ABSTRACT
Phase plane techniques have been used to demonstrate that identifiable neurones of Lymnaea stagnalis may be characterized by their action potential trajectories.
These trajectories are consistent from preparation to preparation, but may be altered by strong synaptic inputs or by injection of current into neurones.
There is a pronounced hysteresis in the maximum rates of depolarization
and the maximum rates of repolarization 
of many cells following a period of hyperpolarization.Curves relating
and 
to injected current have been constructed for each neurone type investigated and may be used to identify the neurones.We conclude that phase plane portraits of action potentials together with measures of
and 
are useful in identification of neurones when structural criteria are not available.
