Biology Reference
In-Depth Information
NEURO
NAL STRUCTURE AND ORGANI
ZATION
Ascaris
and Other Parasites
The Ascaris nervous system promised to be ideal for testing the
hypothesis that a comprehensive functional description of the “wiring
diagram” (or the “connectome”), including the anatomical connectivity
and physiological properties of neurons and synapses, would be suffi-
cient to predict behavior. In addition to being large enough for electro-
physiological and biochemical experiments, Ascaris neurons are few in
number (298 in adult females) and exhibit an unusually simple branching
pattern. However, after assembling an extensive description of the
structure and function of the A. suum motor nervous system, and creating
a model that predicted the mechanism for generating locomotory
behavior, further experiments showed that the hypothesis was wrong; the
structural features, together with detailed measurements of the physio-
logical properties of neurons and their synapses, did not correctly predict
behavior
something crucial was missing. At least part of what was
missing was the influence of neuromodulators that alter the properties of
the neurons and/or synapses thereby regulating the functional connec-
tivity of neurons.
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Indeed, A. suum possesses a large and complex
array of neuromodulators; in addition to small molecular weight trans-
mitters like dopamine and serotonin, there is in excess of 250 neuropep-
tides. One of these Ascaris neuropeptides (designated AF1) was the first to
be tested in this way in nematodes; it exerted its paralyzing action by
disrupting dorsoventral signaling in the inhibitory motor neurons.
Although many other nematode neuromodulators have been studied
physiologically, still the majority remain to be explored.
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Neuroanatomy
A. suum has a long and distinguished history in neuroanatomy. Hesse
1
and Goldschmidt
2
made detailed studies of the A. suum nervous system at
the turn of the last century. Hesse described general features of the
system, including the head ganglia, nerve ring, and nerves. Goldschmidt
studied the neurons in the head, and found that the number of neurons in
different individuals was identical (162 in the head ganglia, including the
retrovesicular ganglion at the anterior end of the ventral nerve cord).
Furthermore, their relative position and morphology (cell body size,
shape, and cytoplasmic inclusions) enabled him to recognize each of the
neurons in different individuals. He concluded that their nervous
systems, at least at this level of analysis, were identical, and he was the
first to elaborate the concept of the “Identified Neuron,” which has been
so important in cellular neurobiology, especially in invertebrates. Of
course, Goldschmidt used light microscopy, and illustrated his findings
