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This Article Full Text Full Text (PDF) Supplemental Data All Versions of this Article: 586/7/1849    most recent jphysiol.2007.150151v1 Services Email this article to a friend Similar articles in this journal Similar articles in PubMed Alert me to new issues of the journal Download to citation manager Citing Articles Citing Articles via HighWire Google Scholar Articles by Schmidt-Hieber, C. Articles by Bischofberger, J. PubMed PubMed Citation Articles by Schmidt-Hieber, C. Articles by Bischofberger, J. Related Collections Neuroscience

¹ Physiological Institute 1, University of Freiburg, Hermann-Herder-Strasse 7, D-79104 Freiburg, Germany

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 23°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 20–30 µm 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 34°C. Modelling of AP propagation in detailed cable models of granule cells suggested that a 4 times higher Na⁺ channel density (1000 pS µm^–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.

(Received 18 December 2007; accepted after revision 5 February 2008; first published online 7 February 2008)
Corresponding author J. Bischofberger: Physiologisches Institut, Universität Freiburg, Hermann-Herder-Str. 7, D-79104 Freiburg, Germany.  Email: josef.bischofberger{at}uni-freiburg.de

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