The functional implications of the differential compartmentalization of Shaker and Shal channels along a pyloric neuron. The diagram represents the PD neuron innervating the PD muscle. The shaded areas represent the regions of the neuron that contain membrane-bound Shal channels but no membrane-bound Shaker channels. The area corresponds exactly to the somatodendritic compartment, which lies in the peripheral layer and neuropil of the STG. The box positioned next to the neurites contains spontaneous, simultaneous intracellular recordings from the somata of each of the six identified pyloric cell types (Miller, 1987). Shal, but not Shaker, channels contribute to the large, rhythmic oscillations in membrane potential that are generated in, and influenced by, the somatodendritic compartment. The circle with the grid represents the region in which membrane-bound Shaker and Shal channels are not detectable. This region most likely contains the initial segment molecular fence that prevents diffusion of membrane proteins (Winckler and Mellman 1999; Winckler et al., 1999). The primary spike initiation zone might lie in, or immediately proximal to, this region, suggesting that somatodendritic Shal channels primarily mediate the effect of the I_A on spike timing and frequency. The unshaded area represents the region in which membrane-bound Shaker channels predominate. This region corresponds to the distal axonal compartment located in stomatogastric nerves. A typical extracellular recording from this compartment is shown above the axon. Shaker channels most likely contribute to spikes that are not propagated on a depolarizing wave but on a flat hyperpolarized membrane potential (Marder and Selverston, 1992). The stippled box represents the muscle. Thequestion mark signifies that Shaker and Shal are both present at the NMJ, but their locations in the component membranes are not known.