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Fig. 8. Proposed role of the CCA-1 T-type channel in the pharyngeal action
potential. In wild-type worms, action potentials are triggered by the influx
of cations through the EAT-2-containing acetylcholine gated channel. The small
rise in membrane potential resulting from cation influx is sufficient to
activate the CCA-1 T-type channel. Calcium influx through CCA-1 lifts membrane
potential further, until the EGL-19 L-type calcium channels can be activated.
In the absence of CCA-1 function, some EPSPs fail to activate EGL-19 channels,
or activate them more slowly than in wild-type worms. When both MC EPSPs and
CCA-1 function are absent, the pharynx employs an alternative method, possibly
a leak conductance and an elevation of resting membrane potential, to trigger
EGL-19 activation.
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