Monday, February 13, 2012

Neurons tuned like the strings of a harp

The auditory brainstem of the boring-old-chicken is actually home to some fascinating neurons.

Key West rooster, taken by me.

The Nucleus Laminaris (NL) is a group of coincidence-detecting neurons which receive indirect input from both ears and is located in the bird auditory brainstem.

NL neurons show a peculiar dendrite pattern.  These bipolar neurons fall into the particular category of football shaped cells which have dendrites coming out the top and bottom of their cell body. The cell body (soma) of these neurons are about the same size, but depending on where they are in the NL, the cells have either short, medium or long dendrites. 

The ones near the midline have a bunch of short stubby little dendrites.
Figure 2B from Smith and Rubel, 1979

If they are a little further out from the midline, they have longer dendrites.
Figure 3B from Smith and Rubel, 1979
and finally if they are furthest from the middle, they have fewer and much longer dendrites.
Figure 10A Smith and Rubel 1979
all together this makes a gradient from short to long dendrites.  
From Figure6 Smith and Rubel 1979
The big question here is "Why?"

What is the purpose of having stubby or extended dendrites like this?  Well, even in 1979 when Smith and Rubel reconstructed these neurons, they knew that these neurons had a special answer to the "form and function" question.

The amazing thing about these neurons is that they are 'tuned' to respond maximally to specific frequencies (sound waves).  And just like strings on an instrument, the cells with shorter dendrites respond to higher frequencies and the cells with longer dendrites respond to lower frequencies. 

Why is this? Dendrites don't actually vibrate like strings, but there must be some reason for a cell with short dendrites to respond to higher frquencies and a cell with long dendrites to respond to low frequencies. 

The answer lies in what the Nucleus Laminaris actually does. In the next post we'll venture into the wilds of computational neuroscience and explore the reason behind this strange connection between dendrite shape and cell function. 

ResearchBlogging.orgSmith DJ, & Rubel EW (1979). Organization and development of brain stem auditory nuclei of the chicken: dendritic gradients in nucleus laminaris. The Journal of comparative neurology, 186 (2), 213-39 PMID: 447882

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