Fig. 2. Patch-clamp recordings of sling neurons demonstrate that these cells fire action potentials. Current-clamp recordings in response to intracellular injection of depolarizing current pulses in (A,B) identified sling cells and (C) wedge cells. (A) Responses to 55 pA (black) and 75 pA (red), 100-msecond long current pulses of a sling cell at E18. The Na⁺-dependent spike (solid arrow) is suppressed after 8 minutes of TTX application, and the Ca²⁺-dependent spike (dotted arrow) is abolished in the absence of Ca²⁺ (0 Ca²⁺). At P2 (B) sling cells evoke TTX-sensitive Na⁺ spikes. Inset depicts spontaneous action potentials recorded, at rest, from the same cell; all spontaneous spikes were reversibly abolished by TTX. By contrast, identified glial wedge cells (C) respond to depolarizing currents with nearly linear increases in voltage responses, and evoke no regenerative spikes. (D) Phase-interference contrast micrograph showing the sling lying directly underneath the corpus callosum (CC). A patch pipette (P) is in place to record from an identified sling cell. Sling cells were identified morphologically directly below the corpus callosum as they form a stream of tightly connected cells, with their leading processes pointing toward the midline (on the left). Only one focal plane is visible in this image, therefore surrounding sling cells are not in focus. Scale bar: 20 µm.