Frequency coding of positional information by an identified neuron, the AP cell, in the leech, Whitmania pigra.

The synaptic connections from pressure-sensitive receptors (P cells) to identified neurons of unknown function (known as anterior pagoda or AP cells) were used to study the way in which leeches process information about the position of a mechanical stimulus on its skin. We elicited spikes in P cells by injecting current intracellularly while recording from AP neurons. The postsynaptic responses consisted of an increase in impulse frequency. We show here that the AP neuron can encode positional information in terms of the frequency of its action potentials. Thus, the AP neuron can serve as an indicator of integrative mechanisms used in the processing of sensory information that is important for the behavior of the animal.