A dendritic spine is a small membranous protrusion on a neuron that receives input from a synapse of another neuron. A spine is typically located on peripheral dendrites, but can also be located on proximal dendrites or even the cell body.
Synapses on a post-synaptic spine are usually from a pre-synaptic axon, but dendrite-to-dendrite synapses also exist. Most synapses on dendrites are excitatory, but again, there are exceptions1. Morphologically, most spines are shorter than 2 micrometers in length and are comprised of a bulbous 'head' and a thinner 'neck' that connects the head of the spine to the shaft of the dendrite.
Dendritic spines are highly dynamic structures. Long-term potentiation, thought to be cellular a representation of learning and memory, can induce the formation of spines2,3. Spine formation can be very rapid and spatially discrete.
Recent evidence using precise application of glutamate on a dendrite has shown that spines can form within seconds of application and within 1 micrometer of the applied area4. Thus, dendritic spines show high electrochemical compartmentalization, which allows for input-specific plasticity.
Relevance to autism:
Autism has been hypothesized as a disorder of connectivity, and dendritic spines and associated synaptic transmission seem to be critical components of this dysregulation.
Typical synaptic development is not linear; rather there is an overgrowth of synapses early in development, followed by a pruning of many synapses6. This pruning is thought to improve the signal-to-noise ratio of information transferred within neuronal circuits.
Tissue from people with autism and fragile X syndrome seem to have unusually high spines densities, presumably due to a failure of proper synaptic pruning. It may be that this deficit in the brain's normal fine-tuning process contributes to the difficulties people with autism have with language or face processing.
== References ==
Engert F. and T. Bonhoeffer Nature 399, 66-70 (1999) (2)
Maletic-Savatic M. et al. Science. 19, 1923-7 (1999) (3)
Kwon H.B. and B.L. Sabatini Nature 474, 100-4 (2011) (4)
Durand C.M. et al. Mol. Psychiatry Epub ahead of print (2011) (5)
Rakic P. et al. Science 232, 232-5 (1986) (6)