Experimental calyx data used to constrain model parameters
| Calyx volume1-a | 400 μm3* | ‡; Helmchen et al., 1997 |
| Calyx membrane surface1-a | 2400 μm2* | ‡ |
| Calyx height1-a,1-b | 400 nm* | ‡ |
| Percentage of calyx surface identified as AZ1-a | 2%* | ‡ |
| Number of AZs1-a,1-c | 500-600* | ‡ |
| Diffusional separation of any AZ to its nearest neighbor1-a | 200–1000 nm (median = 400 nm) | ‡ |
| Size of AZs (approximated as circular patches of varying radii)1-a,1-d | 0.052 ± 0.024 μm2 (SD) mean radius, 125 nm | ‡; also see Rowland et al., 2000 |
| Total Ca2+ influx during single, physiological AP | 0.93 pC = 12 μm in 400 μm3 | Helmchen et al., 1997 |
| Endogenous Ca2+ binding ratio | 40 (2.5% of ions remain free) | Helmchen et al., 1997 |
| Δ[Ca2+] during single AP1-e | From 50 (resting) to 379 nm | Helmchen et al., 1997 (Fig.7B) |
| Whole-cell Ca2+ current (ICa) | Peak = 2.41 nA half-widtht0.5 = 383 μsec | Borst and Sakmann, 1998 (see Fig. 6A for time course) |
| Voltage-dependent gating of Ca2+ channels and channel current1-f | Two-gate Hodgkin–Huxley model | see Borst and Sakmann (1998)for parameter values |
| Ca2+ sensitivity of release1-g | Five-site independent, model A | Bollmann et al., 2000 (Fig. 1) |
| Five-site cooperative, model B | Schneggenburger and Neher, 2000 | |
| Pr, calyx of undialyzed calyx1-h | Model A: 25% (of 800 vesicles) | Bollmann et al., 2000 |
| Model B: 10% (of 2000 vesicles) | Schneggenburger and Neher, 2000 |
↵* Parameters do not (directly) affect calculations of local [Ca2+] transients or Pr but volume average [Ca2+] and estimates of channel density only (see Materials and Methods, Discussion).
↵‡ Sätzler, Söhl, Bollmann, Borst, Frotscher, Sakmann, and Lübke, unpublished data.
↵F1-a Anatomical parameters taken from an EM-based three-dimensional reconstruction of a single calyx (rat, postnatal day 9). Average anatomy during postnatal days 8–10 may deviate from this example. Parameters for the model calyx in Table 1are chosen such that they are internally consistent (e.g., not combining above-average whole-cell Ca2+ current with below-average calyx volume).
↵F1-b “Height” means the average distance between membrane facing the cleft and that opposite the cleft; consider calyx as a thin disk with surface 1000 μm2 at top/bottom, height ∼400 nm, and volume 400 μm3.
↵F1-c Corresponding to an average of 1.3 (Release Model A) or 3.3 (Release Model B) readily releasable vesicles docked per AZ.
↵F1-d The value is smaller than that reported by Sätzler, Söhl, Bollmann, Borst, Frotscher, Sakmann, and Lübke (unpublished data). The latter includes uneven surface of the AZs, which does not contribute to diffusional distance.
↵F1-e 379 nm is the equilibrium [Ca2+] after adding 12 μm free Ca2+ to the calyx (with ATP and endogenous fixed buffer).
↵F1-f For all simulations, single Ca2+ channel currents were predicted with a two-gate HH model calibrated to the calyx (see Materials and Methods). Only for the nonperiodic grid topography, a third gate was added to the channel model (see Property II).
↵F1-g All conclusions presented in this paper were tested with two release models for the calyx: a kinetic release model with five independent binding sites (Release Model A) and one with five cooperative (sequential) binding sites (Release Model B). The number of quanta released during physiological APs was the same in both studies (200). Figure 1 shows a direct comparison of the Ca2+ sensitivities of the models.
↵F1-h Pr, calyxdenotes the release probability during single AP, defined throughout this paper as the total number of vesicles (as fraction of the readily releasable pool) that are released in response to a single AP (phasic release only).