Resonance in the PY Cell

The property that describes the neuron's preferred response to inputs of injected current is called resonance. To produce resonance, a cell must have two requirements: a low-pass filter (attenuates high frequencies) and a high-pass filter (attenuates low frequencies). The neuron whose resonance frequency we wish to determine is the PY neuron of the STNS (stomatogastric nervous system) of Cancer borealis. The STNS consists of rhythmic neural networks that control the movement of a crustacean's foregut. Noting that all neurons exhibit a low-pass filter, we investigate the high-pass filter of the PY neuron, and its properties. The PY neuron has an I h (hyperpolarizing activating current) channel as its high-pass filter. Thus, for the PY neuron to exhibit resonance, it must have an I h channel (or any other high-pass filter) in addition to the inherit low-pass filter property. To inspect if PY has resonance we used the current clamp method and injected a sinusoidal current that sweeps through several frequencies. This current, also called ZAP, produced a voltage response that we can analyze via an impedance versus frequency graph. Resonance was not seen in the preliminary experiments so we decided to use 10 mM CsCl to block I h , to verify that I h did not cause an unnoticeable resonant peak. However, resonance was still not seen, so we injected an artificial I h current via dynamic clamp. Under this method, resonance was produced by increasing the conductance of I h. To model these results we used the Hodgkin-Huxley equation and created the same scenario. Under low conductance, resonance was not evident, but when conductance increased the PY neuron had frequency preference. This leads us to believe that while the PY neuron has an I h current, its conductance is too low to produce resonance. The probable cause for this, is that PY cells are small in size relative to the other STG cells, resulting in less I h channels per unit membrane.