Fig. 7.

Appearance of voltage-dependent Na⁺ and Ca²⁺ currents in circus cells from neurectoderm. Voltage-clamped currents were elicited during the first 6 hr in culture by a 30 msec voltage step from a holding potential of −100 mV to 0 mV. A, Inward currents are not detected in a circus cell at 1.5 hr in culture (left top). Both rapidly inactivating and sustained components of inward current are present at 6 hr (left middle) in some circus cells, whereas other circus cells demonstrate only the rapidly inactivating component (left bottom) or no inward currents at all (not shown). A round cell examined at 6 hr does not demonstrate inward currents (right top), whereas a neuron examined at 6 hr has both rapidly inactivating and sustained currents (right bottom). B, Pharmacological blockade of rapidly inactivating and sustained inward currents. The rapid component of the inward current is blocked by 1 μg/ml TTX, and the remaining sustained component is blocked by further addition of 2 mm Ni²⁺ in a circus cell (left) and in a neuron (right). Voltage-clamp protocol as in A. C, D, Aggregate measurements ofI_Na and I_Caduring early development. Current densities (pA/μm²) of individual circus cells are plotted against time in culture. Mean peak-to-peak recording noise is denoted by the shaded horizontal bar.