Axonal Na+ channels ensure fast spike activation and back-propagation in cerebellar granule cells.
In most neurons, Na+ channels in the axon are complemented by others localized in the soma and dendrites to ensure spike back-propagation. However, cerebellar granule cells are neurons with simplified architecture in which the dendrites are short and unbranched and a single thin ascending axon travels toward the molecular layer before bifurcating into parallel fibers. Here we show that in cerebellar granule cells, Na+ channels are enriched in the axon, especially in the hillock, but almost absent from soma and dendrites. The impact of this channel distribution on neuronal electroresponsiveness was investigated by multi-compartmental modeling. Numerical simulations indicated that granule cells have a compact electrotonic structure allowing excitatory postsynaptic potentials to diffuse with little attenuation from dendrites to axon. The spike arose almost simultaneously along the whole axonal ascending branch and invaded the hillock the activation of which promoted spike back-propagation with marginal delay (<200 micros) and attenuation (<20 mV) into the somato-dendritic compartment. These properties allow granule cells to perform sub-millisecond coincidence detection of pre- and postsynaptic activity and to rapidly activate Purkinje cells contacted by the axonal ascending branch.
Diwakar
Shyam
S
Department of Physiological and Pharmacological Sciences, Via Forlanini 6, University of Pavia, I-27100 Pavia, Italy.
Magistretti
Jacopo
J
Goldfarb
Mitchell
M
Naldi
Giovanni
G
D'Angelo
Egidio
E
D000200:Action Potentials / Q000502:physiology*; D000818:Animals; D000831:Animals, Newborn; D001369:Axons / Q000502:physiology*; D055592:Biophysical Phenomena; D002531:Cerebellum / Q000166:cytology*; D025802:Electric Capacitance; D004558:Electric Stimulation / Q000379:methods; D018690:Excitatory Amino Acid Agonists / Q000494:pharmacology; D066298:In Vitro Techniques; D015640:Ion Channel Gating / Q000187:drug effects / Q000502:physiology; D008564:Membrane Potentials / Q000502:physiology; D051379:Mice; D008959:Models, Neurological; D016202:N-Methylaspartate / Q000494:pharmacology; D009474:Neurons / Q000166:cytology* / Q000502:physiology; D018408:Patch-Clamp Techniques; D015222:Sodium Channels / Q000502:physiology*; D009435:Synaptic Transmission / Q000187:drug effects / Q000502:physiology
D018690:Excitatory Amino Acid Agonists; D015222:Sodium Channels; D016202:N-Methylaspartate