Structure-Function Relationship of Dendritic Spines: Is the Striatum an Exception?
The idea that neuronal structure predicts function goes back to Santiago Ramón y Cajal, the father of modern neuroscience. Cajal observed that neuronal dendrites are covered with little protrusions, termed "spines". He believed that these were the sites where neurons connect. And it is indeed nowadays known that dendritic spines host "synapses", the functional connections between neurons.
Dendritic spines are very plastic, despite their rigid appearance. Long-term potentiation (LTP) is the activity-dependent strengthening of a synapse and widely believed to be the basis of learning and memory. Experimentally inducing LTP in a single dendritic spine leads to its concomitant enlargement. Spine morphology has therefore been subsequently used for classification and functional interpretation. Common classifications are "mushroom", "stubby", and "thin" spine, among which "mushroom" often corresponds to a "mature" and "thin" to an "immature" synapse.
Most experiments leading to the formulation of this "structure-functional relationship" were done on pyramidal cells (PCs). While being the main neuron in hippocampus and cortex (both closely associated with memory and learning), these neurons are, however, only one of many types in the brain.
My research focuses on the striatal spiny projection neuron (SPN), whose main characteristic is the high density and number of dendritic spines. I observed that the striatum often does not play by the same rules as cortex and hippocampus. For example, SPN spines appear more static, yet a large percentage would classify as "thin", hence "immature". Furthermore, spines of SPNs show no obvious correlation between size and synaptic strength. This raised the question whether the "structure-functional relationship" really applies to all neurons, and the SPNs in particular. Dendritic spines clearly come in different forms and sizes. But what do morphological differences mean, if they do not mirror synaptic strength?
Fieblinger, T. and Cenci, M. A. (2015). "Zooming in on the small: the plasticity of striatal dendritic spines in L-DOPA-induced dyskinesia." Mov. Disord. 30: 484-493.
Fieblinger, T., Graves, S. M., Sebel, L. E., Alcacer, C., Plotkin, J. L., Gertler, T. S., Chan, C. S., Heiman, M., Greengard, P., Cenci, M. A., and Surmeier, D. J. (2014). "Cell type-specific plasticity of striatal projection neurons in parkinsonism and L-DOPA-induced dyskinesia." Nat. Commun. 5: 5316.