High-amplitude positive spikes recorded extracellularly in cat visual cortex

Area of Science:

• Neuroscience
• Computational Neuroscience
• Biophysics

Background:

• Extracellular recordings capture neuronal electrical activity, known as action potentials (APs) or spikes.
• Understanding the biophysical basis of APs is crucial for interpreting neural recordings.

Purpose of the Study:

• To simulate the shape and amplitude of extracellular action potentials (APs) using biophysical models of cat visual cortex neurons.
• To compare simulation predictions with experimentally recorded APs from the cat's primary visual cortex.

Main Methods:

• Detailed reconstructions of single neurons from the cat's visual cortex were used to build biophysical models.
• Simulated APs were compared with extracellular spikes recorded from numerous neurons across all cortical layers.

Main Results:

• The majority of recorded spikes were biphasic with a dominant negative peak (-0.11 mV mean amplitude).
• A minority of APs exhibited a dominant positive peak (+0.54 mV mean amplitude), with the largest recorded in layer 5.
• Simulations indicated pyramidal neurons could generate negative peaks up to -1.5 mV but positive peaks rarely exceeded 0.5 mV.
• Juxtacellular patch recordings did not produce spikes with large positive peaks.

Conclusions:

• A significant discrepancy exists between simulated and recorded extracellular action potentials, particularly concerning large positive peaks.
• This gap suggests incomplete understanding of either the spike-generation process in pyramidal neurons or the biophysics of extracellular recording, or both.