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364 NEUROTRANSMITTERS
more arms that extend from the muscle bag
towards the nerve cord in the hypodermis. At
their termination, each muscle arm branches
into processes that join with those from other
arms to form a syncytium, or functional network,
around the nerve cord. The individual
processes are separated from each other by
tight junctions, which ensure the electrical
coupling between muscle cells. Synapses are
seen at the junction between the terminal
divisions of the muscle arm and the nerve.
Pharyngeal muscle
The pharynx of feeding stages of nematode
larvae and adults is an electrically coupled
group of special muscle cells that form a muscular
tube at the head region (Figure 15.7). The
pharynx varies in precise shape with the species
of nematode, and pumps food from the surrounding
medium into the intestine. In Ascaris
it is a large organ, nearly 1 cm in length. The
electrophysiology of pharyngeal muscle was
described initially by Del Castillo and Morales.
It is different from that of most other excitable
cells because it has a negative potential that
repolarizes the cells. The triradiate lumen is
closed when the muscle is relaxed, and contraction
opens the lumen.
There are two types of synaptic potential
recorded in the pharyngeal muscle. There is
the depolarizing postsynaptic potential and
a hyperpolarizing postsynaptic potential. We
have used a two-microelectrode recording technique
to record the effects of L-glutamate on
pharyngeal muscle and found that it produces
hyperpolarization associated with an increase
in chloride conductance (Figure 15.7).
Ascaris nervous system
The neural anatomy of A. suum, like that of
other nematodes, is consistent and relatively
simple. According to Stretton, there are only
FIGURE 15.7 Diagram of an Ascaris pharynx preparation for two-micropipette current clamp recording. A current
injection pipette (I) is placed in the pharyngeal muscle while a voltage recording pipette (V) records membrane
potential and response to injected current for conductance measurement. Two additional pipettes are
placed extracellularly for the application of drugs, including neurotransmitters.
BIOCHEMISTRY AND CELL BIOLOGY: HELMINTHS