Thursday, January 26, 2012

The "Human Neuron", not so special after all?

Von Economo neurons, a set of neurons classified by their elongated, 'spindle-like' shape, were once thought to belong only to humans and great apes. This uniqueness, as you might imagine, encouraged extensive speculation about what this neuron does.  Do they make you smart? Do they process emotions? social cues? future planning?

Not that extensive speculation is a bad thing, it's just that it is easy to jump into the deep end and assume that because something is unique to humans, it is what makes humans unique.

You may have guessed where this is going.  It turns out that this type of cell is not unique to humans and great apes.  It is found in whales, hippos, zebras, manatees, and elephants.

Figure 8. Butti et al., 2011

This doesn't mean that the cells don't process emotions or social cues, it just means that they are not unique to humans.

There are studies showing that Von Economo neurons are reduced in post-mortem human brains afflicted with certain diseases, such as schizophrenia and autism (reviewed in Butti et al., 2011)

But according to Butti et al.
"It is important to note that, despite the previously discussed hypotheses on the possible functional role and implication of VENs in neuropsychiatric disorders, direct evidence has yet to be found."
These cells are obviously difficult to study because no 'research animal' has them.  As far as I am aware, no one knows where they project in the brain or what inputs feed onto them.  And I don't think anyone has been able to investigate their spiking patterns. 

The real question that these cells bring up for me is 'what does it mean to be a class of cell'? why does it matter that these cells have elongated spindle-like bodies, when they are chemically similar to some pyramidal cells in the mouse brain. Butti et al. point out that VENs have specific peptide expression of GRP and NMB, and that pyramidal cells in the mouse cortex also express  GRP and NMB.

If we define type of neuron by chemical signature, then these are not necessarily  unique cells. It is only their specific shape which is special and even that is common to many large animals. Maybe this type of neuron is expressed in all mammals, but in the large ones it needs to have a large spindle shape to work more efficiently. 
source Butti C, Santos M, Uppal N, & Hof PR (2011). Von Economo neurons: Clinical and evolutionary perspectives. Cortex; a journal devoted to the study of the nervous system and behavior PMID: 22130090

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