Figure 1.
NGF reduces activity-dependent slowing of conduction in the mechano-insensitive fibres in a time-dependent manner.
(A) Experimental set-up. Schematic representation of the setup for electrophysiological recordings in the pig skin, in vivo. Close-up of the dissected saphenous nerve, placed in a groove on a metal mirror. A pair of microelectrodes is used for intradermal stimulation in the innervation territory of the saphenous nerve. The nerve endings are stimulated with repetitive rectangular pulses (0.5 ms duration; 0.25 Hz frequency). NGF was injected intradermally in the saphenous nerve innervation territory (5 injections per hind limb, 20 µl per injection site; total dose: 2 or 8 µg), 4 to 7 days before recording day. (B) ADS patterns during stimulation at 2 Hz for 3 min are shown for mechano-insensitive afferents (CMi; circles, left panel) and for mechano-sensitive afferents (CM; triangles, right panel) in control situation (ctrl.) or at 4 to 7 days after NGF treatment (days indicated by numeric value after “_”. Number of units is given in parentheses. Changes are shown relative to the initial latency recorded after pause. Statistical differences between NGF-treated versus control units (ANOVA, repetitive measurements with LSD post hoc test) are marked by asterisks (*).
Figure 2.
Functional properties of default CMi model.
(A) Axon model geometry. A branch of length (L) 2 cm and diameter (D) 0.25 µm is connected to a parent axon (L: 10 cm; D: 1 µm) via a cone (L: 0.5 cm, linearly increasing diameter from 0.25 to 1 µm) (B–E) Protocols used to validate the models. Arrows indicate how results should change for the model to represent a CM rather than a CMi fibre. (B) Activity-dependent slowing (ADS). Stimulation with 360 pulses at 2 Hz results in maximal slowing ADSmax. (C) Activity-dependent conduction failure. 2 Hz with 1.2× threshold current. Nfail is the pulse number for which failures first occur. (D) Supernormality (SN) induced by 80 pulse pairs (ISI = 50 ms) given at 1 Hz. SN1max is defined as the latency shift of the 80th pulse pair. (E) Recovery-cycle protocol with background frequency 0.5 Hz. 100 pre-pulses (not shown) followed by pulse pairs of varying inter-stimulus intervals (ISI). Maximal supernormality (SN2max) is measured at the maximal negative latency shift.
Figure 3.
Effect of changing ion channel and pump densities, resting membrane potential and branch length on slowing, conduction failure and initial conduction velocity.
Bars indicate that a parameter change led to a model function more (green), respectively less (red) like a CM fibre. See text for details on the vertical dotted score lines. (A) Activity-dependent slowing protocol. Bars calculated as ADSmax/ADSmax,DEF, where ADSmax,DEF is ADSmax in the default model. (B) Activity-dependent conduction failure protocol. Bars calculated as Nfail/Nfail,DEF, where Nfail,DEF is Nfail in the default model. (C) Initial conduction velocity (CV). Bars calculated as CV/CVDEF, where CVDEF is CV in the default model.
Figure 4.
Effect of changing ion channel and pump densities, resting membrane potential and branch length on supernormal conduction.
Same colour coding as in fig. 3. See text for details on the vertical dotted score lines. Supernormality protocols were used for calculating SN1max (A) and SN2max (B). Bars calculated as (SN1max/IL)/(SN1max,DEF/ILDEF) (A) and (SN2max/IL)/(SN2max,DEF/ILDEF) (B), where IL denotes the initial latency.
Figure 5.
Results from all stimulation protocols weighed together by use of the scoring system.
Large bars should be interpreted as strong candidates for explaining what change makes a CMi fibre more CM-like (green) or unlike (red). The results suggest that, compared to CMi fibres, CM fibres have less Kdr channels, more NaV1.7 channels, etc. (A): Exponentially increasing score lines (as in Figures 3 and 4): ±A·(j/N)2, with j = 1,2,…,N. For ADSmax: A = 3.5, N = 4; For Nfail: A = 25, N = 2; For CV: A = 6, N = 3; For SN1max: A = 40, N = 2; For SN1max: A = 60, N = 2. (B): Like (A) but with fewer exponentially separated score lines: For ADSmax: N = 3; For Nfail: N = 1; For CV: N = 2; For SN1max: N = 1; For SN1max: N = 1. (C): Similar to (A) but with linearly separated score lines: ADSmax: ±1,2,3,4; Nfail: ±10,20; CV: ±2,4,6; SN1max: ±20,40; SN2max: ±30,60.
Figure 6.
Example simulations with one selected parameter change modifying the CMi model (black, default values) to display CM fibre like function (green).
Compared to the default CMi model (black), the CM-like model (green) has a shorter branch length (1 rather than 2 cm), 50% lower Kdr channel density and 10% higher pump density. Figures show how (A) ADS, (B) conduction failures, (C) SN1 and (D) SN2 were affected by these changes.
Figure 7.
Correlation of data from Figures 3 and 4 across stimulation protocols.
(A–D) Correlations between pairs of functional properties. In panel (D), the lines connect data points from models where the channel densities of NaV1.7 (solid, negative correlation) and NaV1.8 (dashed, positive correlation) were modified.