

In vivo responses of paired giant mechanoreceptor neurons in Aplysia abdominal ganglion

Two neurons with cell bodies symmetrically located in the abdominal ganglion and giant axons in the left (L1) and right (R1) pleurovisceral connectives of Aplysia californica were examined in vivo and in vitro. Direct stimulation of R1 and L1 in the intact animal does not elicit any observable behavior, suggesting that they are neither motoneurons nor command neurons. These cells respond in vivo to sudden onset mechanical stimulation of widespread regions of the body. R1 and L1 spikes are initiated in at least three different loci: (1) the peripheral axon in the foot, (2) the neuropil of the pleural and/or pedal ganglion, and (3) the neuropil of the abdominal ganglion. Furthermore, R1 and L1 probably have two different mechanisms for spike initiation: (1) sensory (foot), and (2) synaptic (abdominal and/or head ganglia). The different loci for spike initiation account for the bidirectional conduction of R1 and L1 spikes. As sensory (mechanoreceptor) neurons, R1 and L1 have peripheral axons in the ipsilateral posterior pedal nerve, show low threshold responses to stimulation of the ipsilateral posterior foot, they are rapidly adapting their responses do not decrease with repetition, and they are not blocked by high Mg++/low Ca++ solutions. As synaptically-driven neurons, R1 and L1 have widespread bilateral responsiveness, their responses decrease with repetition and their inputs are blocked with high Mg++/low Ca++ solutions. These neurons integrate sensory and synaptic inputs and conduct bidirectionally, however, their output connections must be specified before their behavioral function can be understood.