close
Skip to main page content
U.S. flag

An official website of the United States government

Dot gov

The .gov means it’s official.
Federal government websites often end in .gov or .mil. Before sharing sensitive information, make sure you’re on a federal government site.

Https

The site is secure.
The https:// ensures that you are connecting to the official website and that any information you provide is encrypted and transmitted securely.

Access keys NCBI Homepage MyNCBI Homepage Main Content Main Navigation
. 2010 Jul 29;5(7):e11868.
doi: 10.1371/journal.pone.0011868.

Axons amplify somatic incomplete spikes into uniform amplitudes in mouse cortical pyramidal neurons

Na Chen  1 , Jiandong Yu, Hao Qian, Rongjing Ge, Jin-Hui Wang
Affiliations

Axons amplify somatic incomplete spikes into uniform amplitudes in mouse cortical pyramidal neurons

Na Chen et al. PLoS One. .

Abstract

Background: Action potentials are the essential unit of neuronal encoding. Somatic sequential spikes in the central nervous system appear various in amplitudes. To be effective neuronal codes, these spikes should be propagated to axonal terminals where they activate the synapses and drive postsynaptic neurons. It remains unclear whether these effective neuronal codes are based on spike timing orders and/or amplitudes.

Methodology/principal findings: We investigated this fundamental issue by simultaneously recording the axon versus soma of identical neurons and presynaptic vs. postsynaptic neurons in the cortical slices. The axons enable somatic spikes in low amplitude be enlarged, which activate synaptic transmission in consistent patterns. This facilitation in the propagation of sequential spikes through the axons is mechanistically founded by the short refractory periods, large currents and high opening probability of axonal voltage-gated sodium channels.

Conclusion/significance: An amplification of somatic incomplete spikes into axonal complete ones makes sequential spikes to activate consistent synaptic transmission. Therefore, neuronal encoding is likely based on spike timing order, instead of graded analogues.

PubMed Disclaimer

Conflict of interest statement

Competing Interests: The authors have declared that no competing interests exist.

