Action potential generation in an anatomically constrained model of medial superior olive axons.
Neurons in the medial superior olive (MSO) encode interaural time differences (ITDs) with sustained firing rates of >100 Hz. They are able to generate such high firing rates for several hundred milliseconds despite their extremely low-input resistances of only few megaohms and high synaptic conductances in vivo. The biophysical mechanisms by which these leaky neurons maintain their excitability are not understood. Since action potentials (APs) are usually assumed to be generated in the axon initial segment (AIS), we analyzed anatomical data of proximal MSO axons in Mongolian gerbils and found that the axon diameter is <1 μm and the internode length is ∼100 μm. Using a morphologically constrained computational model of the MSO axon, we show that these thin axons facilitate the excitability of the AIS. However, for ongoing high rates of synaptic inputs the model generates a substantial fraction of APs in its nodes of Ranvier. These distally initiated APs are mediated by a spatial gradient of sodium channel inactivation and a strong somatic current sink. The model also predicts that distal AP initiation increases the dynamic range of the rate code for ITDs.
Department Biology II, Graduate School of Systemic Neurosciences, and Department Biology I, BioImaging Zentrum, Ludwig-Maximilians-Universität München, D-82152 Planegg-Martinsried, Germany.
Simon
S
Lehnert
Marc C
MC
Ford
Olga
O
Alexandrova
Franziska
F
Hellmundt
Felix
F
Felmy
Benedikt
B
Grothe
Christian
C
Leibold
D000200:Action Potentials; D000818:Animals; D001369:Axons / Q000378:metabolism / Q000502:physiology* / Q000648:ultrastructure; D001933:Brain Stem / Q000166:cytology / Q000502:physiology; D005849:Gerbillinae; D008959:Models, Neurological; D011901:Ranvier's Nodes / Q000502:physiology; D015222:Sodium Channels / Q000378:metabolism; D013569:Synapses / Q000502:physiology
D015222:Sodium Channels
action potential; axon; coincidence detection; interaural time difference; sound localization