Biology Reference
In-Depth Information
observed gradual reduction in the amplitude of left or right movements
from the head posteriorly. There are also lateral nerves that extend from
head to tail along each lateral line.
The Importance of Motor Neuron Commissures
The dorsoventral commissures in the body of A. suum had been noticed
by Hesse,
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but it was only after reconstruction from serial sections that the
individual types of neurons were recognized, and it was shown that the
dorsoventral commissures were all branches of motorneurons.
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There are
seven types that occur in five repeating subunits, named “segments,”
along the length of the worm. Anatomical and electrophysiological
experiments showed that there were both excitatory and inhibitory motor
neurons, and that there were separate motor neurons controlling the
dorsal and ventral muscles. The dorsal motor neurons were named DE1,
DE2, DE3, and DI, and the ventral motor neurons VE1, VE2, and VI. Each
segment contains 11 motor neurons, with four of the neuronal types (DE1,
VE1, VE2, and VI) present twice in each segment, and DE2, DE3, and DI
only once. The cell bodies of all these motor neurons are in the ventral
nerve cord; five types of neurons (DE1, DE2, DE3, DI, and VI) have
dorsoventral commissures, so there are seven commissures per segment.
Extensive anatomical studies at both light and electron microscope levels
showed that all the motor neurons had output regions where they
synapsed onto muscle and other neurons, and input regions (dendrites)
where they received synapses from other neurons. For neurons with
commissures, these two regions were in different nerve cords. Another
interesting finding was that the sole input to inhibitory neurons was from
the opposite excitors (DE1, DE2, and DE3 to VI, and VE1 and VE2 to DI).
These synapses mediate reciprocal inhibitory activity between dorsal and
ventral musculature.
Physiology of Neurons
The early physiological characterization of the motor neurons made use
of the anatomical reproducibility of the position of the commissures, so that
dissected preparations could be made in which the dorsal and ventral
halves of the animal were linked only by a single commissure (validated
by serial section reconstruction). This allowed single neurons to be stim-
ulated and their effects on muscle to be recorded, leading to the initial
characterization of dorsal/ventral and excitatory/inhibitory motor
neurons.
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In 1989, Ralph Davis made a crucial contribution;
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in a slit-open
piece of the worm he carefully dissected away the muscle cells that overlie
a commissure to allow microelectrode penetration for recording and/or
stimulation. He did control experiments to show that the integrity of the
neurons was not compromised by the dissection, which was important
since one of his key discoveries was that the motor neurons did not