Figures

Figure 1
Figure 1. Sequential action potentials vary in their amplitudes.
Intracellular recording of spikes was done from mice in vivo (A). B) Sequential spikes evoked by current pulses decrease in amplitudes when the intensity of input currents are raised from 0.25 nA (blue traces) to 0.55 nA (red traces). C) Spontaneous sequential spikes decrease progressively in amplitudes. D) shows the comparison of sequential spikes evoked by current pulses 0.25 nA (blue bars) and 0.55 nA (red bars). E) shows the distribution in the amplitudes of spontaneous spikes in (A).
Figure 2
Figure 2. Sequential spikes are initiated at the soma when input intensity increases.
In A–C, whole-cell recoding was done at the soma and axon of identical neurons when the paired-current pulses were injected into the soma (blue traces in A–B). A–B) show the comparisons in the time of spike initiation (TSI) at the soma (blue traces) and axon (reds), when the intensities of second pulse were low (A) and high (B). TSI for the soma appears moving in front of the axon when stimulus intensity is raised. C) shows the difference of TSI at the soma and axon versus the normalized stimulus intensity, in which the negative values of TSIS-TSIA indicate somatic spikes ahead of axonal ones. D–E) computation-simulated sequential spikes at the soma (blue traces) and axon (reds) when stimulus intensities are 0.2 and 0.8 nA, respectively, in which the strong stimuli switch somatic spikes with decreased amplitudes being ahead of axonal ones. F) illustrates the quantitative data of TSIS-TSIA for simulated spikes 1–4 under stimulus intensities at 0.2 (white bars), 0.5 (light grays) and 0.8 nA (dark grays), in which the negative values of TSIS-TSIA indicate somatic spikes ahead of axonal ones.
Figure 3
Figure 3. The amplitudes of presynaptic spikes do not influence uEPSC amplitude, probability and patterns.
The pair-recording was conducted on presynaptic pyramidal neurons and postsynaptic GABAergic cells. A–C) uEPSCs (top traces) are induced by two spikes in a presynaptic neuron (bottom traces). The first spike is normal, and the subsequent spikes are the smallest spikes at ARP (A), incomplete spikes during RRP (B) and complete spikes after RRP (C). Dark green traces show uEPSC two induced by spike two; light blue traces in (A) show only uEPSC-1 induced by spike one. D–F) Three plots show relationships between the second spike amplitudes in presynaptic neurons versus uEPSC-2 amplitudes (D), probability (E) and uEPSC2-uEPSC1 (F), respectively, from the experiment in A–C. G–I) The averaged data (n = 7) show that the second spike amplitudes in presynaptic neurons do not affect uEPSC-2 amplitude (G), probability (H) and patterns (I). Pair-pulses below panels A–C show the injected currents inducing two presynaptic spikes.
Figure 4
Figure 4. QX-314 lowers spike amplitudes in presynaptic axons and uEPSCs.
QX-314 (0.5 mM) was infused into presynaptic neurons through the recording pipettes, when pair-recordings were conducted on presynaptic pyramidal neurons and postsynaptic GABAergic cells. A–B) uEPSCs (top traces) are induced by two spikes in presynaptic cell (bottom traces) before (A) and after (B) QX-314 infusions. The amplitudes of both spikes and uEPSCs are reduced after infusing QX-314. C–E) Statistical analyses from this experiment show relationships between spike amplitudes and uEPSC amplitudes (C), probability (D) or uEPSC2-uEPSC1 (E). Presynaptic QX-314 infusion decreases spike amplitudes and uEPSCs in a parallel manner. F–H) The presynaptic infusions of QX-314 lower spike amplitudes as well as uEPSC amplitude (F) and probability (G) in a linear correlation manner (r2 values are in the range of 0.73–0.91, p<0.01, n = 13), except for the patterns (H). Black symbols denote spike one versus uEPSC one, and open symbols are spike two versus uEPSC two.
Figure 5
Figure 5. The axons convert somatic sequential spikes to large and constant levels.
A–B) Whole-cell recordings were conducted at the soma and axonal bleb on the same neurons simultaneously, in which somatic and axonal spikes were induced by somatic current pulses (3 ms). The delay and intensity of the second pulse were adjusted to induce the second somatic spikes in different amplitudes just after ARP. Except for no spike two (blue trace), somatic spike amplitudes vary referred to axonal spikes. C) illustrates the ratios of the second spikes to the first ones in their amplitudes (Action Potential-2/-1) that are soma-evoked (*) and axon-corresponded for the sample in A–B. D) shows the ratios of the second spikes in the smallest amplitude of soma-evoked spikes (*) to the first spikes versus in the axon-corresponded ones (Spike-2 in the smallest amplitude is divided by Spike-1), 0.33±0.05 for soma-evoked spikes and 0.54±0.02 for axonal ones (n = 11, p<0.001), indicating that the axon amplifies somatic spikes. E) shows the standard deviation of the second spikes in amplitude that are soma-evoked (*) and axon-corresponded, i.e., 6.55±1.31 for soma-evoked spikes and 1.95±0.59 for axonal ones (n = 11, p = 0.007), indicating that the axon makes somatic spikes uniform. F) a diagram shows simultaneous recoding in axonal bleb and soma of a single neuron. Asterisk (*) denotes the loci of evoking action potentials.
Figure 6
Figure 6. The refractory periods of sequential spikes are shorter at the axons than the soma.
Whole-cell recording was conducted at the soma and axonal bleb on the same neurons simultaneously. A–B) A sample of soma-axon pairs shows the measurement of the refractory periods of action potentials evoked at axon and soma, respectively, in which current pulses are 3 ms. Dash lines indicate shorter ARP at the axon (A) than the soma (B). C) shows axonal ARP is 6.86 ms and somatic ARP is 8.14 ms in this example. D) Quantitative data show shorter axonal ARP (7.99±0.21 ms) than somatic ARP (8.7±0.26 ms, filled symbols, n = 13, p<0.05) averaged from individual pairs (open symbols). Asterisk (*) denotes the loci of evoking action potentials.
Figure 7
Figure 7. The comparison of refractory period, current amplitude and open probability of voltage-gated sodium channels (VGSC) at the axon vs. soma.
Pair-recordings in cell-attached model were conducted at axonal bleb and soma on the same neurons. Two depolarization pulses (5 ms) at thresholds for VGSC activation are given, and the delay of pulse two is adjusted to measure absolute refractory period (ARP) for VGSC reactivation. A) Single VGSCs reopen 4.5 ms after ARP at the axon (red line), and 7 ms after ARP at the soma (blue). B) shows that the averaged ARP for axonal VGSCs and somatic ones are 5.45±0.2 ms and 8.45±0.54 ms, respectively (n = 11, p<0.01). VGSCs also were activated by 60 ms depolarization pulse. C) shows the currents of single VGSCs recorded on the soma (left) versus axonal bleb (right). D) illustrates the averaged amplitudes of VGSC currents on the soma (left, 1.35 pA) and axonal bleb (right, 7.3 pA) E) shows the distributions in the amplitudes of VGSC currents recorded on the soma (blue bars) and axonal bleb (reds). F) shows the opening probability of VGSCs recorded on the soma (left, 0.055±0.015) and axonal bleb (right, 0.127±0.013; p<0.001).
Figure 8
Figure 8. The axons function as spike initiation at AIG and propagation by local circuit currents (established theories in left panels) as well as spikes' amplification (our new finding in right panels).
The properties of voltage-gated sodium channels (VGSC) at the axons include shorter absolute refractory period (ARP), higher opening probability (Po) and bigger currents (i), compared with somatic ones. These properties plus local circuit current constitute a comprehensive view for the amplification and propagation of sequential spikes securely at the axons. AIG is an abbreviation for axonal initial segment.
See this image and copyright information in PMC

References

    1. Chen N, Chen SL, Wu YL, Wang JH. The refractory periods and threshold potentials of sequential spikes measured by whole-cell recordings. Biochemical and Biophysical Research Communications. 2006;340:151–157. - PubMed
    1. Shu YS, Hasenstaub A, Duque A, Yu YG, McCormick DA. Modulation of intracortical synaptic potentials by presynaptic somatic membrane potential. Nature. 2006;441:761–765. - PubMed
    1. Alle H, Geiger JRP. Combined analog and action potential coding in hippocampal mossy fibers. Science. 2006;311:1290–1293. - PubMed
    1. Chen N, Zhu Y, Gao X, Guan S, Wang J-H. Sodium channel-mediated intrinsic mechanisms underlying the differences of spike programming among GABAergic neurons. Biochemical and Biophysical Research Communications. 2006;346:281–287. - PubMed
    1. Rieke F, Warland D, De Ruyter van Steveninck R, Bialek W. Spikes: Exploring the neural codes. In: Rieke F, editor. Cambridge, , MA: MIT; 1998. pp. 1–395.

Publication types

LinkOut - more resources