216 — A single spectrum of neuronal identities across thalamus

The thalamus acts as the first stop in the brain for sensory signals, projecting its own neurites outward into cortex. Though we have known for a long time that the thalamus performs this relaying function, it wasn’t well-understood what the thalamus did beyond act as a switchboard (which is such a delightfully antiquated term itself that it shows you the last time the ‘canonical models’ were defined).

To shed light on this question, Phillips & Schulmann et al generated a “near-comprehensive” genetic transcriptome of thalamic neurons. This transcriptome revealed three families of thalamic neuron profiles, “primary,” “secondary,” and “tertiary,” with each profile occupying its own region of thalamus along the ear-to-ear axis (with the intermediate cells exhibiting signs of both profiles).

This differentiation is not solely the result of different thalamic mechanisms for different sensory modalities: That is, the authors controlled for the cortical targets of the thalamic neurons and still discovered genetic differences between the profiles.

The primary neuron profile was most commonly associated with fast firing rates (coding for channels with high-speed kinetics). From this, it became clear that action potential width widened as you progress from primary to tertiary.

Each of these profiles project to a particular region or set of regions of cortex, suggesting that thalamus is acting more like a first layer of filtering than a relay. This makes sense based upon the action potential width measure alone, because this would provide the equivalent of a high-pass, low-pass, and the combination of both with regard to the Nyquist frequency of a stimulus.