1 Introduction 2+ i [1,2] 2+ i [3,4] 2+ 2+ i 2+ 2+ i 2+ 2+ 2+ i 2+ [5] 2+ [6] [7–11] 2+ 2+ 2+ 2+ i 2+ i 2+ [12–15] [e.g. 13,14,16] 2+ 2+ 2+ 2+ 2+ 2+ 2+ 2+ 2+ 2+ 2+ 2 −1 2 Materials and methods 2.1 General [14,16] 2+ 2+ i 2.2 Preparation Helix aspersa −1 2.3 Solutions 2 2 2 2 2 3 2 2 4 2 2.4 Microelectrodes [16,17] [16] 2 2 [15] V Ca V m 2+ i [14] 2.5 Estimating the BAPTA intophoresis transport number V Ca V Ca V Ca −1 2.6 Data collection and analysis V m V Ca [14,16] n p 2.7 Computational modelling 2+ [18–21] 2+ 2+ 2+ 2+ 2+ 2+ [9,22] 2+ 2+ [20] 2+ D Ca 2 −1 D app: (1) D app = D Ca + D e κ e + D b κ b 1 + κ e + κ b , [7] κ e b 2+ D e D b 2+ D Ca 2 −1 [23] 2+ κ tot κ e κ b 2+ κ tot 2+ [24] (2) ∂ [ C a 2 + ] i ∂ t = D app 1 r 2 ∂ ∂ r r 2 ∂ ∂ r [ C a 2 + ] i + r sv j basal , app + r sv j in , app + r sv j out , app + J seq , app , j in app j out app J seq,app j basal app 2+ r sv −1 j −2 −1 J −1 2+ i r t (3a) j in , app = − I Ca , app 1 2 F S , 2+ I Ca,app I Ca,app I Ca κ tot S 2 F 2+ [25] (3b) j out , app = − P out , app [ C a 2 + ] i , P out,app −1 P out,app P out κ tot (3c) J seq , app = − k seq , app ( [ C a 2 + ] i − [ C a 2 + ] s ) , k seq app −1 k seq,out k seq κ tot 2+ s 2+ [26] j basal,app 2+ j basal,app P out,app 2+ s 2.8 Microelectrode model 2+ 2+ (4) E Ca = R T 2 F ln [ C a 2 + ] i [ C a 2 + ] CASM + E 0 , E Ca R T 2+ 2+ CASM E 0 2+ V Ca 2+ V Ca 2+ i [14] V Ca (5) ∂ V Ca ∂ t = E Ca − V Ca τ CASM τ CASM 2+ i 2.9 Integration and parameter fitting [27] (6a) [ C a 2 + ] i ( r , t ) = j Ca , app r ∑ n = 0 ∞ A n τ n sin 2 α n r d ( 1 − e − t / τ n )   0 ≤ t < t Ca (6b) [ C a 2 + ] i ( r , t ) = j Ca , app r ∑ n = 0 ∞ A n τ n sin 2 α n r d ( 1 − e − t / τ n ) e − ( t − t Ca ) / τ n   t ≥ t Ca A n τ n (7) A n = 2 α n   sin ( α n ) α n − sin ( α n ) cos ( α n ) (8) τ n = 4 α n 2 D app d 2 + k seq , app − 1 α n (9) τ P α   cos ( α ) + ( τ D − τ P )   sin ( α ) τ D d 2 D τ p d P d τ 0 d P out τ 0 d 2 πD app [22] r sv (2) S 2+ 3 Results 3.1 Changes in measured calcium with depth of penetration Fig. 1 V m V Ca V Ca V m V m V Ca Fig. 1 2+ 2+ i V Ca Fig. 1 2+ i V Ca V Ca V Ca V Ca V Ca Fig. 2 V Ca Fig. 1 2+ V m 2+ Fig. 3 2+ i 2+ i V Ca 2+ i 2+ i Fig. 3 V Ca Figs. 1 and 3 V Ca V Ca 2+ i [13] V Ca Fig. 3 V Ca V Ca −1 Fig. 3 3.2 The effect of removing external calcium 2+ i 2+ Fig. 4 V Ca V Ca V Ca 3.3 2 2 [28,29] 2+ 2 2+ [16,30] i 2 2+ i 2+ 2 [28] i V Ca 2 3.4 The role of calcium stores. V Ca 2+ i 2+ [31] Fig. 5 V Ca V Ca p n p 2+ p Fig. 5 2+ i p 2+ i 2+ 3.5 The effect of BAPTA injection 2+ [32] [33] 2+ V Ca 2+ Fig. 6 V Ca V Ca V Ca V Ca Fig. 6 2+ V Ca V Ca V Ca [14] 3.6 2+ I Ca,app 2+ D app 2+ 2+ P out,app k seq app 2 2+ Fig. 7 D app 2 −1 k seq app −1 P out,app −1 2 τ CASM 2+ Fig. 7 D app 2 −1 2 −1 [34] 2 −1 [24] 2 −1 Aplysia californica [23] 2 −1 [35] 2 −1 [36] 2+ i D app D app . 2 −1 Fig. 7 D app 2 −1 2+ 2+ i 2+ 2+ 2+ 2+ 2+ 2+ 2+ [24] 2+ V Ca [33] 2+ i 2+ Fig. 7 2+ 2+ 2+ D e 2 −1 2+ κ e 2+ 2+ [25] 2+ D app 2 −1 D app 2 −1 D app 2 −1 D b 2 −1 2+ [37–39] 4 Discussion 2+ 2+ 2+ 2 −1 4.1 The problem of leakage + 2+ [40] + i + + i + i + i [12] i 2+ i [15,17] 2+ i 2+ i 2+ i V Ca [14] V Ca Fig. 1 Fig. 3 V Ca 4.2 2+ 2+ 2+ i V Ca −1 2+ 2+ [41,16] 2+ [41] [16] 2+ V Ca 2+ i 2+ 2+ 2+ 2+ [42] 2+ i 2+ i 4.3 2+ 2+ 2+ + 2+ [43] 2+ i o [16] 2+ 2+ [44,45] 2+ [24] 2+ 2+ 2+ 2+ 2+ [46] 4.4 Calcium transients change systematically with depth V Ca 2+ 2+ 2+ −1 [8,12] 2+ [10] Fig. 1 Fig. 4 2+ 2+ [47] 4.5 Calcium diffusion in large snail neurones without added buffer 2+ 2+ 2 2+ 2+ Fig. 8 2 −1 2 −1 4.6 2+ 2+ 2+ 2+ 2+ 2+ 2+ 2+ [48] 2+ 2+