Well, this announcement is interesting: A giant neuron found wrapped around entire mouse brain:
A new digital reconstruction method shows three neurons that branch extensively throughout the brain, including one that wraps around its entire outer layer. The finding may help to explain how the brain creates consciousness.
The neurons are found in a section of the brain called the claustrum, which is buried under the neocortex. It hasn’t been studied very much, for several practical reasons, but it’s been the focus of intense interest by a couple really important people.
First the problems with studying it. As Psychology Today blogger Joel Frohlich explains:
the strange anatomy of the claustrum means it cannot easily be lesioned without also damaging many neighboring brain structures. For this reason, doctors have yet to meet a patient whose claustral lesion or injury can be clearly associated with a behavioral or cognitive deficit.
This is significant because a lot of what we know about the brain’s function has been discovered through studies of people with brain damage. However, there are a few studies that are pretty suggestive. In particular, one 2015 article examines
the effects of claustrum lesions on consciousness in 171 combat veterans with penetrating traumatic brain injuries. Additionally, we studied the effects of claustrum lesions and loss of consciousness on long-term cognitive abilities. Claustrum damage was associated with the duration, but not frequency, of loss of consciousness, indicating that the claustrum may have an important role in regaining, but not maintaining, consciousness.
It’s also the case that
scientists cannot easily lesion the claustrum of an animal without damaging other areas of the brain. As a result, no experiment to date has clearly established a functional role for the brain’s most curious piece of anatomy.
There’s also the problem of tracing the extent of a neuron’s connections, which when “using conventional methods is a massive task. Researchers inject individual cells with a dye, slice the brain into thin sections and then trace the dyed neuron’s path by hand. Very few have been able to trace a neuron through the entire organ.”
Further, as Christof Koch explains in a 2014 Scientific American article, conventional brain scanning tools literally can’t see it:
the smallest spatial features distinguishable through positron-emission tomography or functional MRI, two of the most widely used imaging techniques, are two to three millimeters across, bigger than the claustrum’s width.
So why’s it worth studying?
Koch and his former collaborator Francis Crick (who won a Nobel Prize for his work on DNA) has been interested in the claustrum for some time: they published an article about it in 2005. (It was Francis Crick’s final paper, in fact.) At the time they noted that “Its anatomy is quite remarkable in that it receives input from almost all regions of cortex and projects back to almost all regions of cortex.” They went on to make a case for “its possible relationship to the processes that give rise to integrated conscious percepts”– in other words, it helps the brain coordinate the different activities that give rise to consciousness. As one of their colleagues put it,
Crick and Koch liken the claustrum to the conductor of an orchestra, who is responsible for binding the performances by individual musicians into an integrated whole that can be much more than the sum of the parts. The neuroanatomical connections of the claustrum, then, just match with the ‘conductor’ required to bind together the various disparate components of the conscious experience represented in many different brain regions.
For this study,
Koch and his colleagues engineered a line of mice so that a certain drug activated specific genes in claustrum neurons. When the researchers fed the mice a small amount of the drug, only a handful of neurons received enough of it to switch on these genes. That resulted in production of a green fluorescent protein that spread throughout the entire neuron. The team then took 10,000 cross-sectional images of the mouse brain and used a computer program to create a 3D reconstruction of just three glowing cells.
Koch has argued that the claustrum is providing the coordinating functions between different brain regions that are one necessary component of consciousness. This latest study suggests that the claustrum’s reach is indeed very wide: not just around the brain and different functional areas, but through the body as well (which is where the other two neurons extend).
This doesn’t prove his case, but it does provide some physical evidence that the claustrum could be performing an important role in consciousness.