trans

NEUROTRANSMITTERS IN NEMATODES 365
approximately 300 neurons in A. suum. Sensory
and motor neurons are concentrated at
the head and the tail of the nematode. The
rest, about 90 motor neurons, are arranged in
repeated segments along the length of the body;
they supply the innervation to the somatic
muscle cells. Figure 15.8 illustrates a segment
of the dorsal and ventral nerve cord with
connecting commissures (three to the left and
one to the right). Each segment has 11 motor
neurons that are excitatory (cholinergic) or
inhibitory (GABAergic). This layout is believed
to be very similar in all species of nematode.
Due to an elementary system of reciprocal
innervation of the neural circuits, excitatory
impulses to the ventral muscle are associated
with inhibitory impulses to the dorsal muscle,
and vice versa. This combination of signals
results in contraction of one block of muscle
whilst the opposing muscle mass relaxes, allowing
the nematode to progress forwards or backwards
with a wave-like motion.
Ascaris physiology
Membrane potential
The membrane potential of Ascaris suum
muscle is near 30 mV, and is relatively insensitive
to changes in the ionic composition of
FIGURE 15.8 A ‘segment’ of the dorsal and ventral nerve cord of Ascaris, with 11 motor neurons and six longitudinal
neurons. There are three right-hand commissures and one left-hand commissure in each segment. The diagram
represents one of seven repetitive segments found in Ascaris. The filled, •, cells are excitatory motoneurons
with acetylcholine as the fast transmitter and empty, O, cells the inhibitory motoneurons with GABA as the fast
transmitter. FaRP peptides may act as co-transmitters. Each motoneuron is classified by the location of the cell
body, and the shape and distribution of its processes. All cell bodies are found in the ventral cord. The excitatory,
cholinergic outputs, , are shown, as are the inhibitory GABAergic ouputs, . The inputs, are also shown. Dorsal
excitatory motoneurons: DE1, DE2, DE3. Ventral excitatory neurons: V1, V2. Dorsal inhibitory neurons: DI. Ventral
inhibitory neurons: IV. V1 and V2 do not have commissures. All commissures are right-handed except DE3.
BIOCHEMISTRY AND CELL BIOLOGY: HELMINTHS