Figure 1. Effects of changes in external Ca²⁺ on action potentials in post-hearing spiral ganglion neurons.

A We used the perforated-patch configuration to evoke electrical activity from apical, rapidly adapting spiral ganglion neurons in 2 mM extracellular Ca²⁺ (left panel, in black), and after (middle panel, in light gray) bath perfusion of extracellular solution containing ~100 μM Ca²⁺. The magnitude and duration of the injected current were 0.2 nA and 100 ms, respectively. On the right panel, we superimposed evoked action potentials recorded in 2 mM Ca²⁺ and 100 μM Ca²⁺-free solutions. The peaks of the action potentials were (in mV): 28 ± 3 in 2 mM Ca²⁺, and 10 ± 6 in 100μM Ca²⁺ bath solutions (n = 8; p < 0.05), and the maximum right slopes were (mV/ms); −99 ± 16 in 2 mM Ca²⁺, and −78 ± 4 in 100 μM Ca²⁺ bath solution (n = 8; p < 0.05). B Evoked action potentials were elicited with a 0.05 nA current for ~100 ms from a basal, slowly adapting, SGN in a bath solution containing 2 mM Ca²⁺ (left panel,) and after a bath perfusion of ~100 μM Ca²⁺solution (middle panel). The right panel shows a comparison of action potential profiles in the two treatment conditions. The dashed lines show 0 mV levels. The peaks of the action potentials were (in mV): 37.0 ± 7 in 2 mM Ca²⁺, and 16 ± 3 in 100 μM Ca²⁺ bath solutions (n = 9; p < 0.05), and the maximum right slopes were (mV/ms): −149 ± 1 in 2 mM Ca²⁺, and −83 ± 15 in 100 μM Ca²⁺ bath solution (n = 9; p < 0.05). C Shown is the characteristic resting membrane potential (rmp) of a 3-month old apical SGN. Reduction of external Ca²⁺ from 2 mM to ~10 nM produced modest hyperpolarization of the rmp. The summary data show that the rmp for apical neurons was (control ) −58 ± 6 mV and (10 nM Ca²⁺) −61 ± 4 mV (n = 9; p = 0.12). The effect of reduced Ca²⁺ was reversible after washout with control solution. D Injection of 0.2 nA current produced single spikes in apical SGNs. E Exemplary current-clamp recordings from SGNs from the basal turn of the cochlea showed the rmp in control solutions and after application of 10 nM Ca²⁺-solution. The effects of 10 nM Ca²⁺ solution were reversible. Summary data from 15 basal SGNs show that the rmp in control solution was: −55 ± 5 mV and in 10 nM-Ca²⁺ solution was: −64 ± 3 mV (n = 15; p < 0.05). F–H The spike frequency of adult basal SGNs was heterogenous. F–G show data from the same neuron after injection of 0.2 nA current for ~200 ms (F) and ~5 s (G), respectively. In 10 nM external Ca²⁺, the spike frequency plummeted by ~5-fold compared to control (from 17 ± 7 Hz (control) to 3 ± 2 Hz (10 nM Ca²⁺: n = 9; p < 0.05). Upon washout the spike frequency was restored to 12 ± 7 Hz (n = 9; p = 0.08). H In slowly adapting SGNs that fire unabatedly after injecting 0.2 nA for 5 s, the spike frequency was reduced by ~4-fold after application of 10 nM- Ca²⁺solutions. From control to 10-nM Ca²⁺ solution the spike frequency changed from 49 ± 10 Hz (control) to 12 ± 6 Hz (10-nM Ca²⁺: n = 11; p < 0.05). The firing frequency after washout was 39 ± 14 Hz (n = 11; p = 0.11).