Dendritic spines

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 spines^2,3. 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.

Mutations in several genes strongly implicated in autism have resulted in altered dendritic spine morphology, including SHANK3⁵ and FMR1.

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.