Biology Reference
In-Depth Information
with drawings. The descriptions of the neuronal cell bodies are remark-
ably accurate and still valid. He also attempted to trace neuronal
processes within the nerve ring and nerve cords. In most cases he did not
connect identified nerve processes with identified cell bodies, and these
diagrams are not informative. In addition, he saw areas where the
processes of different cells apparently “melted” together and he inter-
preted these as sites of cytoplasmic continuity between the neurons, in
support of the reticular theory of the nervous system (along with Golgi, in
joint opposition to Ramon y Cajal and his followers who believed in the
Neuron Doctrine). Ultrastructural datasets generated using electron
microscopy are not consistent with this explanation.
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Areas of apparent
“melting” are sites of synaptic contact, where the membranes of neigh-
boring neuronal processes have complex interdigitations so that the
membranes appear indistinct as they are observed in the light microscope
at different focal planes within a section. When these same sections are
re-sectioned for electron microscopy, they have the structure of typical
chemical synapses.
Other investigators reported on other aspects of the nervous system.
Voltzenlogel described the 30 neurons of the tail ganglia.
32
Hesse
1
described the pattern of dorsoventral commissures in the rest of the body;
Otto
33
recognized a chain of neuronal cell bodies, presumed to be motor
neurons, in the ventral nerve cord. To describe the morphology of the
motor neurons in the ventral and dorsal nerve cords required the analysis
by light microscopy of serial sections of complete worms.
8
Even for
relatively short individual worms, it still required the analysis of 10,000
serial 10
m
m sections per worm. Reconstructions revealed a repeating
pattern of motor neurons, comprising a basic set of five repeats of 11
motor neurons. In addition, the anterior and posterior ends of the ventral
cord depart subtly from the basic repeat pattern, and “extra” neurons are
present, so that the ventral cord includes a total of 72 motor neurons in
females. Within the basic 11 neuron repeat, there are seven types of motor
neuron, and light microscopy and subsequent electron microscopy
showed that four types innervate dorsal muscle and three types innervate
ventral muscle. The neuronal commissures, which connect the ventral
cord and dorsal cord, have been crucial experimentally, as described
below.
Finally, there are 14 neurons in the two lateral lines, and 20 neurons
in the pharynx, giving a grand total of neurons in female A. suum of 298
(
Figure 6.1
), close to the total of 302 found in the C. elegans hermaphrodite.
The four additional neurons in C. elegans are sensory neurons in the head.
The neurons give rise to neurites that project into the nerve ring
and into several longitudinal nerve cords. The major nerve cords are
the ventral and dorsal cords (VC and DC) which contain the axons that
make neuromuscular synapses onto the ventral or dorsal musculature,
