Red-eared turtles

For the first time since a short lecture series on motor control in my second year of undergraduate (and a few throwaway comments by machine learning enthusiasts on biological substrates for supervised learning), my attention is turning to the cerebellum. For the next four months I'll be modelling tonic inhibition in cerebellar granule cells, but I felt that since I knew so little about the cerebellum as whole I should cast a broad review of general anatomy, network structure, functional roles, paradigms, etc. 

One of the first things that struck me was the high degree of conservation across species, where practically every modern vertebrate has some form of cerebellum. It appears to differentiate in quite an early embryonic stage from the spinal cord, making it perhaps the oldest distinct brain structure that we still have today. Whatever it is, it is certainly a bit heavier than a few nematode ganglia emerging from some barely recognisable proto-spine.

Nevertheless, this high degree of conservation has meant there are a wide array of species that can be used as model organisms for the cerebellum. It was the red-eared turtle that first took my attention though, since a highly influential model of cerebellar granule cells by Grabbiani et al. (1994) comprises a mixture of ion channels whose properties were experimentally derived from the aforementioned turtle species, and from rats. It seemed odd that a model would be based off of multiple species. Little did I know that this was barely the half of it. Ultimately, I found other models to be based off of a mess of data collected from rats, guinea pigs, turtles, frogs, and goldfish, being applied to data gathered from knockout strains of mice. Our basic understanding of the anatomy even comes from Ramon y Cajal's work with pigeons. I have no idea how I would justify this to an even remotely skeptical biologist.

Anyway, this piqued my curiosity and I wanted to look further into why in particular the red-eared turtle had been used as a model organism. Apparently, it was for four reasons:

1. Conservation of anatomy (as already discussed)

2. The shell could easily be clamped to afford high mechanical stability

3. Resilience to hypoxia

4. Lissencephalic brain

The resilience to hypoxia is rather remarkable. At low temperatures, the red-eared turtle can survive for weeks without oxygen, and even at high temperatures can survive for a number of hours. This extremely efficient anaerobic metabolism and protection from low oxygen conditions means the brain would be very well preserved and rather stable after dissection and slicing, making electrophysiological experiments far easier.

Lissencephaly, or 'smooth brain', simplified the anatomy of the cerebellum. In mammals, the cerebellum is highly folded and so localising implanted electrodes for in vivo study was difficult (especially in the 1970s, when it seemed that much of this work with turtles was carried out). This allowed greater confidence in the reliability of experiments being performed, since the researcher could provide more assurance that their recordings were empirically valid.

As such, the red-eared turtle was for a time a highly favoured biological model of the cerebellum. This seems to have died away in the last 20 years or so, especially since the rise of genetically modifiable mouse strains. Even so, I'm still astounded at how such an unrelated species could provide such insight into a whole brain structure. It also begs the question at why we still know so little about the cerebellum, when we have such a diverse set of models to work with.