Ten pairs of protrusions, called accessory lobes (ALs), exist at the lateral sides of avian lumbosacral spinal cords. Histological evidence has shown that neurons are present in AL and behavioral evidence suggests that AL acts as a sensory organ of equilibrium during bipedal walking. However, there is little functional evidence to indicate that cells in AL have neuronal functions. To elucidate this point, we developed a method to dissociate cells from chick AL and made electrophysiological recordings with the whole-cell patch clamp technique. Cells dissociated by enzymatic digestion from chick AL contained two major types of cells. One was round with clear cytosol and the other had a round cell body, rich cytosolic structures and some processes. Rapidly activating inward currents and slowly activating outward currents were recorded in response to depolarizing pulses to -10 mV under the voltage clamp configuration only from the latter type of cells. TTX at 100 nM inhibited the inward current by 85%, indicating the functional expression of TTX-sensitive voltage-gated Na(+) channel (VGSC). Activation and inactivation kinetics of the inward currents in AL cells were similar to those of mammalian VGSC. The VGSC-expressing AL cells generated action potentials in response to depolarization under the current clamp configuration. These results clearly indicate that functional neurons expressing fast inactivating and TTXsensitive VGSC which generate action potentials exist in the AL of the chick. These lines of cellular evidence clearly indicate that functional neurons exist in ALs and further support the proposal that the chick ALs function as the sensory organ of equilibrium.