It was only about 20 years ago when the neuroscience community discovered that dendrites can possess active channels. At that time Dr. Terrence Sejnowski’s team sought to address an apparent paradox seen in recordings from neocortical pyramidal cells. It was clear that these neurons possessed voltage dependent sodium channels on their dendrites that promoted and sustained the propagation of action potentials but they rarely enabled the origin of spikes in the dendrites. December 1995 they published the paper “A Model of Spike Initiation in Neocortical Pyramidal Neurons” in Neuron that described a model pyramidal neuron that took these findings into account and suggested mechanisms underlying the observations.

Figure 1. Recordings from the soma (black), axon initial segment (purple), and dendrite (green) of the electronically reconstructed layer 5 pyramidal neuron (at right) during a current pulse applied to the neuron’s main dendritic apical trunk 416 microns from the soma. Run the demo.hoc file in NEURON to reconstruct the associated paper’s Figure 3.

Note: The pyramidal cell model associated with this paper runs in NEURON and is available from the SenseLab ModelDB repository in the Spike Initiation in Neocortical Pyramidal Neurons (Mainen et al 1995) record. This was one of the models used in the research described in the paper “Sensitivity to perturbations in vivo implies high noise and suggests rate coding in cortex” (published July 1, 2010 in Nature) discussed in yesterday’s blog post “Coding in the Brain, Paper Bloat, and the Need to Change the Way Papers are Published“.

Sejnowski’s team found that the axon initial segment was critical for spike initiation. A high density of sodium channels in the initial segment provided a very large source current that even supplied a large fraction of the depolarizing current observed in the soma during an action potential. Regardless of the site of stimulation, spikes were initiated at the axon initial segment and subsequently invaded the soma and dendrites.

Note: The cerebral cortex model by Mainen and Sejnowski (1996) extended the standard model described here. For information on their 1996 model please see my blog post “Brain Modeling Using NEURON: Superficial Pyramidal, Deep Pyramidal, Aspiny, and Stellate Neurons.”


Other related blog posts:

Brain Modeling Using NEURON: Superficial Pyramidal, Deep Pyramidal, Aspiny, and Stellate Neurons

Dendritic Signal Processing Simulated Using NEURON

Coding in the Brain, Paper Bloat, and the Need to Change the Way Papers are Published