Action potential initiation and propagation in hippocampal mossy fibre axons

Abstract

Dentate gyrus granule cells transmit action potentials (APs) along their unmyelinated mossy fibre axons to the CA3 region. Although the initiation and propagation of APs are fundamental steps during neural computation, little is known about the site of AP initiation and the speed of propagation in mossy fibre axons. To address these questions, we performed simultaneous somatic and axonal whole-cell recordings from granule cells in acute hippocampal slices of adult mice at approximately 23 degrees C. Injection of short current pulses or synaptic stimulation evoked axonal and somatic APs with similar amplitudes. By contrast, the time course was significantly different, as axonal APs had a higher maximal rate of rise (464 +/- 30 V s(-1) in the axon versus 297 +/- 12 V s(-1) in the soma, mean +/- s.e.m.). Furthermore, analysis of latencies between the axonal and somatic signals showed that APs were initiated in the proximal axon at approximately 20-30 mum distance from the soma, and propagated orthodromically with a velocity of 0.24 m s(-1). Qualitatively similar results were obtained at a recording temperature of approximately 34 degrees C. Modelling of AP propagation in detailed cable models of granule cells suggested that a approximately 4 times higher Na(+) channel density ( approximately 1000 pS mum(-2)) in the axon might account for both the higher rate of rise of axonal APs and the robust AP initiation in the proximal mossy fibre axon. This may be of critical importance to separate dendritic integration of thousands of synaptic inputs from the generation and transmission of a common AP output.

Figure 3. The AP initiation site is located in the proximal mossy fibre axon

A, AP recorded at the soma (black trace) and in the axon (red trace) evoked by a brief somatic current injection. B, plot of AP latencies against distance from soma, measured in proximal dendritic shafts, axon hillocks or axonal blebs after brief somatic current injection. The continuous curve represents a function fitted to the data (see Methods, eqn (1)). The most negative latencies were measured at ∼20 μm distance from the soma. C and D, same as in A and B, but current was injected into the axon. E, the upper traces (V(t)) show the same AP as in A. The lower traces show the differentiated voltage signal (). F, plot of the maximal rate of rise (dV/dt_max) of APs against distance from soma. The black circles represent binned averages with error bars. Axonal APs had a significantly higher rate of rise than somatic APs (n = 22; P < 0.01).

Figure 1. Robust axonal AP initiation is independent of the site of current injection

A, projection of a stack of fluorescence images taken from a hippocampal granule cell. The cell was filled with biocytin during whole-cell recording and subsequently labelled with FITC–avidin. B, the traces show an AP evoked by brief current injections to the soma, recorded simultaneously in an axonal bleb (red trace) and in the soma (black trace). C, the same AP as in B was plotted at an expanded time scale. D and E, same cell as in B, but the current was injected into the axon. F and G, summary bar graphs comparing the AP amplitudes (F) and half-durations (G) in the soma and axon. H, summary bar graph showing the AP latencies between axon and soma.

Figure 2. Robust axonal AP initiation following synaptic stimulation

A, the traces show an AP evoked by low-intensity extracellular synaptic stimulation of the lateral perforant path (pp). B, bar graph showing AP latencies between axon and soma when currents were injected into the soma or after extracellular synaptic stimulation of the perforant path (n = 5; P > 0.5). C, the traces show an AP evoked by high-intensity extracellular synaptic stimulation. D, bar graph showing AP latencies between axon and soma with low- or high-intensity synaptic stimulation (n = 4).

Figure 4. Simulation of AP initiation and propagation in a detailed compartmental model of a granule cell

A, shape plot of a granule cell (cell 7 from Schmidt-Hieber et al. 2007) generated with NEURON. B, the bar graph shows the maximal rate of rise and the maximal rate of decay of APs. Experimental data (white bars) were taken from recordings at a distance of 20–30 μm from soma (mean distance from soma: 24 ± 1 μm; max. slope during rise: 297 ± 12 V s⁻¹ (soma), 485 ± 49 V s⁻¹ (axon); max. slope during decay: 70 ± 2 V s⁻¹ (soma), 59 ± 2 V s⁻¹ (axon)). Models were obtained using either non-uniform peak Na⁺ conductance densities () in the somatodendritic and axonal domains (black bars) or a uniformly distributed (grey bars). C, traces show a simulated AP recorded at the soma (black trace) and in the axon (red trace) at 24 μm distance from the soma. D, simulated AP latencies were plotted against distance from soma. The most negative latencies were observed at ∼28 μm distance from soma. In addition to an intact axon (continuous curve), results from simulations using axons with variable lengths and a bleb (diameter: 2.5 μm; length: 3 μm) at the end are shown as dashed curves. The dotted curve connects the end points of these lines. Grey points represent measured data. E, traces show simulated APs evoked by excitatory synaptic conductance changes (6 synapses distributed along a dendritic path; see inset). was either non-uniformly (left traces) or uniformly (right traces) distributed in the somatodendritic and axonal domains.