Max Planck Institute for Dynamics and Self-Organization -- Department for Nonlinear Dynamics and Network Dynamics Group
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The encoding bandwidth of the cortical gateway

Somatosensory information enters the primary sensory cortex through thalamic axons that innervate spiny stellate (SpSt) cells in cortical layer 4. We showed earlier [1], that the population of layer 2/3 pyramidal cells (Pyr), the main target of SpSt cells output, can encode input changes in less than a millisecond. Their large dendritic tree might significantly contribute to this ability [2]. It is intuitively clear that the entry site for thalamic information, i.e. SpSt cells, should have an encoding bandwidth at least as wide as those of later processing stages, i.e. layer 2/3 and layer 5 Pyr cells. However, SpSt cells feature the smallest dendritic compartment of all excitatory cortical neurons, which leads to the expectation that they have a severly limited bandwidth. This discrepancy between a required wide bandwidth at the entry level and an expected severely limited bandwidth prompted us to explore whether SpSt cells employ compensatory mechanisms to retain a high bandwidth of information encoding in the absence of large dendrites.

We find that SpSt cells employ two compensatory mechanisms to obtain a superior bandwidth for information encoding. Firstly, the sub-threshold synaptic input fluctuates much slower in SpSt cells1 than in Pyr cells and that leads to an enhanced encoding (Figure 2 B, C) . This is the first report of such a mechanism employed in neural encoding. Secondly, the detrimental effects of small dendritic tree are reduced by a reduction in soma size.

With a combination of experimental and theoretical approaches, we revealed how different biophysical principles are at play to allow for ultra-fast processing of somato-sensory input during cortical processing.

PIC

Figure 1: Spiny stellate cells are the smallest excitatory cells in the cortex

 

PIC

 

Figure 2: A SpSt cells’ encoding bandwidth is only marginally smaller than those of Pyr cells. B Synaptic currents change much slower in SpSt cells. C SpSt population rate rapidly encodes input changes embedded in fluctuations. An ideal observer was faster to detect a step in the input (not shown) when the background fluctuations had a longer correlation time (blue).

1. Due to the presence of an unconventional NMDA receptor, operating already at subthreshold voltages [3]

 

 

[1]    T. Tchumatchenko, F. Wolf et al. J. Neurosci. 31, 34 12171 (2011)

[2]    G. Eyal, et al. J. Neurosci. 34, 24 8063(2014)

[3]    I.A. Fleidervish A.M. Binshtok, M.J. Gutnick Neuron 21 1055 (1998)


Members working within this Project:

 Andreas Neef 
 Fred Wolf